We report on the first beta gallium oxide (beta-Ga2O3) crystal feedback oscillator built by employing a vibrating beta-Ga2O3 nanoresonator as the frequency reference for real-time middle ultraviolet (MUV) light detection. We fabricate suspended beta-Ga2O3 nanodevices through synthesis of beta-Ga2O3 nanoflakes using low-pressure chemical vapor deposition (LPCVD), and dry transfer of nanoflakes on microtrenches. Open-loop tests reveal a resonance of the beta-Ga2O3 device at ~30 MHz. A closed-loop oscillator is then realized by using a combined optical-electrical feedback circuitry, to perform real-time resonant sensing of MUV irradiation. The oscillator exposed to cyclic MUV irradiation exhibits resonant frequency downshifts, with a measured responsivity of $mathscr{R}$ = -3.1 Hz/pW and a minimum detectable power of delta Pmin = 0.53 nW for MUV detection.
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