Optical properties of a new IR converter for laser beam analysis

This work presents the temporal and spacial resolution of a new infrared (IR) converter based on thermal radiation emission. Using this converter, it is possible to measure the intensity distribution of laser beams with a wavelength between 1 and 20 μm. For this purpose, the laser radiation (for example, 10.6 μm) is converted into a wavelength coverage of 800–1100 nm. In the actual converter thin metal foils provide the basis of this method. The metal foils are heated to a temperature of 600–800 K. The emitted radiation of the foils defers into the near-infrared (NIR) area, thus enabling detection by camera systems based on silicate. Additional heat input of the laser results in a local temperature increase, and then the increase in radiation intensity can be measured. Typical thicknesses of converter metal foils are <5μm. Foil materials with a low thermal conductivity, good absorption of the measured laser beams, and a high melting temperature are particularly suitable. These parameters are well shown by using stainless steels, such as INOX (stainless steel 1.4310 CrNi steel). Using this material, it is possible to gain a maximum spatial resolution of 250 μm and a temporal resolution of 12.5 Hz, by a measurement range from 1 to 100 W/cm². The maximum measured intensity is 125 W/cm².     

XUV radiation transfer through high-Z metal foil in a laser-produced plasma

Aluminum and gold foils have been irradiated with 1.05-μm, 100-psec laser pulses at an intensity of 3 × 10¹⁴ W/cm². A spatially resolved spectrum of XUV radiation from a rear-side plasma has been observed in the wavelength range from 10 to 100 Å. The spectral intensity for the Al foil decays exponentially with foil thickness and goes to zero at 3 μm. The intensity for the Au foil also decays exponentially up to 1 μm but remains almost constant from 1 to 6 μm. This result for the Au foil indicates that radiation heat conduction plays an important role in energy transport through high-Z plasmas.     

Nonlinear rotation of the polarization of the intense femtosecond laser radiation in BaF<Subscript>2</Subscript>

The dependence of the polarization plane rotation angle in the BaF₂ crystal on the intensity of the femtosecond laser radiation with 0.62 μm wavelength in the ω = ω + ω ? ω process on cubical nonlinearity χ⁽³⁾ is investigated. An anomalous increase in the efficiency of the cross-polarized radiation generation at I > 2 × 10¹² W cm² is observed for the first time. The 10% efficiency of the orthogonal component generation at the ～3 × 10¹² W cm² intensity is obtained.     

Stimulated photon conversion into pseudo-scalar bosons

An analogy was established between Raman scattering in crystals and the conversion of photons into axions and vice versa. The conditions of light emission observation in experiments such as “Light Shining Through Wall” performed in recent years are analyzed. It is shown that the intensity of secondary light emission resulting from the “conversion-reconversion” process can be significantly increased by employment of the stimulated emission instead of spontaneous one, using intense (∼10¹² W/cm²) ultrashort (∼100 ps) laser pulses of the visible (0.5 μm) region as exciting radiation sources.     

Growth of high quality non-linear optical crystal zinc germanium phosphide for Mid-infrared optical parametric oscillator

ZnGeP₂ single crystals were grown from Vertical Bridgman method. High-quality near-stoichiometric ZnGeP₂ single crystals were obtained in the diameter of 30 mm and length of 120 mm. The results showed that after thermal annealing of the crystals the optical absorption coefficient was below 0.03 cm^?1 at 2.05 μm, and ～0.02 cm^?1 at 3–8 μm. The low absorption loss ZnGeP₂ samples with dimension of 6 × 6 × 18 mm³ were cut from the annealed ingots for 3–5 μm optical parametric oscillation (OPO) experiments. For OPO experiment, we obtained up to 8.7 W output in the 3–5 μm wavelength range (with signal of 3.80 μm and idler of 4.45 μm, respectively) pumped by a 16.3 W 2.05 μm Tm,Ho:GdVO₄ laser at pulse repetition rate of 10 kHz, which corresponded to a conversion efficiency of 53.4% and slope efficiency of 64.8%, respectively.     

Particular features of the transmission of laser radiation with wavelength 0.438 µm and intensity (3–7)·1014 W/cm2 through an undercritical plasma from polymer aerogels

The velocities of energy transport in an undercritical plasma of polymer aerogel with and without copper nanoparticles were measured. Transmission of the laser light through targets of different thicknesses such as submicron three-dimensional polymer networks with densities below the critical value (0.13–0.52 N _cr) for a wavelength of 0.438 μm and intensity of (3–7)·10¹⁴ W/cm² at a half-height pulse duration of 0.32 ns was studied. The transfer of a heating laser radiation was registered on the rear side of the target. It ranged from a level of ∼0.5% for the thickness of a low-density layer of 400 μm and density of 9 mg/cm³ (mass per unit square of 0.36 mg/cm²) up to 50–60% for a thickness of 100 μm and density of 2.25 mg/cm³ (mass per unit square of 0.02 mg/cm²). The time dependences of the optical emission from the rear side of the targets were measured. They appear to be indicative of the plasma dynamics in two-layer targets (polymer foam on Al foil) and enable the estimation of the absorption depth for the laser light in an undercritical plasma. __________ Translated from Preprint No. 8 of the P. N. Lebedev Physical Institute, Moscow (2007).     

Widely tunable monochromatic THz sources based on phase-matched difference-frequency generation in nonlinear-optical crystals: A novel approach

We review our up-to-date result on the development of widely tunable monochromatic THz sources, implemented based on difference-frequency generation (DFG) in GaSe, ZnGeP₂, and GaP. Using a GaSe crystal, the output wavelength was tuned in the range from 66.5 μm to 5664 μm (from 150 cm^?1 to 1.77 cm^?1) with the highest peak power 389 W. This tuning range is the widest ever produced for a continuously tunable and coherent tabletop THz source. Moreover, the conversion efficiency 0.1% is also the highest ever achieved for a tabletop system. On the other hand, based on DFG in a ZnGeP₂ crystal, the output wavelength was tuned in the ranges 83.1–1642 μm and 80.2–1416 μm for two phase-matching configurations. The output power has reached 134 W so far. Finally, using a GaP crystal, the output wavelength was tuned in the range 71.1–2830 μm, whereas the highest peak power was 15.6 W. The advantage of using GaP over GaSe and ZnGeP₂ is that crystal rotation is no longer required for wavelength tuning. Instead, one just needs to tune the wavelength of one mixing beam within the bandwidth of as narrow as 15.3 nm.     

Generation of nonclassical light upon resonant excitation of a semiconductor microcavity

Strong correlations in the fluctuations of the intensity of emission from a semiconductor microcavity under resonant laser excitation are observed. The intensity correlation function exhibits an unusual oscillatory behavior with an unexpectedly long oscillation period and decay time. The visibility of the correlation function reaches 0.81. Long oscillation times are attributed to the Rabi frequency characterizing weak coupling between the electromagnetic field of the semiconductor microcavity mode and long-lived exciton states localized by the random potential of the quantum well. For a laser excitation power density of 400 W/cm², the power density of the radiation emitted by the microcavity is 12 W/cm², which corresponds to the total flux of nonclassical light of 1.5 × 10¹⁵ photons/s from an excited spot 50 μm in diameter. Thus, a microcavity can serve as a bright emitter of nonclassical light.     

红外激光对可见光CCD成像系统的干扰

采用连续波红外激光对可见光面阵CCD成像系统进行了干扰实验,观察到饱和串扰、全屏饱和等干扰现象,并测到串扰阈值小于2.0×102W/cm2,全屏饱和阈值为1.2×104W/cm2。通过分析CCD输出电压、视频信号,提出基于有效干扰面积的干扰效果评估方法,并利用该方法对干扰面积与激光功率密度、辐照时间之间的关系进行了半定量分析。结果显示:激光功率密度对干扰效果的影响较大,而辐照时间在0.5~2.0 s之间时,辐照时间对CCD干扰效果的影响不明显。     

Electric conductivity of nanocluster PbTe structures with controlled topology: Manifestation of macroscopic quantum effects

Methods of laser modification allow us to observe macroscopic quantum phenomena in nanostructured (cluster) materials. The laser synthesis of nanoparticles/nanoclusters with different topologies in semiconductor PbTe samples is performed via direct laser modification of thin films under the action of continuous laser radiation with a wavelength of 1.06 μm and a power density of ~10⁵ W/cm². Nanoparticles with bimodal distribution in lateral dimensions are obtained on the surfaces of the samples. The electrophysical properties of such structures can be controlled as desired by modifying their topology. Variations in electric properties depending on the particle location density are demonstrated. The results are interpreted based on the existence of quantum coherent processes with tunneling transitions and hopping conductivity. This approach is promising for the fabrication of elements and devices in optoelectronics and photonics based on new physical principles, and of different hybrid optoelectrical schemes.     

靶的厚度对激光产生的x射线输运到靶后能谱的影响

利用一维辐射流体动力学程序MULTI数值模拟研究了功率为10¹⁴W/cm²、脉冲宽度为1ns、波长为0.35μm的短脉冲强激光辐照不同厚度的平面Au靶时，靶厚度对靶背面x射线能谱结构和辐射强度的影响. 关键词： 激光等离子体 辐射流体力学 x射线转换     

Photoionization mass-spectrometry of benzene and benzaldehyde molecules with an excimer KrF laser

A laser photoionization time-of-flight mass-spectrometer has been developed and used to investigate the photoionization and photofragmentation of benzene and benzaldehyde molecules by an excimer KrF laser radiation at the wavelength of 249 nm in the intensity range from 5·10⁴ to 5·10⁹ W/cm². It has been found that at low laser intensity ions formed by two-step photoionization are most abundant in mass spectra. By increasing laser intensity an extensive fragmentation of molecules, up to C⁺ ions, was observed. The maximum ionization yield of benzaldehyde comes to 0.1% of the number of molecules in the photoionization volume and, according to calculations, to 10% for benzene molecules, when the radiation intensity is 5·10⁹ W/cm².     

Generation of harmonics of femtosecond radiation from the surface of aluminum targets

The results of a study of the generation of harmonics from a laser plasma resulting from the interaction of radiation of femtosecond duration (λ=1.06 μm, t=475 fs, and I～2×10¹⁷ W cm^?2) with aluminum targets are presented. The observed frequency shift of harmonics to the short-wavelength region (1.6 and 5.1 nm for the second and fifth harmonics, respectively) is determined by a collisionless absorption resulting from an anomalous skin effect. The efficiencies of conversion into the second and fifth harmonics in an s-polarized pumping field were lower than the conversion efficiencies in a p-polarized pumping field by a factor of eight and a factor of two, respectively (for intensities I<10¹⁷ W cm^?2). With a further increase in the pumping intensity, these values decreased to 0.8 and 0.5, respectively. The mechanisms of such behavior of the conversion process are considered.     

Narrow bandgap mid-infrared photodetectors based on InAsSbP quantum dots

The results of investigations of mid-infrared photodetectors based on InAsSbP quantum dot (QD) grown on InAs(100) substrate by modified liquid phase epitaxy are presented. The atomic force microscope measurements have shown that the surface density of grown QDs is (4–8) × 10⁹ cm^?2. Also, the morphology and crystalline quality of grown QDs are investigated by a scanning tunneling microscope. Photodetectors based on n-InAs(100) substrate with InAsSbP QDs on its surface were fabricated in the form of a photoconductor cell. The photoresponse spectrum extended up to 4 μm was observed. The optical properties of fabricated structures were investigated under He–Ne laser irradiation with wavelength of 1.15 μm. It was found that the relative surface conductance increases by 16% at power density of 0.15 W/cm². Capacitance hysteresis with maximal remnant capacitance of 2.17 nF at 10³ Hz was observed as well.     

高峰值功率脉冲激光与石英光纤耦合的空气击穿问题

 研究了15 MW峰值功率脉冲激光与600 μm芯径石英光纤耦合中存在的空气击穿现象。对聚焦区域的空气击穿现象进行了理论和实验研究,测得空气击穿阈值为0.79×10⁹ W/cm²。测得固体介质的激光损伤阈值为2.12×10⁹ W/cm²,与理论计算结果相符。提出了七合一光纤耦合器用于解决空气击穿的办法,实验测得7根光纤并束的耦合效率为67.21%。结果表明光纤耦合器可有效解决15 MW峰值功率脉冲激光与600 μm芯径石英光纤的耦合。     

Laser-induced cavities and solitons in overcritical hydrogen plasma

A picosecond CO₂ laser was used successfully in a number of experiments exploring advanced methods of particle acceleration [1]. Proton acceleration from gas-jet plasma exemplifies another advantage of employing the increase in laser wavelength from the optical to the mid-IR region. Recent theoretical- and experimental-studies of ion acceleration from laser-generated plasma point to better ways to control the ion beam’s energy when plasma approaches the critical density. Studying this regime with solid-state lasers is problematic due to the dearth of plasma sources at the critical electron density ∼10²¹ cm⁻³, corresponding to laser wavelength λ = 1 μm. CO₂ laser offers a solution. The CO₂ laser’s 10 μm wavelength shifts the critical plasma density to 10¹⁹ cm⁻³, a value attainable with gas jets. Capitalizing on this approach, we focused a circular polarized 1-TW CO₂ laser beam onto a hydrogen gas jet and observed a monoenergetic proton beam in the 1–2 MeV range. Simultaneously, we optically probed the laser/plasma interaction region with visible light, revealing holes bored by radiation pressure, as well as quasi-stationary soliton-like plasma formations. Our findings from 2D PIC simulations agree with experimental results and aid in their interpretation.     

Trace-gas sensor based on mid-IR difference-frequency generation in PPLN with saturated output power

The development and characterization of a compact pulsed mid-IR laser source for sensitive on-line trace-gas analyses in the 3–4 μm wavelength range is reported. The source is based on an advantageous difference-frequency mixing configuration in periodically poled LiNbO₃ (PPLN) with a cw external-cavity diode laser (ECDL; 810–830 nm) for broad and accurate tunability and a diode-pumped passively Q-switched Nd:YAG laser (1064 nm) for high mid-IR peak power. With 5 mW cw pump and 4.7 kW signal peak power incident on a 19 mm long PPLN crystal, a maximum of 360 μW mid-IR peak power was generated. The narrowband (∼150 MHz) radiation was saturated by a factor of 15 compared with the nonsaturated case due to depletion of the pump laser radiation. This results in a very high amplitude stability of the generated mid-IR power and thus in a high detection sensitivity. A minimum detectable absorption coefficient of 2.8×10⁻⁸ cm⁻¹ was achieved in combination with a 36.2 m multipass cell in an averaging time of 20 s, as demonstrated by on-line analyses of formaldehyde traces near 3.53 μm.     

Stochastic (white noise) analysis of resonant absorption in laser generated plasma

We suggest to use stochastic differential equations for laser-plasma problems. Using this technique we solve analytically the motion of the electrons at the critical layer and calculate the density profile scaling length (L). For an Nd laser flux of 10¹⁷ W/cm² we estimate L ≈ 0.15 μm, while for a CO₂ laser flux of 10¹⁵ W/cm² we get L ≈ 1.5 μm.     

基于差频中红外激光的痕量气体高分辨光谱检测研究

研究了基于差频光源的高分辨中红外激光光谱检测系统,差频中红外光源使用两台近红外半导体激光器作为种子光源,采用PPLN晶体作为非线性混频器件,结合准相位匹配技术实现了3.2~3.7 μm中红外相干光源输出,最大差频输出功率约为1 μW.以CH₄为例检验了系统的高分辨红外光谱检测特性,选择CH₄分子3 028.751 cm^-1 v₃基频吸收线作为分析谱线,10 cm光程的检测限为0.8 ppm.光谱数据分析表明,系统检测限主要受到标准具光学噪音的限制.     

X-ray emission from aluminium under intense, ultrashort irradiation

We present studies of X-ray emission from aluminium under picosecond and femtosecond irradiation in the intensity range 10¹²-10¹⁵ W cm^-2. We use a new and simple method to measure spectrally resolved absolute X-ray yields. It is shown that the X-ray yields can be obtained for arbitrary levels of X-ray flux. We present details of the variation of the absolute yields as a function of wavelength, intensity, polarization and pulse duration of the incident laser radiation. Electron temperatures in the keV range are observed at 10¹⁵ W cm^-2 with femtosecond laser pulses. Received 12 August 1999 and Received in final form 29 November 1999