Tom L. Stocker

Reflectivity Enhancement of Semiconductors by Q‐Switched Ruby Lasers

Reflectivity enhancement of single‐crystal silicon, germanium, gallium arsenide, and indium antimonide and polycrystalline boron and cadmium selenide was observed. The reflectivity variations were produced by irradiating the semiconductor with the output of a Q‐switched ruby laser (6943 A) with pulses of 30 and 10 nsec. The reflectivity variations were monitored with a pulsed argon‐ion laser (mainly 4880 and 5145 A). The effect of surface damage at high power levels produced an irreversible reflectivity decrease. Polished samples of molybdenum and tungsten, when irradiated by the Q‐switched ruby‐laser light, exhibited irreversible decreases in reflectivity due to surface damage and reversible variations, which were attributed to elastic deformation. Reflectivity enhancement of semiconductors was attributed to the formation on the semiconductor surface of a liquid film with metallic properties.

Effect of electron-hole recombination processes on semiconductor reflectivity modulation

The increases in the reflectivity of germanium, silicon, GaAs, GaSb, In As, InSb, CdTe and CdSe have been observed by use of a ruby laser to produce the electron-hole plasma that is responsible for the reflectivity changes and to probe the reflectivity changes. Observations were made with the ruby laser light incident at the pseudo-Brewster angle in order to measure small increases in reflectivity. In the case of the III-V and II-VI semiconductors, the increase in reflectivity was found to exhibit a linear dependence when plotted as a function of the square root of the incident light intensity. Germanium and silicon exhibited a linear dependence of the increase in reflectivity on the incident light intensity. These results agree with the calculated dependence that ascribes the differences between the two groups of semi-conductors to the fact that the recombination of electrons and holes is a direct process in the III-V and II-VI compounds and in germanium and silicon occurs mainly by trapping of the free carriers at impurity centres.

Mode Selection Properties of Segmented‐Rod Giant Pulse Lasers

The segmented‐rod ruby laser structure has been operated as a giant pulse laser by use of a Kerr cell to provide the required Q switching. The giant pulse output, peak power of 106 W/cm2 and pulse width of 30 nsec, has been analyzed by use of Fabry‐Perot etalons. The number of oscillating modes is shown to be similar to that previously observed in pulsed operation. Good agreement is found between the measured and computer‐calculated frequency differences. The computer calculations presented indicate that the best performance can be expected when the two laser rod segments are of approximately equal length.

Giant pulse laser operation with semiconductor mirrors

Abstract : Giant optical pulses have been produced in the operation of lasers in which one reflector is a polished optically flat semiconductor and the other is a partially reflecting multiple film dielectric mirror. Giant pulse operation is initiated when the laser light is sufficiently intense to produce an increase in semiconductor reflectivity by inducing a high carrier concentration. Results of giant pulse operation with ruby lasers using germanium, silicon, boron, GaAs, GaSb, InAs, and InSb and Nd3+ glass lasers using germanium, InAs and InSb are presented. These results are contrasted with operation of lasers using dielectric mirrors of comparable reflectivity. In all cases, the semiconductors were severely damaged after one or a few giant pulses. Much reduced semiconductor damage was observed when InAs and particularly InSb were used at oblique incidence. Giant pulse operation was observed with semiconductor mirrors (germanium, InAs and InSb) cooled to 100 K. The comparative performance of the semiconductor giant pulse lasers can be understood on the basis of a model in which the dominant loss mechanism is due to the recombination of electron-hole pairs. (Author)

Power‐Dependent Frequency Shifts in Ruby Lasers at 77°K

Ruby lasers, when operated near threshold at 77°K with a low Q cavity (one end silvered for maximum reflectivity and the other end uncoated) oscillated at two frequencies. The frequency differences near threshold, measured with Fabry‐Perot etalons, were in good agreement with the splitting of the R1 doublet, previously measured by microwave methods. As the power of the laser increased, the frequency difference between the oscillating components [2Ē → 4A2(±32, ±½)] decreased. These results are accounted for on the basis of saturation broadening of overlapping Lorentzian lines. The observed variations in the splitting are in good accord with the theory. The rise in temperature of the ruby rods produced a negligible change in the splitting of the R1 doublet. The same mechanism which produces the variation in the splitting should broaden the output spectrum of the laser.

THERMAL EFFECTS IN SEMICONDUCTOR REFLECTIVITY ENHANCEMENT

The time dependence of the enhanced reflectivity of semiconductors, produced and monitored by a pulsed argon ion laser, showed that the electron‐hole plasma responsible for the enhanced reflectivity was thermally generated.

9C10 - Laser operation with liquid semiconductor mirrors

Q -switched laser operation has been observed using a liquid selenium mirror as one of the reflectors in a ruby laser. Detailed measurements of the changes in reflectivity of liquid selenium under ruby laser giant pulse excitation were obtained using a pulsed argon ion laser to monitor the reflectivity variations. With liquid selenium, an initial decrease in reflectivity was observed which was followed by oscillations. The initial decrease was attributed to deformation of the liquid surface by heating. The Q -switched behavior is attributed to the periodic deformation and heating of the liquid selenium mirror.

Recombination Radiation of Boron

The band gap fluorescence of boron has been observed for the first time using Czochralski-grown, high-purity samples. A 30-mW, cw, helium-neon laser was used to excite the fluorescence and a high-speed, quartz-prism spectrograph was used to record the emitted radiation. The peak intensity of the luminescence (recombination radiation) at 80°K was found to occur at 1. 55 ± 0. 03 eV.

Linewidths of the R′ Optical Transitions of Chromium Ions in Ruby

New experimental measurements of the temperature dependence of the R′ fluorescence of ruby (77°–450°K) have disclosed features not previously observed. Direct phonon transitions to nearby levels contribute less and indirect Raman processes contribute more to the R3′ linewidth than had previously been reported. Direct phonon relaxation from the R1′ and R2′ levels to the 2E level contributes appreciably to linewidth of their fluorescence to the ground state.