Acetonitrile (CH3CN) infrared absorption cross sections in the 3 μm region

High resolution infrared absorption cross sections of acetonitrile have been determined from spectra recorded in the 3 μm spectral region using a Bruker IFS 125 HR Fourier transform spectrometer (FTS) and a multipass White cell. The eleven synthetic air-broadened acetonitrile spectra were recorded at a resolution of 0.015 cm⁻¹ (calculated as 0.9/MOPD (Maximum Optical Path Difference), the Bruker definition of resolution) over a range of different temperatures and pressures that are representative of conditions in the Earth's atmosphere (50-760 Torr and 207-296 K). Intensities were calibrated using infrared spectra recorded at the Pacific Northwest National Laboratory (PNNL). These new cross sections will enable satellite retrievals of acetonitrile in the 3 μm region from atmospheric spectra recorded by satellite instruments, such as the ACE (Atmospheric Chemistry Experiment)-FTS.     

Generation of tunable infrared radiation in rubidium titanyl phosphate crystal for the optical measurement of number density, absorption cross-section of methane gas

Tunable near-infrared radiation has been generated in a rubidium titany1 phosphate (RTP) crystal by employing non-collinear difference-frequency mixing (DFM) technique. The input radiation sources are Nd:YAG laser radiation and its second harmonic pumped dye laser radiation. For the generation of 2.0 radiation, the maximum value of the conversion efficiency (quantum) obtained in the process is 49% from the dye (0.6945 μm) to the infrared (2.0 μm) radiation in the 7.9-mm-long crystal. The generated tunable mid-infrared radiation has been used to measure the number density, absorption cross-section and minimum detectable concentration of methane gas in its 2ν₃ band in a multi-pass cell at 30.075 Torr pressure. The number density and column density of the methane molecules are found to be 1.068×10¹⁸ cm⁻³ and 3.02×10²¹ cm⁻², respectively, whereas the minimum still-detectable concentration at 1.658 μm wavelength is estimated to be 4.523×10¹⁷/cm³.     

Modeling of plasma-controlled surface evaporation and condensation of Al target under pulsed laser irradiation in the nanosecond regime

Phase transition on the surface of an aluminium target and vapour plasma induced by laser irradiation in the nanosecond regime at the wavelengths of 1.06 and 0.248 μm with an intensity of 10⁸-10⁹ W/cm² in vacuum are analysed. Particular attention is paid to the wavelength dependence of the observed phenomena and the non-one-dimensional effect caused by the Gaussian laser intensity distribution. A transient two-dimensional model is used which includes conductive heat transfer in the condensed phase, radiative gas dynamics and laser radiation transfer in the plasma as well as surface evaporation and back condensation at the phase interface. It is shown that distinctions in phase transition dynamics for the 1.06 and 0.248 μm radiation result from essentially different characteristics of the laser-induced plasmas. For the 1.06 μm radiation, evaporation stops after the formation of hot optically thick plasma, can occasionally resume at a later stage of the pulse, proceeds non-uniformly in the spot area, and the major contribution to the mass removal occurs in the outer part of the irradiated region. Plasma induced by the 0.248 μm laser is much more transparent therefore evaporation does not stop but continues in the subsonic regime with the Mach number of about 0.1.     

Widely phase-matched tunable difference-frequency generation in periodically poled LiNbO3 crystal

We report on the development of a laser source in the mid-infrared spectral region based on difference-frequency generation (DFG) in a periodically poled LiNbO₃ (PPLN) crystal. Continuously tunable coherent radiation from 2.75 to 4.78 μm was produced by optical parametric interaction between a diode-pumped monolithic continuous-wave (CW) Nd:YAG laser operating at 1.064 μm and a CW Ti:Sapphire laser tunable from 767 to 871 nm. Temperature-dependent quasi-phase-matched DFG wavelength acceptance bandwidth was studied and characterized. An empiric formula is given to estimate the phase-matched wavelength acceptance bandwidth as a function of the crystal temperature at Λ = 22.5 μm. A large frequency scan of 128 cm⁻¹ (about 78 cm⁻¹ above 1 μW) near 4.2 μm was achieved. The whole absorption spectrum of the P and R branches of the ν₃ band of atmospheric carbon dioxide has been recorded with a single phase-matched frequency scan.     

Localization of nonequilibrium current carriers close to Cr ions in ZnSe:Cr single crystals

The photoluminescence (PL) of Cr-doped ZnSe single crystals is investigated in a temperature interval from 83 up to 297 K and in a wavelengths region from 440 up to 2700 nm. The doping was carried out during a high-temperature annealing of ZnSe crystals in CrSe vapors and in chrome chlorides medium. It is revealed that the doping results in an appearance of both luminescence bands located at 0.54, 0.97, and 2.15 μm and edge luminescence bands located at 454, 457, and 460 nm at 83 K. It is shown that the PL bands located at 457 and 460 nm are caused by the radiative recombination with the participation of holes located on hydrogen-like orbits close to Cr⁺ centers, having a binding energy of 99 meV. The excitons bound with centers responsible for the radiation located at 0.54 μm and having a binding energy of 65-68 meV are considered. The energy of a lattice relaxation at recharge of centers responsible for green radiation is estimated and equals 40-170 meV.     

Synthesis and properties of submicron range CaSO4:Eu particles

CaSO₄:Eu with particle size in submicron range was synthesized. Radiation induced Eu³⁺↔Eu²⁺ conversion as well as thermal conversion was studied. The samples showed thermal conversion above 400 °C. However, no radiation induced conversion in submicron range particles was observed. Particles heated above 400 °C coalesce and when heated at 925 °C bigger particles of 20 μm size were formed. Optical microscopy of these particles reveals red inclusion of about 5 μm inside CaSO₄ particle. It is speculated that the red inclusion is CaS:Eu²⁺.     

Non-destructive quantification of pharmaceutical tablet coatings using terahertz pulsed imaging and optical coherence tomography

Optical coherence tomography (OCT) and terahertz pulsed imaging (TPI) are two powerful techniques allowing high quality cross-sectional images from within scattering media to be obtained non-destructively. In this paper, we report experimental results of using OCT and TPI for quantitatively characterizing pharmaceutical tablet coatings in the thickness range of 10-140 μm. We found that the spectral OCT system developed in-house has an axial resolution of 0.9 μm, and is capable of quantifying very thin coatings in the range of 10-60 μm. The upper limit of 60 μm within the tablet coating and core is owed to the strong scattering of OCT light, which has relatively short wavelengths in the range of 0.5-1.0 μm. On the other hand, TPI utilizes terahertz radiation that has substantially long wavelengths in the range of hundreds of microns, and thus is less prone to the scattering problem. Consequently TPI has been demonstrated to be able to quantify thicker coatings in the range of 40-140 μm and beyond. We concluded that OCT and TPI are two complementary analytical techniques for non-destructive and quantitative characterization of pharmaceutical tablet coatings.     

3D periodic structures grown on silicon by radiation of a pulsed Nd:YAG laser and their field emission properties

Periodic three-dimensional structures were successfully grown on single crystal Si wafers either bare or Au-covered under their exposure to a pulsed radiation of a Nd:YAG laser in vacuum. The structures protrude above the initial wafer surface for 10 μm while their spatial period is about 70 μm. The coupling of the laser radiation to Si surface is related to the thermal non-linear absorption of the near band gap radiation. The structures exhibit an efficient field emission with an average emission current of 5 mA/cm² and is sensitive to the post-treatment of samples. The drawbacks of the emission current densities are discussed.     

A new imaging method for gold-surface adsorbates based on anomalous reflection

A new imaging method is proposed for ultrathin films with a thickness of a few nanometers, based on the anomalous reflection (AR) of gold. In the AR effect, the reflectivity fairly decreases for blue or purple light (380 nm < λ < 480 nm) with the existence of a transparent dielectric layer at a gold surface. Thus, a thin gold film can be used as an imaging platform. Clear AR images are obtained for a microarray of protein (avidin) spots of diameter 120 μm with gaps of size 50 μm between the spots (36 spots/mm²). The resolution of the AR imaging is governed solely by the illumination spot size. AR imaging is a promising technique for high throughput analysis of biomolecular detection in a microarray format.     

Highly nonlinear photonic-crystal fibers for the spectral transformation of Cr: forsterite laser pulses

We demonstrate novel photonic-crystal fibers (PCFs) fabricated of a highly nonlinear glass. Dispersion profiles and nonlinearity of these fibers are tailored with an array of submicron holes in the fiber core. With the PCF structure designed to provide a nonlinearity on the order of 10³ W⁻¹ km⁻¹ at the radiation wavelength of 1 μm and a fundamental-mode dispersion profile with zero group-velocity dispersion around 1.19 μm, unamplified femtosecond Cr: forsterite laser pulses are efficiently frequency-converted into the 540-1000-nm wavelength range through solitonic spectral-transformation mechanisms and four-wave mixing.     

Designing of endlessly single mode polarization maintaining highly birefringent nonlinear micro-structure fiber at telecommunication window by FV-FEM

An endlessly single mode highly polarization maintaining nonlinear microstructure fiber at telecommunication window is reported via full-vector finite element method. By taking three ring hexagonal PCF with suitable fiber parameter such as air hole diameter in cladding region d = 0.8 μm, pitch 2.3 μm and introducing four symmetrical large air holes near core region d′ = 2 μm, single mode (V_eff ≤ π), small effective mode area 2.7 μm², nonlinear co-efficient 44.39 W⁻¹ km⁻¹, high phase birefringence of the order of 10⁻³ and group birefringence of the order of 10⁻⁴ with beat length 0.3 μm at wavelength 1.55 μm are achieved.     

Enhanced field emission from Si doped nanocrystalline AlN thin films

Si doped and undoped nanocrystalline aluminum nitride thin films were deposited on various substrates by direct current sputtering technique. X-ray diffraction analysis confirmed the formation of phase pure hexagonal aluminum nitride with a single peak corresponding to (1 0 0) reflection of AlN with lattice constants, a = 0.3114 nm and c = 0.4986 nm. Energy dispersive analysis of X-rays confirmed the presence of Si in the doped AlN films. Atomic force microscopic studies showed that the average particle size of the film prepared at substrate temperature 200 °C was 9.5 nm, but when 5 at.% Si was incorporated the average particle size increased to ∼21 nm. Field emission study indicated that, with increasing Si doping concentration, the emission characteristics have been improved. The turn-on field (E_to) was 15.0 (±0.7) V/μm, 8.0 (±0.4) V/μm and 7.8 (±0.5) V/μm for undoped, 3 at.% and 5 at.% Si doped AlN films respectively and the maximum current density of 0.27 μA/cm² has been observed for 5 at.% Si doped nanocrystalline AlN film. It was also found that the dielectric properties were highly dependent on Si doping.     

Modeling the infrared radiance of rough sea surface in the infrared windows: 3-5 μm and 8-12 μm

The general formulations for spectral directional emissivity and spectral bi-directional reflectivity are analyzed by using Fresnel's formula and Snell's law in rough sea surface which is simulated by wave facets, whose slopes are changing according to the isotropic Gaussian distribution with respect to surface wind. Then, shadowing effect is taken into account in both emission and reflection of sea surface. On this basis, the mathematic expression is obtained for spectral radiance of rough sea surface, in which the radiance of rough sea surface is considered as a composition of self-emission, reflection of sky and reflection of sun. Finally, calculations for infrared radiance of rough sea surface in bands of 3-5 μm and 8-12 μm are given to illustrate their significant different, which are caused mostly by the dispersion of sun radiation.     

基于高次余弦散射分布的空间卫星可见光特性

基于几何光学和辐射理论,研究了空间卫星的可见光散射特性.空间卫星的背景辐射主要包括太阳的直接辐射和地球及大气的散射辐射,根据目标的结构特性与背景特性建立了空间卫星的几何模型和光照模型.分析目标表面状况,入射到目标表面的光线近似服从高次余弦散射分布,根据能量守恒定理及表面材料的高次余弦散射分布特性建立了目标散射特性的数学模型.通过矢量坐标变换确定太阳、地球、观测卫星在目标本体坐标系下的相对位置关系.根据给定的几何尺寸和表面物性参量仿真获得了目标在探测器入瞳处的能量分布及星等特征,目标本体与太阳帆板在探测器入瞳处的辐照度最大量级均为10-12 W/m2.仿真结果表明太阳帆板在目标特性分析时不可忽略,为空间目标的可见光探测提供参考数据.     

Enhancement of terahertz wave difference frequency generation based on a compact walk-off compensated KTP OPO

A compact, walk-off compensated dual-wavelength KTP OPO near the degenerate point of 2.128 μm pumped by a Nd:YAG pulsed laser is employed as the pump for terahertz (THz) source based on difference frequency generation (DFG) in a GaSe crystal. Coherent THz radiation that is continuously tunable in the range of 81-1617 μm (0.186-3.7 THz) is achieved. An enhancement of 76.7% in average for the THz energies at different wavelengths is realized using the walk-off compensated KTP OPO than the common one. Using a 8 mm-long GaSe crystal, the maximum output THz pulse energy is 48.9 nJ with the peak power of 11 W, corresponding to the energy conversion efficiency of 5.4 × 10^− 6 and the photon conversion efficiency of about 0.09%.     

Scattering from unidirectional ground steel surfaces in the specular direction

Scattering in the specular direction from unidirectional ground steel surfaces having random roughness is studied theoretically and experimentally. The grooves were oriented perpendicular and parallel to the plane of incidence and were illuminated by a light beam that was smaller than the sample size. Expressions for scattering from a one-dimensional rough surface in the specular direction were derived for the both orientations of the grooves. For the same groove orientations, scattering (λ = 0.633 μm) from the ground surfaces was measured in the specular direction at angles of incidence from 6° to 82°. The measured rms roughnesses of the surfaces were 0.096 μm, 0.143 μm, 0.311 μm, and 0.501 μm, respectively. The measured scattering was independent of the orientation of the grooves for the two smoother surfaces and depended on the groove orientation for the other surfaces. Calculations using the derived expressions taking into account the experimental results show that the scattering is independent of the orientation of the grooves if the rms roughness is no larger than ∼0.16 μm.     

A complete study of the line intensities of four bands of CO2 around 1.6 and 2.0 μm: A comparison between Fourier transform and diode laser measurements

Atmospheric carbon dioxide is a key specie for the Earth climate. Two spectral windows at 1.6 μm and 2.0 μm are of particular interest for the in situ and remote monitoring of carbon dioxide from satellite, balloon or airborne platforms using infrared absorption spectroscopy. A precise knowledge of the line strengths is a prerequisite for an accurate concentration retrieval. In this paper, we have revisited in the laboratory the (30⁰1)_III←(00⁰0) and (30⁰1)_II ← (00⁰0) bands of CO₂ near 1.6 μm and the (20⁰1)_III ← (00⁰0) and (20⁰1)_II ← (00⁰0) bands near 2.0 μm by implementing both a high-resolution Connes-type Fourier-transform spectrometer and a tunable diode laser spectrometer equipped with several telecommunication-type semiconductor laser devices. Approximately 200 (respectively 18) transitions of CO₂ have been carefully investigated in spectra recorded with the FT spectrometer (respectively with the tunable diode laser spectrometer). The intensity measurements achieved with both instruments are thoroughly compared to previous instrumental determinations, ab-initio calculations and available atmospheric molecular database.     

Ultra-high detectivity room temperature THZ-IR photodetector based on resonant tunneling spherical centered defect quantum dot (RT-SCDQD)

In this paper, a novel structure for THZ-IR photodetector based on resonant tunneling spherical centered defect quantum dot (RT-SCDQD) operating at room temperature is proposed. The proposed structure includes a quantum dot with centered defect following a resonant tunneling double barrier. It is shown that inserting a centered defect leads to considerable enhancement in absorption coefficient at long wavelength in small dot size (1.05 × 10⁶-7.33 × 10⁶ m⁻¹ at 83 μm). This effect guarantees large responsivity of the proposed system for THZ-IR photodetector. In this proposal, intersublevel transitions in related states positioned at mid energies of large conduction-band-offset materials (GaN/AlGaN) are used to depress the thermal effect in dark current. Adding the resonant tunneling double barrier to the quantum dot resolves the basic problem of collecting electrons from deep excited state without applying large bias voltage. Also, employing the RT double barrier reduces the ground state dark current term. Reduction of the dark current and increasing the responsivity yields ultra-high detectivity, 5 × 10¹⁶ and 2.25 × 10⁹ cm Hz^1/2/W at 83 μm, at 83 and 300 K, respectively. Analysis of the proposed structure is done analytically.     

High-efficiency continuous-wave and Q-switched diode-end-pumped multi-wavelength Nd:YAG lasers

We demonstrate high-efficiency diode-end-pumped multi-wavelength Nd:YAG lasers for continuous-wave and Q-switched operation. For the continuous-wave case, the Nd:YAG laser oscillates at 1.06 and 1.3 μm simultaneously; the maximum output power of 2.0 W (M² = 1.3) and 3.6 W (M² = 1.8) have been achieved at the incident pump power of 20.3 W, with the respective average slope efficiencies of 12.0% and 21.4%, for the lines of 1.06 and 1.3 μm, respectively. For the Q-switched operation, we achieve the average output power of 1.3 W (M² = 2.7) at 1.06 μm and 2.0 W (M² = 3.0) at 1.3 μm with the corresponding peak power of 10.2 and 4.2 kW under an incident pump power of 20.3 W.     

Field emission characteristics of thin-metal-coated nano-sheet carbon films

Nano-sheet carbon films (NSCFs) coated with very thin (≈5-nm-thick) metal layers were fabricated on Si wafer chips by means of quartz-tube-type microwave-plasma chemical-vapour-deposition method with hydrogen-methane gas mixture and an electron beam evaporation method. Field emission (FE) current densities obtained at a macroscopic average electric field, E, of ≈10 V/μm changed from 13 mA/cm² for NSCF with no coated metal to 1.7, 0.7 and 30 mA/cm² for Ti-, Al- and Au-coated NSCFs, respectively, while the threshold E varied from 4.4 V/μm for the former one to 5.3, 5.4 and 2.0 V/μm for the corresponding latter ones, respectively. As the FE currents of Au-coated NSCFs tended to be saturated in a higher E region, compared to those of NSCFs with no coated metal, no simple Fowler-Nordheim (F-N) model is applicable. A modified F-N model considering statistic effects of the FE tip structures and a space-charge-limited-current effect is successfully applied to an explanation for the FE data observed in the low and high E regions.