Tunable middle infrared radiation with HgGa2S4 crystal

Optical properties of orange phased HgGa₂S₄ crystal is investigated. Generation of tunable middle infrared radiation by second harmonic of tunable CO₂ laser radiation has been demonstrated. Second harmonic conversion efficiency with respect to other infrared crystal has also been discussed.     

Simulation of parametric oscillation in the submillimeter range at pumping of the ZnGeP2 crystal by a train of 100-ps high-power pulses

Analysis is given for a possibility of singly resonant parametric oscillation in the submillimeter range at synchronous pumping of the ZnGeP₂ crystal by a train of 100-ps second-harmonic pulses from the CO₂ laser with the radiation energy 1.0 J. The calculation shows that using the ZnGeP₂ crystal and the second harmonic of the CO₂ laser with the energy density 1.8 J cm⁻², one can get the peak submillimeter radiation power from 3.6 to 12 MW in the range from 95 to 300 μm (1.0–3.3 THz). The expected peak power values are larger than the experimental ones obtained by other nonlinear optics methods.     

Third harmonic generation of CO2 laser radiation in AgGaSe2 crystal

Generation of third harmonic of CO₂ laser radiation has been obtained in a type-II, ϑ=57° cut 9 mm thick AgGaSe₂ crystal for the first time by sum-frequency-mixing of the fundamental with its second harmonic, the latter being obtained using another type-I, ϑ=55° cut 11 mm thick AgGaSe₂ crystal. The energy conversion efficiencies obtained for second harmonic and third harmonic generations are 6.3% and 2.4% respectively with the input fundamental pump power density of 5.9 MW/cm² only. The wavelength of the fundamental CO₂ laser radiation used for the generation of harmonics is 10.6 μm, P(20) line. A compact TEA CO₂ laser source has been built in the laboratory.     

Pulsed inductive discharge CO2 laser

A pulsed inductive discharge CO₂ laser with a wavelength of 10.6 μm has been created for the first time. The excitation system of a cylindrical pulsed inductive discharge (pulsed inductively coupled plasma) in the gas mixture of CO₂:N₂:He was developed. The temporal and energy parameters of the laser radiation were investigated. The maximum inductive discharge CO₂ laser radiation energy of 104 mJ was achieved. An average power of 3.2 W was obtained at laser generation energy of 65 mJ and pulse repetition rate of 50 Hz. In the cross-section, the laser radiation had the ring shape with an external diameter of 34 mm and thickness of 4-5 mm. The measured divergence of laser radiation was 12 mrad.     

An all-diode-laser-based, dual-cavity AgGaS2 cw difference-frequency source for the 9–11 μm range

2 ) nonlinear crystal is reported. The device uses a standing-wave dual-resonator scheme to enhance both near-infrared lasers. For 7 mW of 778.2-nm and 275 mW of 842.5-nm input powers, 40 nW of 10.2-μm radiation is generated by a preliminary setup. A μW-level range output power is expected after optimization of the device optical components. This compact tunable source will allow Fabry–Pérot high-resolution Doppler-free spectroscopy of spherical molecules, such as OsO₄ or SF₆, in the 9–11 μm range as an alternative source to frequency-stabilized CO₂ lasers. Received: 17 March 1998     

Decay and recondensation of electron-hole liquid in germanium under CO2 laser pulse irradiation: Investigation by microwave conductivity measurements

In microwave conductivity investigations of photoexcited germanium at low temperatures under CO₂ laser pulse irradiation the evaporation of EHL and e-h plasma formation have been observed. This plasma irreversibly vanishes at high CO₂ laser intensities I_CO2 >4 × 10⁵ W cm^?2 but recondenses at low intensities. It was found that complete and irreversible disappearance of EHL is due to the e-h plasma throw out to the crystal boundaries by phonon wind, generated in 10.6 μm radiation absorption whereas at I_CO2 > 10⁶ W cm^?2 it is connected with the crystal lattice heating over the condensation critical temperature. A theoretical analysis of the CO₂ laser produced phonon wind interaction with e-h plasma is briefly presented. By comparing with experimental data on recondensation process the phonon wind efficiency is estimated.     

A comparative study of second harmonic generation of pulsed CO2 laser radiation in some infrared crystals

A comparative study of second harmonic generation of CO₂ laser radiation in different infrared transmitting crystals like HgGa₂S₄, AgGa_xIn_(1−x)Se₂, sulphur and indium doped GaSe and ZnGeP₂ is reported. It is also shown that sulphur doped GaSe crystal is more efficient than ZnGeP₂ crystal in terms of effective figure of merit.     

Mid-infrared trace gas detection using continuous-wave difference frequency generation in periodically poled RbTiOAsO4

A tunable mid-infrared continuous-wave (cw) spectroscopic source in the 3.4–4.5 μm region is reported, based on difference frequency generation (DFG) in a quasi-phase-matched periodically poled RbTiOAsO₄ (PPRTA) crystal, DFG power levels of 10 μW were generated at approximately 4 μm in a 20-mm long PPRTA crystal by mixing two cw single-frequency Ti:Al₂O₃ lasers operating near 713 nm and 871 nm, respectively, using a laser pump power of 300 mW. A quasi-phase-matched infrared wavelength-tuning bandwidth (FWHM) of ∼12 cm^-1 and a temperature tuning rate of 1.02 cm^-1/°C were achieved. Experimental details regarding the feasibility of trace gas detection based on absorption spectroscopy of CO₂ in ambient air using this DFG radiation source are also described. Received: 23 October 2000 / Revised version: 22 January 2001 / Published online: 27 April 2001     

Infrared phase-conjugate reflection by hot electron-induced optical nonlinearity in n-Ge

We have studied backward-degenerate four-wave mixing at CO₂ laser wavelengths in n-type Ge having a free electron density of N=5×10¹⁶ cm^-3. The phase conjugation due to the redistribution of free electrons between the equivalent valleys was observed. The effect occurs only when the electric field E of the light wave is aligned nonsymmetrically relative to the long axes of the valleys in the crystal, and is related to carrier heating by the infrared radiation. Received: 11 April 2000 / Revised version: 13 September 2000 / Published online: 10 January 2001     

Chemisorption,photodesorption and conductivity measurements on ZnO surfaces

Experimental results of a mass-spectro metric analysis of photodesorption from ZnO single crystals at different temperatures are reported. They provide direct evidence that CO₂ is the only photodesorbed species from both the single crystal and powder samples studied. The CO₂ photodesorption occurs only when the incident photon energy exceeds the ZnO band gap energy. Excellent agreement between the illumination time dependence of the CO₂ photodesorption and surface conductivity data in both single crystals and powder samples strongly suggests a substrate dependent mechanism in which photodesorption occurs by the neutralization of chemisorbed CO₂^? “ion-molecules” by photogenerated holes. In addition, measurements of the chemisorption kinetics of oxygen on ZnO single crystal and powder surfaces are reported. The results are compared with CO₂ and CO chemisorptiun experiments to show that, of these gases, only oxygen chemisorbs from the gas phase. Auger analysis of oxygen saturated and photodesorbed surfaces of ZnO show a significant relation between the carbon content and the photo-desorptive and conductive activities of those surfaces. These observations indicate that impurity carbon atoms on ZnO surfaces can be oxidized by electron capture to produce chemisorbed CO₂^? “ion-molecules’ which will then readily photodesorb by bandgap radiation. This proposed process is discussed together with further supporting evidence.     

Far infrared generation by CO2 lasers frequencies subtraction in a ZnGeP2 crystal

a pulsed radiation power of the order of 1W in the submillimeter range (102–110 µm) was reached for the first time, with the aid of a ZnGeP₂ crystal, at the difference frequency of CO₂ laser radiations.     

CO2 laser cleaning of black deposits formed during the excimer laser etching of polyimide in air

Pulsed CO₂ laser cleaning of black debris formed during the excimer laser ablation of polyimide in air is demonstrated. The 10.6 m CO₂ laser radiation is strongly absorbed in the debris but only weakly absorbed in polyimide thus enabling the clean removal of the debris without damaging the polyimide.     

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).     

Mechanism of phase conjugation via stimulated Brillouin scattering in narrow band gap semiconductors

We develop a theoretical model to study optical phase conjugation via stimulated Brillouin scattering (OPC-SBS) in narrow band gap transversely magnetized semiconductors. Threshold value of pump electric field and reflectivity of the image radiation for the onset of OPC-SBS are estimated. The analysis is applied to both cases viz. centrosymmetric (CS) and non-centrosymmetric (NCS) crystals. Numerical estimates made for n-type InSb crystal at liquid nitrogen temperature duly irradiated by nanosecond pulsed 10.6 μm CO₂ laser shows that high OPC-SBS reflectivity (90%) can be achieved in NCS crystals at moderate pump electric fields if the crystal is used as an optical waveguide with relatively large interaction length (L = 5 mm) which proves its potential in practical applications such as fabrication of phase conjugate mirrors.     

Noncollinear phase-matched four-photon mixing of CO2 laser radiation in germanium

Noncollinear four photon mixing of two TEA CO₂ laser beams in germanium at room temperature has been used to obtain phase-matched generation of step tunable radiation in the 8.7 μm region which is of interest for the uranium isotope separation. Using an 8.3 cm long crystal of germanium, peak output power of ≈10kW (corresponding to 1 mJ per pulse) was obtained at 8.7 μm with 3 MW peak input power from each of the two CO₂ lasers operating at 9.6 μm and 10.6 μm.     

Optical rectification in CdS

Optical rectification non-linear process in CdS single crystals at the Nd-YA1G and CO₂ frequencies is reported. V_OR signal measurements versus θ angle, between radiation E-field and crystal c-axis, give non-zero terms values of the optical rectification tensor. The frequency dispersion effect of this non-linear process is discussed.     

Synchrotron radiation assistant MOCVD deposition of ZnO films on Si substrate

The growth of ZnO film on Si(1 0 0) substrate has been studied with synchrotron radiation (SR) assisted MOCVD method. The diethylzinc (DEZn) and CO₂ are used as source materials, while Nitrogen is employed as a carrier gas for DEZn. With the assistance of SR the ZnO film can be deposited even at room temperature. XRD, SEM and photoluminescence (PL) studies show that the crystal quality of ZnO films grown with the assistance of SR is higher than that of those without SR assistance. The growth mechanism of ZnO film with the SR assistant MOCVD system is primarily discussed.     

Photoacoustic gas sensing employing fundamental and frequency-doubled radiation of a continuously tunable high-pressure CO2 laser

We present the first photoacoustic spectrometer for gas sensing employing both the fundamental and the frequency-doubled radiation of a continuously tunable high-pressure CO₂ laser with room temperature operation. A quasi-phase-matched diffusion-bonded GaAs crystal is used in the system for second-harmonic generation. A pulsed photoacoustic detection scheme with a non-resonant cell, equipped with an 80-microphone array, is employed. The wide continuous tuning range in the fundamental (9.2–10.7 μm) and the frequency-doubled (4.6–5.35 μm) regimes, together with the narrow linewidth of 540 MHz (0.018 cm^-1) for the 10-μm region and of 1050 MHz (0.0315 cm^-1) for the 5-μm region, allow the measurement of gas mixtures, individual species and isotope discrimination. This is illustrated with measurements on NO and CO₂. The measured isotope ratio ¹⁵ NO/¹⁴ NO=(3.58±0.55)×10^-3 agrees well with the literature (3.700×10^-3) and demonstrates the good selectivity of the system. Received: 30 April 2002 / Revised version: 10 June 2002 / Published online: 2 September 2002 RID="*" ID="*"Corresponding author. Fax: +41-1/633-1077, E-mail: sigrist@iqe.phys.ethz.ch     

Effect of gamma irradiation on undoped and uranium doped calcium fluoride crystals

Color centers in undoped and U³⁺-doped CaF₂ crystals induced by γ-irradiation with different doses were studied by differential absorption and Raman spectra. Multiple color centers and conversion among them were caused in undoped CaF₂ with the creation of radiation defects. In U³⁺:CaF₂ crystal, trivalent uranium was demonstrated to act as hole trap in the process of γ-irradiation, which was ionized to tetravalent. This process was accompanied by the formation of F₂⁺ centers, but without additional background absorption due to radiation defects.     

Effects of Na2CO3 flux addition on the structure and photoluminescence properties for Eu-doped YVO4 phosphor

The luminescence properties of (Y_0.9Eu_0.1)VO₄ phosphor with Na₂CO₃ flux prepared using the solid-state reaction were investigated. The XRD patterns show that all of the peaks are attributed to the YVO₄ phase. The best crystallinity was obtained with 2 wt% Na₂CO₃ flux addition. The surface morphology of (Y_0.9Eu_0.1)VO₄ phosphor changed from fluffy to a bar shape structure after Na₂CO₃ flux addition due to the tetragonal crystal system of YVO₄. The calcined powders emit bright red luminescence centered at 618 nm due to the ⁵D₀→⁷F₂ electric dipole transition under an excitation wavelength of 318 nm; its intensity was increased about 15% with 2 wt% Na₂CO₃ flux addition. Red shift behavior was observed for the charge transfer state (CTS) absorption, which was due to the grain size of (Y_0.9Eu_0.1)VO₄ phosphor increasing with increasing flux content. For 2 wt% Na₂CO₃ flux addition, the red emission of the (Y_0.9Eu_0.1)VO₄ phosphor had CIE chromaticity coordinates of (0.66, 0.34), which are very close to the NTSC system standard red chromaticity coordinates of (0.67, 0.33).