Content area
Abstract
A time domain terahertz spectrometer and a bolometer have been used to study the coherent THz radiation emitted from n- and p-type InAs surfaces illuminated by femtosecond near infrared pulses. The magnetic field enhancement of the emitted average power in different polarisations and experimental geometries has been studied and is in qualitative agreement with the predictions of a Drude-Lorentz model for the radiation emitted by surface photocurrents. The dependence of the emitted radiation on the sample temperature, the sample doping density and type and power density of the excitation beam has been studied. The emitted average power was found to decrease rapidly with increasing n-type doping, particularly above doping densities of ~1018 cm-3. A calibrated Golay cell was used to measure the absolute average power emitted under the different conditions, and the peak average power was measured to be 30 <IMG WIDTH=8 HEIGHT=14 ALIGN=MIDDLE SRC="/maths/mu.gif">W for an excitation beam of 1.2 W average power at 765 nm, and for a sample with a carrier density of 1.7x1015 cm-3 at 10 K and under the influence of a magnetic field of 5 T.
A report of InAs having been used as a source of coherent THz radiation for ultrafast spectroscopy, to probe optically excited cyclotron resonance in high resistivity silicon is presented. The masses of the carriers calculated from the cyclotron resonance measurements were found to be m<IMG WIDTH=11 HEIGHT=11 ALIGN=BOTTOM SRC="/maths/perp.gif"> = 0 . 19me and m|| = 0.90me. The temperature dependence of the cyclotron resonance was measured over the range 5 K to 80 K, and a peak is found at ~30 K which can be explained in terms of ionised and neutral impurity scattering at temperatures below 30 K and by phonon scattering above 30 K.
The measurement of small amplitude ferromagnetic resonance oscillations in the time domain in thin films of permalloy(78), iron and cobalt has been achieved by using the time resolved magneto-optic Kerr effect. A stripline device was fabricated to provide an out of plane broadband magnetic pulse with a peak strength of ~5 Oe. The observed frequencies are shown to agree well with the established theory.
