Nonlinear optical laser devices using GaSe

Tunable second-harmonic generation of CO₂ laser radiation and tunable upconversion of infrared radiation from CO₂-laser radiation in a GaSe crystal are demonstrated. The capability of low infrared-signal detection in this crystal is also explored and compared with the AgGaS₂ crystal.     

A Broadband Infrared Laser Source (2.5–17 μm) for Plasma Diagnostics

This paper presents the results of studies aimed at the creation of a hybrid laser system which is composed of a gas lasers and a nonlinear crystal and appreciably broadens and enriches the radiation spectrum of these lasers. A highly efficient conversion (37%) is attained when generating the second harmonic in a ZnGeP₂ crystal owing to an increase in the peak power of CO laser radiation in the mode locking regime. The two-cascade conversion (generation of both sum and difference frequencies) of radiation of a broadband CO laser in the single sample of such nonlinear crystals as ZnGeP₂ and AgGaSe₂ is demonstrated. In this case, the radiation spectrum is broadened by nearly a factor of two, and the number of detected spectral lines grows by a factor of four. The use of a comparatively simple laser system of gas-discharge CO and CO₂ lasers to conversion in AgGaSe₂ results in laser radiation tunable over a set of narrow spectral lines within a range from 2.5 to 16.6 μm (more than two and a half octaves).     

Investigation of the Radiation Mechanism of Microwave Excited Cluster Lamps

Experimental and theoretical investigations towards a better understanding of the radiation mechanisms in WO₂Br₂-discharges, so-called “cluster lamps” are presented. The main purpose of the investigation is to find out whether a major part of the continuum emitted from these discharges, which in the past has been interpreted as thermal cluster radiation, could also originate from molecular band radiation, in particular from triatomic molecules in colder discharge regions. Comparing emission spectra of WO₂Br₂-discharges from different plasma regions, using lamps operated at different vapor pressures and power levels (i.e. non-LTE and LTE-discharges), we conclude that by far the major part of the strong continuum radiation emitted from WO₂Br₂ lamps is indeed thermal radiation of hot tungsten cluster particles. WO— (below 550 nm) and WO₂-molecular radiation (above 650 nm), which is superimposed on the cluster continuum, also contribute to the total radiation output, but to a lesser degree. A crude model for WO₂ molecular emission is presented. Unfortunately, the WO₂ emission component greatly impedes proper fitting of the measured cluster continuum to a thermal cluster model.     

Thermodynamic Properties of an Atom Inside a Kerr Nonlinear Blackbody

Thermodynamic properties of a Kerr nonlinear blackbody (KNB) are investigated. It is found that below a transition temperature T _c , the free energy of a KNB radiation is larger than that in a normal blackbody. At the transition from KNB radiation state to a normal blackbody radiation state, the photon system undergoes a first-order phase transition. The thermodynamic system of an atom interacting with a KNB radiation bath is also investigated by using a thermodynamic perturbation theory. It is found that below a transition temperature T _c , the increment of the free energy of the atom is larger than that in a normal blackbody. Above T _c , the KNB becomes a normal blackbody, and the properties in a KNB turn to be the same as those in normal blackbody radiation.     

A broadband excimer source of visible radiation with barrier discharge pumping

We investigate the characteristics of a broadband cylindrical excimer source of visible radiation with a surface area of 230 cm² excited by a pulse-periodic barrier discharge based on multicomponent mixtures (mercury diiodide and dibromide with helium and admixtures of molecular nitrogen and xenon). The working mixture components were excited by a pulse-periodic (pulse repetition rate 500–5000 Hz, pulse duration ～150 ns) barrier discharge. We detected radiation from excimer HgI* and HgBr* molecules, the second positive system of molecular nitrogen, and mercury and xenon atoms. The amplitude, duration, and trailing edge of the radiation pulses in the HgI₂:HgBr₂:Xe:He and HgI₂: HgBr₂:N₂:He mixtures with admixtures of xenon and molecular nitrogen were found to change compared to the HgI₂:HgBr₂:He mixture. The optimal partial pressure of helium lies within the range 162–195 kPa. The most intense radiation from HgI* and HgBr* molecules (in a ratio of more than 3: 1) is observed in the HgI₂:HgBr₂: Xe:He mixture. The mean and pulse radiation powers are 45 W and 93 kW, respectively, at a pumping pulse repetition rate of 5000 Hz and an efficiency of 30%. We discuss the spectral and temporal characteristics of the radiation source and the dependence of the radiation intensity of excimer molecules of mercury monoiodide and monobromide on the partial pressures of xenon and nitrogen. We point out that the radiation source is of considerable interest for applications in biotechnology and medicine.     

Renal Clearable Luminescent WSe2 for Radioprotection of Nontargeted Tissues during Radiotherapy

High‐energy ionizing radiation is widely used in medical diagnosis and cancer radiation therapy. However, high‐energy radiation can also impose significant damages in healthy tissues during medical treatments via direct DNA damages and indirect damages from production of reactive oxygen species (ROS). Therefore, it is urgent to develop highly effective radioprotectants with low toxicities that can meet the increasing needs for alleviating the adverse effects from cancer radiation therapy and nuclear emergency. In this work, strongly catalytic ultrasmall (sub‐5 nm) cysteine‐protected WSe₂ dots are employed to protect healthy tissues against radiation via diminishing radiation‐induced free radicals. The WSe₂ dots with high surface activities can recover radiation‐induced DNA damages and eliminate the excessive ROS generated from radiation. In vivo experiments confirm that the survival rate of mice treated with WSe₂ dots is significantly elevated with radiation damages postponed under exposure to high‐dose ionizing radiation. Furthermore, the free radicals in major organs and hematological system can be appreciably omitted, suggesting their unique role as free radical scavengers. These WSe₂ dots in ultrasmall size show rapid renal clearance of ≈74% injection dose via urine excretion in 24 h and do not cause any apparent toxicity in vivo for up to 30 d.     

Asymmetry of Hawking Radiation of Dirac Particles in a Charged Vaidya–de Sitter Black Hole

The Hawking radiation of Dirac particles in a charged Vaidya–de Sitter black hole is investigated by using the method of generalized tortoise coordinate transformation. It is shown that the Hawking radiation of Dirac particles does not exist for P₁, Q₂ components, but for P₂, Q₁ components it does. Both the location and the temperature of the event horizon change with time. The thermal radiation spectrum of Dirac particles is the same as that of Klein-Gordon particles.     

The light-enhanced NO2 sensing properties of porous silicon gas sensors at room temperature

The NO2 gas sensing behavior of porous silicon（PS） is studied at room temperature with and without ultraviolet（UV） light radiation.The PS layer is fabricated by electrochemical etching in an HF-based solution on a p ＋-type silicon substrate.Then,Pt electrodes are deposited on the surface of the PS to obtain the PS gas sensor.The NO2 sensing properties of the PS with different porosities are investigated under UV light radiation at room temperature.The measurement results show that the PS gas sensor has a much higher response sensitivity and faster response-recovery characteristics than NO2 under the illumination.The sensitivity of the PS sample with the largest porosity to 1 ppm NO2 is 9.9 with UV light radiation,while it is 2.4 without UV light radiation.We find that the ability to absorb UV light is enhanced with the increase in porosity.The PS sample with the highest porosity has a larger change than the other samples.Therefore,the effect of UV radiation on the NO2 sensing properties of PS is closely related to the porosity.     

Use of metallic and dielectric films for hollow fibers

The problem of transmission of CO₂ laser radiation through hollow fibers and waveguides was studied theoretically and confirmed experimentally. The transmission of the laser radiation through metal and metal-dielectric tubes was measured and compared with the theoretical data based on a ray model solution. This makes possible the investigation of the transmission of the CO₂ radiation through waveguides when the internal wall is covered with a metal or a metal- dielectric film. It was shown theoretically and proved experimentally that the transmission of the CO₂ radiation is possible even through bent waveguides.     

A procedure for the distinction between static and dynamic radiation exposures of personal radiation badges using pulsed optically stimulated luminescence

A method is described for the distinction between the normal, “dynamic” radiation exposure of a personal radiation dosimetry badge during normal wear of the badge, and an abnormal, or “static” exposure of the badge in a fixed position relative to a radiation source. An example of a static exposure would be the intentional (unauthorized), or accidental, exposure of the badge to a radiation source when the badge is not being worn by a person. Other abnormal exposure conditions of interest include when the badge is accidentally partially shielded by (say) coins or other heavy material during incorrect wear of the badge (e.g. in a pocket) such that the usual reading of the badge may reveal a lower-than-expected absorbed dose reading. This paper describes the use of Pulsed Optically Stimulated Luminescence (POSL) as a dosimetric readout mode and the application of this technique to “imaging” of the dose distribution across the face of a large area Al₂O₃:C dosimeter. The technique relies upon the use of a radiation filter with a periodic array of holes, or other weakly absorbing regions, such that the Al₂O₃:C is exposed in a pattern which follows the pattern of the filtration. Subsequent illumination of the Al₂O₃:C using a defocused laser beam reveals a pattern of luminescence emission corresponding to the pattern of the absorbed dose. During static exposure, the absorbed radiation pattern is sharp and clear, whereas during dynamic exposure the absorbed radiation pattern is diffuse. Image processing of the collected POSL emission pattern using a Discrete Fourier Transform produces a spatial frequency spectrum of the collected image from which a factor is calculated which indicates the probability of static versus dynamic exposure. Furthermore, the POSL image obtained (vis-à-vis the absorbed radiation dose pattern) can also reveal accidental shielding. The image collection procedure, the image processing algorithms, and two different methods to determine the probability factor are described. Application of the technique to several exposure conditions is illustrated.     

Experimental investigation of supersonic radiation propagation in low-density plastic and copper-doped foams

1 Introduction The investigation of supersonic radiation wave transporting in low density foam pro-duced by thermal radiation is of crucial importance in inertial confinement fusion (ICF) research[1]. When the intense radiation flux is incident in the media, first, a supersonic heat wave is formed which propagates into the undisturbed material. In time, due to the increasing mass of heated material, it slows down and is overtaken by a shock wave,thus becoming of the ablative type. Normally, …     

Forward light reflection from a moving inhomogeneity

The reflection of monochromatic and quasi-monochromatic pulsed light incident on a moving inhomogeneity in the optical characteristics of a medium having plasma-type dispersion has been analyzed. The velocity V of the inhomogeneity, induced in the medium by an intense laser pulse, has been changed by varying its carrier frequency. It has been shown that the usual back-reflection mode, when the reflected radiation pulse moves in the direction opposite the direction of incident radiation, is implemented only if the velocity V is less than the critical value V _min, which depends on the carrier frequency of the incident radiation pulse. It has been found that reflected radiation moves in the same direction as the incident radiation in a certain range of the velocity V _min < V < V _max (forward reflection). In this case, the reflected radiation pulse begins to lag behind a fast-moving inhomogeneity. When V _max < V < c, where c is the speed of light in vacuum, the group velocity of the incident radiation pulse is less than the speed of inhomogeneity, and there is no reflection. Analytical treatment is supported by numerical simulation.     

Parametric x-ray radiation generated by 5.7-MeV electrons in a pyrolytic-graphite crystal

Parametric x-ray radiation is observed in a mosaic pyrolytic-graphite crystal irradiated with a 5.7-MeV electron beam. The first-order (002) reflection of parametric x-ray radiation is detected in Bragg geometry. In the measured spectra, there is a peak at about 8 keV. The detector used is arranged at an angle of Θ_D=2Θ_B=27° with respect to the electron beam, where Θ_B is the Bragg angle. The target thickness is 0.3 mm. The spectra of parametric x-ray radiation are measured at various angles of target orientation with respect to the axis of the incident-electron beam.     

Characterization and Suppression Techniques for Degree of Radiation Damping in Inversion Recovery Measurements

Radiation damping is a phenomenon in which transverse nuclear magnetization couples with the current in a coil used in nuclear magnetic resonance experiments. This results in an additional magnetic field that increases the relaxation pathway for the magnetization, which then relaxes back to equilibrium more quickly. Radiation damping has been shown to affect longitudinal relaxation time (T₁) measurement in inversion recovery experiments. In this work, we demonstrate that the extent of radiation damping depends upon the T₁ of the sample. Radiation damping difference spectroscopy is used to characterize the severity of radiation damping, while gradient inversion recovery is used for radiation damping suppression in T₁ measurements. At field strength of 9.4 T, for the radiation damping characteristic time (T_rd) of 50 ms, these investigations show non-negligible radiation damping effects for T₁ values greater than T_rd, with severe distortions for T₁ longer than about 150 ms, showing reasonable agreement with the predicted T_rd. We also report a discrepancy between published expressions for the characteristic radiation damping time.     

Optical-pyrometric diagnostics of the state of silicon during nanopulsed laser irradiation

The dynamics of the reflectivity at ?? = 0.53 ??m and the IR radiation of silicon in the wavelength range 0.9?C1.2 ??m is studied under the action of nanosecond ruby laser radiation pulses. When radiation energy density W is lower than the threshold of laser-induced melting of the surface of a semiconductor crystal, the major contribution to the IR radiation emitted by this crystal is made by edge photoluminescence. As the melting threshold is exceeded, the nanosecond dynamics of the detected IR radiation changes from photoluminescence to the thermal radiation of the forming Si phase melt with a high reflectivity. The results of pyrometric measurements of the peak melt surface temperature as a function of W obtained at an effective wavelength ?? _e = 1.04 ??m of the detected IR radiation agree with the data of analogous measurements performed at ?? _e = 0.53 and 0.86 ??m.     

Visible luminescence of Er2O3 induced by CO2 laser radiation with a wavelength of 10.6 µm

The visible luminescence of Er₂O₃ powder induced by CO₂ laser radiation with a wavelength of 10.6 μm is observed and spectrally studied. The mechanisms for the CO₂ laser radiation’s conversion into the luminescence are discussed. The experimental results can be used for the creation of large-screen devices for displaying, visualization, and digital photo and video detection of the intensity distributions, spectra, and interferograms of laser radiation. Original Text ? Astro, Ltd., 2006.     

Circular radiation heat shields with temperature dependent emissivities: transient and steady-state analyses

Radiation heat loss is an important type of heat loss in thermal systems. In this work, a numerical study of the transient response of two circular radiation heat shields inserted between two parallel and circular surfaces of emissivities ε₁ and ε₂ is presented. The same dimensions have been assumed for the two main radiating surfaces and the two radiation shields. The radiation shields are assumed to have different emissivities on their top (ε₃ and ε₅) and bottom ( ε₄ and ε₆) surfaces, and both are assumed to be different but linear functions of temperature. A specific configuration is investigated in detail to highlight the transient temperature and heat transfer characteristics of the system. Some new results for the transient temperature and heat transfer characteristics of the system such as the effect of shield location, shield emissivities, the temperature dependence of shield emissivities, system dimensions, temperatures of the hot and cold surfaces and emissivities of the hot and cold surfaces are presented for future references. It has been observed that increasing the temperature of the first radiation shield by changing a parameter such as surface emissivity or distance between the radiation shield or the temperature of the hot surface, will not necessarily decrease the temperature of the second radiation shield.     

Control of radiation spatial coherence under synchronous amplification in crystalline LiF:F 2 − amplifier

A technique of spatial coherence control, based on the synchronous amplification of a radiation in LiF crystals with F ₂ ^? color centers, is demonstrated. Spatial radiation distributions of stimulated Raman scattering (SRS) in oxide crystals were investigated under picosecond laser excitation. Low spatial radiation coherence was revealed for both the transient and quasi-stationary SRS. Spatially incoherent SRS was transformed to spatially coherent radiation as a result of phase—locked picosecond synchronous laser pumping of nonlinear Raman and LiF: F ₂ ^? crystals and the Stokes radiation amplification in the color center crystal.     

Noninversive partial velocity amplification of radiation by ions due to their rotation in a magnetic field

It is shown that upon the application of an external magnetic field, a gas of ionized particles may experience noninversive partial velocity amplification of radiation by ions due to their Larmor rotation. In this case, virtually all ions may be in the ground state. It may happen that approximately half the number of ions in the medium amplify the incident radiation. The integrated absorption coefficient remains positive due to the enhancement of absorption of radiation by the other half of ions. Noninversive amplification of radiation takes place when the condition ω_c?Γ²/kv _T is satisfied ωw_c is the cyclotron frequency of ions in the magnetic field; Γ is the homogeneous half-width of the absorption line for ions, and kv _T is the Doppler width). In the case of interaction of atomic ions with radiation in the optical range, this corresponds to magnetic fields B?600 G (for the ion mass M～10 amu). Noninversive partial velocity amplification of radiation is a “latent” effect in the sense that it disappears upon averaging over all velocity directions of ions. This effect is associated with the emergence of phase incursion of the induced dipole moment oscillations for ions moving in circular cyclotron orbits, which depends on the ion velocity.     

Weak antilocalization in a three-dimensional topological insulator based on a high-mobility HgTe film

The up-conversion luminescence of Er³⁺ from the ²H_11/2, ⁴S_3/2, and ⁴F_9/2 levels in nanocrystals of Y_0.95(1?x)Yb_0.95xEr_0.05PO₄ (x = 0, 0.3, 0.5, 0.7, 1) orthophosphates activated with Er³⁺ ions has been studied under the excitation of Yb³⁺ ions to the ²F_5/2 level by 972-nm cw laser radiation. Broadband radiation in the wavelength range of 370–900 nm has been observed at certain power densities of exciting laser radiation; this broadband radiation is absent in the case of excitation of the powders under study by pulsed laser radiation with a wavelength of 972 nm at a pulse repetition frequency of 10 Hz and a duration of a pulse of 15 ns. Experimental data indicating that this radiation is thermal in nature have been presented.