Continuous Wave THz Spectroscopy Research Articles

Broadband cw-terahertz spectroscopy for characterizing reactive plasmas

Terahertz spectroscopy is seeing increasing use in analytical chemistry because of its ability to access ground state molecular rotational and vibrational transitions. Experiments reported on the use of THz spectroscopy to the studies of low pressure plasmas have focused on the use of pulsed sources, which have limited spectral resolution. These are suitable in the characterization of plasma parameters such as electron density, but somewhat inadequate in the study of molecular absorption spectra. The recent commercial availability of continuous THz sources with much higher spectral resolution has afforded the detailed probing of molecular absorption features but they have not yet been exploited in studies of low pressure plasma chemistry. One of the major benefits of continuous-wave THz spectroscopy (cw-TS) compared to optical and infrared methods is its applicability to the study of both plasma chemistry as well as parameters, i.e. electron density and collision frequency. In this article, we provide first insights into the use of cw-TS as a plasma diagnostic, and demonstrate the opportunities and challenges by example measurements in a low-pressure argon-methanol radio frequency inductively coupled plasma in which we estimate both the plasma-induced dissociation in the precursor methanol concentration, as well as the electron density and momentum scattering collision frequency.

High-frequency breakdown of the integer quantum Hall effect in GaAs/AlGaAs heterojunctions

The integer quantum Hall effect is a well-studied phenomenon at frequencies below about 100 Hz. The plateaus in high-frequency Hall conductivity were experimentally proven to retain up to 33 GHz, but the behavior at higher frequencies has remained largely unexplored. Using continuous wave THz spectroscopy, the complex Hall conductivity of GaAs/AlGaAs heterojunctions was studied in the range of 69-1100 GHz. Above 100 GHz, the quantum plateaus are strongly smeared out and replaced by weak quantum oscillations in the real part of the conductivity. The amplitude of the oscillations decreases with increasing frequency. Near 1 THz, the Hall conductivity does not reveal any features related to the filling of Landau levels. Similar oscillations are observed in the imaginary part as well, this effect has no analogy at zero frequency. This experimental picture is in disagreement with existing theoretical considerations of the high-frequency quantum Hall effect.

Multilayer Thickness Determination Using Continuous Wave THz Spectroscopy

We present a multilayer thickness measurement system based on optoelectronic continuous wave THz spectroscopy. Due to its wide tuning range, high frequency resolution, and fast data acquisition the system combines micrometer precision with short measurement time. In addition, the presented system is not limited by the 2π uncertainty of previous continuous wave signals and is therefore capable of measuring thick layers. Thus, the presented system and measurement method are a cost effective alternative to THz time-domain systems in the field of thickness measurements.