Abstract
A novel technique for high-resolution 1.5 µ 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 L i Y F 4 : H 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 H 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 H o 3 + ions in L i Y F 4 : H 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)PRBMDO0163-182910.1103/PhysRevB.94.205132.
Highlights
The terahertz (THz) electromagnetic band (0.1 THz to 10 THz) lies in the technically challenging spectral gap between infrared light and microwave radiation [1,2]
The FWHM increases with the illumination broadening, revealing an root mean square (RMS) trend as expected for convolutions of two Gaussian spectral line shape (SLS) [Eq (5a)]
If the observed 0.907 GHz FWHM were to be attributed to a purely Lorentzian hypothesis, the expected FWHM curve would show a considerable departure from the identity asymptote and the observed linewidth, as shown in Fig. 6
Summary
The terahertz (THz) electromagnetic band (0.1 THz to 10 THz) lies in the technically challenging spectral gap between infrared light and microwave radiation [1,2]. Specialized technology for the generation, manipulation, and detection of so-called “T-rays” has come later than for other frequency bands and is still an active area of development. THz radiation is mainly innocuous to live tissue [3], making it attractive for medical diagnostics and material identification for safety and security. Despite the significant absorption of THz radiation by the atmosphere, existing transmission windows allow useful applications in astronomy [4] and telecommunications [5]. Various THz excitations in solids offer promise for quantum measurement and control [6], if compact coherent sources could be realized
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