Publications

Programmably structured plasma waveguide for development of table-top photon and particle sources. Programmable fabrication of longitudinal spatial structures in an optically preformed plasma waveguide in a gas jet was achieved, by using laser machining with a liquid-crystal spatial light modulator as the pattern mask. Fabrication of periodic structures with a minimal period of 200 lm and density-ramp structures with a minimal slope length of 100 lm was attained. The technique is useful for the optimization of various laser-plasma-based photon and particle sources.
Physics of Plasmas 19, 063109 (2012). [reprint]

A canonical approach to the action-angle formalism in quantum mechanics. In classical mechanics, the action and angle variables (J; ) can be found by integrating the momentum p with respect to the coordinate q under the constraint of energy conservation. Because it is not known how to extend Riemann integration to operator functions and variables, the classical method of action-angle formalism cannot be extended to quantum mechanics. We show that by using general quantum canonical transformations, one can transform (p; q) into (J; ), by which one of the conjugate variables in the Hamiltonian is eliminated. This algebraic integration by quantum canonical transformations gives not only the operator relations between (p; q) and (J; ), but also the eigenfunctions of the Hamiltonian and the eigenfunctions of the phase operator. These results offer a new point of view for the action-angle formalism in quantum mechanics.
Chinese Journal of Physics 49, 555 (2011). [reprint]

High-brightness optical-field-ionization collisional-excitation extreme-ultraviolet lasing pumped by a 100-TW laser system in an optically preformed plasma waveguide. Recent study on optical-field-ionization collisional-excitation extreme-ultraviolet lasing of Ni-like krypton at 32.8 nm pumped by a 100-TW laser system with an optically preformed plasma waveguide is reported. By using a 9-mm-long pure krypton plasma waveguide fabricated with the axicon-ignitor-heater scheme, the 32.8-nm extreme-ultraviolet laser provided an average output of 10¹² photons/pulse at pump energy of 1 J, more than one order of magnitude enhancement relative to the previous results with the same scheme at pump energy of 235 mJ. It is also found the far-field pattern of laser beams varies from a single peak profile at low pump energy to an annular profile at high pump energy due to over-ionization of krypton ions at the center of the plasma channel.
Applied Physics B 105, s00340 (2011). [reprint]

Enhancement of injection and acceleration of electrons in a laser wakefield accelerator by using an argon-doped hydrogen gas jet and optically preformed plasma waveguide. A systematic experimental study on injection of electrons in a gas-jet-based laser wakefield accelerator via ionization of dopant was conducted. The pump-pulse threshold energy for producing a quasi-monoenergetic electron beam was significantly reduced by doping the hydrogen gas jet with argon atoms, resulting in a much better spatial contrast of the electron beam. Furthermore, laser wakefield electron acceleration in an optically preformed plasma waveguide based on the axicon-ignitor-heater scheme was achieved. It was found that doping with argon atoms can also lower the pump-pulse threshold energy in this experimental configuration.
Physics of Plasmas 18, 063102 (2011). [reprint]

Relativistic birefringence induced by a high-intensity laser field in a plasma. Field-induced birefringence, also known as cross-polarization wave generation, has played an important role in ultrafast nonlinear optics. In this paper we analyze birefringence induced by relativistic collective motion of electrons driven by a high-intensity laser field. An analytical expression for the phase difference between the parallel and perpendicular polarizations of a weak probe pulse with respect to the polarization of a strong pump pulse as a function of intensity, density, and wavelengths is derived. It is shown that under typical experimental conditions of high-field physics, the effect is well above detection threshold. The analysis is compared with particle-in-cell simulations, and the agreement provides good support for the theory.
Physical Review A 83, 033801 (2011). [reprint]

Generation of intense ultrashort mid-infrared pulses by laser-plasma interaction in the bubble regime. As an intense laser pulse propagates through an underdense plasma, the strong ponderomotive force pushes away the electrons and produces a trailing plasma bubble. In the meantime the pulse itself undergoes extreme nonlinear evolution that results in strong spectral broadening toward the long-wavelength side. By experiment we demonstrate that this process can be utilized to generate ultrashort mid-infrared pulses with an energy three orders of magnitude larger than that was produced by crystal-based nonlinear optics. The infrared pulse is encapsulated in the bubble before exiting the plasma, hence is not absorbed by the plasma. The process is analyzed experimentally with laser-plasma tomographic measurements and numerically with 3-dimensional particle-in-cell simulation. Good agreement is found between theoretical estimation, numerical simulation, and experimental results.
Physical Review A 82, 063804 (2010). [reprint]

Seeding of a soft-x-ray laser in a plasma waveguide by high harmonic generation. A strongly saturated waveguide-based optical-field-ionization soft-x-ray laser seeded by high harmonic generation was demonstrated for Ni-like Kr lasing at 32.8 nm. Compared with the same laser seeded only with spontaneous emission, seeding with high harmonics yields much smaller divergence, enhanced spatial coherence, and controlled polarization. The integration of high harmonic seeding, optically preformed plasma waveguide, and optical-field-ionization pumping forms one of the optimal archetypes of an ultrashort-pulse soft-x-ray laser.
Optics Letters 34, 3562 (2009). [reprint]

Relativistic optical rectification driven by a pulsed Gaussian beam. Relativistic optical rectification is analyzed by utilizing the analytical solutions of the three-dimensional collective electron motion driven by a high-intensity pulsed Gaussian beam. Under the paraxial expansion the slowly varying components in the nonlinear oscillation of electron density and electric current are extracted from the solutions first, then they are used in the Maxwell equations as the source terms to calculate the terahertz electromagnetic fields as functions of the spatiotemporal amplitude of the pulsed Gaussian beam. The analysis compares well with available experimental data and computer simulations, and demonstrates the general scheme of paraxial expansion for the analysis of relativistic nonlinear optical interactions.
Physical Review A 80, 023802 (2009). [reprint]

Reducing radiation exposure in intra-medullary nailing procedures: Intra-medullary endo-transilluminating (iMET). The purpose of the study was to reduce the level of radiation exposure during intra-medullary nailing procedures. A visible light sourcewas inserted into the medullary bone cavity in order to detect the distal interlocking screw holes. The light penetrates out of the bone surface, revealing the position of the screw hole, and this allows the subsequent drilling and placing of the interlocking screw to be free of fluoroscopy. Among the 19 consecutive tibia-fracture patients recruited for this study, no repetition of the drilling procedure or insertion of a transverse interlocking screw was needed. The average time to finish the insertion of one distal interlocking screw was 4.1 1.8 min. It was extrapolated that 13–41% of previous radiation exposure levels could be saved. The non-fluoroscopic approach thus decreases the health hazards that the patients are experiencing as well as those of the surgical team who need to perform such intra-medullary nailing operations on a routine basis.
Injury 40, 1084 (2009). [reprint]

Single-shot soft x-ray digital holographic microscopy with an adjustable field of view and magnification. Single-shot digital holographic microscopy with an adjustable field of view and magnification was demonstrated by using a tabletop 32.8 nm soft-x-ray laser. The holographic images were reconstructed with a twodimensional
fast-Fourier-transform algorithm, and a new configuration of imaging was developed to overcome the pixel-size limit of the recording device without reducing the effective NA. The image of an atomicforce-microscope cantilever was reconstructed with a lateral resolution of 480 nm, and the phase contrast image of a 20 nm carbon mesh foil demonstrated that profiles of sample thickness can be reconstructed with few-nanometers uncertainty. The ultrashort x-ray pulse duration combined with single-shot capability offers great advantage for flash imaging of delicate samples.
Optics Letters 34, 623 (2009). [reprint]

Applications of laser-fabricated plasma structures in laser-wakefield accelerators, x-ray lasers, and plasma nonlinear optics. A general method for fabricating transient plasma structures with high-intensity laser pulses is developed to gain fine control over laser-plasma interactions. These structures have been used as programmable photonic devices in the development of laser-wakefield accelerators, soft X-ray lasers, and plasma nonlinear optics driven by multi-terawatt laser pulses. Plasma ramps are used to control electron injection in laser-wakefield accelerators, plasma waveguides are used to enhance the efficiency of soft X-ray lasers by orders of magnitude, and periodic plasma structures are used to achieve quasi-phase matching in relativistic harmonic generation. By scanning the interaction length with the same plasma-fabrication method, tomographic measurements are carried out to resolve the injection/acceleration process in laser-wakefield accelerators and amplification processes in X-ray lasers and relativistic harmonic generation. A theoretical analysis and a computer simulation are also carried out to provide insightful pictures of these processes. These research works show that by controlling plasma structures with optical fabrication methods, laser-plasma interaction can be engineered to expand and enrich the frontier of high-field physics.
Journal of the Korean Physical Society 53, 3719 (2008). [reprint]

Backward Raman Amplification in a Plasma Waveguide. Backward Raman amplification of a short laser pulse in a plasma waveguide is demonstrated. With a guided seed pulse of 0.8-uJ energy and a pump pulse of 345-mJ energy in a 9-mm-long optically-preformed plasma waveguide, 910-fold energy amplfication is achieved. Heating of the plasma by the long pump pulse is identi¯ed to be a key issue for plasma-waveguide-based backward Raman amplifiers.
Physical Review Letters 101, 065005 (2008). [reprint]

Invariance of the canonical quantization prescription under classical canonical transformations. Dirac's postulate of canonical quantization, [ˆpi, ˆqj ] = −i¯h ij for conjugate canonical variables, has been the most concise and general prescription on how to quantize a classical system. Since classical systems described by variables connected with canonical transformations are equivalent, [ˆpi, ˆqj ] = −i¯h ij must remain invariant under classical canonical transformations. This invariance has not been proved except for the limited class of cascaded infinitesimal transformations. In this paper it is shown that if ( ˆ Pi, ˆQj) are related to (ˆpi, ˆqj)
by a classical canonical transformation, then [ˆpi, ˆqj ] = −i¯h ij implies [ ˆ Pi, ˆQj ] = −i¯h ij . In other words, the canonical quantization prescription is invariant for variables connected with classical canonical transformations.
Chinese Journal of Physics 45, No. 4, 425 (2007). [reprint]

Three-dimensional analysis of collective relativistic electron motion and coherent harmonic generation driven by an intense pulsed Gaussian beam. Collective electron motion driven by intense laser pulses can become highly nonlinear as results of the vXB term in the Lorentz force and the relativistic mass increase effect. These nonlinear effects are the bases of many nonlinear phenomena in high-field physics. Using paraxial expansion we analyzed the three-dimensional collective relativistic motion of electrons driven by an intense pulsed Gaussian beam. We solved the threedimensional velocity field and density field of the electrons, as well as the scalar potential field resulting from the electron density modulation, in terms of the vector potential of the laser pulse in the highly relativistic regime. From the solutions we derived an analytical expression for the nonlinear oscillation of the electric current density. The results are applied to a systematical analysis of coherent relativistic harmonic generation as a demonstration of the applications. Extensive comparison with published experimental data is made, and the remarkable agreement provides a strong support to the analysis presented in this paper.
Physical Review A 76, 063815 (2007). [reprint]

Optical-field-ionization collisional-excitation x-ray lasers with optically preformed plasma waveguide. Optical-field-ionization x-ray lasers in an optically preformed plasma waveguide for pure xenon, krypton, and argon gases, respectively, are achieved. In addition to the 46.9 nm main lasing line for Ne-like argon, the 45.1 and 46.5 nm lasing lines are also observed, indicative of the strong enhancement effect and the large gas density in the plasma waveguide. With this technique multispecies parallel x-ray lasing is also demonstrated in a Kr-Ar mixed-gas waveguide. Extensive experimental results including the pump-energy dependence, the density dependence, and the effects of parameters that control the waveguide fabrication are reported and discussed.
Physical Review A 76, 053817 (2007). [reprint]

Enhancement of high-harmonic generation by laser-induced cluster vibration. Sharp periodic enhancement of high-harmonic generation synchronized to prepulse-induced cluster vibration is observed. Ten-fold enhancement of high-harmonic-generation effciency is achieved by optimal selection of the prepulse-pump delay. This strong correlation between high-harmonic generation and cluster vibration also provides a new tool for studying the vibrational dynamics of nanometer atomic clusters.
Optics Letters 32, 2714 (2007). [reprint]

Dramatic enhancement of optical-field-ionization collisional-excitation x-ray lasing by using an optically preformed plasma waveguide. Dramatic enhancement of optical-¯eld-ionization collisional-excitation x-ray lasing is achieved by using an optically-preformed plasma waveguide. With a 9-mm-long pure krypton plasma waveguide prepared by using the axicon-ignitor-heater scheme, lasing at 32.8 nm is enhanced by 400 folds relative to the case without the plasma waveguide. An output level of 8 X 10¹⁰ photon/shot is reached at an energy conversion efficiency of 2 x 10^-6. The same method is used to achieve x-ray lasing in a gas jet for the high-threshold low-gain transition at 46.9 nm in neon-like argon.
Physical Review Letters 99, 063904 (2007). [reprint]

Degenerate four-wave mixing mediated by ponderomotive-force-driven plasma gratings. Degenerate four-wave mixing mediated by ponderomotive-force-driven plasma gratings is demonstrated in the near-infrared regime. The quadratic dependence of the reflectivity of the probe pulse on plasma density indicates that the mixing is caused by quasi-neutral plasma grating driven by laser ponderomotive force. The experiment verifies that ponderomotive force is an effective means to produce a large-amplitude short-period plasma grating, which has many important applications in ultrahigh-intensity optics. In particular, such a grating is a crucial element for the development of plasma phase-conjugate mirrors that can be used to restore wavefront distortion ubiquitous in nonlinear propagation.
Physical Review E 75, 036403 (2007). [reprint]

Production of a monoenergetic electron bunch in a self-injected laser-wakefield accelerator. Production of a monoenergetic electron bunch in a self-injected laser-wake¯eld accelerator is inves-tigated with a tomographic method which resolves the electron injection and acceleration processes. It is found that all the electrons in the monoenergetic electron bunch are injected at the same location in the plasma column and then accelerated with an acceleration gradient exceeding 2 GeV/cm. The injection position shifts with the position of pump-pulse focus, and no signi¯cant deceleration is observed for the monoenergetic electron bunch after it reaches the maximum energy. The results are consistent with the model of transverse wave-breaking and beam loading for the injection of monoenergetic electrons. The tomographic method adds a crucial dimension to the whole array of existing diagnostics for laser beam, plasma wave, and electron beam. With this method the details of the underlying physical processes in laser-plasma interaction can be resolved and compared directly to particle-in-cell simulations.
Physical Review E 75, 036402 (2007). [reprint]

Optimization of laser propagation in optical-field-ionization plasmas for X-ray laser generation. Optical-field-ionization X-ray lasers are investigated numerically with a three-dimensional wave propagation code considering the effects of photoionization, energy depletion due to ionization, and refraction on pump laser pulses in spatially and temporally varied plasmas. By focusing the pump laser with small f-number optics at the optimal position, simulations show that diffraction and ionization-induced refraction in plasmas are compensated to keep the pump beampropagating at the optimal size for a longer distance. An amplification length as long as 5mm can be achieved in Pd-like xenon and Ni-like krypton X-ray lasers at a pump energy of 160 mJ in 50-fs and 30-fs pulses, respectively. The significant reduction of the pump energy is a desirable step toward low-threshold and practical high-repetition-rate operations.
Applied Physics B 86, 25 (2007). [reprint]

Enhancement of relativistic harmonic generation by an optically-preformed periodic plasma waveguide. Enhancement of relativistic third harmonic generation by using an optically-preformed periodic plasma waveguide was achieved. Resonant dependence of harmonic intensity on plasma density and density modulation parameters was observed, which is a distinct characteristic of quasi-phase matching. The results demonstrate the potential of a modulated plasma waveguide in high-field applications.
Physical Review Letters 98, 033901 (2007). [reprint]

Programmable fabrication of spatial structures in a gas jet by laser machining with a spatial light modulator. Programmable fabrication of longitudinal spatial structures in a gas jet was achieved by using laser machining with a liquid-crystal spatial light modulator as the pattern mask. By this technique single-shot fabrication of arbitrary gas/plasma structures is demonstrated, which establishes the crucial step toward raising the designs and applications of high-¯eld plasma devices to the level of adaptive feedback optimization.
Physics of Plasmas 13, 110701 (2006). [reprint]

Transverse oscillation in laser cavities. Recent experiments on lasers with large Fresnel numbers have demonstrated a close correspondence between the transverse degrees of freedom _transverse modes_ and the two-dimensional harmonic oscillator. Mode patterns that correspond to classical trajectories have been observed. These experiments suggest that such systems may serve as convenient platforms for studying quantum-classical correspondence. But why the paraxial wave equation for the laser leads to two-dimensional harmonic oscillation for the transverse degrees of freedom is yet to be clarified. We use two methods to show that such a correspondence is not a mathematical coincidence: the focusing mirror that stabilizes the laser gives rise to the transverse oscillation, and the reduced equation that governs the transverse degrees of freedom is the Schrödinger equation for the harmonic oscillator. For the fundamental Gaussian mode, the theory remains valid in the weakly nonlinear limit.
Physical Review A 73, 033804 (2006). [reprint]

Constructing Green functions of the Schrodinger equation by elementary transformations. The initial value problem in quantum mechanics is most conveniently solved by the Green function method. Instead of the conventional methods of eigenfunction expansion and path integration, we present a new method for constructing the Green functions systematically. By using suitable elementary transformations, one of the conjugate variables in the Hamiltonian can be eliminated and the Green function for the simplified Hamiltonian can be easily derived. We then obtain the Green function for the original Hamiltonian by the reverse sequence of the elementary transformations. The method is illustrated for the linear potential, the harmonic oscillator, the centrifugal potential, and the centripetal barrier oscillator.
American Journal of Physics 74, 600 (2006). [reprint]

Experimental investigation of the parameter space for optical-field-ionization cluster-jet x-ray lasers. Optical-field-ionization collisional-excitation x-ray lasers in xenon and krypton clustered gas jets were experimentally investigated in detail. Dependence of x-ray lasing on atom density, laser focus position, and laser polarization ellipticity was mapped out to find the optimal lasing conditions, and a tomographic measurement technique was used to provide clear pictures of the x-ray amplification process.
Physical Review A 74, 023804 (2006). [reprint]

Tomography of the injection and acceleration processes that produce a monoenergetic electron beam in a laser wakefield accelerator. A tomographic diagnosis method was developed to systematically resolve the injection and acceleration processes of a monoenergetic electron beam in a laser wakefield accelerator. It was found that all the monoenergetic electrons are injected at the same location in the plasma column and accelerated from 5 MeV to 55 MeV energy in 200-μm distance. This is a direct measurement of the real acceleration gradient in a laser wakefield accelerator, and the experimental data are consistent with the model of transverse wave-breaking and beam loading for monoenergetic electron injection.
Physical Review Letters 96, 095001 (2006). [reprint]

Tomography of high harmonic generation in a cluster jet. Tomographic measurement of high harmonic generation in a cluster jet was demonstrated by programming the cluster density distribution with a laser machining technique. The growth of harmonic energy with the propagation of the pump pulse was resolved by scanning the end of the argon cluster distribution in the path of the pump pulse. A downstream shift of the position of rapid growth and decrease of slope with increasing backing pressure as results of changes in the phase matching condition were observed, which explains the presence of an optimal backing pressure.
Optics Letters 31, 984 (2006). [reprint]

Characterization and control of plasma density distribution for the development of solid-target x-ray lasers. By using deflectometry of a longitudinal probe pulse and reflective interferometry of a transverse probe pulse to resolve the spatiotemporal distribution of the preformed plasma, we characterize and control the plasma density distribution near the target surface for the development of solid-target x-ray lasers. We show that the use of prepulses in an ignitor-heater scheme can increase the scale length of the preformed plasma and how the effect varies with target materials. Many important issues crucial to x-ray lasing such as electron density distribution, electron temperature, and the optimal timing between pumping pulses can be resolved with these methods.
Physical Review E 72, 026407 (2005). [reprint]

Fabrication of periodic transient-density structures in a gas jet with a boundary scale length approaching 10 μm was demonstrated. This was achieved by passing an ultrashort high-intensity laser pulse through a patterned mask and imaging the mask onto the target plane. Gas/plasma density at the laser-irradiated regions drops as a result of hydrodynamic expansion following ionization and heating by the laser pulse. The fabrication of gas/plasma density structures with such a scheme is an essential step in the development of plasma photonic devices for applications in high-field physics.
Physics of Plasmas 12, 070707 (2005). [reprint]

Collisional-excitation soft x-ray laser pumped by optical-field ionization in a cluster jet. An optical-field-ionization soft x-ray laser using a clustered gas jet was demonstrated. Pd-like xenon lasing at 41.8 nm with 95-nJ pulse energy and 5.2-mrad divergence was achieved, indicating near-saturation ampli-fication. By using a prepulse to control the expansion of ionized clusters, it was found that the microscopic uniformity of the plasma is essential for efficient lasing. In addition, the optimal atom density for maximum lasing reported previously is verified to result from the tradeoff between large gain coefficient and short gain length due to ionization-induced refraction.
Physical Review A - Rapid Communication 71, 061804(R) (2005). [reprint]

Spatially localized self-injection of electrons in a self-modulated laser wakefield accelerator by using a laser-induced transient density ramp. By using a laser-induced transient density ramp, we demonstrate self-injection of electrons in a self-modulated laser-wakefield accelerator with spatial localization. The number of injected electrons reaches 1.7x10⁸. The transient density ramp is produced by a prepulse propagating transversely to drill a density depression channel via ionization and expansion. The same mechanism of injection with comparable efficiency is also demonstrated with a transverse plasma waveguide driven by Coulomb explosion.
Physical Review Letters 94, 115003 (2005). [reprint]

Super-resolution bright-field optical microscopy based on nanometer topographic contrast. By using an expectation-maximization maximum likelihood estimation algorithm to improve the lateral resolution of a recently developed non-interferometric wide-field optical profilometer, we obtain super-resolution bright-field optical images of nanometer features on a flat surface. The optical profilometer employs a 365-nm light source and an ordinary objective lens of a 0.95 numerical aperture. For objects of 100 nm thickness, lateral features about l/7 can be resolved in the restored images without fluorescence labeling. Current image acquisition rate is 0.1 frame/sec, which is limited by the brightness of the light source. With a brighter light source, the imaging speed can be fast enough for real-time observation of dynamic activities in the nanometer scale.
Microscopy Research and Technique 65, 180 (2004). [reprint]

A versatile 10-TW laser system with robust passive controls to achieve high stability and spatiotemporal quality. In this paper we discuss the design, construction, and output characteristics of a versatile 10-terawatt Ti:sapphire laser system of high stability and spatiotemporal quality. By pumping the three amplifier stages independently and running at saturation, an energy stability of 1.3% is obtained. Controls over self-phase modulation, high-order dispersion, spatial aberration, and amplified spontaneous emission are done by robust passive methods. Time-bandwidth product of 1.2 times the Fourier transform limit with a temporal contrast larger than 5x10^8 in the -10-ns scale, 2x10^6 in the -100-ps scale, and 10^4 within the -1-ps scale is achieved. The beam can be focused down to 1.2 times of the diffraction limit with 80% energy enclosed in the Gaussian focal spot. Beam pointing stability is <13 microrad. Such high stability and spatiotemporal quality have made possible precision control over extremely nonlinear laser-plasma experiments, and the capability of computerized independent control of prepulse, pump pulse, proble pulse, and on-line diagnoses have made this system highly versatile and reliable.
Applied Physics B 79, 193 (2004). [reprint]

Efficient generation of extended plasma waveguides with the axicon ignitor-heater scheme. An efficient method for generating extended plasma waveguides is developed by using the axicon lens in conjunction with the ignitor-heater scheme. The short-pulse ignitor generates the seed electrons by multiphoton ionization and the long-pulse heater expands the plasma by inverse bremsstrahlung heating and builds up the plasma density barrier by collisional ionization. A 1.2-cm-long plasma waveguide is generated in pure Ar gas with a total energy of only 100 mJ. Evolution of the plasma density profile is measured by time-resolved interferometry to show the waveguide forming process and how it can be optimized.
Physics of Plasmas11, L21 (2004). [reprint]

Control of laser beam propagation and absorption in a nanoplasma gas by programming of transient complex refractive index with a prepulse. By utilizing the intensity- and duration-dependent heating and expansion rate of nanoplasma to generate transient transverse gradient of refractive index, prepulse controlled laser beam propagation is demonstrated. The dynamical response of the macroscopic optical refractive index is traced back to the microscopic polarizability of nanoplasmas experimentally, in accordance with hydrodynamic nanoplasma models. In particular, delay between the prepulse and the main pulse for maximum Rayleigh scattering is found to be longer than that for maximum x-ray emission, supporting the more refined one-dimensional self-consistent hydrodynamic nanoplasma model.
Physical Review E 69, 035403(R) (2004) [reprint]

Optically controlled seeding of Raman forward scattering and injection of electrons in a self-modulated laser wakefield accelerator. Optical seeding of plasma waves and injection of electrons are key issues in self-modulated laser wakefield accelerators. By implementing a copropagating laser prepulse with proper timing, we are able to control the growth of Raman forward scattering and the production of accelerated electrons. The dependence of the Raman intensity on prepulse timing indicates that the seeding of Raman forward scattering is dominated by the ionization-induced wakefield, and the dependence of the divergence and number of accelerated electrons further reveals that the stimulated Raman backward scattering of the prepulse plays an essential role of injecting hot electrons into the fast plasma wave driven by the main pulse.
Physical Review Letters 92, 075003 (2004) [reprint]

Prepulse controlled splitting of relativistically self-guided channel and suppression of Raman forward scattering instability. The effects of laser prepulse on splitting of a relativistically self-guided channel and suppression of Raman forward scattering instability in the propagation of an intense ultrashort laser pulse in an underdense plasma are studied. They are resolved by using probing interferometry, shadowgraphy, and spectrometry. By changing the prepulse intensity, the propagation of the laser beam can be controlled to show self-guiding or channel splitting. The effect of prepulse on Raman forward scattering instability shows that the instability is significantly reduced if the gas target is preionized, identifying ionization wakefield as the seeding mechanism for this process.
Physics of Plasmas 11, 1173 (2004) [reprint]

Fabricating high-aspect-ratio sub-diffraction-limit structures on silicon with two-photon photopolymerization and reactive ion etching. We fabricate sub-micrometer objects with feature sizes about one third of the exposure wavelength using two-photon photopolymerization in an epoxy-based photoresist SU-8. Owing to the high mechanical strength of this photoresist, an aspect ratio as high as 9 is achieved with a 200-300 nm lateral dimension. A simple equation is used to estimate the feature size from the laser parameters such as spot size, exposure time, pulse width, pulse repetition rate, and the material properties including the two-photon absorption coefficient and the exposure threshold dose. Patterns in SU-8 are transferred onto silicon using reactive ion etching, preserving both the feature size and aspect ratio. Vertical sidewalls of the transferred patterns are achieved using the black silicon method.
Applied Physics A 79, 2027 (2004). [reprint]

High-brightness soft x-ray generation under optimized laser-cluster interaction. High-brightness soft x-ray emissions are obtained by optimizing the interaction between high-intensity ultrashort optical pulses and Ar clusters. With only 100-mJ pump energy, the yield at 13.8 nm with 0.1-nm bandwidth reaches 0.3 mJ/pulse, while the yield in the spectral range of 11-20 nm reaches 12 mJ/pulse in 4p solid angle. The relation between optimal pulse durations and cluster sizes are found to conform with the nano-plasma expansion model, while the optimal pump beam size is related to the pulse contrast by the condition that prepulse ionization is sufficiently weak to preserve the local plasma density.
Optics Communications 231, 375 (2004) [reprint]

Sub-diffraction-limit imaging based on the topographic contrast of differential confocal microscopy. Using the nanometer depth sensitivity of differential confocal microscopy, we detect surface features of lateral dimensions smaller than the diffraction limit without fluorescence labeling. The lateral resolution of the topographic images is further enhanced by a maximum-likelihood estimation algorithm. Based on the comparison of signal and noise at high spatial frequency, we estimate the best lateral resolution of the enhanced images to be 0.15l. In addition, on composite samples this technique can simultaneously display sub-diffraction-limit topographic features and reflectivity heterogeneity.
Optics Letters 28, 1772 (2003) [reprint]

Non-interferometric wide-field optical profilometry with nanometer depth resolution. Applying the principle of differential confocal microscopy to wide-field optically sectioning microscopy, we develop a noninterferometric optical profilometer without scanning mechanisms. Depth resolution of 2 nm is achieved with a power-regulated tungsten-halogen lamp as the light source and a 14-bit CCD camera as the detector. The effects of inhomogeneous surface reflectivity are removed from topographic measurements by arithmetic division. The whole system can be constructed on a single silicon chip for use as a miniaturized optical profiler.
Optics Letters 27, 1773 (2002) [reprint]

Dependence of relativistic self-guiding and Raman forward scattering on duration and chirp of an intense laser pulse propagating in a plasma. Relativistic self-guiding and Raman forward scattering of an intense ultrashort laser pulse propagating in an underdense plasma were studied using a terawatt Ti:sapphire laser system. The dependence of these processes on the duration and frequency chirp of the laser pulse was investigated by detuning the pulse compressor. It was observed that the efficiency of Raman forward scattering is enhanced for a positively chirped pulse and diminished for a negatively chirped pulse. In addition, as a result of the dependence on pulse duration and peak power, an optimal duration for Raman forward scattering was found for fixed pulse energy and spectral bandwidth. On the other hand, relativistic self-guiding of the laser pulse was affected by the pulse duration while no dependence on the chirp was observed.
Physics of Plasmas 9, 391 (2002) [reprint]

Investigations of transient Ne-like Fe x-ray lasers pumped by femtosecond laser system. Detailed simulations of a Ne-like Fe X-ray laser pumped by a femtosecond laser system are presented. In this study, we have investigated an efficient multiple pumping pulse configuration including two 150 ps prepulses and one 100 fs main pulse for transient X-ray lasers. The influences of pumping pulse delay, duration, and intensity are studied. Simulation results suggest that a high repetition rate and high gain tabletop X-ray laser at 255 Angstrom can be achieved with a total pumping energy less of than 0.4 J.
Japanese Journal of Applied Physics 40, 2282 (2001) [reprint]

Integration of the Schröedinger equation by canonical transformations. Owing to the operator nature of the quantum dynamical variables, classical canonical transformations for integrating the equation of motion cannot be extended to the quantum domain. In this paper, a general procedure is developed to construct the sequences of quantum canonical transformations for integrating the Schröedinger equations. The sequence is made of three elementary canonical transformations that constitute a much larger class than the unitary transformations. In conjunction with the procedure, we also developed a factorization technique that is analogous to the method of integration factor in classical integration. For demonstration, with the same procedure we integrate nine nontrivial models, including the centripetal barrier potential, the Kratzer's molecular potential, the Morse potential, the Pöschl-Teller potential, the Hulthén potential, etc.
Physical Review A 65, 012104 (2001) [reprint]

Using differential confocal microscopy to detect the phase transition of lipid vesicle membranes. We use differential confocal microscopy, a far-field optical profilometry with 2-nm depth resolution, to monitor the thermal fluctuations and the deformation of the bilayer membranes of lipid vesicles. From the linear relation between the mean-square amplitudes of fluctuations and temperatures we can directly determine the phase-transition temperatures of lipid bilayers. We then employ femtonewton optical force to induce sub-micrometer deformation of the vesicle membranes. From the deformation we obtain the bending rigidity of membranes with a simple geometric analysis. The bending modulus changes by an order of magnitude as the temperature is changed across the transition temperature, such that we can detect the phase transition behaviors of the bilayer structures.
Opt. Eng. 40, 2077 (2001) [reprint]

All-optical measurements of the bending rigidity of lipid-vesicle membranes across structural phase transitions. By exploiting the nanometer sensitivity of the confocal response to the position of an in-focus reflecting surface, we measured the bending rigidity of lipid-bilayer vesicles with a noninvasive all-optical method. The vesicles were weakly deformed with femtonewton optical force, and the bending rigidity was measured continuously from the La through the Pb' to the Lb' phases on the same specimen for the first time. The bending modulus is found to increase by an order of magnitude from the La phase to the Lb' phase, as a result of the increasing area-compressibility modulus and bilayer thickness. The dips of bending modulus give precisely the main-transition and pretransition temperatures, which supports the recently proposed chain-melting model of pretransition.
Phys. Rev. E 64, 020901(R) (2001) [reprint]

Femtosecond self-focusing dynamics measured by three-dimensional phase-retrieval cross correlation. We use a phase-retrieval cross-correlation technique to analyze the spatiotemporal field evolution of self-focused ultrashort pulses. The technique features a new phase-retrieval algorithm based on functional differentiation. Its sensitivity, rapid convergence, and temporal nonreciprocality enable reliable three-dimensional waveform reconstruction. At less than the critical power, the experiments verify conventional description of self-focusing and give a direct proof of the Kerr-lens mode-locking mechanism. In contrast, for pulses with peak power much more than the critical power, nearly uniform self-focusing and quasi-stable single-filament trapping to a universal beam diameter were observed. The trapping can be explained by the saturation of the nonlinear refractive-index change at delta n~7x10^-5. The saturation is verified by an independent cross-polarization modulation measurement.
J. Opt. Soc. Am. B 17, 1626 (2000) [reprint]

Deconvolution of local surface response from topography in nanometer profilometry with a dual-scan method. In profilometric measurements, by scanning the sample twice with a fixed vertical offset, one can separate the signal that comes from surface heterogeneity from the topographic signal. Using differential confocal microscopy, a newly developed open-loop nanometer profilometric technique, we demonstrated this dual-scan method on composite samples and obtained 10-nm depth resolution. This technique can also be applied to other profilometric techniques such as atomic force microscopy and scanning tunneling microscopy.
Optics Letters 24, 1732 (1999) [reprint]

Population diffusion and equipartition in quantum systems of many degrees of freedom. In the interaction picture, population transfer among coupled degrees of freedom is greatly enhanced by resonances. We show that statistically the number of resonances increases rapidly with degrees of freedom, changing the characteristic of population transfer from being bounded to diffusive. From the diffusion rate we derive simple expressions for the time scales of energy relaxation and equipartition. These expressions are supported by a wide range of experimental data. The analysis elucidates quantitatively the dependence of equipartition on resonances.
Physical Review Letters 80, 3682 (1998) [reprint]

Optical measurement of the viscoelastic and biochemical responses of living cells to mechanical perturbation. We have developed an optical method for real-time monitoring of cellular motion in a natural environment with nanometer resolution. From the motion driven by small optical forces, we measured dynamic viscoelastic responses of living cells in the linear reversible region. Cytoplasmic gel-to-sol transition that was due to the disruption of the actin-filament framework was detected, and a linear release of Ca²⁺ from intracellular storage that was related to submicrometer cell deformation was observed. The method was shown to be a powerful tool for studying the natural response of cells to mechanical perturbation.
Optics Letters 23, 307 (1998) [reprint]

Dynamic model of multipass ultrashort-pulse laser amplifiers and its application. We developed a dynamic model for evaluating the gain and amplified spontaneous emission of multipass ultrashort-pulse laser amplifiers. This model takes into account the time dependence of the population inversion due to the time-varying pump power and the evolution of the amplified spontaneous emission. For gain media of a short upper-state lifetime, a time-dependent analysis is essential for the model to extend beyond order-of-magnitude estimation. We verified the model with a femtosecond dye laser amplifier. The calculated output energy is accurate to within 5% of the experimental data. We utilized this model to optimize the conversion efficiency of the dye laser amplifier under low-energy pumping. An efficiency of 2.3% is achieved with an optimal pump energy of only 4 microjoules.
Applied Optics 36, 7802 (1997) [reprint]

Noninterferometric differential confocal microscopy with 2-nm depth resolution. By utilizing the sharp slopes of the axial response curve of confocal imaging, we demonstrate a differential confocal technique for surface imaging with depth resolution as great as 2 nm. Neither servo feedback loop nor lock-in detection are used, hence measurements can be performed at high speed with long working distance. Because the technique uses no interferometric effects, it offers large open-loop dynamic range and is compatible with fluorescence microscopy.
Optics Communications 135, 233 (1997) [reprint]

Energy diffusion due to nonlinear perturbation on linear Hamiltonians. In nonintegrable Hamiltonian systems, energy initially localized in a few degrees of freedom tends to disperse through nonlinear couplings. We analyze such processes in systems of many degrees of freedom. As a complement to the well-known Arnold diffusion, which describes energy diffusion by chaotic motion near separatrices, our analysis treats another universal case--coupled small oscillations near stable equilibrium points. Because we are concerned with the low energy regime, where the nonlinearity of the unperturbed Hamiltonian is negligibly small, existing theories of Arnold diffusion cannot apply. Using probability theories we show that resonances of small detuning, which are ubiquitous in systems of many degrees of freedom, make energy diffusion possible. These resonances are the cause of energy equipartition in the low energy limit. From our analysis, simple analytic equations which relate the energy, the degrees of freedom, the strength of nonlinear coupling, and the time scale for equipartition emerge naturally. These equations reproduce results from large-scale numerical simulations with remarkable accuracy.
Physical Review E 54, 4657 (1996) [reprint]

Femtosecond transform-limited Kerr-lens mode-locked dye lasers. Using SF6 glass plates as intracavity Kerr lenses and double-prism pairs for dispersion compensation, we achieve tunable femtosecond passive mode locking in rhodamine 590 (R6G& and 4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM) dye lasers. The R6G laser produces transform limited 240-500 fs pulses between 577-606 nm, and the DCM laser produces 150 fs transform-limited pulses between 650-671 nm. We use dilute intracavity saturable-absorber jets to make the mode locking self-starting. Characteristics of the pulses and the stability regions of the lasers agree with general theories of passive mode locking.
Optical and Quantum Electronics 28, 1443 (1996)

Imaging through animal tissues with cw diode laser based broadband interferometry. We use a cw superluminescent laser diode, a CCD camera, and broadband interferometry to image millimeter-size objects hidden in 15-mm chicken muscle, and demonstrate that the resolution and penetration depth is comparable to that obtained with femtosecond lasers. Coherent images are recovered from the diffused background by selectively homodyne amplifying the "least scattered light" and by momentum-space filtering. A scattering rejection ratio as large as 1.1e11 (25 mean-free-paths) is achieved. We also investigated the limit of spatial resolution of our method in the diffusive region by random-phase path integration. A scaling relation among the resolution, the penetration depth, and the coherence length of the light source is derived and verified by experiments.
Optics Communications 130, 317 (1996)

Off-axial pulse propagation in graded-index materials with Kerr nonlinearity--a variational approach. We analyze off axial, self-focused pulse propagation in graded-index Kerr materials with the variational method. Equations which determine the characteristics of the beam, namely the optical path, the wavefront curvature, the beam width, and the pulse duration are derived and solved. We show that continuously adjustable negative group-velocity dispersion can be generated by off-axial propagation, and for pulse energy smaller than a critical value, negative group-velocity dispersion and Kerrr nonlinearity warrant the existence of stable spatio-temporal solitary pulses.
Optics Communications 128, 145 (1996)

Tunable multiwavelength synchronized femtosecond pulse trains for ultrafast spectroscopy. By using femtosecond optical techniques including self-phase modulation, spectral filtering and dispersion compensation, we constructed a tunable multiwavelength synchronized femtosecond light source from a cw modelocked femtosecond Ti:sapphire laser, and experimentally characterized its performance. Synchronized femtosecond pulses of different central frequencies can be selected independently within a 60-nm FWHM band. Pulse duration can also be programmed from 200 fs to 100 fs by controlling the bandwidth. Synchronization between pulses of different wavelengths are experimentally shown to be within a few femtoseconds. We demonstrate applications of the light source in ultrafast pump-probe spectroscopy with a sensitivity as great as 1/100000 in transient transmission measurements.
Optics Communications 124, 505 (1996)

Experimental study on the geometric group-delay dispersion in graded-index media. We use the highly sensitive method of spatially coherent white-light interferometry to measure the geometric group-delay dispersion in graded-index media. Our measurements confirm previous analyses that the geometric dispersion produced by off-axis beam propagation is anomalous (negative), hence may be utilized to eliminate the ubiquitous positive dispersion in optical systems. Fabrication of large index-gradient waveguides optimal for dispersion compensation is discussed.
Applied Optics 35, 2610 (1996)

Spatio-temporal solitary pulses in graded-index materials with Kerr nonlinearity. We analyze spatio-temporal pulse propagation in graded-index Kerr materials with the variational method, and find that if the path of propagation provides negative group-velocity dispersion, stable solitary pulses exist for pulse energy smaller than a critical value. Spatial and temporal degrees of freedom cannot be analyzed separately due to their coupling through the Kerr nonlinearity. Our unified analysis not only elucidates the hidden coupling, but also clarifies relations between parameters of the solitary pulse. The analysis is verified by direct simulation of the paraxial wave equation.
Optics Communications 119, 167 (1995)

Imaging and profiling surface microstructures with noninterferometric confocal laser-feedback. Exploiting the sensitivity and the self-aligning feature of the confocal laser-feedback technique and the convenience of superluminescent laser diodes, we developed an optical method for imaging and profiling surface microstructures with a depth resolution as great as 20 nm. The incoherent, noninterferometric nature of the technique enables fast open-loop operation and large dynamic range. Measurements of calibrated semiconductor surface microstructures and optical ridge waveguides are demonstrated.
Applied Physics Letters 66, 2022 (1995) [reprint]

Characteristics of a femtosecond transform-limited Kerr-lens mode-locked dye laser Using an SF6 glass plate as the intracavity Kerr medium and a double-prism pair for dispersion compensation, we developed a femtosecond transform-limited passively mode-locked dye laser. Self-starting mode locking is achieved with a dilute intracavity TCVEBI saturable-absorber jet. Within a 50% power drop the tuning range is 577-606 nm. Pulse characteristics of the laser agree with theoretical predictions based on the Ginzburg-Landau equation.
Optics Letters 19, 975 (1994) [reprint]

Measurements of the self-starting threshold of Kerr-lens mode locking lasers. We measured the self-starting threshold of passive Kerr-lens mode locking dye lasers and Ti:sapphire lasers by varying the concentration of the intracavity dilute dye saturable absorbers which start the mode locking. From the threshold absorber concentration, we determined the strength of the intracavity pulse broadening effects which counteract the pulse shortening mechanisms in the self-starting stage. Experimental results agree well with theories based on phase diffusion and mode-pulling.
Optics Letters 19, 566 (1994) [reprint]

Nanometer-resolution distance measurement with a noninterferometric method. We developed a noninterferometric technique for high-resolution distance measurement (optical ranging). The technique utilizes the fact that the wavelength of a broadband cw laser can be changed by external feedback. By placing the ranging target near the focal point of a microscope objective, we make the feedback from the target--hence the laser wavelength, v ry sensitive to the target position. The target position is determined from the laser wavelength with great resolution. In our experiments a 20-nm resolution is obtained. The resolution is limited by the instrumental resolution and mechanical stability of the experimental set-up.
Applied Optics 33, 113 (1994)

Rejection of stochastic background noise in low-level pulsed light scattering experiments. Background noise is a common problem in many low level light scattering experiments. An electronic scheme for reducing the stochastic noise in low level light detection is reported. This scheme has been applied successfully in Raman experiments to reject the noise generated by laser induced fluorescence and afterpulses of high gain photomultipliers.
Review of Scientific Instruments 64, 2550 (1993)

Wavelength-tunable passive mode locking of dye lasers by use of intracavity optical Kerr effect. Using intracavity optical Kerr effect, we achieve self-starting wavelength-tunable passive mode locking in a Rhodamine 590 dye laser. With an external grating-pair for group-velocity-dispersion compensation, 1.1-ps pulses of about twice the bandwidth of the Fourier transform limit are obtained. Mode locking is started by a dilute saturable absorber jet, and the wavelength is tuned from 579 to 602 nm with a birefringent filter.
Optics Letters 18, 1247 (1993)

A spatially coherent white-light interferometer based on a point fluorescent source. We developed a point-fluorescent-source based white-light interferometer for high resolution reflectometry, range-gating imaging, and group-velocity dispersion measurement. The laser-pumped point fluorescent source has 9 mW of power and a spatial coherence of 0.97, which allows it to be used just like a laser beam. Due to its 40-nm FWHM spectral width, the width of its temporal autocorrelation is only 19 fs, corresponds to that of 14-fs Gaussian pulses.
Optics Letters 18, 678 (1993)

Imaging through random scattering media by using cw broadband interferometry. Imaging of an object hidden in random scattering media is achieved using cw broadband ("white-light") interferometry. The light source is an inexpensive, easy-to-operate, superluminescent diode laser. An efficient image enhancing algorithm is developed to eliminate the effect of phase noise in the interferometer and enhance the recovered image. Sub-millimeter spatial resolution is achieved.
Optics Letters 18, 546 (1993)

Optical soliton in graded-index waveguides. We show that optical solitons in a broad spectral range can be excited by propagating an off-axis Gaussian beam in nonlinear graded-index waveguides. Due to periodical variation of both beam size and dispersion coefficient in graded-index waveguides, the solitions belong to a newly discovered class called "guiding-center solitons". In planar graded-index waveguides, solitons may be arranged to interact in both temporal and spatial domain, thus the waveguide may serve as a novel platform for soliton interactions.
Optics Letters 18, 266 (1993)

Adjustable negative group-velocity dispersion in graded-index lenses. An analysis of group-velocity dispersion in graded-index lenses is presented. The analysis shows that continuously adjustable negative group-velocity dispersion up to hundreds of square femtosecond can be produced by propagating the optical beam off the axis of a graded-index lens. Comparing with the well known prism-pair and grating-pair methods for producing negative dispersion, the method described in this letter has the advantages of being 100-fold smaller in physical size, low insertion loss, and being compatible with integrated optics.
Optics Letters 17, 1177 (1992)

Analysis of passive additive-pulse mode-locking with eigenmode theory. The equation of motion for the coupled cavities of additive-pulse mode-locking is solved with the eigenvalue method. Pulse evolution and self-starting condition for passive operation is analyzed. It is shown that the laser configuration is equivalent to an intracavity interferometer, and a phase modulation across the pulse is the essential element for the mode-locking. With the method developed in this paper, transient pulse evolution of an initial seed pulse can be calculated, and thereby optimized. It is found that the pulse shortening rate is linear with respect to the number of round trips. The duration for the initial pulse to shorten to its steady state is proportional to its initial width, and inversely proportional to the nonlinear coefficient. The effect of dynamic gain saturation on the self-starting condition is also analyzed. The result shows that it would be difficult to achieve self-starting additive-pulse mode-locking in the color-center and dye lasers, due to their large emission cross sections. Single-cavity configurations of additive-pulse mode-locking are proposed. It is shown that they are mathematically equivalent to the couple-cavity one.
IEEE J. Quantum Electron. 28, 562 (1992)

Self-starting issues of passive self-focusing mode-locking. An analysis of passive mode-locking with intracavity self-focusing effect is presented, and the self-starting issue is addressed. The analysis shows the initial pulse shortening force of this mechanism is too weak to start the mode-locking from mode-beating or noise, and how it can be greatly enhanced with dilute dye saturable absorber in the cavity. This analysis also shows how the pulse width evolution is related to the "pulse shortening force" in cw mode-locking lasers.
Optics Letters 16, 1689 (1991)

Theory of passive additive-pulse mode-locking. The laser configuration of newly developed additive-pulse mode locking, also known as coupled-cavity mode locking, can be viewed as an intracavity interferometer. By solving the equation of motion of the two coupled cavities, a mathematical description of the self-starting mechanism is obtained. With this method, transient pulse evolution of an initial seed pulse can be calculated, and thereby optimized. The structure of the equation of motion suggests new single cavity configurations of additive-pulse mode locking, and the same method of analysis can be applied to them.
Optics Letters 16, 1104 (1991)