The concentration of the electromagnetic field inside WG dielectric resonators  has been emphatized as a cause of sensitivity enhancement in the techniques of ESR spectroscopy . This work is an experimental study dedicated to the detection of optical and infrared radiation utilizing as a thermal detector a small polycristalline sample of high-Tc superconducting material placed inside a WG resonator and using the techniques of microwave measurements . The experimental set-up, as depicted in Fig 1, consisted essentially in a simple transmission-cavity spectrometer operated in the frequency range from 18 to 26.5 GHz (K-band). The resonator was a 20 mm alumina disk, 3 mm thick, excited in the WGH modes of resonance with dielectric rod waveguides, and supported by a liquid nitrogen cold finger in high vacuum conditions. The transmitted power was detected by a crystal receiver. A few tens of μg of YB2C3O7-δ powder , was pressed at the bottom of a .9 mm hole, in a region of high electromagnetic field intensity. The hole was drilled in radial direction into the curved surface of the disk and was about 1.5 mm deep. A microwave sweep oscillator of 10 dBm of output power was fixed at the resonant frequency of the loaded resonator, the temperature being sligthly lower than the critical temperature of the superconducting material. The transmitted power level was recorded while a red-laser beam was directed inside the hole. The recorded signals, as reported in Fig 2, give indications about the response time, and the sensitivity attainable. The upper trace represents the transmitted signal variation obtained illuminating the resonator hole with a non focalized 1 mw CW red laser beam. The rise time of the signal was of τ=.5 s , and it was found to be proportional, within limits, to the sample mass. The voltage signal was proportional to the incident microwave power. The lower trace represents the transmission curves of the resonator as a function of time: with "laser on" (the first two peaks), and with "laser off" (the three peaks to the rigth) . These curves were obtained by sweeping repeatedly the microwave source +/−10 MHz around the frequency of 24.4 MHz, i.e. the resonant frequency of the WG disk resonator with laser "off'. The resonant frequency shift under "laser on" conditions was of 4 MHz and the relative Q-factor variation was of about 17%. Both the resonant frequency and the Q-factor diminished as the sample temperature raised over Tc. The observed signals are attributable to the phase transition of the superconducting sample under the heating effect of light. Chopped laser signals up to 10 Hz could be recorded in a straigth video- detection scheme, with a signal to noise ratio of 10. The loaded Q-factor was always of the order of 103. The quasi black-body absorption characteristic of the powder charged hole, acting much as a light-trap, makes the device capable of detecting infrared radiations in a wide range of wavelengths. The maximum voltage "responsivity" (at 0 hertz) was of about 200 V/W , with a voltage noise less than of .1 mV/(Hz)1/2. Experiments with higher Q-factor dielectric resonators and epitaxially deposited thin superconducting films for characterization purposes at microwave frequencies are in project.