What Is Submillimeter Wave Technology?

Submillimeter wave technology encompasses the devices, circuits, and systems that operate in the frequency range from roughly 300 gigahertz to 3 terahertz, corresponding to wavelengths between 0.1 and 1 millimeter. This spectral region, often called the terahertz (THz) gap, lies between microwave electronics on one side and infrared photonics on the other. It was historically called a gap because conventional electronic and photonic device technologies performed poorly in this range, making practical sources and detectors difficult to build. Advances in compound semiconductor epitaxy, quantum cascade lasers, and high-electron-mobility transistors over the past two decades have substantially closed this gap, enabling a growing set of applications.

The physical behavior of electromagnetic waves in the submillimeter band is distinctive. Many molecular gases and solid materials exhibit characteristic absorption and reflection features at terahertz frequencies, which makes this band valuable for spectroscopic identification of chemical compounds. At the same time, many common packaging materials such as plastics, paper, and dry textiles are transparent or semitransparent at THz frequencies, enabling concealed-object imaging. The propagation loss in the atmosphere from water vapor absorption is substantial but varies significantly with frequency, humidity, and path length.

Submillimeter Wave Circuits and Devices

Building circuits at terahertz frequencies requires transistors with transit frequencies well above the operating frequency. Indium phosphide high-electron-mobility transistors (InP HEMTs) and heterojunction bipolar transistors (HBTs) currently achieve transit frequencies above 1 THz, enabling amplifiers, mixers, and oscillators at submillimeter wavelengths in purely electronic form. Schottky diode multiplier chains, which multiply a lower-frequency microwave signal up to terahertz frequencies, are mature technology used in scientific and space-based instruments. NASA Jet Propulsion Laboratory research on THz electronics documents submillimeter-wave receiver technology developed for atmospheric remote sensing instruments.

THz Spectroscopy

Terahertz time-domain spectroscopy (THz-TDS) is the most widely used THz measurement technique. A femtosecond laser pulse drives a photoconductive switch to generate a broadband THz pulse, which propagates through a sample and is then detected by a time-gated receiver. Fourier transforming the time-domain waveform yields the amplitude and phase spectrum, from which the complex refractive index of the sample is extracted. This technique provides simultaneous amplitude and phase information across a broad bandwidth without the need for a separate reference measurement. NIST's terahertz research program develops calibration standards and measurement methods for THz spectroscopy that support both scientific and industrial users.

Terahertz Imaging

THz imaging systems form spatial maps of THz amplitude or phase by raster-scanning a sample through a focused beam or by using focal plane array detectors. Because many materials are semi-transparent at THz frequencies and exhibit distinct spectral signatures, THz imaging can reveal concealed features, identify materials by their absorption spectra, and detect defects in layered structures. Transmission and reflection geometries are both used depending on whether the sample can be accessed from both sides. Resolution is limited by diffraction to approximately one wavelength, which is on the order of 0.1 to 1 millimeter, so THz imaging is suited to macroscopic structures rather than microscale features.

Submillimeter Wave Communication

The wide bandwidths available in the submillimeter wave region make it attractive for very high data rate wireless links. A carrier at 300 GHz can support tens of gigabits per second of throughput with modest modulation complexity, which could support wireless fronthaul and backhaul for dense 5G and 6G deployments. IEEE 802.15.3d is a wireless standard that defines physical layer specifications for 100 Gbps links at 300 GHz, representing the first international standardization effort focused on the THz communications band.

Applications

• Security screening systems at airports and public venues use THz imaging to detect concealed weapons under clothing.
• Pharmaceutical quality control uses THz time-domain spectroscopy to identify polymorphic forms of active ingredients in tablets.
• Astronomy and atmospheric remote sensing instruments use submillimeter receivers to study molecular emission lines in the interstellar medium and to profile water vapor in Earth's atmosphere.
• Non-destructive testing uses THz imaging to inspect composite structures, coatings, and packaged electronics for internal defects.
• Advanced wireless backhaul links use 300 GHz carriers to deliver multi-gigabit throughput over short distances between base stations and the core network.