## Saturday, October 17, 2020

### Abstract-Ultra-high-resolution software-defined photonic terahertz spectroscopy

Rodolfo I. Hermans, James Seddon, Haymen Shams, Lalitha Ponnampalam, Alwyn J. Seeds, and Gabriel Aeppli

 Experiment schematic: a monochromatic telecommunications wavelength laser feeds an optical frequency comb generator (OFCG) with exact tunable spacing. A programmable wavelength selective switch (WSS) selects two bands 12 peaks apart, amplified and mixed in uni-traveling-carrier photo-diode (UTC-PD). A 200 GHz beat frequency is transmitted through horn antennas and lenses through a continuous-flow liquid helium cryostat with thin polypropylene windows and LiYF4:Ho3+${\mathrm{L}\mathrm{i}\mathrm{Y}\mathrm{F}}_{4}:{\mathrm{H}\mathrm{o}}^{3+}$ sample. The received signal is down-converted using a sub-harmonic mixer and measured using a microwave spectrum analyzer.
https://www.osapublishing.org/optica/abstract.cfm?uri=optica-7-10-1445

A novel technique for high-resolution $1.5\phantom{\rule{thickmathspace}{0ex}}\text{µ}\mathrm{m}$ 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 ${\mathrm{L}\mathrm{i}\mathrm{Y}\mathrm{F}}_{4}:{\mathrm{H}\mathrm{o}}^{3+}$, 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 ($<100\phantom{\rule{thickmathspace}{0ex}}\mathrm{H}\mathrm{z}$ full-width at half maximum) frequency-domain spectroscopy with quality factor $Q>2×{10}^{9}$ 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 ${\mathrm{H}\mathrm{o}}^{3+}$ ions in ${\mathrm{L}\mathrm{i}\mathrm{Y}\mathrm{F}}_{4}:{\mathrm{H}\mathrm{o}}^{3+}$. 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 ${}^{5}{I}_{8}$ manifold complementing the results reported in Phys. Rev. B 94, 205132 (2016) [CrossRef .

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