The last frontier in materials characterisation

Situated between infrared and millimeter regions of the electromagnetic spectrum, Terahertz (THz) energy has been referred to as the “last frontier” in materials characterisation.

The THz gap is an area of widespread interest due to its both its promise in new materials exploration, and the challenge to date in creating and applying THz energy.

While much of the THz intrigue is centered on imaging for security and medical applications, Terahertz characterisation efforts have already yielded insights into new materials.

Numerous results include the characterisation of dielectric materials for high-frequency and waveguiding applications, chemical identification of different polymorphs in pharmaceutical applications, and non-contact measurements of mobility and doping levels in semiconductors.

Terahertz characterisation efforts have already yielded insights into new materials

Many new THz technologies have emerged in the past 20 years, but these security or imaging-focused approaches are not readily leveraged for use in materials characterisation. THz sources such as quantum cascade and carbon dioxide lasers offer high terahertz power but only at discrete frequencies.

Tunable electronic sources such as frequency-multiplied Schottky Diodes and backward-wave oscillators operate over limited spectral slices, and multiple modules are needed to cover sufficient spectral bandwidth. THz Time Domain Spectroscopy (THz-TDS) and photoconductive mixing (photomixing) are more conducive for spectroscopy as both techniques offer broad, continuous, and coherent spectral coverage.

But with recent advances in THz sources and methods, avenues have been opened for THz frequency characterisation of solid state materials such as semiconductors, molecular solids, and magnetic materials.

Today, THz-TDS systems are the most common THz source used in these studies. For THz-TDS, optical pulses from a mode-locked, femtosecond laser are transformed into ultrashort (or THz), electromagnetic pulses by a GaAs photoconductive switch (PCS), semiconductor or nonlinear crystal. The THz pulse then reflects from or propagates through a sample before being focused onto a THz detector- typically a second PCS device or an electro-optic crystal.

The emerging characterisation possibilities associated with the THz regime are intriguing

Photomixing is a cost-effective alternative to the pulsed laser solutions used in THz-TDS sources. These devices operate by illuminating narrow gaps in the DC-biased electrodes of a PCS with the combined output of two, single-colour diode lasers tuned to different wavelengths. The THz output frequency can be tuned by varying the wavelength of the lasers. Coherent detection is achieved by mixing the CW-THz signal incident on a second PCS with the same optical radiation used to drive the emitter.

While other optical characterisation technologies using visible light, IR, and even millimeter waves have already been developed, the emerging characterisation possibilities associated with the THz regime are intriguing to physicists, electrical engineers, and materials scientists because a number of important electronic and magnetic phenomena including cyclotron resonances in semiconductors and antiferromagnetic magnons align with THz frequencies.

Coupling developments in THz spectroscopy with variable temperature and magnetic environments can enable more detailed characterisation of a broader range of materials. THz energy phenomena and mechanisms which are indistinguishable from the background at room temperatures can emerge as the material approaches cryogenic temperatures.

Magnetic properties and correspondingly THz response can significantly vary with temperature and magnetic field. While researchers in the field of THz technologies have been actively pursuing characterisation methods in magnetic and cryogenic environments, complex experimental configurations have inhibited penetration of THz characterisation techniques into the materials development community.

To this end, Lake Shore Cryotronics is developing a turnkey, CW THz-based characterisation solution, similar to a Hall Measurement System, specifically focused on the electronic and magnetic materials characterisation application needs of researchers.

Demonstrated value can emerge from strong collaborations between the terahertz and materials development communities

The system could help researchers evaluate the growth and quality of novel electronic materials such as graphene, semiconductor heterostructures, and even high-temperature superconductors with non-contact mobility measurements. Hall measurements are the standard characterisation technique to evaluate mobility and resistivity in electronic materials.

Researchers can spend considerable time and effort attaching electrical leads and sometimes patterning the material into Hall bar structures for these measurements. The intent of the system is to provide a sample environment and complete system integration for non-contact THz materials characterisation so that researchers have more time to focus on material development instead of attaching electrical leads, continually aligning THz optics or writing control and analysis software.

Wide-spread adoption of THz characterisation in the materials community will require demonstrated value to a broad range of materials classes and further reduction in the cost of THz hardware. Commercial interest in terahertz for imaging applications will likely provide favourable cost conditions for future THz spectroscopy applications, while demonstrated value can emerge from strong collaborations between the terahertz and materials development communities.