Publication

Journal	Optica
Volume (Issue)	7(10)
Page(s)	1445 - 1445
Title of proceedings	Optica
DOI	10.1364/optica.397506

A novel technique for high-resolution 1.5µm photonics-enabled terahertz (THz) spectroscopy using software control of the illumination spectral line shape (SLS) is presented. The technique enhances the performance of a continuous-wave THz spectrometer to reveal previously inaccessible details of closely spaced spectral peaks. We demonstrate the technique by performing spectroscopy on LiYF4:Ho3+, a material of interest for quantum science and technology, where we discriminate between inhomogeneous Gaussian and homogeneous Lorentzian contributions to absorption lines near 0.2 THz. Ultra-high-resolution (<100Hz full-width at half maximum) frequency-domain spectroscopy with quality factor >2×109 is achieved using an exact frequency spacing comb source in the optical communications band, with a custom uni-traveling-carrier photodiode mixer and coherent down-conversion detection. Software-defined time-domain modulation of one of the comb lines is demonstrated and used to resolve the sample SLS and to obtain a magnetic field-free readout of the electronuclear spectrum for the Ho3+ ions in LiYF4:Ho3+. In particular, homogeneous and inhomogeneous contributions to the spectrum are readily separated. The experiment reveals previously unmeasured information regarding the hyperfine structure of the first excited state in the 58 manifold complementing the results reported in Phys. Rev. B 94, 205132 (2016)