Characterization of laser-diode end-pumped intracavity frequency doubled, passively Q-switched and mode-locked Nd:YVO4 laser

Simultaneous Q-switching and mode-locking in a laser-diode end-pumped intracavity frequency doubled Nd:YVO₄/KTP green laser using Cr⁴⁺:YAG saturable absorber is experimentally demonstrated. The influence of the initial transmission (T₀) of the Cr⁴⁺:YAG crystal on the Q-switched mode-locked green pulses as well as on the average green power is characterized by using Cr⁴⁺:YAG crystal with various T₀. The effect of T₀ on the pulse build-up time in intracavity second harmonic configuration is theoretically investigated. It was found that the depth of modulation for the mode-locked pulses is greatly improved at the second harmonic wavelength as compared to that for the fundamental wavelength. The average pulse duration of the individual mode-locked pulse for the second harmonic beam measured to be less than 500 ps with a repetition rate of 400 MHz.     

39.5% conversion of low-power Q-switched Nd:YAG laser radiation to 266 nm by use of a resonant ring cavity

A simple ring cavity consisting of mirrors reflecting at the second harmonic is used to generate the fourth harmonic of low-power Q-switched Nd:YAG laser pulses. A large beam waist of 1 mm minimized the effects of double refraction and thermal loading of the nonlinear crystals. By simulating the low-power pulses with a small portion of the output from a 30-Hz flash-lamp-pumped Q-switched Nd:YAG laser, we obtained IR-UV energy conversion as great as 39.5% to yield 84-mW average power at 266 nm from 213 mW of single-mode 1064-nm radiation.     

Dependence of photoemission efficiency on the pulsed laser cleaning of Tungsten photocathodes, part 1: Experimental

3+ :YAG laser with 5^th harmonic generator, generating 16 ps duration pulses at 213 nm, with energies up to 0.5 mJ. Experimental results concerning the action of laser pulses, as well as the effects of residual pressure on the cleanliness of the photocathodes surface are presented and discussed. Influence of laser pulses and residual pressure on the work function of the metal are also investigated. Received: 15 April 1996/Accepted: 5 November 1996     

Perforation of biomodels and biotissues by powerful laser pulses

In vitro perforation of gelatin-based biomodels and myocardium tissue by powerful CO₂ and YAG:Er laser pulses is considered. Features of the thermal and shock-wave effects on the walls of the laser channel are investigated. Patterns of laser channeling in gelatin by a single YAG:Er laser pulse were studied.     

Enhanced four-wave mixing in a hollow-core photonic-crystal fiber

Hollow-core photonic-crystal fibers are shown to substantially enhance four-wave mixing (FWM) of laser pulses in a gas filling the fiber core. Picosecond pulses of Nd:YAG fundamental radiation and its second harmonic are used to generate a signal at the frequency of the third harmonic by the FWM process 3omega = 2omega + 2omega - omega. The efficiency achieved for this process in a 9-cm-long, 13-microm-hollow-core-diameter photonic-crystal fiber, designed to simultaneously transmit a two-color pump and the FWM signal, is shown to be approximately 800 times higher than the maximum FWM efficiency attainable with the same laser pulses in the tight-focusing regime.     

Laser resonance external photoelectric effect in F 2 − color centers of LiF crystals

The laser-resonance two-step external photoelectric effect is revealed in the F ₂ ^– color centers of LiF crystals exposed to nanosecond laser pulses. The first photoionization step uses IR radiation tunable within the resonance absorption band of the color centers and the second step the second harmonic of a Nd:YAG laser. The photoionization cross sections of the F ₂ ^– centers are estimated.     

Generation of tunable picosecond pulses in the medium infrared by down-conversion in AgGaS2

Picosecond pulses from a mode-locked Nd:YAG laser and a traveling-wave dye laser are mixed in an AgGaS₂ crystal to generate pulses at the difference frequency. The dye laser is tunable between 1200 nm and 1460 nm resulting in a tuning range of the parametric pulses from 3.9 μm to 9.4 μm. The spectral bandwidth is quite narrow. A value of Δ?=6.5 cm^-1 was measured which is constant over the whole tuning range. Several percent of the Nd:YAG laser photons are converted to infrared photons. Pump pulses of 21 ps give parametric pulses of 8 ps.     

High-peak power,passively Q-switched,composite, all-poly-crystalline ceramics Nd:YAG/Cr<Superscript>4+</Superscript>:YAG laser and generation of 532-nm green light

Output performances of passively Q-switched, composite Nd:YAG/Cr⁴⁺:YAG lasers that consisted of bonded, all-poly-crystalline ceramics Nd:YAG and Cr⁴⁺:YAG are reported. Laser pulses at 1.06 μm with 2.5-mJ energy and 1.9-MW peak power are obtained from a 1.1-at % Nd:YAG/Cr⁴⁺:YAG ceramics that was quasi-continuous-wave (quasi-CW) pumped with a diode laser. Single-pass frequency doubling with LiB₃O₅ (LBO) nonlinear crystal at room temperature yielded green laser pulses at 532 nm of 0.36-mJ energy and 0.3-MW peak power, with a conversion efficiency of 0.27.     

Diode pumped Ho:YAG and Ho:LuAG lasers, Q-switching and?second harmonic generation

Direct diode pumped Ho:YAG generated laser pulses at 2.12???m with an optical to optical slope efficiency of 0.24. Ho:YAG and Ho:LuAG laser rods were evaluated with both wide and narrow bandwidth pump diodes. The laser wavelength varies with the level of pumping and optical design. This variation was found to be predictable. Second harmonic at 1.06???m was produced in a 6.0?mm long BBO crystal.     

Synchronized picosecond pulse generation with a synchronously pumped dye laser and a compressed Nd: YAG laser for non-degenerate transient grating experiments

We have set up a laser system which simultaneously provides synchronized picosecond pulses in the visible and at a wavelength of 1.06 m with a repetition rate of 76 MHz. The set-up consists of a dye laser synchronously pumped by the second harmonic of a cw mode-locked Nd:YAG laser and a fiber-grating compressor for the fundamental wavelength of the Nd;YAG laser. Crosscorrelation measurments reveal the time jitter between the two pulse trains to be less than 10 ps. As a first application we have performed non-degenerate transient grating experiments in semiconductors. The non-degenerate technique allows to use excitation energies well above the bandgap energy and to separate non-linear refractive index effects from photoinduced absorption or transmission changes.     

Ablation experiments on polyimide with femtosecond laser pulses

Some applications of polymer films require the microstructuring of partly uneven substrates. This cannot be achieved by conventional photolithography, usually performed with ultraviolet short-pulse lasers (excimer, fourth harmonic Nd:YAG). When processing thermally sensitive or undoped polymers with low optical absorption, the use of femtosecond laser pulses can improve the ablation precision, also reducing the heat-affected zone. Therefore, a Ti:sapphire laser system was employed to perform ablation experiments on polyimide (PI). The irradiated areas were evaluated by means of optical and scanning electron microscopy. Highly oriented ripple structures, which are related to the polarization state of the laser pulses, were observed in the cavities. The relationship between the ablation threshold fluence and the number of laser pulses applied to the same spot is described in accordance with an incubation model.     

Double-pulse laser ablation applied to reactive PLD method for synthesis of carbon nitride film: A second laser shot delay

Carbon nitride films were deposited using ablation of graphite target by second harmonic radiation of Nd:YAG laser in nitrogen atmosphere. To produce high hardness films, the deposited particles should have sufficient kinetic energy to provide their efficient diffusion on a substrate surface for formation of crystal structure. However, a shock wave is arisen in ambient gas as a consequence of laser plasma explosive formation. This shock wave reflected from the substrate interacts with plume particles produced by the first laser pulse and decreases their kinetic energy. This results in decrease of film crystallinity. To improve film quality, two successive laser pulses was proposed to be used. At adjusting time delay, the particles induced by the second pulse wilt serve as a piston, which will push forward both stopped particles ablated by the first pulse and arisen from chemical reactions in ambient gas. An X-ray photoelectron spectroscopy (XPS) analysis of deposited films has shown an increase of content of sp ³ carbon atoms corresponding to crystalline phase, if double-pulse configuration is employed. The luminescence of excited C₂ and CN molecules in laser plume at different distances from the target was studied to optimize the delay between laser pulses.     

High-power diode-directly-pumped tenth-order harmonic mode-locked TEM00 Nd:YVO4 laser with 1 GHz repetition rate

A high-efficiency high-power diode-directly-pumped tenth-order harmonic mode-locked TEM₀₀ Nd:YVO₄ laser with 1 GHz repetition rate was first demonstrated. The maximum output power was 10.4 W with optical-optical efficiency of 41.8% and slope efficiency of 78.1%, respectively, the pulse width was about 30 ps at the output power of 9.6 W. Based on the large third-order nonlinearity of Nd:YVO₄, the tenth-order harmonic mode-locked pulses were induced by the intensity-dependent Kerr effect and the cooperative action of counter-propagating pulses colliding in the laser crystal for a colliding-pulse-modelocking-like cavity. The pulses were further modulated by a semiconductor saturable absorber mirror.     

Observation of an icelike state in supercooled water and its destruction by laser pulses

An icelike state of water supercooled to ?1°C was found using Raman spectra excited by second harmonic pulses from a Nd:YAG laser. The spectrum of the OH stretching vibrations of this state was found to be similar to the spectrum of hexagonal (Ih) ice. The icelike state is destroyed by laser pulses of a moderate intensity (1 MW/cm²) with a repetition frequency of 1 Hz, and the water returns to its initial state, with no icelike clusters, within five minutes through an aperiodic process of variation in the concentration of hydrogen bonds without changing its macroscopic temperature.     

A nonlinear correlation method using multiphoton ionization for the measurement of uv ultrashort pulses

A new nonlinear correlation method which makes it possible to measure the duration of uv ultrashort pulses has been proposed. In this method the multiphoton ionization of gases is used as a nonlinear process and the ion current caused by the process is detected. We have also experimentally demonstrated its usefulness in the uv region by observing the second- and the third-order auto-correlation profiles of picosecond pulses at 2661 Å (the fourth harmonic of a mode-locked YAG laser), using NO and Xe gases, respectively.     

Non-contact subsurface temperature measurements following mid-infrared laser irradiation

A difficult challenge in laser processing at nanosecond time scales is monitoring substrate temperature in the laser focal volume, particularly for mid-infrared laser irradiation where the absorption depth is relatively large and the attained temperatures are often relatively low. Here, we describe time-dependent measurements of the subsurface temperature of a target material following absorption of pulsed mid-infrared (MIR) laser irradiation, by detecting the luminescence from micron-size ceramic phosphor particles (Gd-doped YAG:Ce) embedded in the target material at a concentration of up to 10 %. Temperature calibrations were obtained by measuring the luminescence decay of the probe particles in an oil-bath heater. A silica-nanoparticle film was irradiated by an Er:YAG laser operating in a free-running mode over a fluence range up to but below the ablation threshold, while the third harmonic of the Nd:YAG laser excited the luminescence of the probe particles. From the temperature calibrations, it was possible to infer the thermal history of the target as a function of time delay between the Er:YAG and Nd:YAG laser pulses.     

$90^{\circ}$ phase-matched parametric frequency conversion in AgGa1-xInxS2

AgGa_1-xIn_xS₂ with x=0.14±0.01 was found to be 90° phase-matchable for type-I difference-frequency generation (DFG) by mixing the dual-wavelength pulses emitted from an electronically tuned Ti:sapphire laser. Infrared radiation continuously tunable over the range of 4.80–6.98 μm was generated by independently varying the two wavelengths in the 705–932 nm spectral range, and 4.04 μm radiation by mixing a Nd:YAG laser with the Ti:sapphire laser. In addition, this material was found to be noncritically phase-matchable for the second harmonic generation (SHG) of CO₂ laser radiation at 10.591 μm at 203 °C. Sellmeier equations that reproduce well these experimental data are presented. PACS  42.65.-k; 42.65.ky     

Time-resolved photo-electron spectroscopy on mass-selected metal clusters using a regenerative femtosecond amplifier up to 100 Hz

A femtosecond photo-electron experiment is described which provides excellent conditions for measuring time-resolved photo-electron spectra of free, monodispersed cluster anions using repetition rates up to 100 Hz. Cluster anions are synthesized in an electric arc and subsequently cooled in a helium carrier gas. A time-of-flight spectrometer is used for mass separation of the negatively charged clusters. The kinetic energy of the photo-electrons is analyzed by a magnetic-bottle time-of-flight spectrometer, which guarantees a maximum collection efficiency. Femtosecond laser pulses are generated by a seeded regenerative Ti:Sa amplifier, which is externally pumped with the second harmonic of a diode-pumped solid-state Nd:YAG laser. A retroreflector mounted on a computer-controlled translation stage serves as a reproducible time delay of the probe pulse. The high energy output of the laser pulses (∼3 mJ) in combination with the variable repetition rate and the high stability of the amplified pulses provide excellent conditions for recording pump-probe photo-electron spectra of mass-separated cluster anions even at the fairly low ion density of pulsed plasma cluster sources. First results on the electron dynamics of the Pt₃ ^- cluster demonstrate the reliability of the whole system. Further experimental investigations will concentrate on electron-relaxation processes in transition- and noble-metal clusters as well as on the nuclear and transition-state dynamics of chemically reacted adsorbate clusters. Received: 7 January 2000 / Published online: 7 August 2000     

基频和三倍频Nd:YAG激光诱导熔石英损伤特性

 采用光电探测器和数字示波器检测散射光脉冲信号,研究了基频和三倍频Nd:YAG激光诱导熔石英损伤过程,给出了泵浦光和探针光的散射光光电信号;比较了基频和三倍频激光作用下熔石英烧蚀斑显微照片,并分析了其损伤机理。结果显示:在ns脉冲激光作用下,熔石英损伤均发生在泵浦激光脉冲峰值附近,且基频光作用下损伤开始时间点比三倍频作用下早;在多脉冲或高能量激光辐照下,检测到了等离子体闪光信号,等离子体闪光发生在时间延迟21 ns附近。基于Keldysh理论计算了基频光和三倍频光作用下,熔石英光致电离速率同激光强度的关系。     

Coupled-cavity passively Q-switched two-Stokes microchip laser

Experimental and theoretical studies of the coupled-cavity diode-pumped Nd:YAG/Cr:YAG microchip lasers with intracavity Raman conversion of laser pulses in a Ba(NO₃)₂ crystal into two Stokes pulses have been made. Two lasers with a different cavity length have been investigated. The minimal pulse durations at the 2nd Stokes wavelength were ??100 ps in the short-cavity laser at pulse energy of 5???J, and the pulse repetition rate reached 20?C24?kHz. The laser and Stokes pulse dynamics, as well as the spatial intensity distribution of the laser and the 1st Stokes beams at the output mirror have been recorded. A model describing such coupled-cavity microchip Raman lasers has been developed. The numerically simulated laser and Stokes pulse dynamics, and the calculated pulse energy, duration, and repetition rate are in good agreement with the experimental data.