595-nm Pulsed Laser Research Articles

Evaluation of Crystalline Volume Fraction of Laser-Annealed Polysilicon Thin Films Using Raman Spectroscopy and Spectroscopic Ellipsometry.

We investigated the crystallinities of poly silicon (poly Si) annealed via green laser annealing (GLA) with a 532-nm pulsed laser and blue laser annealing (BLA) with 450-nm continuous-wave lasers. Three-dimensional heat transfer simulations were performed to obtain the temperature distributions in an amorphous silicon (a-Si) thin film, and GLA and BLA experiments were conducted based on the thermal simulation results. The crystallinity of annealed poly Si samples was analyzed using Raman spectroscopy and spectroscopic ellipsometry. To evaluate the degree of crystallization for the annealed samples quantitatively, the measured spectra of laser-annealed poly Si were fitted to those of crystalline Si and a-Si, and the crystal volume fraction (fc) of the annealed poly Si sample was determined. Both the Raman spectroscopy and ellipsometry showed consistent results on fc. The fc values were found to reach >85% for optimum laser power of GLA and BLA, showing good crystallinity of the laser-annealed poly Si thin films comparable to thermal furnace annealing.

Comparative study on crystallinity of laser-annealed polysilicon thin films for various laser sources

In this study, we compared the crystallinities of polycrystalline silicon thin films annealed using green laser annealing (GLA) with a 532-nm pulsed laser, near-ultraviolet laser annealing (NULA) with a 355-nm pulsed laser, and blue laser diode annealing (BLDA) with 450-nm continuous-wave (CW) lasers. Three-dimensional heat transfer simulations were performed to obtain the temperature distributions in a 100 nm-thick amorphous silicon layer, and optimum laser conditions were determined for each laser annealing. GLA, NULA, and BLDA experiments were conducted based on the thermal simulation results, and the crystallinity of the annealed samples was quantitatively analyzed via spectroscopic ellipsometry. The analysis results indicated that GLA and BLDA resulted in good crystallinity that was comparable to the result of furnace annealing, whereas NULA resulted in a relatively poor crystal quality. The difference in the crystallinities produced by the annealing lasers could be explained using the thermal simulation results. This study provides an insight into the optimum laser annealing conditions for realizing high-quality poly-silicon thin films.

Strong-field ionization of water: Nuclear dynamics revealed by varying the pulse duration

Polyatomic molecules in strong laser fields can undergo substantial nuclear motion within tens of femtoseconds. Ion imaging methods based on dissociation or Coulomb explosion therefore have difficulty faithfully recording the geometry dependence of the field ionization that initiates the dissociation process. Here we compare the strong-field double ionization and subsequent dissociation of water (both H$_2$O and D$_2$O) in 10-fs and 40-fs 800-nm laser pulses. We find that 10-fs pulses turn off before substantial internuclear motion occurs, whereas rapid internuclear motion can take place during the double ionization process for 40-fs pulses. The short-pulse measurements are consistent with a simple tunnel ionization picture, whose predictions help interpret the motion observed in the long-pulse measurements.

Intensity modulation of the population of atomic excited states by using near-infrared femtosecond laser pulses

We have theoretically investigated the modulation structures of the excited-state population of an argon atom exposed to a linearly polarized 800-nm laser pulse in the tunneling region. The numerical result identifies a nearly opposite phase modulation structure between the low-lying n = 4–7 states and over all higher $$n>7$$ states. This phase-dependent alternating population phenomenon is found to be related to the periodic channel-closing effect that results from ac-Stark-shifted multiphoton resonances. Moreover, the angular momentum states attributed to different parities have a similar alternating effect, which confirms a multiphoton process in the tunneling region. We introduce a mechanism of two-photon $$\Lambda $$ -type Raman transitions involving Rydberg and continuum states with a simultaneous redistribution of states to account successfully for this observed out-of-phase angular momentum state population.

Optical amplification from high vibrational states of ionized nitrogen molecules generated by 800-nm femtosecond laser pulses.

We experimentally investigated the interaction between nitrogen molecules and intense femtosecond laser pulses. When irradiated by an 800-nm pump laser and a delayed 355-nm seed laser, the spectral lines around 353.3 nm and 353.8 nm are observed to be greatly amplified, no matter whether the pump laser is circularly or linearly polarized. The two spectral lines correspond to the transition of N2+ (B, ν' = 5 → X, ν = 4) and N2+ (B, ν' = 4 → X, ν = 3), respectively. In comparison with the spectral lines related with ground vibrational states of nitrogen molecular ion, the observed amplification exhibits different polarization dependence of the pump laser. This distinctive change can be explained by the population variation of high vibrational states caused by the pump laser with different polarizations.

Laser writing of CsPbBr3 nanocrystals mediated by closely-packed Au nanoislands

Metal halide perovskites have attracted great interest in recent years owing to their excellent luminescence properties. The low formation energy makes it possible to synthesize perovskites of different types via direct laser writing. Here, we report on the formation of metal halide perovskite (CsPbBr3) nanocrystals coupled to the surface plasmon resonances of gold (Au) nanoparticles by irradiating a polymer film doped with the precursors (CsBr and PbBr3) and coated on a thin Au film. The Au film, which is composed of closely-packed Au nanoislands deposited on a silica substrate, act as not only the absorber of 800-nm femtosecond laser pulses but also the heat source for synthesizing and annealing of CsPbBr3 nanocrystals. The formation of CsPbBr3 nanocrystals in the area of laser irradiation was confirmed by Raman scattering spectra measurements. In addition, closely-packed Au nanoislands are transformed into isolated Au nanoparticles which exhibit surface plasmon resonances at ~530 nm. The CsPbBr3 nanocrystals coupled to isolated Au nanoparticles emit two-photon-induced luminescence at ~513 nm, which is enhanced by the surface plasmon resonances through the Purcell effect. An optimal laser power for direct laser writing was determined and the decomposition of CsPbBr3 nanocrystals was observed if a large laser power was employed. Our results indicate the possibility for the formation and annealing of perovskite nanocrystals coupled to surface plasmon resonances by one-step laser writing and open new horizon for their practical applications in color display and optical memory.

Laser speckle contrast imaging system using nanosecond pulse laser source.

.Significance: Nanosecond-pulsed laser has proven to be used to obtain the velocity of blood using the speckle contrast method. Without the scanning time, it has potential for achieving fast two-dimensional blood flow images in a photoacoustic imaging system with the same pulsed laser.Aim: Our study aimed to evaluate the qualities of regional cerebral blood flow (rCBF) obtained in a laser speckle contrast imaging (LSCI) system using continuous wave (cw) and nanosecond pulse laser sources.Approach: First, a LSCI system consisting of a cw laser with a wavelength of 632.8 nm and a cw laser/nanosecond pulse laser with a wavelength of 532 nm was developed. This system was used to obtain rCBF images of mouse in vivo with two different laser sources.Results: Continuous wave lasers (532 and 632.8 nm) show different imaging characteristics for rCBF imaging. The rCBF images obtained using 532-nm nanosecond pulse laser showed higher resolution than those using 532-nm cw laser. There was no significant difference in the results using nanosecond pulse laser among various pulse widths or repetition rates.Conclusions: It is proved that a nanosecond pulse laser could be used for LSCI.

Enhancement of THz Generation by Two-Color TW Laser Pulses in a Low-Pressure Gas

We identify experimental conditions for efficient generation of THz pulses by high power near IR laser radiation: 2 TW, 30 fs, 800-nm laser pulses with their second harmonic were slightly focused in a low-pressure gas cell in such a way to avoid multiple filamentation and excessive ionization. This two-color scheme yields a microjoule level of THz pulses which is two orders of magnitude higher than the signal generated in the atmosphere of ambient air.

Temperature-sensing characteristics of polarization-maintaining fiber Bragg grating inscribed directly by 800-nm femtosecond laser pulses

Abstract A temperature-sensing system based on a polarization-maintaining fiber Bragg grating (PM-FBG) with dual-wavelength peaks was proposed and demonstrated experimentally. First-order bow-tie- and panda-type PM-FBGs with 3-mm gratings were inscribed by a 800-nm femtosecond laser by the point-by-point method, in which the laser was focused onto the fiber core through the fiber coating and cladding. The FBG arrays were directly fabricated by the femtosecond laser writing technique. The PM-FBGs with dual-wavelength reflection-spectrum peaks were scanned by a cost-effective interrogator system using modulated-grating Y-branch (MG-Y) tunable laser to sample the wavelength center position. In the temperature-sensing experiment, the sensitivity of the bow-tie-type PM-FBG exceeded 14 pm/°C within the 30–90 ℃ range. The proposed PM-FBGs and its interrogation system can be widely used in the field of fiber sensing and fiber laser.

Optimal spectral phase control of femtosecond laser-induced up-conversion luminescence in Sm3+:NaYF4 glass

The spectral phase of the femtosecond laser field is an important parameter that affects the upconversion (UC) luminescence efficiency of dopant lanthanide ions. In this work, we report an experimental study on controlling the UC luminescence efficiency in Sm3+:NaYF4 glass by 800-nm femtosecond laser pulse shaping using spectral phase modulation. The optimal phase control strategy efficiently enhances or suppresses the UC luminescence intensity. Based on the laser-power dependence of the UC luminescence intensity and its comparison with the luminescence spectrum under direct 266-nm femtosecond laser irradiation, we propose herein an excitation model combining non-resonant two-photon absorption with resonance-mediated three-photon absorption to explain the experimental observations.

Spatial distribution on high-order-harmonic generation of an H2 + molecule in intense laser fields

SynopsisHigh-order-harmonic generation (HHG) for the H2 + molecule in a 3-fs, 800-nm few-cycle Gaussian laser pulse is investigated by solving the time-dependent Schrödinger equation. The spatial distribution on HHG is demonstrated and the results present the recombination process of the electron with the two nuclei. And there is little possibility of the recombination of the electron with the nuclei around the origin z = 0 a.u. and equilibrium internuclear positions z = ±1.3 a.u.. The electron-nuclear wave packet and ionization probability are investigated to explain the physical mechanism.

Effect of laser intensity on quantum trajectories in the macroscopic high-order harmonic generation**Project supported by the National Key Research and Development Program of China (Grant No. 2017YFA0403300), the National Natural Science Foundation of China (Grant Nos. 11627807, 11774175, 11534004, 11774129, and 11604119), the Fundamental Research Funds for the Central Universities of China (Grant No. 30916011207), the Jilin Provincial Research Foundation

We macroscopically investigate the effect of the laser intensity and gas density on quantum trajectories in the high-order harmonic generation of Ne atoms irradiated by few-cycle, 800-nm laser pulses. The time–frequency profile of the harmonics shows that the long quantum trajectory is dominant at both lower and higher gas densities for a low laser intensity. At high laser intensities, the long quantum trajectory plays an important role for lower gas densities, while the short quantum trajectory is dominant at higher gas densities. An analysis of the phase mismatch for high-order harmonic generation shows that the primary emission of the quantum trajectories is determined by dynamic changes in the laser electric field during the propagation process.

Electronic-coherence-mediated molecular nitrogen-ion lasing in a strong laser field

We report a combined theoretical and experimental investigation on the unambiguous role of electronic coherence underlying the generation of laserlike coherent emission from ${{\mathrm{N}}_{2}}^{+}$ ions launched in a strong laser field. The coherence manifests itself by giving rise to a dramatic enhancement of ${{\mathrm{N}}_{2}}^{+}$ laserlike radiation with two temporally separated pump laser pulses centered at 800- and 1580-nm wavelengths, respectively. We further show that coherent control of ${{\mathrm{N}}_{2}}^{+}$ lasing in a subfemtosecond timescale can be achieved by replacing the single 800-nm laser pulse with a pair of 800-nm laser pulses of adjustable time delay. The experimental results are well reproduced in theory using a three-level quantum model of ${{\mathrm{N}}_{2}}^{+}$ ions under the density matrix representation.

Terahertz Wave Modulation by Pre-plasma Using Different Laser Wavelength

We report the terahertz (THz) wave modulation from a pre-plasma using different laser wavelengths, which is intersected orthogonally to the two-color laser filament produced by 800-nm laser pulse. When the pre-plasma exists, the THz radiation excited by the two-color field decreases significantly and the modulation depth increases with the increasing modulation pulse wavelength. Moreover, the amplitude reduction at high frequency in THz spectrum and the THz wave polarization change also have the modulation pulse wavelength dependence. These results can be explained by a photocurrent model considering wavelength-dependent ionization rate. The work contributes to further understand the theoretical mechanism of THz wave generation and enrich the practical application of ultrafast THz modulator.

Dynamics of secondary contamination from the interaction of high-power laser pulses with metal particles attached on the input surface of optical components.

We investigate the interaction of 355-nm and 1064-nm nanosecond laser pulses with nominally spherical metallic particles dispersed on the input surface of transparent substrates or high-reflectivity (HR) multilayer dielectric coatings, respectively. The objective is to elucidate the interaction mechanisms associated with contaminant-induced degradation and damage of transparent and reflective optical elements for high-power laser systems. The experiments involve time-resolved imaging capturing the dynamics of the interaction pathway, which includes plasma formation, particle ejection, and secondary contamination by droplets originating from the liquefied layer of the particle. The results suggest that HR coatings are more susceptible to secondary contamination by liquid droplets produced by the particles because of the different geometry of excitation and the location of plasma initiation. Modeling results focus on better understanding the melting of the particle surface, leading to ejections of liquid droplets and the pressure applied to the substrate, leading to mechanical damage.

Subfemtosecond-resolved modulation of superfluorescence from ionized nitrogen molecules by 800-nm femtosecond laser pulses.

Superfluorescence emission around 391 nm is generated when nitrogen molecules are irradiated by a strong 800-nm pump laser and a delayed seed laser. The emission corresponds to the transition between N2+(B2Σu+,ν″=0) and N2+(X2Σg+,ν=0). When another weak 800-nm probe laser is injected and scanned after the pump laser, the superfluorescence intensity is observed to exhibit periodical modulation. The period is determined to be ~2.63 fs, corresponding to the transition frequency between N2+(A2Πu,ν'=2) and N2+(X2Σg+,ν=0). Based on theoretical derivation, these observations can be attributed to the laser-induced population transfer and polarization variation between the relevant electronic states of ionized nitrogen molecules.

Transparent capacitive micromachined ultrasonic transducers (CMUTs) for photoacoustic applications

Integration of acoustic and optical techniques prompted the need for transparent ultrasonic transducers to guide the light through the transducer and improve the signal to noise ratio. In the presented paper, capacitive micromachined ultrasound transducers (CMUTs) using glass substrate and indium-tin-oxide electrodes were fabricated by adhesive wafer bonding technique presenting a transparency of up to 82% in the visible range. A receive sensitivity of 65.5 μV/Pa was measured with noise equivalent sensitivity of 95 Pa. Capacity of the produced CMUTs for photoacoustic imaging was also demonstrated by successfully detecting the photoacoustic signal from an aluminum foil target, which was irradiated by a 532-nm pulse laser transmitted through the CMUT. The centre frequency of the detected photoacoustic signal was at 2 MHz with 52.3% -6-dB fractional bandwidth.

Sono-photoacoustic theranostics using phase-changing contrast agents

Phase-change contrast agents are low boiling point liquid perfluorocarbon droplets that can be vaporized to form larger microbubbles. Droplet vaporization can be used for both imaging and therapy. Sono-photoacoutics is a non-linear imaging method that uses simultaneous optical and acoustic pulses to activate phase-change contrast agents. By combining photothermal heating from a laser pulse and the negative pressure from an acoustic pulse, lower droplet activation thresholds are achieved than from either source alone. In this study, we demonstrate in an in vitro model that sono-photoacoutics activation of polypyrrole coated perfluorocarbon droplets can be used to disrupt 2-cm long fibrin clots to restore flow. Polypyrrole coated droplets under 200 nm in diameter were introduced upstream and allowed to diffuse into the clot. A 1.24-MHz single element transducer coaxially aligned with a 1064-nm pulsed laser was used to scan the clot, activating any agents within the clot. Agent activation was quantified using passive cavitation detection, while flow was monitored using a digital balance. Our results show that the droplets can freely diffuse into fibrin clots and SPA activation can restore approximately 25% of the flow.

Generation of a single-cycle pulse using a two-stage compressor and its temporal characterization using a tunnelling ionization method

A single-cycle laser pulse was generated using a two-stage compressor and characterized using a pulse characterization technique based on tunnelling ionization. A 25-fs, 800-nm laser pulse was compressed to 5.5 fs using a gas-filled hollow-core fibre and a set of chirped mirrors. The laser pulse was further compressed, down to the single-cycle limit by propagation through multiple fused-silica plates and another set of chirped mirrors. The two-stage compressor mitigates the development of higher-order dispersion during spectral broadening. Thus, a single-cycle pulse was generated by compensating the second-order dispersion using chirped mirrors. The duration of the single-cycle pulse was 2.5 fs, while its transform-limited duration was 2.2 fs. A continuum extreme ultraviolet spectrum was obtained through high-harmonic generation without applying any temporal gating technique. The continuum spectrum was shown to have a strong dependence on the carrier-envelope phase of the laser pulse, confirming the generation of a single-cycle pulse.

Angle dependence of strong-field single and double ionization of carbonyl sulfide

We have studied, experimentally and theoretically, the ionization probability of carbonyl sulfide (OCS) molecules in intense linearly polarized 800-nm laser pulses as a function of the angle between the molecular axis and the laser polarization. Experimentally, the molecules are exposed to two laser pulses with a relative time delay. The first, weaker pulse induces a nuclear rotational wave packet within each molecule such that the ensemble exhibits preferential alignment in the laboratory frame at specific times. The second, stronger pulse induces ionization, and the variation in single and double ionization yields is measured as a function of the delay between the two pulses. The angular dependence of the ionization yield is extracted by fitting the delay-dependent yields to a sum of delay-dependent moments of the rotational wave packet's angular distribution. We compute these same angular-dependent strong-field ionization yields for OCS using time-dependent density-functional theory (TDDFT). For the single-ionization case, our measurements agree well with TDDFT calculations and with previous experiments. Furthermore, analysis of the simulated one-body density reveals that, when averaged over a laser cycle, the resulting hole is delocalized across the molecule for light polarized perpendicular to the molecular axis and mostly localized on the sulfur for parallel polarization. This suggests that preferential molecular alignment is a key parameter for controlling charge migration dynamics initiated by strong-field ionization. For double ionization, the agreement between experiment and theory is less compelling, reflecting the substantial challenges of computing double-ionization yields using TDDFT methods.