FIR gain measurements in cw laser pumped CH3OD

Direct measurements of small signal and saturated gain in cw laser pumped CH₃OD are reported for three lasing transitions of 57 m, 82 m and 103 m. The 57 m transition has a measured gain of 0.6/m, the strongest gain in methanol reported so far. Moreover small gain saturation of this line makes it to be one of the strongest known cw FIR laser lines.     

Tunable CW laser oscillation of NdP5O14 at 1.3 μm

Low-threshold tunable cw laser oscillation is achieved at the⁴ F _3/2⁴ I _13/2 transition of NdP₅O₁₄. Continuous tunign over 1.8% of the central wavelenght is possible with 55 mW of absorbed pump power. At gain line maximum near 1.32 , the output power exceeded 1 mW. Pump threshold as low as 8 mW was observed. Reliable cw operation of the 1.3 m NdP₅O₁₄ laser pumped by a 15 mW AlGaAs laser diode is demonstrated.     

High repetition rate CH4 laser

A 16-m CF₄ laser oscillator has operated at 1 kHz in a cooled static cell. Pump energies required from the low pressure, Q-switched, cw discharge CO₂ laser were as low as 60 J. The laser cavity employed a multiple-pass off-axis path resonator in a ring configuration. CF₄ laser power at 615 cm^–1 and a 1-kHz repetition rate exceeded 300 W.     

Efficient vertical-cavity lasers

Recent experimental and theoretical results from strained InGaAs/GaAs quantum-well vertical-cavity surface-emitting lasers indicate that relatively high-power, efficient devices are possible. On 10 m square devices cw output powers of more than 3mW at room temperature and 0.4 mW at 100°C are observed. Broad-area, 100 m diameter devices gave pulsed outputs of over 0.5 W and 60 m diameter devices provided more than 12 mW cw. The device parameters have been modelled and found to be consistent with theoretical predictions. The results generally indicate that considerable device improvements are still possible by reducing internal losses, series resistance and voltage and improving heat sinking. With current internal loss levels 30 cm^–1, three InGaAs quantum-wells have been found to be optimum.     

Reactive pulsed laser deposition and laser induced crystallization of SnO2 transparent conducting thin films

Transparent conducting SnO₂ thin films with a thickness between 1000–2000 Å were deposited on glass, quartz and silicon substrates using standard pulsed laser deposition techniques with two different targets (Sri and SnO₂) and with three different laser wavelengths (1.06, 0.532 and 0.266 ) from a Q-switched Nd: YAG laser. Tin dioxide films with optical transmission over most of the visible spectrum exceeding 80% were obtained using a Sn target and a background oxygen pressure of 20 Pa. The electrical resistivity () depended strongly on the substrate temperature during deposition, with the lowest values of of about 10^–2 -cm obtained when the substrate was maintained at 400°C during deposition. Using SnO₂ targets, predominantly amorphous phase SnO₂ films were deposited on Si substrates and then transformed into polycrystalline Sn₃O₄ by laser induced crystallization ( = 1.06 m). Whereas these later films were essentially non-conducting as deposited ( > 400 -cm), the electrical resistivity was permanently reduced after laser induced crystallization by a factor greater than 1000 to a value of approximately 4 × 10^–1 -cm.     

Formaldehyde sensor using interband cascade laser based quartz-enhanced photoacoustic spectroscopy

A novel continuous-wave mid-infrared distributed feedback interband cascade laser was utilized to detect and quantify formaldehyde (H₂CO) using quartz-enhanced photoacoustic spectroscopy. The laser was operated at liquid-nitrogen temperatures and provided single-mode output powers of up to 12 mW at 3.53 m (2832.5 cm^-1). The noise equivalent (1) detection sensitivity of the sensor was measured to be 2.2×10^-8 cm^-1W(Hz)^-1/2 for H₂CO in ambient air, which corresponds to a detection limit of 0.6 parts in 10⁶ by volume (ppmv) for a 10 s sensor time constant and 3.4 mW laser power delivered to the sensor module. PACS 42.62.Fi; 72.50.+b     

Experimental study of laser induced decomposition based processing of a brittle plastic,CR-39 (allyl diglycol carbonate)

A cw CO₂ laser, coupled with an astigmatism free beam focussing mirrors arrangement is used for processing a brittle plastic, CR-39 without producing cracks, vents or chips. The processing is based on the formation of volatile products of laser-induced decomposition in the plastic. Threshold fluence for the decomposition (found to be independent of the power density and beam residence time) in CR-39 at=10.6m is determined to be 25 J cm^–2 and the decomposition threshold power density for cw irradiation 2.1±0.5 W cm^–2. The depth and width of the tapered laser processed region are observed to increase with power density and beam residence time. The widths attain a steady state value of 1 mm at beam residence time above 65 ms, for a fixed power density (2.5×10⁴ W cm^–2) and sheet thickness (250 m). Taper angle of the edges decreases with increasing power density and/or beam residence time. The heat affected zone (measured in crossed polarisers) around the processed region is found to extend with increasing beam residence time but remains unaffected on changing power density. The results are discussed in terms of the optical and thermophysical properties of CR-39 and the parameters of the interacting laser beam.     

The spectrum of a quasiperiodic Schrödinger operator

The spectrum (H) of the tight binding Fibonacci Hamiltonian (H _mn= _m,n+1+ _m+1,n + _m,n v(n),v(n)= ((n–1)), 1/ is the golden number) is shown to coincide with the dynamical spectrum, the set on which an infinite subsequence of traces of transfer matrices is bounded. The point spectrum is absent for any , and (H) is a Cantor set for 4. Combining this with Casdagli's earlier result, one finds that the spectrum is singular continuous for 16.On leave from the Central Research Institute for Physics, Budapest, Hungary     

Short pulse scattering from a laser produced plasma

Experiments are reported on 1.06m scattering from a laser produced plasma. Generally in such experiments scattering is observed on the shock wave front at 90° from the incident beam. In our set-up the plasma is expanding in a Fabry-Perot resonator and we note a strong emission at 1.06m.     

Positron annihilation anomaly in the high-temperature superconductor YBa2Cu3O7−x

We have measured the ac susceptibility of a wire with a Nb core (1.27 mm diam.) and a Cu cladding (0.37 mm thickness) atT50 K andB0.1 mG. Due to its proximity to Nb, the Cu becomes fully superconducting. From the data we find a breakdown fieldH _b =1.2 (mG) and a coherence length =2.2T ^–1/2 (m) for the Cu, as well as a field penetration depth -34T ^1/2 (m) at the Cu/Nb interface.     

New cascade laser transitions in CH2F2 pumped with the 9R32 line of a cw CO2 laser

New cascade laser transitions of¹²CH₂F₂ at 172.50m, 208.83m, 220.44m, 223.99m and 250.61m are reported. A waveguide FIR laser was pumped with a quasi cw¹²C¹⁶O₂ laser operating on the 9R32 line. Together with the already known lines at 184.3m, 196.1m and 235.9m, the laser lines can be assigned to rotational transitions in the ₉ vibrational band of¹²CH₂F₂ and to refill transitions of the vibrational ground state ₀.     

Laser damage in silicon photodiodes

Thermal damage of silicon photodiodes exposed to intense optical radiation is investigated. Damage thresholds of Si photodiodes irradiated by 1.06m laser pulses are reported for values of irradiation time,, ranging from 10^–8 to 1s. Threshold laser irradiation produces visible microscopic damage and a permanent degradation in photoresponse. The loss of responsivity is associated with degradation of the detector diode characteristics due to laser-induced heating. The time and wavelength dependence agree with the predictions of a thermal model which treats a semi-infinite material irradiated by a Gaussian laser beam. The energy density thresholds are independent of for short irradiation times and asymptotically approach a limiting behaviour which increases as for long times. They are given by the empirical relationE ₀=65[1+217/tan^–1(258)^1/2] J cm^–2 for 1.06m radiation. The thresholds at short irradiation times of detectors damaged by 1.06m radiation are about 25 times larger than those of detectors exposed to 0.6943m radiation. The greater susceptibility at 0.6943m is attributed to a larger optical absorption coefficient.     

Strong far-infrared laser action in carbonyl fluoride and vinyl fluoride

Intense far-infrared laser action is reported for carbonyl fluoride and vinyl fluoride. Eleven new lines with wavelengths between 339 m538 m were obtained by optically pumping carbonyl fluoride with numerous lines of the 10.4 m band of a cw CO₂ laser. Twenty-three lines with wavelengths in the range 172 m783 m were detected when pumping the recently discovered efficient FIR laser molecule vinyl fluoride. In addition, three very weak new lines were found using 1.1-difluoroethylen.     

Four-mirror imaging system (magnification +1/5) for ArF excimer laser lithography

Research was conducted on the axially symmetric four-mirror system (magnification +1/5) for use in optical lithography using an ArF excimer laser beam. The initial design is derived from an extensive numerical calculation that makes the sum of the third-order aberration coefficients very small _j=I ^V |S _j |<10_-6. Using an optimization method (damped least-squares method) the finite aberrations are reduced; then, to obtain the diffraction-limited performance, three surfaces are aspherized. The final system has NA=0.38 for the ArF excimer laser line (=0.193 m) and depth of focus of 1.2 m over a 2.6×2.6 cm² object field. Nearly all rays fall within the Airy disk in the image plane. The resolution is 540 cycles mm^-1 at MTF=0.5 level for axial object point. We consider that the present four-mirror system may be further refined for use in soft x-ray lithography.     

74% Slope efficiency from a diode-pumped Yb3+:LiNbO3:MgO laser crystal

An optimization of the laser action performance from a diode-pumped Yb³⁺-doped LiNbO₃:MgO crystal has been carried out. In this sense, efficient laser action at 1.06 m when pumping with a fiber-coupled laser diode at 980 nm has been demonstrated, achieving laser slope efficiencies as high as 74%. The influence of output mirror transmittance on both pumping threshold and laser slope efficiency has been investigated, and the parameters of relevance in laser dynamics (emission cross section and optical losses) have been determined. Under the experimental conditions leading to maximum slope efficiency, the pump power at threshold was 300mW, and the pump-to-laser conversion efficiency was 40%. PACS 42.55.Xi; 42.55.Rz; 42.60.Lh     

CW and pulsed FIR-CO2 hybrid laser with improved efficiency and amplitude stability at short FIR wavelengths

For the first time stable cw output of a FIR-CO₂ hybrid laser has been achieved at wavelengths shorter than 300 m. Due to the saturable absorber characteristic of the FIR laser gas, cw or pulsed emission is observed on both, the CO₂ and the FIR laser output, depending on the operating conditions. Results are reported on different resonant lines in CH₃OH and the 67 m Raman line in NH₃. The good efficiency of this laser is also demonstrated by the excitation of two new emission lines in CH₃OH, namely 49 and 56 m, pumped by 9R(22) and 9R(24), respectively.On leave from the Institute of Physics, University of Beograd YU-11000, Beograd, Yugoslavia     

Spectral investigation of cw rf-pumped atomic Xe laser with a slab geometry

A radiofrequency excited atomic Xe slab laser with an active volume of 2 × 10 × 300 mm³ using a quartz envelope containing the laser-gas mixture shows a stable cw performance with an output power of almost 1 W. The free-running system oscillates, depending on gas composition and density, on several lines between 1.73 and 3.51 m. Line competition phenomena are observed. Single-line oscillation yields more than 500 mW.     

Synthetic proustite (Ag3AsS3): A summary of its properties and uses

There is at present great interest in crystals which are suitable for efficient mixing at optical frequencies. Such a crystal should be acentric, have a wide transmission band, have a large non-linear coefficient and permit phase matching. Proustite possesses such properties [1], and this paper summarises the information that has been obtained to date at R R E on these properties and their use in up-converting 10m radiation to the visible, on the available optical quality, on the electro-optic effect, and on electrical conduction in the crystal; some general observations on the type of damage caused to proustite by laser beams are given.     

GalnAsP-InP double-heterostructure lasers

GaInAsP-InP double-heterostructure lasers which are the most promising optical sources for the long wavelength region have been investigated. In the crystal growth, it was found that the liquidus and solidus isotherms are affected by the orientation of InP source and substrate crystals in the liquid phase epitaxial growth of GaInAsP alloys. The unit cell of the GaInAsP epitaxial layer is tetragonally deformed due to the interface lattice misfit such that the lattice constant parallel to the wafer surface is invariant across the interfaces in the GaInAsP-InP wafers. Two kinds of stripe lasers, planar stripe and buried lasers, have been fabricated by use of GaInAsP cap layer for good p-type contact. The planar stripe lasers of 10,15 and 20 m wide stripe operated in fundamental-transverse mode with 20–30% differential external efficiencies per facet. The 15 m wide stripe laser showed good mode characteristics to operate in a fundamental-transverse mode up to the cw output power of 22 mW per facet and in a single longitudinal mode over a wide range of currents. The buried stripe laser operated in a fundamental-transverse mode with the threshold current as low as 30 mA by making the stripe width narrower than 2 (m. The laser succeeded in cw operation at 100 °C because of its low threshold current and low thermal resistance.The authors would like to thank K. Noda, N. Kuroyanagi and Y. Furukawa for their encouragement. Thanks are also due to H. Kano for permission to use his results of buried stripe lasers prior to publication, and S. Ando for his technical assistance.