Showing papers by "Hoi Sing Kwok published in 1983"
TL;DR: In this article, the design and construction of a CO2 laser-triggered plasma shutter is described and an inexpensive pyroelectric detector and a novel needle-resistor spark gap, requiring an applied voltage of less than 500 V, are employed.
Abstract: Design and construction of a CO2 laser‐triggered plasma shutter is described. An inexpensive pyroelectric detector and a novel needle‐resistor spark gap, requiring an applied voltage of less than 500 V, are employed. The total electronic delay can be made to be less than 15 ns with subnanosecond jitter. This plasma shutter finds applications in producing picosecond CO2 laser pulses and in optical isolation.
15 citations
TL;DR: In this article, the melting of heavily doped silicon (>1018 cm−3) was studied using both nanosecond and picosecond laser pulses and evidence for melting was presented and the duration of the melting was measured at intensities below the damage threshold.
Abstract: CO2 laser‐induced melting of heavily doped silicon (>1018 cm−3) was studied using both nanosecond and picosecond laser pulses. Evidence for melting is presented and the duration of the melting was measured at intensities below the damage threshold. Dense plasma formation was observed before melting occurred. The melt durations were considerably longer than reported previously for visible laser pulses of comparable durations, indicating a very deep (≳1 μm) molten layer had been achieved.
7 citations
TL;DR: The observation of fast reproducible electrical signals from silicon photodiodes when exposed to high power CO2 laser radiation, which is reported to be strong enough to induce melting of the substrate.
Abstract: The CO2 laser is not capable of producing a dense semiconductor plasma by direct ionization of valence band electrons in most semiconductors. However, at sufficiently high intensities, strong nonlinear interactions can occur that may lead to significant heating of the substrate. Intense thermal heating of existing free carriers may also be possible because of the high absorption cross section at 10.6 μm. These interactions may in fact be strong enough to induce melting of the substrate. It has been demonstrated in InSb where two-photon absorption occurs and highly doped silicon where gradual thermal runaway heating is possible. In all the above cases, the semiconductor is first heated, then melting occurs followed finally by a surface spark formation (laser-induced damage). In this Letter, we wish to report the observation of fast reproducible electrical signals from silicon photodiodes when exposed to high power CO2 laser radiation. The power of the laser was kept low enough so that the diodes did not suffer any degradation. The experimental arrangement was straightforward. A well-characterized hybrid TEA low pressure double-discharge CO2 laser was used in the experiment. It produces smooth 60-nsec single-longitudinal mode CO2 laser pulses with an amplitude fluctuation of ±5%. The laser pulse was directed onto the reverse-biased silicon photodiode without focusing. The photodiode signal was then ac coupled to a fast oscilloscope for observation. The laser intensity was controlled by inserting a calibrated CaF2 attenuator into the beam path. The metal can and window protecting the silicon chip were removed to expose the chip for direct illumination. Two commercially available photodiodes were used in these experiments: Silicon Detector Corp.'s model SD 0411222011 and a Radio Shack model TIL 413 PIN diode. The manufacturer's specified response time for the former was 5 nsec, Fig. 1. Upper traces: CO2 laser pulse; lower traces: electrical responses of the photodiodes to 5 MW/cm of CO2 laser intensity. (a) SDC diode; (b) Radio Shack diode; (c) SDC diode with double-mode laser pulse input. Horizontal scale: 50 nsec/div.
5 citations