1. A system for sensing a signal beam from a sample at a first frequency, the system comprising: a difference-frequency generator to mix the signal beam with an optical pump beam so as to produce an optical idler beam at a second frequency equal to a difference between the first frequency and a frequency of the optical pump beam; a filter, in optical communication with the difference-frequency generator, to at least partially attenuate and/or reflect the optical pump beam and to transmit the optical idler beam; and a spectrometer, in optical communication with the filter, to detect plural spectral components of the optical idler beam in parallel at the second frequency so as to yield an indication of a spectroscopic measurement of the sample at the first frequency, wherein the first frequency is in a range from about 100 GHz to about 10 THz.