Μm Laser Pulses Research Articles

High-power passively Q-switched 2 μm all-solid-state laser based on a Bi_2Te_3 saturable absorber

By using the ultrasound-assisted liquid phase exfoliation method, Bi2Te3 nanosheets are synthesized and deposited onto a quartz plate to form a kind of saturable absorber (SA), in which nonlinear absorption properties around 2 μm are analyzed with a home-made mode-locked laser. With the as-prepared Bi2Te3 SA employed, a stable passively Q-switched all-solid-state 2 μm laser is successfully realized. Q-switched pulses with a maximum average output power of 2.03 W are generated under an output coupling of 5%, corresponding to the maximum single-pulse energy of 18.4 μJ and peak power of 23 W. The delivered shortest pulse duration and maximum repetition rate are 620 ns and 118 kHz under an output coupling of 2%, respectively. It is the first presentation of such Bi2Te3 SA employed in a solid-state Q-switched crystalline laser at 2 μm, to the best of our knowledge. In comparison with other 2D materials suitable for pulsed 2 μm lasers, the saturable absorption performance of Bi2Te3 SA is proved to be promising in generating high power and high-repetition-rate 2 μm laser pulses.

Laser-plasmas in the relativistic-transparency regime: Science and applications.

Laser-plasma interactions in the novel regime of relativistically induced transparency (RIT) have been harnessed to generate intense ion beams efficiently with average energies exceeding 10 MeV/nucleon (>100 MeV for protons) at “table-top” scales in experiments at the LANL Trident Laser. By further optimization of the laser and target, the RIT regime has been extended into a self-organized plasma mode. This mode yields an ion beam with much narrower energy spread while maintaining high ion energy and conversion efficiency. This mode involves self-generation of persistent high magnetic fields (∼104 T, according to particle-in-cell simulations of the experiments) at the rear-side of the plasma. These magnetic fields trap the laser-heated multi-MeV electrons, which generate a high localized electrostatic field (∼0.1 T V/m). After the laser exits the plasma, this electric field acts on a highly structured ion-beam distribution in phase space to reduce the energy spread, thus separating acceleration and energy-spread reduction. Thus, ion beams with narrow energy peaks at up to 18 MeV/nucleon are generated reproducibly with high efficiency (≈5%). The experimental demonstration has been done with 0.12 PW, high-contrast, 0.6 ps Gaussian 1.053 μm laser pulses irradiating planar foils up to 250 nm thick at 2–8 × 1020 W/cm2. These ion beams with co-propagating electrons have been used on Trident for uniform volumetric isochoric heating to generate and study warm-dense matter at high densities. These beam plasmas have been directed also at a thick Ta disk to generate a directed, intense point-like Bremsstrahlung source of photons peaked at ∼2 MeV and used it for point projection radiography of thick high density objects. In addition, prior work on the intense neutron beam driven by an intense deuterium beam generated in the RIT regime has been extended. Neutron spectral control by means of a flexible converter-disk design has been demonstrated, and the neutron beam has been used for point-projection imaging of thick objects. The plans and prospects for further improvements and applications are also discussed.

Ablation spot area and impulse characteristics of polymers induced by burst irradiation of 1 µm laser pulses

The ablation spot area and impulse characteristics of various polymers were experimentally investigated against burst irradiation of Nd: YLF laser pulses with a pulse repetition frequency of 1kHz, wavelength of 1047nm, temporal pulse width of 10ns, and single-pulse fluence of 6.1J/cm2 to 17.1J/cm2. The dependences of ablation area on the pulse energy from 0.72 to 7.48mJ and the number of pulses from 10 pulses to 1000 pulses were investigated. In order to characterize their impulse performance as a function of fluence, which should not depend on ablation material, an effective ablation spot area was defined as that obtained against aluminum, 1050A, as the reference material. An impulse that resulted from a single burst of 200 pulses was measured with a torsion-type impulse stand. Various impulse dependences on the fluence, which were not readily predicted from the optical properties of the material without ablation, were obtained. By fitting the experimentally measured impulse performance to Phipps and Sinko's model in the vapor regime, the effective absorption coefficient with laser ablation was evaluated, thereby resulting in three to six orders of magnitude larger than that without ablation. Among the polymers examined using polytetrafluoroethylene (PTFE) as the best volume absorbers, the highest momentum coupling coefficient of 66 μNs/J was obtained with an effective absorption coefficient more than six times smaller than that of the other polymers.

Near- and mid-IR ultrashort laser pulse filamentation in a molecular atmosphere: a comparative analysis

Self-focusing and filamentation of high-power pulsed laser radiation along a spectrally selective absorbing air path are numerically simulated in three spectral ranges centered at 0.8, 3.9, and 10.6 μm. A detailed comparative analysis of the main pulse parameters in the filamentation area is performed to examine the potential of various laser sources for atmospheric optics. We found that the radiation of mid-IR lasers (3.9 and 10.6 μm) forms the longest filamentation region and the widest supercontinual spectrum in comparison with near-IR radiation. Filamentation of the 3.9 μm laser pulse results in the best spatially continued plasma channel, while the 10.6 μm pulse retains the widest spectral composition under the conditions of strong molecular absorption in the atmosphere.

Quasi-stationary convection in a periodic-pulsed optical discharge in high pressure rare gas

Unusual convection flows were observed in stabilized pre-breakdown phase of the periodic-pulsed optical discharge (POD) called “quiet” POD. The discharge was a relatively weakly glowing plasma filament sustained by focused λ = 1.064 μm laser pulses with repetition rate of fr = 50÷100 kHz at the intensity several times below than that required for the optical breakdown to occur. No strong shock waves or irregular turbulence around the discharge were observed, in contrast to breakdown types of POD. Significant laser beam refraction measured in the beam cross-section behind the discharge zone was explained by the gas heating in the discharge up to 10 kK, providing high gradients of gas density and refraction index. Intense convective flow was detected on the schlieren images as thermal traces of the laser-induced gas streams flowing from the discharge zone, directed mainly normally to the optical axis. Repeated relaxation of the gas expanding after being rapidly heated by the laser pulse is proposed to explain the effect. The periodic-pulsed discharge located in the elongated beam waist generates an anisotropic heated region with gas streams and vortices, which may form the observed regular convective flow at the late stages of expanding.

13.5 mJ polarized 2.09 μm fiber-bulk holmium laser and its application to a mid-infrared ZnGeP2 optical parametric oscillator

A high pulse repetition frequency (PRF), high energy Ho:YAG laser directly pumped by a Tm-doped fiber laser and its application to a mid-infrared ZnGeP2 (ZGP) optical parametric oscillator (OPO) is demonstrated. The maximum polarized 2.09 μm laser pulse energy is 13.46 mJ at a PRF of 1 kHz. The corresponding peak power reaches 504 kW. In a double-resonant ZGP-OPO, a maximum mid-infrared laser pulse energy of 1.25 mJ, corresponding to a peak power of 79 kW, is accomplished at a PRF of 3 kHz. The nonlinear conversion efficiency reaches 41.7%. The nonlinear slope efficiency reaches 53.3%.

Strong-field ionization of clusters using two-cycle pulses at 1.8 μm.

The interaction of intense laser pulses with nanoscale particles leads to the production of high-energy electrons, ions, neutral atoms, neutrons and photons. Up to now, investigations have focused on near-infrared to X-ray laser pulses consisting of many optical cycles. Here we study strong-field ionization of rare-gas clusters (103 to 105 atoms) using two-cycle 1.8 μm laser pulses to access a new interaction regime in the limit where the electron dynamics are dominated by the laser field and the cluster atoms do not have time to move significantly. The emission of fast electrons with kinetic energies exceeding 3 keV is observed using laser pulses with a wavelength of 1.8 μm and an intensity of 1 × 1015 W/cm2, whereas only electrons below 500 eV are observed at 800 nm using a similar intensity and pulse duration. Fast electrons are preferentially emitted along the laser polarization direction, showing that they are driven out from the cluster by the laser field. In addition to direct electron emission, an electron rescattering plateau is observed. Scaling to even longer wavelengths is expected to result in a highly directional current of energetic electrons on a few-femtosecond timescale.

Integration of helicity-control and pulse-modulation for vortex laser based on a black phosphorus plate.

Using a home-made black phosphorus plate (BPP) as handedness controller and Q-switch modulator synchronously, a ~1.6 µm pulsed vortex laser with well-determined handedness is demonstrated in this letter. Stable vortex pulses of LG0, + 1, LG0,-1, LG0, + 2 and LG0,-2 modes were respectively achieved from compact resonant cavities in this experiment. Such pulsed vortex laser should have promising applications in various fields based on its simple structure, controllable handedness, and carried orbital angular momentum.

Bulk crystal growth and characterization of 2.5–5.0 µm mid-infrared laser crystals

Mid-infrared waveband lasers have demonstrated important applications in the fields of atmosphere pollution monitoring, gas flow monitoring, ocean detection, laser medicine, military confrontation and basic scientific researches etc. In this paper, we review the research development of bulk crystal growth, physicochemical, spectral and laser properties of 2.5–5.0 μm waveband mid-infrared laser crystals in recent years. A serial of gallate and aluminate crystals with low phonon energy were grown by the Czochralski method, such as Er3+ activated and rare earth ions co-doped Gd3Ga5O12(GGG), SrGdGa3O7, SrLaGa3O7, CaYAlO4 and CaGdAlO4 crystals, Ho3+ activated and rare earth ions co-doped GGG crystals, as well as Dy3+:YAlO3 crystals, and 2.5–3.4 μm waveband emissions were obtained in these crystals. High quality Dy3+:KPb2Cl5, Cr2+:CdSe and Cr2+:Fe2+:CdSe crystals were grown by the Bridgman method, and 2.5–5.0 μm waveband emissions were obtained in these crystals. The physicochemical and spectral properties of the above crystals were studied systematically, with focus on analyses of the concentration effect of activated ions Er3+, the sensitization mechanism of the co-dopant sensitized ions and the impact of the depopulated ions on the inhibition of self-termination effect. The fluorescence dynamic of the activated ions within the mid-infrared waveband, the energy transfer efficiencies between activated ions and co-dopant sensitization ions, as well as between the activated ions and co-dopant depopulation ions were calculated, and the energy transfer mechanism was discussed. Based on the results of spectroscopic investigations, the concentrations of Er3+, the types and the concentrations the co-dopant sensitization ions were optimized in different kinds of crystals. The laser properties of Er3+ activated GGG, CaYAlO4 crystals were presented. Finally, 2.7–2.8 µm multi-wavelength CW or pulsed lasers were achieved in Er:GGG, Er,Pr:GGG and Er:GGG/GGG composite crystals end-pumped by 965 nm LD or in Cr,Er:YSGG crystals by Xenon lamp, furthermore, the single longitudinal mode microchip laser was achieved in Er:GGG crystal for the first time.

Resonant cavities for efficient LT-GaAs photoconductors operating at λ = 1550 nm

We show that photoconductors based on low-temperature-grown GaAs (LT-GaAs) can be efficiently operated by 1.55 μm telecom wavelength by using metallic mirror based optical cavities. Two different semi-transparent front mirrors are compared: the first one is a thin gold layer, whereas the second one consists of a gold grating. Light absorption in grating mirror based optical cavities is numerically, analytically, and experimentally investigated allowing for an appropriate optical design. We show a 3 times improvement of the LT-GaAs photoconductor photoresponse by using, as front mirror, the gold grating once compared with the thin gold layer. It reaches around 0.5 mA/W under continuous wave, whereas a transient photoresistivity (Ron) as low as 5 Ω is deduced from dc photocurrents measured under femtosecond pulsed laser excitation. This work paves the way to efficient and reliable optoelectronics systems for GHz or THz waves sampling driven by 1.55 μm pulsed lasers widely available.

Spatiotemporal light bullets and supercontinuum generation in β-BBO crystal with competing quadratic and cubic nonlinearities

We experimentally study filamentation and supercontinuum generation in a birefringent medium [beta-barium borate (β-BBO) crystal] pumped by intense 90 fs, 1.8 μm laser pulses whose carrier wavelength falls in the range of anomalous group velocity dispersion of the crystal. We demonstrate that the competition between the intrinsic cubic and cascaded-quadratic nonlinearities may serve as a useful tool for controlling the self-action effects via phase matching condition. In particular, we found that spectral superbroadening of the ordinary polarization is linked to three-dimensional self-focusing and formation of self-compressed spatiotemporal light bullets that could be accessed within a certain range of either positive or negative phase mismatch. In the extraordinary polarization, we detect giant spectral shifts of the second harmonic radiation, which are attributed to a light bullet-induced self-phase matching.

Mid-infrared supercontinuum generation in telecommunication multimode fiber by nanosecond 2.054 μm laser pulses

We demonstrated a broadband mid-infrared (mid-IR) supercontinuum (SC) directly generated in telecommunication multimode fiber (MMF) pumped by 2.054 μm nanosecond laser pulses. The characteristics of mid-IR SC, including the output spectrum, pulse energy, and optic-to-optic conversion efficiency, were exhaustively investigated at different pump powers and fiber lengths. The nonlinear processes for SC generation in fiber were analyzed, and the frequency shift of the modulation instability (MI) was theoretically calculated, which was close to the experimental data. The output SC pulse shapes were measured in different output powers, and no split was found. The SC laser beam was nearly diffraction limited with an M2 =1.15 in 2.1 μm measured by the traveling knife-edge method, and the laser beam spot was monitored by an infrared vidicon camera.

Controlling Chemical Reactions by Short, Intense Mid-Infrared Laser Pulses: Comparison of Linear and Circularly Polarized Light in Simulations of ClCHO+ Fragmentation

Enhanced mode selective fragmentation of oriented ClCHO(+) → Cl + HCO(+), H + ClCO(+), HCl(+) + CO with linear polarized intense mid-IR pulses was demonstrated in our previous computational study ( J. Phys. Chem. Lett. 2012 , 3 , 2541 ). Simulations of angle-dependent strong field ionization of ClCHO indicate the ionization rate in the molecular plane is nearly twice as large as perpendicular to the plane, suggesting a degree of planar alignment can be obtained experimentally for ClCHO(+), starting from neutral molecules. Classical trajectory calculations with a 4 cycle 7 μm laser pulse (peak intensity of 1.26 × 10(14) W/cm(2)) show that circularly polarized light with the electric field in the plane of the molecule deposits more energy and yields larger branching ratios for higher energy fragmentation channels than linearly polarized light with the same maximum field strength. These results suggest circularly polarized mid-IR pulses can not only achieve control on reactions but also provide an experimentally accessible implementation.

Single- and multi-wavelength Nd:YAlO3 lasers at 1328, 1339 and 1364 nm

We report on the diode-pumped continuous-wave (CW) and passively Q-switched laser operation of a c-cut Nd:YAlO3 (YAP) laser crystal. CW laser operation is demonstrated for the first time with 0.61W output power on a low-gain emission line at 1364nm by using a 0.08-mm glass etalon. Dual-wavelength laser operation is also achieved at 1328 and 1340nm as well as at 1340 and 1364nm with maximum output powers of 0.87 and 0.83W, respectively. Using graphene oxide as saturable absorber, stable Q-switching is obtained at 1339nm with a maximum average output power of 0.43W, a pulse width of 380ns and a repetition rate of 76.9kHz, thus laser pulses with 14.7W peak power and 5.6μJ pulse energy. This work presents the first graphene oxide based Q-switched Nd:YAP laser and shows the effectiveness of graphene oxide as saturable absorber for the generation of 1.3μm laser pulses.

高峰值功率中红外2.8 μm脉冲光纤激光器

高峰值功率中红外2.8 μm脉冲光纤激光器

High-power, mid-infrared, picosecond pulses generated by compression of a CO₂ laser beat-wave in GaAs.

We report on the generation of a train of ∼2 ps, 10 μm laser pulses via multiple four-wave mixing and compression of an infrared laser beat-wave propagating in the negative group velocity dispersion region of bulk GaAs and a combination of GaAs and NaCl crystals. The use of a 200 ps, 106 GHz beat-wave, produced by combining laser pulses amplified on the 10P(20) and 10P(16) transition of a CO₂ laser, provides a novel method for generating high-power, picosecond, mid-IR laser pulses at a high repetition rate. By using 165 and 882 GHz beat-waves, we show that cascaded phase-mismatched difference frequency generation plays a significant role in the four-wave mixing process in GaAs.

Low-loss, robust fusion splicing of silica to chalcogenide fiber for integrated mid-infrared laser technology development.

We demonstrate a low-loss, repeatable, and robust splice between single-mode silica fiber and single-mode chalcogenide (CHG) fiber. These splices are particularly difficult to create because of the significant difference in the two fibers' glass transition temperatures (∼1000°C) as well as the large difference in the coefficients of thermal expansion between the fibers (∼20×10(-6)/°C). With 90% light coupled through the silica-CHG fiber splice, predominantly in the fundamental circular-symmetric mode, into the core of the CHG fiber and with 0.5 dB of splice loss measured around the wavelength of 2.5 μm, after correcting only for the Fresnel loss, the silica-CHG splice offers excellent beam quality and coupling efficiency. The tensile strength of the splice is greater than 12 kpsi, and the laser damage threshold is greater than 2 W (CW) and was limited by the available laser pump power. We also utilized this splicing technique to demonstrate 2 to 4.5 μm ultrabroadband supercontinuum generation in a monolithic all-fiber system comprising a CHG fiber and a high peak power 2 μm pulsed Raman-shifted thulium fiber laser. This is a major development toward compact form factor commercial applications of soft-glass mid-IR fibers.

Origins of Very-Low and Zero-Energy Electron Structures in Strong-Field Ionization with Intense Mid-IR Pulses

SynopsisIntense long wavelength (λ ≥ 2 μm) laser pulses enable experiments in the tunneling ionization regime (γ ≪ 1) and reveal surprising low electron energy features, which can not be described with the strong-field approximation (SFA). These features, universal for all target species, include the low-energy structure (LES), the very-low-energy structure (VLES) and the zero-energy structure (ZES). Using full 3D electron-ion coincidence detection in combination with our ultrafast 160 kHz mid-IR source, we reveal the entire 3D momentum spectrum well below 1 eV. Quantum and classical simulations allow for an interpretation of the LES, VLES and of the newly identified ZES.

Studies of ablated plasma and shocks produced in a planar target by a sub-nanosecond laser pulse of intensity relevant to shock ignition

AbstractThe effect of laser intensity on characteristics of the plasma ablated from a low-Z(CH) planar target irradiated by a 250 ps, 0.438 µm laser pulse with the intensity of up to 1016W/cm2as well as on parameters of the laser-driven shock generated in the target for various scale-lengths of preformed plasma was investigated at the kilojoule Prague Asterix Laser System (PALS) laser facility. Characteristics of the plasma were measured with the use of 3-frame interferometry, ion diagnostics, an X-ray spectrometer, andKαimaging. Parameters of the shock generated in a Cl doped CH target by the intense 3ω laser pulse were inferred by numerical hydrodynamic simulations from the measurements of craters produced by the shock in the massive Cu target behind the CH layer. It was found that the pressure of the shock generated in the plastic layer is relatively weakly influenced by the preplasma (the pressure drop due to the preplasma presence is ~10–20%) and at the pulse intensity of ~1016W/cm2the maximum pressure reaches ~80–90 Mbar. However, an increase in pressure of the shock with the laser intensity is slower than predicted by theory for a planar shock and the maximum pressure achieved in the experiment is by a factor of ~2 lower than predicted by the theory. Both at the preplasma absence and presence, the laser-to-hot electrons energy conversion efficiency is small, ~1% or below, and the influence of hot electrons on the generated shock is expected to be weak.

Multi-strata subsurface laser die singulation to enable defect-free ultra-thin stacked memory dies

We report the extension of multi-strata subsurface infrared (1.342 μm) pulsed laser die singulation to the fabrication of defect-free ultra-thin stacked memory dies. We exploit the multi-strata interactions between generated thermal shockwaves and the preceding high dislocation density layers formed to initiate crack fractures that separate the individual dies from within the interior of the die. We show that optimized inter-strata distances between the high dislocation density layers together with effective laser energy dose can be used to compensate for the high backside reflectance (up to ∼ 82%) wafers. This work has successfully demonstrated defect-free eight die stacks of 25 μm thick mechanically functional and 46 μm thick electrically functional memory dies.