New Frontiers in Modern Resonator Spectroscopy

New opportunities due to the advancements in the techniques and methods of broadband resonator spectroscopy in the millimeter and submillimeter range (also known as the terahertz region) are considered in this paper. An upgraded variant of the spectrometer produced at the Institute of Applied Physics, Russian Academy of Sciences (Nizhny Novgorod, Russia) possessing a sensitivity to the absorption coefficient of approximately 4 × 10
^-9
cm
^-1
is presented. The spectrometer allows the continuous recording of gas spectra in the 45-500 GHz frequency range (six waveguide subbands) at pressures of 10-1000 torr. A fully automated spectrum recording within the waveguide subband reduces the overall time required for the experiment by almost an order of magnitude. Examples of recording the spectra of oxygen and water vapor resonance lines, as well as of continuum absorption, including the rotationally resolved water dimer spectrum, are presented. It is demonstrated that the uncertainty of measuring the broadening parameter of diagnostic atmospheric lines may be reduced to 0.1%, which meets the stringent present-day requirements of remote sensing of the atmosphere. The physically attainable ultimate sensitivity of broadband microwave resonator spectrometers of various types is discussed. Prospects for further development of the spectrometer and its application for solving fundamental and applied problems of spectroscopy are analyzed.