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 1.5µm$1.5\text{µ}\mathrm{m}$ 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 LiYF4:Ho3+${\mathrm{L}\mathrm{i}\mathrm{Y}\mathrm{F}}_4:{\mathrm{H}\mathrm{o}}^{3+}$, 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 (<100Hz$<100\mathrm{H}\mathrm{z}$ full-width at half maximum) frequency-domain spectroscopy with quality factor 𝑄>2×109$Q>2\times {10}^9$ 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 Ho3+${\mathrm{H}\mathrm{o}}^{3+}$ ions in LiYF4:Ho3+${\mathrm{L}\mathrm{i}\mathrm{Y}\mathrm{F}}_4:{\mathrm{H}\mathrm{o}}^{3+}$. 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 5𝐼8${}^5{I}_8$ manifold complementing the results reported in Phys. Rev. B 94, 205132 (2016) [CrossRef]  .

Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Abstract-Exact frequency and phase control of a terahertz laser

Reshma A. Mohandas, Lalitha Ponnampalam, Lianhe Li, Paul Dean, Alwyn J. Seeds, Edmund H. Linfield, A. Giles Davies, and Joshua R. Freeman

Schematic diagram of the experimental arrangement. EDFA, erbium doped fibre amplifier; Tx, photomixer emitter; Rx1 and Rx2, photomixer receivers; PLL, phase lock loop; Δ𝜑$\mathrm{\Delta }\phi$, variable delay line. Electrical connections are shown in black, optical fiber and IR connections in red, and terahertz connections in green.

https://www.osapublishing.org/optica/abstract.cfm?uri=optica-7-9-1143

The accuracy of high-resolution spectroscopy depends critically on the stability, frequency control, and traceability available from laser sources. In this work, we report exact tunable frequency synthesis and phase control of a terahertz laser. The terahertz laser is locked by a terahertz injection phase lock loop for the first time, with the terahertz signal generated by heterodyning selected lines from an all-fiber infrared frequency comb generator in an ultrafast photodetector. The comb line frequency separation is exactly determined by a Global Positioning System-locked microwave frequency synthesizer, providing traceability of the terahertz laser frequency to primary standards. The locking technique reduced the heterodyne linewidth of the terahertz laser to a measurement instrument-limited linewidth of <1Hz$<1\mathrm{H}\mathrm{z}$, robust against short- and long-term environmental fluctuations. The terahertz laser frequency can be tuned in increments determined only by the microwave synthesizer resolution, and the phase of the laser, relative to the reference, is independently and precisely controlled within a range ±0.3𝜋$\pm 0.3\pi$. These findings are expected to enable applications in phase-resolved high-precision terahertz gas spectroscopy and radiometry.

Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Abstract- Injection locking of a terahertz quantum cascade laser to a telecommunications wavelength frequency comb

Joshua R. Freeman, Lalitha Ponnampalam, Haymen Shams, Reshma A. Mohandas, Cyril C. Renaud, Paul Dean, Lianhe Li, A. Giles Davies, Alwyn J. Seeds, and Edmund H. Linfield

https://www.osapublishing.org/optica/abstract.cfm?uri=optica-4-9-1059

High-resolution spectroscopy not only can identify atoms and molecules but also can provide detailed information on their chemical and physical environment and relative motion. In the terahertz frequency region of the electromagnetic spectrum, where many molecules have fundamental vibrational modes, there is a lack of powerful sources with narrow linewidths that can be used for absorption measurements or as local oscillators in heterodyne detectors. The most promising solid-state source is the THz frequency quantum cascade laser (QCL), however, the linewidth of this compact semiconductor laser is typically too broad for many applications, and its frequency is not directly referenced to primary frequency standards. In this work, we injection lock a QCL operating at 2 THz to a compact fiber-based telecommunications wavelength frequency comb, where the comb line spacing is referenced to a microwave frequency reference. This results in the QCL frequency locking to an integer harmonic of the microwave reference, and the QCL linewidth reducing to the multiplied linewidth of the microwave reference, <100  Hz$<100\mathrm{Hz}$. Furthermore, we perform phase-resolved detection of the locked QCL and measure the phase noise of the locked system to be −75  dBc/Hz$-75\mathrm{dBc}/\mathrm{Hz}$ at 10 kHz offset from the 2 THz carrier.

Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.