Tunable Antenna-Coupled Intersubband Terahertz (TACIT) Detectors

Introduction:

The Sherwin group has designed, fabricated, and is currently testing a novel terahertz (a.k.a. far-infrared or sub-mm wave) detector. This detector combines high sensitivity and high speed by taking advantage of the unique properties of quantum well structures in semiconductors.

The device structure is similar to a transistor. Current flows from source to drain and there is an electrostatic gate above and below the active region. The resistance between the drain and source increases dramatically when THz light is incident on the device.

(a) TACIT detector cross section showing active rection and contacts, with vertical THz electric field and transverse temperature sensing current. (b) 3D rendering of TACIT architecture with twin slot dipole antenna. (Inset: SEM image of fabricated device). (c) SEM image of TACIT detector, antenna, and leads with microstrip filters. (d) Experimental configuration. Silicon lens couples incident radiation to microfabricated antenna structure.

How it works:

The channel or active region consists is an double quantum well. The intersubband electon level separation is about 6meV, the energy of a 1.5THz photon. The energy spacing can be tuned by applying a DC bias between the top and bottom gate. This tunablility allows the detector to discriminate between frequencies, a property useful for interstellar astronomy and chemical sensing.

Now think of the incident THz light as an extremely high frequency radio wave. An waveguide structure connects the top and bottom gate of the detector to a lithographically patterned antenna. Rather than modulating the current, the light is absorbed by electrons in the quantum wells. As the electron gass heats up the moblity is reduced causing and increase in resistance.

The detector takes advantage of four unique properties of quantum wells:

1. Property: Large oscilator strengh and high density of states
   Effect: High quantum efficiency
2. Property: Electron mobility is a strong function of temperature.
   Effect: High sensitivity
3. Property: Very fast thermal relaxation
   Effect: High speed
4. Property: Intersubband absorption shifts with DC electric field.
   Effect: Frequency Tunablility

The spectral width of the absorption peak should be narrow enough for use as a direct detector, but broad enough for use as a mixer with a several-GHz IF bandwidth.

We design the layered semiconductor quantum well structures with our collaborators in the Materials Department, and they grow the wafers for us. We design the detector structure and develop a fabrication process to produce that structure. Optical testing is done at the optimal device temperature of 77 K (-321 degrees Fahrenheit).

Outlook:

Technologies in the Terahertz frequency range (1-10 THz, or 300-30 µm wavelengths) have lagged technologies in the nearby microwave and infrared frequency ranges. NASA has historically been a dominant funding source and driver for technological development, so the state of the art Terahertz detectors have been designed for astronomical observing. As these detectors mature, and move from being one-of-a-kind research tools to being commercially available products, they will be used for many other applications. Future applications may include chemical spectroscopy, industrial process monitoring, and security (clothing is transparent to THz).

Future generations of the TACIT detector may be sensitive enough to detect the arrival of single photons, which would allow them to be used to read out the results from proposed implementations of a quantum computer.

Our current THz detector research is focused on a a variant of the TACIT detector which is designed to work at room temperature, and will operate with many of the same advantages of the TACIT detector, without the need for liquid cryogens or thermoelectric cooling.

Early TACIT detector utlizing a log periodic antenna.

Research sponsored by NASA.

Terahertz Detector Publications

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