  We present proof-of-operation for a new method of electron thermometry using
microwave impedance of a hafnium micro-absorber. The new method leads to an
ultimate THz-range detector suitable for microwave readout and frequency
division multiplexing. The sensing part of the device is a hot-electron-gas
absorber responding to the incident radiation by variation of its impedance
measured at probing frequency about 1.5 GHz. The absorber is a microbridge made
from hafnium (Tc = 375 mK, RN = 30 Ohm) sized 2.5 um by 2.5 um by 50 nm and
integrated with a planar 600-700 GHz antenna placed near the open end of a
quarter-wave CPW resonator (Q-factor about 10^4). All elements of the circuit,
except the microbridge, are made from 100-nm thick Nb, including the resonator,
which is weakly coupled to a throughput line. The device was tested at 50-350
mK smoothly responding with its transmission coefficient S21 to applied
microwave power at the resonance frequency. We have found that the power
absorbed by the bridge fits to the model of hot electron gas, P=k(Te^n-Tph^n)
(n = 5...6). The idle NEP down to about 10^-18 W/Hz^(-1/2) and the
corresponding cross-over temperature for photon background about 5 K are
estimated from the measured data. The saturation power of about 1 pW and
possibility of moderate gain are anticipated for a practicable device operating
at temperature 200 mK. Since the optimum readout frequency is found exactly at
the resonance, the detector is insensitive to most phase instabilities at the
probing frequency.
