Current Research


HEB (Hot Electron Bolometric) mixers are made of type II superconductors. In HEBs the incident thermal radiation is absorbed by the quasi-particles in the micro bolometer. There are two types of HEB mixers:

  • Phonon-cooled (or lattice-cooled) HEB (PHEB) mixers (NbN).
  • Diffusion-cooled HEB (DHEB) mixers (Nb).

The thermal energy in phonon-cooled HEBs is extracted from the hot quasi-particles by interaction with phonons that is followed by the escape of the phonons into the substrate (see Figure 1a).

In the diffusion-cooled HEBs, the hot quasi-particles diffuse into the metallic contacts and subsequently cool down (see Figure 1b).

(a)

(b)

Figure 1. Cooling mechanisms for: (a) a phonon-cooled HEB. (b) a diffusion cooled HEB.

 

PHEB mixers are fabricated from thin NbN films (3-5 nm thick) deposited on either silicon or magnesium oxide substrates.

Typical PHEB mixers are 1 um long by 4 um wide. Smaller devices (typically 0.4 um long by 4 um wide) are written using e-beam lithography.

A quasi-optical scheme is used to couple the incoming radiation to the HEB mixer consisting of a silicon lens and a monolithic antenna designed to operate at terahertz frequencies (see Figure 2). A Far Infrared (FIR) laser is used as the LO source.

 

(a)

(b)

Figure 2. (a) An illustration of the quasi-optical coupling scheme. (b) An SEM picture of a twin-slot antenna.

 

The best measured noise temperatures of PHEB in our laboratory are 440 K at 620 GHz, 500 K at 1.56 THz, and 1,100 K at 2.24 THz. We have recently measured IF gain bandwidth for a PHEB device of 3 GHz and noise bandwidth of 5 GHz, both at 1.56 THz.

Examples of ongoing research projects related to PHEB performed by the group include: theoretical studies of noise processes in HEB mixers, HEB stability studies under different biasing conditions, antenna and electromagnetic analysis of the quasi-optical configurations, integration of HEB mixers with MMIC amplifiers and circuitry, and development of HEB focal plane arrays.