The Terahertz (THz) region is the range of the electromagnetic spectrum comprised in between the microwaves and the infrared, with frequencies extending from 0.3 to 10 THz (i.e. wavelength of 1 mm to 30 µm). The interest for THz light is driven by a variety of possible technological applications. One example is that THz light can be used for security screening, e.g. to detect concealed weapons or plastic explosive, since it penetrates several nonconducting materials (clothing, paper, wood, plastic) whereas it is absorbed by metals. In addition, many substances of interest for security and defense exhibit THz spectral fingerprints.
In the healthcare domain, THz light could enable new medical imaging capabilities, including oncology for epithelial cancer detection and wound inspection through bandages, since its propagation in living tissues is highly sensitive to the water content of cells. THz radiation has photon energies many orders of magnitude below x-rays but care has to be taken that electric fields are maintained below threshold levels to prevent damage to DNA and other biological materials. Below such thresholds, THz radiation has been shown to be safe to use on epithelium tissue and ophthalmology applications.
Furthermore, THz radiation has potential in the short range/high-bandwidth telecommunications field, since the Earth's atmosphere has sufficient transparency in selected spectral windows especially below 0.5 THz.
An integrated THz technology (comprising emitters, detectors and I/O electronics), which is essential to achieve the required performances and reduce the device costs for the above-mentioned mass-market applications, is still missing. The development of a Si-based THz technology, nonetheless, could ultimately respond to these market requirements, since Si-based microelectronics accounts for >98% of the semiconductor global market, granting mass-production capabilities and low prices at volume.
In this perspective, the realization of an electrically pumped Si-based THz laser would be a long sought-after scientific and technological breakthrough.
By addressing significant material science challenges with innovative experimental and theoretical approaches, the FLASH project targeted the demonstration of a cost-effective and compact THz quantum cascade laser (QCL) integrated on Si. QCLs are unipolar devices exploiting carrier transitions between subband states in or between quantum wells (QWs) to generate population inversion and lasing. Lasing in the THz region can potentially be achieved by means of suitable band-structure engineering of selected semiconductor heterostructures. While QCLs based on III-V materials have already been demonstrated and are commercially available, a Si-based THz QCL is still elusive. FLASH’s team proposed to leverage on the electronic transitions occurring between L-valley energy subbands formed in the conduction band of Ge QWs featuring Ge-rich GeSi barriers to achieve this ambitious goal and demonstrate an industrial-viable technological THz platform comprising a Si-based THz source, low-loss waveguides, and integrated MEMS optics.
