Far-field patterns of electron-beam pumped semiconductor lasers

Laser action is described for total internal reflection modes such that the far-field pattern is a circular ring surrounding the crystal in a plane perpendicular to the rectangular cavity faces. A number of recent experiments have shown that this far-field pattern can be observed in GaAs samples as well as in ZnO and CdS. Experiments are reported for single crystals at a temperature of 77°K pumped by a low-voltage electron beam using 100-ns low-duty cycle pulses. Photographs of the far-field pattern show a narrow line extending 360° around the axis of the crystal and having an angular width of 5 to 10°. Photographs of the near-field pattern indicate that the laser light is emitted in quadrants at the four corners of the rectangular cavity, combining at large distances to produce a 360° pattern. Experimental results on a number of CdS crystals of various thicknesses show that the voltage threshold for lasing (at constant current density) for the total internal reflection mode varies approximately as the 0.7 power of the crystal thickness. Losses from total internal reflection are negligible, and for CdS and ZnO platelets absorption losses are also so small that lasing can occur when the penetration depth of the electrons is only 1/40 of the total crystal thickness. Because of these low losses, lasing has been observed in ZnO and CdS at 2.5 keV and a current density of about 5 A/cm
²
for crystal platelets approximatley 2 microns thick. Since the penetration of a 2.5 keV electron is only about 500 Å, this indicates that surface losses in CdS and ZnO with as-grown surfaces are also small. A similar thickness dependence is reported for GaAs doped with
1.5 \times 10^{19}
Zn atoms/cm
³
, but absorption losses appear to be higher. Thus a 2-micron-thick crystal of GaAs pumped with an electron beam of the same current density as above lases with a voltage threshold of 10 keV. The threshold voltages measured for ZnO, CdS, and GaAs are considerably lower than any previously reported for electron-beam pumped semiconductor lasers.