Terahertz spectroscopy and laser induced infrared emission spectroscopy of nitromethane and optical properties of laser-induced carriers on semiconductor surfaces probed by a 10.6 micron wavelength carbon dioxide laser.

摘要

This work consists of two parts, (1) Terahertz (THz) spectroscopy and laser-induced infrared emission spectroscopy of nitromethane and (2) optical properties of laser-induced carriers on semiconductor surfaces probed by a 10.6 mum wavelength CO2 laser. In the spectroscopic study of nitromethane, previously unreported low resolution rotational-torsional spectra in the THz frequency were obtained by a Bruker IFS 66 v/S Fourier transform spectrometer. The acquired spectra were then compared with a calculation based on a rotational-torsional Hamiltonian which includes centrifugal distortions and rotational-torsional coupling terms. Even though the constants used in the calculation were a result of fitting the microwave spectrum, a discrepancy was observed between the calculated and the experimentally obtained spectrum.;In addition, gaseous nitromethane was irradiated with a c.w. CO 2 laser (∼20 W cm-2 intensity, 10.6 mum wavelength) and the laser-induced steady state emission spectrum was analyzed with the IFS 66 v/S spectrometer. The laser-induced emission spectrum showed the characteristics consistent with the laser-heated thermal emission. The decay constant of the emission followed by a 100 ms CO2 laser pulse was measured with a pyroelectric detector and determined to be 0.3 s.;In part II, several polycrystalline semiconductors [silicon (Si), germanium (Ge), gallium arsenide (GaAs), and cadmium telluride (CdTe)] were irradiated with a 150 Ps Nd:YAG laser (532/1064 nm wavelength) and induced changes in the optical properties were monitored by measuring the time-resolved reflectance and transmittance of a low power CO2 laser incident on the samples at the Brewster angle. The experimental results showed a sub-nanosecond increase in the reflectance and a longer increase in the absorption as a result of electron-hole pairs (i.e. carriers) generated by absorption of the incident Nd:YAG laser pulses.