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1. Radio frequency transmitter based on a laser frequency comb

   https://doi.org/10.1073/pnas.1903534116  

   Piccardo, Marco; Tamagnone, Michele; Schwarz, Benedikt; Chevalier, Paul; Rubin, Noah A.; Wang, Yongrui; Wang, Christine A.; Connors, Michael K.; McNulty, Daniel; Belyanin, Alexey; et al (April 2019, Proceedings of the National Academy of Sciences)

   Since the days of Hertz, radio transmitters have evolved from rudimentary circuits emitting around 50 MHz to modern ubiquitous Wi-Fi devices operating at gigahertz radio bands. As wireless data traffic continues to increase, there is a need for new communication technologies capable of high-frequency operation for high-speed data transfer. Here, we give a proof of concept of a compact radio frequency transmitter based on a semiconductor laser frequency comb. In this laser, the beating among the coherent modes oscillating inside the cavity generates a radio frequency current, which couples to the electrodes of the device. We show that redesigning the top contact of the laser allows one to exploit the internal oscillatory current to drive a dipole antenna, which radiates into free space. In addition, direct modulation of the laser current permits encoding a signal in the radiated radio frequency carrier. Working in the opposite direction, the antenna can receive an external radio frequency signal, couple it to the active region, and injection lock the laser. These results pave the way for applications and functionality in optical frequency combs, such as wireless radio communication and wireless synchronization to a reference source. 

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2. Widely tunable room temperature semiconductor terahertz source

   https://doi.org/10.1063/1.4902245  

   Lu, Q Y; Slivken, S; Bandyopadhyay, N; Bai, Y; Razeghi, M (November 2014, Applied Physics Letters)

   We present a widely tunable, monolithic terahertz source based on intracavity difference frequency generation within a mid-infrared quantum cascade laser at room temperature. A three-section ridge waveguide laser design with two sampled grating sections and a distributed-Bragg section is used to achieve the terahertz (THz) frequency tuning. Room temperature single mode THz emission with a wide tunable frequency range of 2.6–4.2 THz (∼47% of the central frequency) and THz power up to 0.1 mW is demonstrated, making such device an ideal candidate for THz spectroscopy and sensing. 

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3. Nanopatterned parametric oscillators for laser conversion beyond an octave

   https://doi.org/10.1364/OPTICA.545158  

   Brodnik, Grant_M; Liu, Haixin; Carlson, David_R; Black, Jennifer_A; Papp, Scott_B (February 2025, Optica)

   Many uses of lasers place the highest importance on access to specific wavelength bands. For example, mobilizing optical-atomic clocks for a leap in sensing requires compact lasers at frequencies spread across the visible and near-infrared. Integrated photonics enables high-performance, scalable laser platforms. However, customizing laser-gain media to support wholly new bands is challenging and often prohibitively mismatched in scalability to early quantum-based sensing and information systems. Here, we demonstrate a tantalum pentoxide microresonator optical-parametric oscillator (OPO) that converts a pump laser to an output wave within a frequency span exceeding an octave. We control phase matching for oscillation by nanopatterning the microresonator to open a photonic-crystal bandgap on the mode of the pump laser. The photonic crystal splits only the pump mode and preserves the broader mode structure of the resonator, thus affording a single parameter to control output waves across the octave span using a nearly fixed frequency pump laser. We also demonstrate tuning the oscillator in free-spectral-range steps, more finely with temperature, and minimal additive frequency noise of the laser-conversion process. Our work shows that nanophotonic structures offer control of laser conversion in microresonators, bridging phase-matching of nonlinear optics and application requirements for laser designs. 

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4. Pockels laser directly driving ultrafast optical metrology

   https://doi.org/10.1038/s41377-025-01872-4  

   Xue, Shixin; Li, Mingxiao; Lopez-rios, Raymond; Ling, Jingwei; Gao, Zhengdong; Hu, Qili; Qiu, Tian; Staffa, Jeremy; Chang, Lin; Wang, Heming; et al (May 2025, Light: Science & Applications)

   Abstract The invention of the laser unleashed the potential of optical metrology, leading to numerous advancements in modern science and technology. This reliance on lasers, however, also introduces a bottleneck for precision optical metrology, as it requires sophisticated photonic infrastructure for precise laser-wave control, leading to limited metrology performance and significant system complexity. Here, we take a key step toward overcoming this challenge by demonstrating a Pockels laser with multifunctional capabilities that elevate optical metrology to a new level. The chip-scale laser achieves a narrow intrinsic linewidth down to 167 Hz and a broad mode-hop-free tuning range up to 24 GHz. In particular, it delivers an unprecedented frequency chirping rate of up to 20 EHz/s and an exceptional modulation bandwidth exceeding 10 GHz, both of which are orders of magnitude greater than those of existing lasers. Leveraging this laser, we successfully achieve velocimetry at 40 m/s over a short distance of 0.4 m, and measurable velocities up to the first cosmic velocity at 1 m away—a feat unattainable with conventional ranging approaches. At the same time, we achieve distance metrology with a ranging resolution of <2 cm. Furthermore, for the first time to our knowledge, we implement a dramatically simplified architecture for laser frequency stabilization by directly locking the laser to an external reference gas cell without requiring additional external light control. This approach enables long-term laser stability with a frequency fluctuation of only ±6.5 MHz over 60 min. The demonstrated Pockels laser combines elegantly high laser coherence with ultrafast frequency reconfigurability and superior multifunctional capability. We envision its profound impact across diverse fields including communication, sensing, autonomous driving, quantum information processing, and beyond. 

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5. Thermal-light heterodyne spectroscopy with frequency comb calibration

   https://doi.org/10.1364/OPTICA.440389  

   Fredrick, Connor; Olsen, Freja; Terrien, Ryan; Mahadevan, Suvrath; Quinlan, Franklyn; Diddams, Scott_A (February 2022, Optica)

   Precision laser spectroscopy is key to many developments in atomic and molecular physics and the advancement of related technologies such as atomic clocks and sensors. However, in important spectroscopic scenarios, such as astronomy and remote sensing, the light is of thermal origin, and interferometric or diffractive spectrometers typically replace laser spectroscopy. In this work, we employ laser-based heterodyne radiometry to measure incoherent light sources in the near-infrared and introduce techniques for absolute frequency calibration with a laser frequency comb. Measuring the solar continuum, we obtain a signal-to-noise ratio that matches the fundamental quantum-limited prediction given by the thermal photon distribution and our system’s efficiency, bandwidth, and averaging time. With resolving power $R\sim <\#comment/>{10}^6$, we determine the center frequency of an iron line in the solar spectrum to sub-MHz absolute frequency uncertainty in under 10 min, a fractional precision 1/4000 the linewidth. Additionally, we propose concepts that take advantage of refractive beam shaping to decrease the effects of pointing instabilities by $100\times <\#comment/>$, and of frequency comb multiplexing to increase data acquisition rates and spectral bandwidths by comparable factors. Taken together, our work brings the power of telecommunications photonics and the precision of frequency comb metrology to laser heterodyne radiometry, with implications for solar and astronomical spectroscopy, remote sensing, and precise Doppler velocimetry. 

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