1. Active Element

Phonon-cooled HEB mixers are fabricated from thin NbN films on either silicon or MgO substrates. We have fabricated devices using UV lithography and e-beam writing. Devices fabricated with UV lithography at UMass/Amherst are typically 1 mm long and 4 mm wide whereas the smaller devices, written with e-beam at Chalmers University, are 0.4 mm long and 4 mm wide.

2. Quasi-Optical Coupling

We have implemented a quasi-optical coupling scheme (Figure R1a).  This method uses an elliptical silicon lens and a monolitic antenna to couple the terahertz frequencies to the device fabricated at the terminals of the antenna. A typical antenna design for linearly polarized signal is the twin-slot antenna configuration as can be seen in Figure R1b.




Figure 1. (a) Quasi-optical coupling scheme. (b) Twin-slot antenna.


3. Mixer Block

TREND's mixer block can be seen in Figure R2. The biasing circuitry is incorporated into the mixer block.

Figure R2. Mixer block and DC/IF circuit board.


4. Cryogenics

NbN HEB devices are superconducors with typical critical temperatures of below 10 K. The mixer block is mounted in a cryostat operating at arround 4 K.

 5. Local Oscillator

The Local Oscillator (LO) used with the TREND instrument is made of an FIR laser, which is pumped by CO2 CW laser. Figure R3 shows the TREND LO laser.

Figure R3. TREND LO laser.


6. IF Amplifier

The IF signal coming out of the HEB mixer is amplified by a cryogenically-cooled low noise amplifier. Indium phosphide (InP) High Electron Mobility Transistors (HEMT) have become an excellent choice for cryogenic LNAs because of their low noise, high frequency response, and superior cryogenic behavior.

7. Optical Path

An elaborate optical setup was implemented at the telescope in order to focus both the LO beam and the sky beam onto the HEB mixer. Different optical configurations for two different polarizations are illustrated in Figure R4.




Figure R4. Illustrations of the optical paths for:
(a) 1.27 THz signal. (b) 1.46 THz signal.