July 20, 2009

THz detection and Imaging by nanometer size FETs at room temperature

Room temperature image into an envelope with millimeter resolution.

At Arxiv, "Field Effect Transistors for Terahertz Detection: Physics and First Imaging Applications" THz detection and imaging has been demonstrated by nanometer size FETs at room temperature. Other work has enabled terahertz resolution to be increased to the nanometer level. In Japan, there is work to combine carbon nanotubes and quantum dots into a room temperature terahertz video camera.

Experimental and theoretical results clearly indicate that nanometer transistors are promising candidates for a new class of efficient THz detectors. The natural next step is the realization of real-time imaging THz cameras. To understand whether FETs are the best candidates for this purpose, let us briefly consider other approaches that have already demonstrated their potential in THz real-time recording systems.

The simplest way is using a commercial infrared 160×120 element microbolometer camera. Although the device is designed for wavelengths of 7.5-14 μm, it retains the sensitivity to the THz radiation delivered by optically pumped molecular THz laser. It was shown that in a transmission-mode THz images can be obtained at the video rate of 60 frames/s; signal-to-noise ratio is estimated to be 13 dB for a single frame of video at 10 mW power. An essential step in scaling down the dimensions of a real-time imaging system is the replacement of the optically-pumped laser by a quantum cascade laser. For instance, a quantum cascade laser operating at 4.3 THz with the power of 50 mW allowed reaching the signal-to-noise ratio of 340 at 20 frames/s acquisition rate and an optical NEP of 320 pW.

In another promising approach based on a thin-film absorber upon a silicon
nitride membrane, with thermopile temperature readout produced with the CMOS
technology, a 5 ms thermal time constant of the detector, together with the noise equivalent power of 1 nW/Hz1/2 enables the real-time imaging at 50 frames/s with a signal-to-noise ratio of 10 for an optical intensity of 30 μW/cm2. Very recently, THz images below 1 THz at room temperature were recorded using InGaAs-based bowties diodes with a broken symmetry. The operation principles rely on a nonuniform carrier heating in a specific diode structure merging an antenna concept for coupling of the radiation and a high mobility 2DEG as an active medium. The response time was found to be less than 7 ns, the NEP of about 5.8 nV/√Hz, the sensitivity in the range of 6 V/W, and the dynamic range of about 20 dB at the bandwidth of 100 MHz.

In this context, FETs can be regarded as the most promising option

An electrical current applied to the metamaterial – a hybrid structure of metallic split-ring resonators – controlled the phase of a terahertz (THz) beam 30 times faster and with far greater precision than a conventional optical device, the researchers report in the journal Nature Photonics.
The metamaterial devised by the research team electronically controlled the flow of terahertz radiation over roughly 70 percent of the frequency band – not simply at the points of maximum or minimum frequency

Roberto Merlin of the University of Michigan has devised a different way of making a superlens that promises to focus light more efficiently, and to an even smaller spot – perhaps 500 times smaller than light's wavelength.

Terahertz (THz) near-field microscopy can resolve 40 nanometers based on THz scattering at atomic force microscope tips.

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