About the Technology
Time-domain versus continuous wave
Since the early 1990’s, time-domain THz (TDS) measurements have been used to characterize the properties of numerous electronic and magnetic materials. Most efforts with continuous wave (CW) THz, however, have been focused on characterizing chemical compounds, typically under ambient conditions.
Lake Shore is developing the methodologies to enable variable temperature CW-THz transmission measurements of electronic and magnetic materials in two distinct sample types — semiconducting wafers (like InAs or InP) and thin conductive films supported by an insulating substrate (like ZnO/sapphire, graphene on silicon, or 2DEGs). By replacing TDS technology with less costly, higher resolution CW spectroscopy, instrumentation cost can be reduced by 50 to 75%, opening the technology to a much broader market. In addition, no commercial THz systems currently exist that enable cryogenic (4 K) and high field (9 T) measurements.
Time-domain THz spectroscopy
In TDS, optical pulses from a mode-locked, femtosecond laser are split into two optical paths. The first path leads to an emitter where the optical pulses are transformed into ultrashort, electromagnetic (or THz) pulses by a photoconductive switch (PCS), semiconductor, or nonlinear crystal. For transmission measurements, the THz pulse then propagates through a sample before being focused onto a THz detector — typically a second PCS device or an electro-optic crystal. Optical pulses diverted from the emitter path are time-delayed with a mechanical or fiber optic delay stage and are used to drive the ultrafast THz detector. The time delay of the detector pulse is varied and the electric-field amplitude of the THz waves is derived from the delay-dependent PCS photocurrent or optical polarization. Fourier transformation of the THz pulse yields the frequency dependent transmission of the sample. The spectral resolution of the TDS technique is typically 3 to 10 GHz and limited, because of the reciprocal nature of the Fourier Transform, by the extent of the delay time trace.
Continuous wave THz spectroscopy
Photoconductive mixing or photomixing is a cost-effective alternative to the pulsed laser solutions inherent to TDS sources. Photomixing also provides the largest continuously tunable range among CW radiation sources. THz photomixers operate by illuminating narrow gaps in the DC-biased electrodes of a PCS with a fraction of the combined output of two, single-color diode lasers. The mixed optical beams induce an oscillation in the photocurrent at the (THz) difference frequency between the two diode lasers. A broadband antenna and silicon lens convert THz frequency photocurrent oscillations in the PCS to propagating THz beams. The THz output frequency can be tuned by varying the wavelength of the lasers. Coherent detection is achieved by mixing the CW signal incident on a second PCS with the same optical radiation used to drive the emitter.