Search

The laser-induced damage threshold (LIDT) was measured for a

A search for a new Z^{'} gauge boson associated with (un)broken B-L symmetry in the keV-GeV mass range is carried out for the first time using the missing-energy technique in the NA64 experiment at the CERN SPS. From the analysis of the data with 3.22×10^{11} electrons on target collected during 2016-2021 runs, no signal events were found. This allows us to derive new constraints on the Z^{'}-e coupling strength, which, for the mass range 0.3≲m_{Z^{'}}≲100 MeV, are more stringent compared to those obtained from the neutrino-electron scattering data.

The refractive indices and the absorption coefficients of β-BBO, LBO, and KTP nonlinear optical crystals have been measured in the wavelength range of 0.5–3 mm at temperatures of 298 and 78 K. The absorption coefficients of all crystals at a wavelength of 1 mm do not exceed 3.5 cm–1 and fall with cooling. The anisotropy of the absorption coefficients of crystals increases significantly with decreasing wavelength ( 2 mm, it almost disappears. Upon cooling, the absolute values of the absorption coefficients fall below 0.2 cm–1. Based on the measured refractive indices, the collinear phase-matching curves are simulated numerically. The theoretical feasibility of IR (1.064 μm) laser radiation conversion into the millimeter-wave range is demonstrated.

The possibility of generating high power terahertz radiation in a nonlinear crystal of zinc germanium phosphide (ZnGeP2) with pumping in a short-wavelength absorption shoulder of 0.67–2.5 μm is studied in detail. High power femtosecond Ti:Sapphire laser system operating at wavelengths of 800 and 950 nm are considered as a pump source. A peak terahertz radiation power of ~1 kW is obtained.

We analyse the functional capabilities of new crystals, BaGa2GeS6 (BGGS) and BaGa2GeSe6 (BGGSe), which are used for nonlinear optical frequency conversion in their transparency range. The wavelengths at which maximum conversion efficiencies can be obtained and the tuning range for difference-frequency generation are found. It is shown that there are wavelength combinations at which the effective nonlinearity coefficient varies only slightly in a wide frequency band.

This paper presents the results of research on parameters of the THL-100 hybrid laser system of the visible spectrum with a view to modernizing it, in order to achieve the boundary parameters required for the pumping of non-linear crystals and production of the unprecedented THz radiation powers. The laser system consists of the Start-480M Ti:Sa starting complex and photodissociation XeF(C-A) amplifier with a 24 cm aperture. After modernization, the starting complex provides the spectrally limited radiation pulse of 60 fs at the first harmonic (950 nm) at energy of 25 mJ and 50 fs at the second harmonic (475 nm) at energy of 10 mJ. In the case of the positively chirped pulse with duration of 50 ps, the energy at the second harmonic is 3 mJ. When amplifying the negatively chirped radiation pulse in the XeF(C-A) amplifier at the wavelength of 475 nm with duration of 1.8 ps, the power of 40 TW is reached, and the maximum energy is 2 J. When amplifying the positively chirped pulse with duration of 50 ps, the highest energy of 3 J is achieved.

A study is performed of using such oxide nonlinear crystals as beta barium borate, lithium triborate, and lithium tetraborate to create powerful sources of terahertz radiation that work in windows of atmospheric transparency. It is shown that in terms of combined properties, such oxide crystals can provide greater efficiency and power output in the terahertz range than the widely used semiconductor and organic crystals.

The paper reports the efficient terahertz generation using nonlinear optical rectification process and comparative studies of optical, dielectric, and surface scattering properties of 1.0 mm thin GaSe: S crystal in transmission/reflection modes. We employed 800 nm wavelength of 140 fs and 50 fs pulse durations obtained from Ti: Sapphire oscillator and amplifier systems at 80 MHz and 1 kHz repetition rates, respectively, and evaluated the THz generation potential and optical properties such as refractive index, absorption coefficients, relative permittivity in transmission mode. In the next step, the crystal was subjected to a commercial THz system of 0.3 picosecond pulses at 100 MHz repetition rate in reflection geometry and once again recorded the similar parameters and compared with the transmission mode data. In addition, these data were used to measure the scattering losses from the surface of the crystal in terms of surface roughness, optical impedance, and Fresnel’s reflection coefficients. The maximum generated THz power was of the order of 4.5 μW with conversion efficiency (η) ∼ 2.2 × 10−3%. The measured THz efficiency was found equivalent to 20 mm thick GaSe crystal used in the difference frequency mixing technique. Finally, we have measured the Rayleigh roughness factor (g) of the crystal surface at different THz frequencies. When g < 1 (for small roughness) the Fresnel reflection loss factor becomes 0.75 at 1.4 THz frequency range which helps to enhance the generated signal.

The current most stringent constraints for the existence of sub-GeV dark matter coupling to Standard Model via a massive vector boson $A^\prime$ were set by the NA64 experiment for the mass region $m_{A^\prime}\lesssim 250$ MeV, by analyzing data from the interaction of $2.84\cdot10^{11}$ 100-GeV electrons with an active thick target and searching for missing-energy events. In this work, by including $A^\prime$ production via secondary positron annihilation with atomic electrons, we extend these limits in the $200$-$300$ MeV region by almost an order of magnitude, touching for the first time the dark matter relic density constrained parameter combinations. Our new results demonstrate the power of the resonant annihilation process in missing energy dark-matter searches, paving the road to future dedicated $e^+$ beam efforts.

The laser-induced damage threshold (LIDT) was measured for a Z n G e P 2 crystal exposed to 0.3–9.5 ps 1030-nm laser pulses. Single-pulse LIDT fluence was ∼ 0.22 J / c m 2 for the laser pulse widths of 0.3–3.5 ps and increased until 0.76 J / c m 2 for 9.5-ps pulses. Multi-pulse LIDT fluence for 0.3-ps pulses at repetition frequencies in the range of 100 Hz–1 kHz was ∼ 0.053 J / c m 2 and decreased further at higher, multi-kHz, pulse repetition frequencies. The coating of the Z n G e P 2 crystal surface with an anti-reflection multi-layer thin film increased the multi-pulse LIDT by one order of magnitude, up to 0.62 J / c m 2 (about 2 T W / c m 2 ). The significant increase in LIDT coupled with a decrease in reflection losses provides a way to cardinally improve efficiency of frequency conversion of popular 1-µm ultrashort pulses into mid- and far-IR ranges with a thin AR-coated Z n G e P 2 crystal sample.

The optical properties and hardness of GaSe, GaSe:Al, GaSe:S:Al, and GaSe:Al:O crystals are studied to create effective terahertz radiation generators. Doped crystals differ from pure GaSe in high mechanical and optical properties at a optimum doping level. Oxygen doping allows wide ranging changes in birefringence in the terahertz range.

A study is performed of the generation of ultrashort THz pulses in Ti, Ni, and TiNi plates, compared to Au, Ag, Cu plates. The possibility of using them as photosensitive materials in bolometers is considered. Such sources can be used in atmosphere monitoring systems.

The use of nonlinear oxide crystals for radiation generation and conversion into terahertz (THz) range using the nonlinear crystal optics methods is a new promising direction that has actively been developed in the last few years. For the first time, the possibility is considered of effective frequency doubling of millimeter radiation in the wavelength range of 400–2500 m in borate crystals. The phase matching angles are calculated. It is shown that o + o → e and e + o → e interactions can be realized in LN and β-BBO crystals, and only o + o → e interactions are possible in LB4 crystals. The walk-off angle and the spectral and angular matching bandwidths are calculated. The maximal efficiency of the second harmonic generation is estimated taking into account the measured optical damage threshold of 150 TW/cm2 for the LB4 crystal and 100 TW/cm2 for the β-BBO crystal. This shows advantages of application of the oxide crystal in the THz range of the spectrum.

Numerical analysis of function capabilities for nonlinear optical frequency conversion (harmonic generation, sum- and difference-frequency generation, parametric oscillation and amplification) in new nonlinear crystals BaGa 2 GeS 6 and BaGa 2 GeSе 6 in their transparency range was performed. The most efficient frequency mixing schemes were defined.

THz wave generation was investigated in nonlinear ZnGeP2 crystals with the fs pumping at 800 nm and 950 nm. Transmission and absorption spectra were analyzed. Refractive indices of the ZnGeP2 in the THz range were measured and approximated in the form of Sellmeier equations. Phase-matching conditions were calculated. Calculated conditions are well correlated with experimental data.

Recently, the ATOMKI experiment has reported new evidence for the excess of e+e- events with a mass ∼ 17 MeV in the nuclear transitions of 4He, that they previously observed in measurements with 8Be. These observations could be explained by the existence of a new vector X17 boson. So far, the search for the decay X17 → e+e- with the NA64 experiment at the CERN SPS gave negative results. Here, we present a new technique that could be implemented in NA64 aiming to improve the sensitivity and to cover the remaining X17 parameter space. If a signal-like event is detected, an unambiguous observation is achieved by reconstructing the invariant mass of the X17 decay with the proposed method. To reach this goal an optimization of the X17 production target, as well as an efficient and accurate reconstruction of two close decay tracks, is required. A dedicated analysis of the available experimental data making use of the trackers information is presented. This method provides independent confirmation of the NA64 published results [1], validating the tracking procedure. The detailed Monte Carlo study of the proposed setup and the background estimate show that the goal of the proposed search is feasible., The European Physical Journal C, 80 (12), ISSN:1434-6044, ISSN:1434-6052

We present the thorough studies of optical properties of BiB3O6 (BIBO) crystal in the millimeter-wave (subterahertz) range. We observe a large birefringence Δn = nZ -nX = 1.5 and the values of absorption coefficients of all three axes to be less than 0.5 cm−1 at the frequency of 0.3 THz. The difference from visible range in angle ϕ between the dielectric axis z and crystallophysical axis X is found to be more than 6°. The simulated phase-matching curves in the xz plane of the crystal show the optimal value of the angle θ to be around 25.5°±1° for an efficient millimeter-wave generation under the pump of 1064 nm laser radiation.

Results from model studies of the possibility of creating a source of high-power picosecond pulses of terahertz radiation via optical rectification and phase-matched down conversion of femtosecond pulses of Ti:Sapphire lasers in a nonlinear LBO crystal are presented. Modified Sellmeier equations are used in calculations. It is shown that the lengths of coherence for the generation of THz radiation at frequencies higher than 0.5 THz are more than 0.5 mm, allowing the technologically simple production of periodic structures. The maximum length of coherence is achieved for s–s type interactions in the XY plane. Phase-matched s–f type down conversion is possible only in the XY plane, including noncritical spectral matching conditions. Maximum efficiency can be expected for three-wave interactions with the polarization of interacting waves parallel to Z axis.

Phase-matching tuning of second harmonic and sum frequencies generation in ZnGeP2 crystal was studied with CO laser in mid-IR range. The experiments and calculations showed a promising of this technique for the spectrum control of frequency converted radiation.

We report the results of the analysis of the functional possibilities of sum and difference frequency generation in the transparency range of a PbIn6Te10 crystal. Regions of angular and frequency noncritical phase-matching, as well as maximum values of the effective nonlinearity coefficient are determined.

Monoclinic and cubic Ga2S3 crystals were obtained by Bridgman and flux methods. For the first time optical properties are measured in the bulk samples including THz range. The transparency range 0.44–25 μm is recorded. Ga2S3 crystal demonstrated 20 times higher light induced damage threshold compared to GaSe. No phonon absorption peaks are found in the THz range at wavenumbers below 100 cm−1. IR and THz optical, as well as other physical properties render Ga2S3 among the prospective materials for THz applications.

Comparative model analysis of the possibilities of down-conversion into the terahertz (THz) frequency range of radiation of all-solid-state femtosecond Ti:Sapphire laser systems and hybrid systems based on the same type lasers and equipped with amplifiers on XeF(C-A) and XeCl(C-A) molecules of excimer gases with harmonic generators on nonlinear LBO crystals is performed. It is established that when the crystal is cooled to a temperature of 80 K, the coefficient of optical losses in the frequency range from 0.2 to 1.5 THz linearly decreases in the direction of the X axis by 5–6 times down to 0.1–0.2 cm–1. It is shown that the use of hybrid laser systems as pumping sources will allow the power of generated THz pulses to be increased from 3.5 to 20 times.

We report the method and results of a complex analysis of phase-matching and nonlinear properties for biaxial crystals of all point groups of symmetry, which determine their functional possibilities in solving various problems of nonlinear frequency conversion of laser radiation.

The effect of doping with sulfur, indium, tellurium, aluminum, erbium, and silver on the mechanical properties of nonlinear GaSe crystals grown by a modified Bridgman method with the rotation of a thermal field is studied. GaSe:S crystals have the best optical properties and GaSe:Al crystals have the highest microhardness. Under identical experimental conditions, we revealed an additive influence of double doping on the properties of GaSe: the crystals doped with sulfur and aluminum demonstrate the maximum efficiency of laser radiation frequency conversion and the maximum hardness. The increased hardness is supported by the appearing ability of the crystals to undergo cracking under the action of high-intensity radiation, which is characteristic of glass.

The bulk crystals of LiBa12(BO3)7F4 were obtained from two flux compositions by the top seeded solution growth method. Crystals grown from BaCO3:BaF2:H3BO3:Li2CO3 = 3:3:3:1 charge were found to have a distinct linear dichroism effect at 0.35–0.65 and around 0.73 μm. This suggests application of the material in the production of polarizing filters and attenuators for visible range lasers.

Optical properties of $\beta$ -BBO crystals i. e. absorption coefficient and refractive index dispersions, were studied using a custom-made terahertz time-domain spectrometer (THz-TDS) in the range of 0.2-2.0 THz at the room temperature (RT) of 293 K and at liquid nitrogen (LN) temperature of 81 K. The measured ordinary and extraordinary wave refractive indices were approximated in the form of Sellmeier equations. Phase-matching conditions for the IR- THz and THz- THz frequency conversion were calculated.

Broadband three-stage frequency conversion of multiline sub-microsecond CO laser radiation in a single sample of ZnGeP

the spectrum of sum frequency generation when converting multiline CO laser radiation in ZnGeP 2 under noncritical spectral phase-matching was thorougly studied. This spectrum was found to be a broadband comb of multiline groups with complicated fine substructure that is attractive for atmospheric spectroscopy.

Optical properties of β-BBO were studied in the range of 0.2 – 2.5 THz at RT and 81 K by THz–TDS. Recorded dispersions were approximated in the form of Sellmeier equations. Phasematched down-conversion, as well as SHG within the THz range, are found possible. Efficient optical rectification into long wavelength region was experimentally observed.

Creation and application of the remote imaging spectrometer based on high power nanosecond terahertz source with standoff detector is reported. 2D transmission images of metal objects hided in nonconductive (dielectric) materials were recorded. Reflection images of metal objects mounted on silicon wafers are recorded with simultaneous determination of the wafer parameters (thickness/material).

The thermo-optic coupling process of second harmonic generation was numerically simulated in ZnGeP2 crystals pumped by a pulsed CO2 laser at the wavelength of 9.6 μm, under the strong and weak cooling conditions. The conversion efficiencies, temperature distributions were calculated during the evolution of the thermo-optic coupling. The results showed that the thermooptic coupling was weak in the strong cooling condition, which nearly did not disturb the conversion processes and temperature distribution, while in the weak cooling case, the temperature distribution showed a great influence on the conversion efficiency and light intensity. Finally, it was found that compensation of the phase mismatch induced by the thermal effect can well recover the conversion efficiency.

GaSe crystals with 0, 0.07, 0.38, 0.67, 2.07, 3, and 5 wt % Te are grown. GaSe:Te (≤2.07 wt %) crystals are suitable for nonlinear optics applications. The optimum doping level is 0.07 wt %. This minimizes optical losses and increases the damage threshold by ≥20%, hardness by 33%, and the efficiency of femtosecond Ti:Sa laser frequency conversion into the 8.8–24 μm and 0.2–3 THz ranges by ≥50%.

Long bases RT THz standoff spectrometer is designed. GaSe 91 S 0.09 :Al(0.03 at.%) DFG of near IR lasers was seeded by THz emission from electronic system. THz generation efficiency was improved by the factor of ≥10 at principally reduced emission bandwidth, and pulse magnitude variation decreased for at least of 4–5 times.

We launched into a development of a new stand-off sensing system that can detect atmospheric and hazardous gases in real atmosphere utilizing THz technology. Narrow line width

Centimeter-sized crystals of GaSe have been grown by a modified vertical Bridgman method from a melt with content 0.01, 0.05, 0.1, 0.2, 0.5, 1, and 2 at.% Al and 0.025, 0.1, 0.5, 1, and 2 at.% Er using heat field rotation. Their physical properties have been investigated and compared with the properties of doped crystals with content 0.1, 0.5, 1, 2, 3, 5, 7, and 10.2 mass% S, 0.01, 0.1, 0.5, 1, 2, 3, and 5 mass% In, and 0.01, 0.1, 0.5, 1, and 2 mass% Te, grown by the same method and by the conventional Bridgman method. Optimal levels of doping of GaSe crystals with Al and Er have been established here for the first time to be equal to 0.01–0.05 at.% Al and ~0.5 at.% Er, respectively, of the growth charge.

The paper reports a first growth of the Al-doped GaSe crystals by modified technology with heat field rotation. These crystals shown from 2 to 3 times lower absorption coefficient at the maximal transparency range to that in crystals grown by conventional Bridgman technology. Possibility of the identification of optimal doping level in grown crystals by analysis of shape of the exciton absorption peak and intensity of the absorption shoulder at the transmission edge was demonstrated. The optimal doping can be attributed to the concentration of Al in the growth charge between 0.02 and 0.05 at.%. The result of the identification is confirmed by frequency conversion experiments: CO2 laser SHG and optical rectification of fs pulses.

Temperature dependences of nonlinear ZnGeP2 crystal indices within the wavelength range of 5.0–6.2 μm (around noncritical spectral phase-matching) were examined with a frequency-selective Q-switched CO laser. The correct dependences were found and applied for numerical study of the spectrum tuning by the crystal temperature changing under the broadband CO laser sum frequency generation. The latter was experimentally studied for the crystal temperature range from the room one up to 145 °C. Requirements for ZnGeP2 crystal temperature changing needed for broadband frequency conversion of 5–7.5 μm radiation were formulated.

A two-cascade frequency conversion of CO-laser radiation is demonstrated in a single sample of a nonlinear ZnGeP2 crystal. The crystal is pumped by a repetitively pulsed cryogenic lowpressure CO laser operating on ~150 vibration – rotational transitions in the wavelength range 5.0 – 7.5 μm, which corresponds to the frequency range of a half octave. In the first conversion cascade, generation of second harmonic and sum frequencies of various pairs of CO-laser radiation give ~350 emission lines in the wavelength range 2.5 – 3.7 μm. In the second cascade, by mixing the radiation converted in the first cascade with the residual radiation of the CO laser we have obtained ~90 lines in the range 4.3 – 5.0 μm and more than 80 lines in the range 7.5 – 8.3 μm. Thus, using a single sample of the nonlinear ZnGeP2 crystal pumped by the radiation of a single CO laser we have produced a source of broadband (more than one and a half octaves) laser radiation, simultaneously operating at ~670 lines in the wavelength range 2.5 – 8.3 μm.

Modified GaSe single crystal was grown from the melt with charge composition GaSe + 10 mass% of AgGaSe2. Lattice structure, visible to mid-IR and further THz range optical properties, as well as Raman spectra were studied in details. The grown crystal was identified as epsilon-GaSe:Ag (0.04 mass%). This silver content in GaSe has resulted in 6% decreased non-linearity that was over compensated in CO2 laser SHG efficiency by vanished of bulk damages and 10-20% improved surface damage threshold. About 30% increased microhardness is promising for cut and polishing at arbitrary direction and epsilon-GaSe:Ag applications in out-of-door systems. (C) 2012 Elsevier B.V. All rights reserved.