Harald G. L. Schwefel

We demonstrate laser refrigeration of a millimeter‐scale crystalline whispering‐gallery‐mode (WGM) resonator fabricated from 10% :YLF, operating at atmospheric pressure. Under resonant pumping at 1031 nm, the resonator exhibits a measurable temperature reduction of K below room temperature with an input power of only 100 mW, corresponding to a temperature drop rate of about 4.8 K/W. Although the present undercoupled configuration yields an effective cavity enhancement factor close to unity, these results represent the experimental demonstration of optical refrigeration in a crystalline WGM resonator. This work establishes crystalline WGMs as a viable platform for thermally stable rare‐earth doped photonic devices, and with optimized coupling conditions, significantly deeper cooling and improved performance are expected for applications in precision photonics and quantum technologies.

Dynamically tunable terahertz (THz) devices are of great interest for applications in wireless communication, information encryption, and biomedical imaging. In this paper, we propose a dynamic and stretchable THz metasurface lens fabricated via 3D aerosol jet printing. Based on Pancharatnam–Berry phase, eight optical meta-atoms covering the phase shift range of 0–2 π are employed to construct a phase distribution of a lens. Combined with a multi-level quantitative phase compensation, the metasurface lens achieves focus compensation between focal spots during the stretching process. Experimental results show that the metasurface lens can maintain the target focal length under a stretching ratio of up to s = 115%. This work also investigates the feasibility of 3D aerosol jet printing for THz metasurfaces, providing an insight into the development of low-cost THz optical devices.

The development of technologies based on electromagnetic resonant structures usually relies on precise understanding of the spatial form of the supported modes. In millimeter-sized resonators that support millimeter-wave modes, this is often done numerically. However, simulations often fail to accurately predict the resonance frequencies and mode numbers. Such discrepancies can arise from uncertainties in the dielectric properties of the materials or inaccuracies in the geometric characterization of the structures. Here, we experimentally map the spatial distribution of terahertz modes in a silicon disk resonator by perturbing the mode with a metal needle scanned across the disk. This allows us to identify the modes, and hence to design a perturbative periodic structure to selectively manipulate specific modes. This technique could be useful for targeted tuning of such modes for spectral engineering.

In this study, we build upon our earlier research on distance calibration using Newton’s rings [ Opt. Lett. 47 , 6053 ( 2022 ) 10.1364/OL.474539 ] in a disc-shaped whispering gallery mode resonator (WGMR). Here, we use a glass trapezoidal prism coupler to evanescently trap light in a disc of calcium fluoride (CaF ₂ ) WGMR. While sweeping the distance between the coupler and the WGMR, the back of the trapezoidal prism is illuminated with a red-green-blue (RGB) lamp. Microscopic color images of the Newton’s rings formed between the resonator and the coupler are captured using a camera. The resonator-prism distance is measured by fitting the color images via phase extraction from the Newton’s rings. This modified method removes the ambiguity inherent in the interferometric distance measurement with monochromatic light and allows for the determination of distance with a single image, which results in a distance error of a few nanometers. We show that by including a position reference, we can further improve the accuracy to less than a tenth of a nanometer. Additionally, this technique eliminates the need for filters and offers more precision, owing to broad spectrum coverage of the RGB imaging, better signal-to-noise ratio, and three distinct color channels.

Lithium tetraborate (LB4) is a nonlinear crystal that has very low optical absorption in the visible and ultraviolet range. Hence, it is a very interesting material for nonlinear conversion experiments, particularly in whispering gallery mode (WGM) resonators. Here, we show theoretically that second-harmonic generation is possible over a wide range of pump wavelengths (488nm to 3320nm) in an x-cut LB4 WGM resonator. We also demonstrate this experimentally at three different pump wavelengths (1560nm, 795nm, and 517nm). We find a reasonable conversion efficiency of 0.2%/mW for the generation of UV light at 258.5nm. This is possible due to the high-quality factors on the order of 5×10⁷ and the use of the so-called selective coupling method.

Lithium tetraborate (LB4) is a lithium borate compound known for its exceptional linear and nonlinear optical properties, including a wide transparency range (0.16–3.5 μm) and high Raman gain. Here, a millimeter-sized LB4 whispering gallery mode (WGM) resonator with a record quality factor of 2.0×10⁹ at 517 nm was fabricated using single-point diamond cutting. Pumped with about 10 mW at 517 nm, it demonstrated four cascaded stimulated Raman scattering (SRS) peaks (537–608 nm), with the first-order SRS achieving a threshold of 0.71 mW and 7.2% slope efficiency. To our knowledge, this marks the first LB4 WGM resonator Raman laser.

Second-order nonlinear optical processes allow the conversion of electromagnetic signals to a higher frequency domain. Such so-called "upconversion" processes have been of interest for the optical detection of terahertz signals for decades. To enhance the nonlinear interaction, many investigations have focused on using high quality resonators, which improve the upconversion efficiency but limit the bandwidth. Here, we present a hybrid scheme that takes advantage of the high optical quality of a crystalline resonator while allowing the upconversion of signals at multiple frequency bands. We observed upconversion of six THz channels with MHz bandwidth, centered around 0.157 THz, and separated by about 10 GHz. The highest photon conversion efficiency at 0.157 THz was observed to be 3 × 10⁻⁶ per milliWatt of optical pump power. By numerically analyzing the landscape of nonlinear coupling in such resonators, we establish the conditions under which the demonstrated scheme can serve as an efficient multi-channel upconverter.