Room-temperature terahertz detector

In space, faint terahertz (THz) waves in the frequency range of 0.1–10.0 THz are the dominant type of electromagnetic waves and provide crucial information about astronomical phenomena such as galaxy formation. The question is how to detect such radiation? So far, near-quantum-limited heterodyne THz detectors based on cryogenically cooled superconducting mixers have been used to detect weak THz waves, but device bandwidths on the order of 100 GHz are not sufficient to obtain meaningful spectra.

Credit: Springer Nature Ltd

Now, Ning Wang and co-workers from the University of California, Los Angeles and the California Institute of Technology in the USA have developed a plasmonic photomixer (pictured) that operates over a much broader bandwidth of 0.1–5.0 THz at room temperature (Nat. Astron. https://doi.org/10.1038/s41550-019-0828-6; 2019).

The plasmonic photomixer consists of a conventional photomixer and two nanoscale Ti/Au gratings that cover an 8 × 8 μm² active area with a tip-to-tip gap of 1 μm. The photomixer is integrated with a logarithmic spiral antenna on a low-temperature-grown GaAs substrate. During operation, it is pumped by two wavelength-tunable lasers with central wavelengths of 780 nm and 785 nm to provide a tunable optical beat frequency ω_beat in the THz region from 0.1 THz to 5.0 THz.

The Ti/Au gratings are designed to locally enhance the optical pump intensity through the excitation of surface plasmon waves. The drift photocurrent induced by THz waves and the pump beam has three frequency components at ω_THz, ω_beat + ω_THz and ǀω_beat – ω_THzǀ, here the last component is referred to as the intermediate frequency (IF). By tuning ω_beat and recording the IF output power, the received THz spectrum is extracted over a broad frequency range determined by the logarithmic spiral antenna bandwidth.

The spectral resolution of the THz detectors based on the plasmonic photomixer is determined by the bandwidth of the bandpass filter and the linewidth of the optical pump beam. The down-converted IF spectrum from 0.55 THz to 1 GHz exhibits a linewidth of 1 kHz full-width at half-maximum.

The IF output and noise powers of the plasmonic photomixer have a quadratic and a linear dependence on the optical pump power, respectively. By controlling the photomixer’s signal-to-noise ratio through the optical pump power level, THz detection with quantum-level sensitivities without cryogenic cooling was achieved.

The plasmonic photomixer can also be integrated with polarization-sensitive antennas to determine small anisotropies in the interstellar radiation polarization, which are crucial for understanding shock processes in the interstellar medium originating from supernova explosions and stellar winds.