Closing the Terahertz Gap

http://www.npl.co.uk/quantum-phenomena/quantum/research/closing-the-terahertz-gap

NPL scientists and their collaborators develop ultra-sensitive terahertz detectors, methods and instrumentation for their calibration.

Practically the whole spectrum of electromagnetic radiation, spreading from single-Hz audio frequencies to zetta (10²¹) Hz gamma rays, is already heavily utilized by mankind. The only underused band lies in the tera (10¹²) Hz range between microwaves and visible light and is dubbed “the terahertz gap”. Physics, astronomy, cosmology, chemistry, biology, medicine, environmental monitoring, homeland security are only some of the fields which would benefit from reliable and efficient technologies to generate, shape and detect terawaves.

In collaboration with physicist from Royal Holloway University of London we are developing a sensor and a tunable source of terahertz radiation.

Principle of THz photon detection

At the heart of the novel technology lies a quantum dot (QD) – a nanoscale reservoir of electrons cut out of a two-dimensional electron gas (2DEG) in a GaAs/AlGaAs semiconductor heterostructure. In dark conditions roughly a thousand electrons are confined within the dot by negative electrostatic potential of gate electrodes. A terahertz photon arriving at the dot knocks an electron out of the dot. This minute change in the electric charge can be sensed by another ultra-sensitive nanoscale device – an aluminium single-electron transistor, SET, which is closely coupled with the QD. The SET responds by a large change in its electric conductance, which can be measured using conventional electronics. In our measurements the arrival of each terahertz photon is heralded by a telegraph-like change in the measured SET signal. More photons means more frequent switches. The noise-equivalent power of this device is 10^-19W/Hz^1/2.

An atomic force microscope image
of a QD-SET detector 

No conventional means are good enough to calibrate new generation terahertz detectors, such as our QD – SET detector. That is why we are developing a versatile source of monochromatic radiation tunable in a wide frequency range and based on a superconducting Josephson junction. The frequency of radiation emitted by such junction is strictly proportional to the voltage across the junction, each THz corresponding to slightly less than two millivolts (483.6 GHz/mV). This range is only accessible with devices made of high-T_csuperconductors, such as YBa₂Cu₃O₇ or Bi₂Sr₂CaCu₂O₈. We have made prototype emitters using Josephson junctions fabricated on a sapphire bicrystal substrate. The junction is attached to a broadband self-complementary log-periodic antenna to ensure good radiation efficiency. The same type of antenna will be attached to the QD – SET detector.

An optical micrograph of the THz source

We have recently joined a team of universities and companies from a dozen European countries to win a European grant Teraeye for terahertz research and development. EU funding will allow the international team to make the next development step – to build and test a prototype passive terahertz imager. The imager is similar to a camera, so sensitive that it can catch single photons in the terahertz range to scan and identify specific products by electromagnetic radiation. While other Teraeye team members will build different aspects of the imager, NPL’s role is to provide calibration and test the imager. Although the Teraeye project specifically targets security applications, such as detection of concealed weapons, explosives, and dangerous chemical and biological substances, imaging in the Terahertz frequency band may also have benefits for biomedical screening, radio-astronomy, and for communicating and processing quantum information.

Our recent papers

• H. Hashiba, V. Antonov, A. Ya. Tzalenchuk, P. Kleinschmid, S. Giblin, and S. Komiyama ”Isolated quantum dot in application to terahertz photon counting” Physical Review B 73, 081310(R)
• P. Kleinschmidt, S. Giblin, A. Ya. Tzalenchuk, H. Hashiba, V. Antonov, and S. Komiyama “Sensitive detector for a passive terahertz imager” J. Appl. Phys. 99, 114504 (2006)
• P. Kleinschmidt, S. Giblin, V. Antonov, H. Hashiba, L. Kulik, A. Ya. Tzalenchuk, and S. Komiyama “A Highly Sensitive Detector for Radiation in the Terahertz Region” IEEE Instrumentation and Measurement 56, 463-467 (2007)

For more information about Tera-Hz Metrology work at NPL visit the THZ Metrology in Electromagnetics pages.