Heterodyne Quasi-Optical Spectrometer

This instrument that we refer to as the quasi-optical heterodyne spectrometer is a multi-frequency instrument that can be used both for cw- and pulsed EPR. It consists of a multi-frequency quasi-optical millimeter-wave bridge coupled to solid state millimeter wave sources and employs Schottky-diode mixers as principal detectors.

The detection bandwidth is ~1 GHz and it is suitable for both transient and pulsed EPR. Phase sensitive detection is used with simultaneous detection of the in- and out-of-phase signals. The instrument is also equipped with RF sources and amplifiers for pulsed and cw Electron-Nuclear Double Resonance experiments to measure unresolved hyperfine interactions.

Normally the instrument is used without resonator, which allows rather large samples up to a volume of ~200 microliters. The instrument is equipped with a single axis rotator with an accuracy of 0.5 degrees. The maximum single crystal dimensions for the standard rotator are 2x2x2 mm, but larger samples can also be accommodated.

In CW mode, field modulation is used. In pulsed mode, without resonator π/2 pulse lengths are of the order of 400 ns. For single crystals a simple HE11 resonator can be used, which reduces these pulse lengths to about 100 ns.

Available frequencies are 120, 240, 266, 316, 336, and 395 GHz. The sample temperature can be controlled over the range of 1.5K to 400K.

The latest addition source and detector operate at 258-267 GHz with a power around 250 mW, resulting in pulse lengths as short as 200 ns without resonator and considerably shorter with a simple resonator. In the near future (late 2026) an Arbitrary Waveform Generator will be employed to generate arbitrary excitation pulse sequences.