How 50 Years of Technology Development Has Transformed Millimeter-THz Astronomical Spectroscopy

Millimeter wavelength astronomical spectroscopy started about 50 years ago, with the detection of carbon monoxide in interstellar space. The detection of massive clouds of molecular material in the Milky Way answered a long-standing question of from what stars form, but understanding the formation, structure, and evolution of these clouds demanded ever more sensitive receivers operating throughout the millimeter/submillimeter//THz range. Fractional frequency resolution R>3x10
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is required to measure the motions in star-forming regions, with the result that heterodyne systems have become the dominant technology for detailed kinematic studies. To unravel the complexity of chemistry in molecular clouds, higher sensitivity was required. To achieve this, the Schottky diodes initially used as nonlinear elements were first cooled to reduce noise, but then replaced by superconductor insulator superconductor (SIS) and hot electron bolometer (HEB) mixers. Noise temperatures dropped from many thousands of K to < 100 K. Local oscillators (LOs) evolved from vacuum tubes to solid state devices. Large efforts were devoted to the development of frequency multipliers which allow mixers up to several THz to operate, starting with spectrally pure, tunable sources below 100 GHz. These allowed unbiased "spectral scans" and better use of observing time. The quantum cascade laser LO has allowed exploiting frequencies to 5 THz and beyond. Single antennas grew by more than an order of magnitude in collecting area. Interferometric arrays providing unprecedented angular resolution have allowed detailed observations of sources ranging from planet-forming disks to star-forming regions in distant galaxies. The interplay between technology and astronomical spectroscopy has been hugely productive, and continues, with new types of mixers for individual pixels and sophisticated focal plane arrays being developed for ground and space-based applications.