We employ spectroscopic ellipsometry in the terahertz (0.2 to 1.5 THz) and the mid-infrared (9 to 50 THz) spectral range for the non-contact, non-destructive optical determination of the free-charge-carrier properties of low-doped Silicon bulk and thin film structures. We find that carrier concentrations as low as 1015 cm−3 in thin films can be unambiguously determined. We envision ellipsometry in the THz spectral range for future non-contact, non-destructive monitoring and control applications.

Rapidly evolving plasmas represent a challenging environment for both study and control. Density, collision frequency and temperature fluctuations can change over orders of magnitude on time scales of one ns with spatial features less than one cm and thus are not amenable to conventional continuous-wave diagnostic techniques such as microwave or mm-wave interferometry. We have developed a new technique for studying plasmas undergoing rapid nonequilibrium changes that uses THz time-domain spectroscopy (THz-TDS) in conjunction with optical fluorescence imaging. The advantages of using THz pulses lie in the fact that the broad bandwidth of a THz pulse contains frequency components both above and below the plasma frequency allowing a single ps-duration pulse to carry away information about the complex path-integrated susceptibility. Transverse fluorescence gives us a model of the longitudinal plasma distribution and using a novel rms error-minimization technique we can recover the real and imaginary parts of the susceptibility with <5 mm spatial and, potentially, ps time resolution (we are currently limited by S/N considerations to averaging over several THz pulses and thus obtain 40 ns resolution). From this we obtain the electron density and collision frequency, spatially and temporally resolved, with dynamic range >103. The principle of this new technique will be discussed along with results on a pulsed DC-discharge plasma. We will also present some new ideas such as concurrent molecular spectroscopy and computed tomography.

Pulsed THz wave spectroscopy using air as the THz wave emitter with the excitation of femtosecond laser provides intense (>100 kV/cm) and broadband THz waves (usable bandwidth from 0.3 to 11 THz). Using the air-biased-coherent-detection (ABCD) method, air can also coherently detect pulsed THz waves over a broadband spectrum. By utilizing these two technologies, we developed a prototype THz air photonics time-domain reflection spectrometer, and applied it on many materials in normal reflection geometry. Optical properties of CaCO3 crystals and several other samples in the THz range were studied. The system provided a signal-to-noise ratio (SNR) over 1000:1, with 0.1 cm−1 frequency resolution. The results acquired from both transmission and reflection measurements were then compared.