Mid-infrared ultra-high-Q resonators based on fluoride crystalline materials

Anisotropy Dependence of Material Deformation Mechanisms in Nanoscratching Monocrystalline BaF2: Experiments and Atomic Simulations

Monocrystalline barium fluoride (BaF2), known for its exceptional optical properties in the infrared spectrum, exhibits anisotropy that influences surface quality and material removal efficiency during ultraprecision machining. This research explores the impact of anisotropy on the deformation and removal mechanisms of monocrystalline BaF2 by integrating nanoscratch tests with molecular dynamics (MD) simulations. Nanoscratch tests conducted on variously oriented monocrystalline BaF2 surfaces using a ramp loading mode facilitated the identification of surface cracks and a systematic description of material removal behaviors. This study elucidates the effect of crystal orientation on the ductile-brittle transition (DBT) of monocrystalline BaF2, further developing a critical depth prediction model for DBT on the (111) crystal plane to reveal the underlying anisotropy mechanisms. Moreover, nanofriction and wear behaviors in monocrystalline BaF2 are found to be predominantly influenced by scratch direction, crystal surface, and applied load, with the (110) and (100) planes showing pronounced frictional and wear anisotropy. A coefficient of friction model, accounting for the material's elastic recovery, establishes the intrinsic relationship between anisotropic friction and wear behaviors, the size effect, and scratch direction. Lastly, MD modeling of nanoscratched monocrystalline BaF2 reveals the diversity of dislocations and strain distributions along the (111) [-110] and [-1-12] crystal directions, offering atomic scale insights into the origins of BaF2 anisotropy. Thus, this study provides a theoretical foundation for the efficient processing of fluorine-based infrared optic materials exhibiting anisotropy.

Highly Sensitive Force Sensor Based on High-Q Asymmetric V-Shaped CaF2 Resonator

Whispering gallery mode (WGM) resonators have high-quality factors and can be used in high-sensitivity sensors due to the narrow line width that allows for the detection of small external changes. In this paper, a force-sensing system based on a high-Q asymmetric V-shaped CaF2 resonator is proposed. Based on the dispersion coupling mechanism, the deformation of the resonator is achieved by loading force, and the resonant frequency is changed to determine the measurement. By adjusting the structural parameters of the asymmetric V-shaped resonator, the deformation of the resonator under force loading is improved. The experimental results show that the sensitivity of the V-shaped tip is 18.84 V/N, which determines the force-sensing resolution of 8.49 μN. This work provides a solution for force-sensing measurements based on a WGM resonator.

High-accuracy optical vector network analyzer for optical notch and bandpass responses

A high-accuracy optical vector network analyzer (OVNA) based on optical carrier-suppressed double sideband (CS-DSB) modulation is proposed and experimentally demonstrated. The ±1st-order sideband signals are generated by CS-DSB modulation and then pass through the symmetric optical device under test (DUT). The band-stop or band-pass responses can be realized by detecting and processing the double frequency of the driven RF signal. Compared with the conventional symmetrical DSB-based OVNA, the measurement accuracy is improved by eliminating the errors caused by the even-order sidebands, and the complexity is reduced as the proposed method with only one step measurement can avoid the complex postprocessing. In addition, the optical carrier is aligned to the center frequency of the DUT by employing the Pound-Drever-Hall (PDH) technique, which provides stable measurement. At the same time, the limitation that the band-pass responses cannot be measured by the traditional single-sideband (SSB)-based OVNA is overcome. Additionally, accurate magnitude and phase responses of the DUT near the optical carrier can be also achieved since the proposed OVNA is optical filter-immune. The proposed method is theoretically analyzed and verified by experiment. A Fabry-Perot (FP) interferometer serves as the symmetric DUT; the band-stop responses in a frequency range of 6 GHz are obtained with a resolution of 1.2 MHz; and the band-pass responses with the range from 0 to 13 GHz offsetting the optical carrier are also obtained. The measurement time can reach up to 30 min with high stability. The proposed OVNA offers enhanced accuracy and a stable approach for applications in photonic systems and other innovations.

Crystalline germanium high-Q microresonators for mid-IR

High-quality-factor optical microresonators have become an appealing object for numerous applications. However, the mid-infrared band experiences a lack of applicable materials for nonlinear photonics. Crystalline germanium demonstrates attractive material properties such as high nonlinear refractive index, large transparency window including the mid-IR band, particularly long wave multiphonon absorption limit. Nevertheless, the reported optical losses in germanium microresonators might not allow the potential of the Ge-based devices to be revealed. In this study, we report the fabrication of germanium microresonators with radii of 1.35 and 1.5 mm, exhibiting exceptional quality factors (Q-factors) exceeding 20 million, approaching the absorption-limited values at a wavelength of 2.68 µm. These Q-factors are a hundred times higher than previously reported, to the best of our knowledge. We measured the two-photon absorption coefficient combined with free-carrier absorption leveraging the high-Q of the resonators (obtained βTPA = (0.71 ± 0.12) · 10-8 m/W at 2.68 µm). This research underscores the potential of whispering gallery mode microresonators as valuable tools for measuring absorption coefficients at different wavelengths, providing a comprehensive analysis of various loss mechanisms. Furthermore, the exceptional Q-factors observed in germanium microresonators open intriguing opportunities for the advancement of germanium-based photonics within the mid-infrared spectral band.

Sandwich structure magnetometer with a high sensitivity and signal-to-noise ratio based on an ultrahigh-Q CaF2 resonator

Whispering gallery mode (WGM) resonators with an ultrahigh quality (Q) factor provide an extremely high resolution for high-precision sensing. However, it is difficult to use them directly in magnetic sensors because of the transparency to the magnetic field. In this paper, a sandwich structure consisting of a neodymium iron boron magnet and calcium fluoride resonator with a Q factor of 10⁹ is proposed. The experimental results show that, compared with the conventional magnetometer, the signal-to-noise ratio of the optical WGM magnetometer reaches 62 dB, with the direct current sensitivity of 42.59 MHz/mT and the AC sensitivity of 794p T H z ^-1/2 at 42 kHz.

Ultrahigh-Q WGM microspheres from ZBLAN for the mid-IR band

The advantages of high-quality-factor (high-Q) whispering gallery mode (WGM) microresonators can be applied to develop novel photonic devices for the mid-infrared (mid-IR) range. ZBLAN (glass based on heavy metal fluorides) is one of the most promising materials to be used for this purpose due to low optical losses in the mid-IR. We developed an original, to the best of our knowledge, fabrication method based on melting of commercially available ZBLAN-based optical fiber to produce high-Q ZBLAN microspheres with the diameters of 250 to 350 μm. We effectively excited whispering gallery modes in these microspheres and demonstrated high quality factor both at 1.55 μm and 2.64 μm. Intrinsic quality factor at telecom wavelength was shown to be (5.4 ± 0.4) × 10⁸ which is defined by the material losses in ZBLAN. In the mid-IR at 2.64 μm we demonstrated record quality factor in ZBLAN exceeding 10⁸ which is comparable to the highest values of the Q-factor among all materials in the mid-IR.

Simultaneous generation of a broadband MIR and NIR frequency comb in a GaP microring

Midinfrared (MIR) optical frequency combs are of great significance as broadband coherent light sources used in extensive areas such as coherent communications and molecule detections. Conventional MIR combs are usually restricted in size and power, while most microcombs are focused in the near-infrared (NIR) region because of the limited accessible Q-factor of microrings and the poor performances of available pumps. In this paper, we numerically demonstrate the simultaneous generation of a broadband MIR and NIR comb in a GaP microring with an additive waveguide. The achieved octave-spanning (1890-4050 nm) MIR microcomb at a low pump power of 34 mW can be effectively converted to the second-harmonic NIR comb covering 1120-1520 nm with separate dispersion optimization of the ring cavity and straight waveguide. The proposed system has the advantage of simple structure and low power threshold, which could find potential in highly integrated MIR optical sources and related applications.

Mid-infrared optical parametric oscillation spanning 3.4-8.2μm in a MgF2microresonator

Mid-infrared optical parametric oscillators (OPOs) offer a compelling route for accessing the 'molecular fingerprint' region and, thus, can find intensive applications such as precision spectroscopy and trace gas detection. Yet it still remains rather a challenge to realize broadband mid-infrared OPOs within a single cavity, usually limited by strict phase-matching conditions for wide spectral coverage and available pump power for adequate frequency generation. Here, we report the mid-infrared parametric oscillation spanning from 3.4 to 8.2μm, based on four-wave mixing in a high-QMgF₂microresonator with optimized dispersion. The center wavelength at 4.78μm is determined by the continuous tunable quantum cascade laser source, which contributes to effective expansion towards longer wavelength, as well as systemic miniaturization with smaller pump module. Such results could not only shed light on new ultimates of crystal and other microresonators, but also inspire explorations on their growing potentials in near future.

Midinfrared Tunable Laser with Noncritical Frequency Matching in Box Resonator Geometry

Monolithic optical parametric oscillators extend laser frequencies in compact architectures, but normally guide and circulate all pump, signal, and idler beams. Critical frequency matching is raised among these resonances, limiting operation stability and continuous tuning. Here, we develop a box resonator geometry that guides all beams but only resonates for signal. Such noncritical frequency matching enables 227 GHz continuous tuning, with sub-10 kHz linewidth and 0.43 W power at 3310 nm. Our results confirm that monolithic resonator can be effectively used as a tunable laser including midinfrared wavelength, as further harnessed with methane fine spectral measurement at MHz accuracy.

Chalcogenide Glass Microfibers for Mid-Infrared Optics

With diameters close to the wavelength of the guided light, optical microfibers (MFs) can guide light with tight optical confinement, strong evanescent fields and manageable waveguide dispersion and have been widely investigated in the past decades for a variety of applications. Compared to silica MFs, which are ideal for working in visible and near-infrared regions, chalcogenide glass (ChG) MFs are promising for mid-infrared (mid-IR) optics, owing to their easy fabrication, broad-band transparency and high nonlinearity, and have been attracting increasing attention in applications ranging from near-field coupling and molecular sensing to nonlinear optics. Here, we review this emerging field, mainly based on its progress in the last decade. Starting from the high-temperature taper drawing technique for MF fabrication, we introduce basic mid-IR waveguiding properties of typical ChG MFs made of As2S3 and As2Se3. Then, we focus on ChG-MF-based passive optical devices, including optical couplers, resonators and gratings and active and nonlinear applications of ChG MFs for mid-IR Raman lasers, frequency combs and supercontinuum (SC) generation. MF-based spectroscopy and chemical/biological sensors are also introduced. Finally, we conclude the review with a brief summary and an outlook on future challenges and opportunities of ChG MFs.

Batch Fabrication of High-Quality Infrared Chalcogenide Microsphere Resonators

Optical microsphere resonators working in the near- and mid-infrared regions are highly required for a variety of applications, such as optical sensors, filters, modulators, and microlasers. Here, a simple and low-cost approach is reported for batch fabrication of high-quality chalcogenide glass (ChG) microsphere resonators by melting high-purity ChG powders in an oil environment. Q factors as high as 1.2 × 10⁶ (7.4 × 10⁵ ) are observed in As₂ S₃ (As₂ Se₃ ) microspheres (≈30 µm in diameter) around 1550-nm wavelength. Smaller microspheres with sizes around 10 µm also show excellent resonant responses (Q ≈ 2.5 × 10⁵ ). Based on the mode splitting of an azimuthal mode in a microsphere resonator, eccentricities as low as ≈0.13% (≈0.17%) for As₂ S₃ (As₂ Se₃ ) microspheres are measured. Moreover, by coupling ChG microspheres with a biconical As₂ S₃ fiber taper, Q factors of ≈1.7 × 10⁴ (≈1.6 × 10⁴ ) are obtained in As₂ S₃ (As₂ Se₃ ) microspheres in the mid-infrared region (around 4.5 µm). The high-quality ChG microspheres demonstrated here are highly attractive for near- and mid-infrared optics, including optical sensing, optical nonlinearity, cavity quantum electrodynamics, microlasers, nanofocusing, and microscopic imaging.

Mid-infrared Raman lasers and Kerr-frequency combs from an all-silica narrow-linewidth microresonator/fiber laser system

Mid-infrared (mid-IR) lasers have great applications in bio-molecular sensing due to strong vibrational fingerprints in this wavelength range. However, it is a huge challenge to realize mid-IR lasers in conventional silica materials. Here, we demonstrate the generation of mid-IR Raman lasers and Kerr-frequency combs from an all-silica microresonator/fiber laser system. A single wavelength narrow-linewidth laser at ∼2 µm is first realized by using an ultrahigh Q-factor silica whispering-gallery-mode (WGM) microresonator as mode-selection mirror, and thulium-doped silica fiber as gain medium. Due to the strong intensity enhancement in the microresonator itself, multiple third-order nonlinear optical effects are observed, which include stimulated Stokes and anti-Stokes Raman scattering, and (cascaded) four-wave-mixing (FWM). The stimulated Stokes and anti-Stokes Raman scattering shift the initial 2 µm narrow-linewidth laser to as far as ∼2.75 µm and ∼1.56 µm, respectively. While the cascaded FWM helps to form a Kerr-frequency comb with a broad bandwidth of ∼900 nm and a mode spacing of twice of the microresonator free-spectral-range. This work offers a simple and effective route to realize all-silica mid-IR lasers based on enhanced optical nonlinearity in WGM microresonators.

Universal light-guiding geometry for on-chip resonators having extremely high Q-factor

By providing an effective way to leverage nonlinear phenomena in integrated devices, high-Q optical resonators have led to recent advances in on-chip photonics. However, developing fabrication processes to shape any new material into a resonator with extremely smooth surfaces on a chip has been an exceptionally challenging task. Here, we describe a universal method to implement ultra-high-Q resonators with any new material having desirable properties that can be deposited by physical vapor deposition. Using this method light-guiding cores with surface roughness on the molecular-scale are created automatically on pre-patterned substrates. Its efficacy has been verified using As₂S₃, a chalcogenide glass that has high-nonlinearity. The Q-factor of the As₂S₃ resonator so-developed approached the propagation loss record achieved in chalcogenide fibers which were limited by material losses. Owing to the boosted Q-factor, lasing by stimulated Brillouin scattering has been demonstrated with 100 times lower threshold power than the previous record.

Widely separated optical Kerr parametric oscillation in AlN microrings

Here, we report $ {\chi ^{(3)}} $χ⁽³⁾-based optical parametric oscillation (OPO) with widely separated signal-idler frequencies from crystalline aluminum nitride microrings pumped at $ 2\,\,\unicode{x00B5}{\rm m} $2µm. By tailoring the width of the microring, OPO reaching toward the telecom and mid-infrared bands with a frequency separation of 64.2 THz is achieved. While dispersion engineering through changing the microring width is capable of shifting the OPO sideband by $ \gt {9}\;{\rm THz}$>9THz, the OPO frequency can also be agilely tuned in the ranges of 1 and 0.1 THz, respectively, by shifting the pump wavelength and controlling the chip's temperature. At high pump powers, the OPO sidebands further evolve into localized frequency comb lines. Such large-frequency-shift OPO with flexible wavelength tunability will lead to enhanced chip-scale light sources.

High-Q diamond microresonators in the long-wave infrared

High quality factor (Q) photonic devices in the room temperature thermal infrared region, corresponding to deeper long-wave infrared with wavelengths beyond 9 microns, have been demonstrated for the first time. Whispering gallery mode diamond microresonators were fabricated using single crystal diamond substrates and oxygen-based inductively coupled plasma (ICP) reactive ion etching (RIE) at high angles. The spectral characteristics of the devices were probed at room temperature using a tunable quantum cascade laser that was free space-coupled into the resonators. Light was extracted via an arsenic selenide (As₂Se₃) chalcogenide infrared fiber and directed to a cryogenically cooled mercury cadmium telluride (HgCdTe) detector. The quality factors were tested in multiple microresonators across a wide spectral range from 9 to 9.7 microns with similar performance. One example resonance (of many comparables) was found to reach 3648 at 9.601 µm. Fourier analysis of the many resonances of each device showed free spectral ranges slightly greater than 40 GHz, matching theoretical expectations for the microresonator diameter and the overlap of the whispering gallery mode with the diamond.

Octahedrally coordinated single layered CaF2: robust insulating behaviour

Using first-principles calculations, the structural, vibrational, and electronic properties of single-layered calcium fluoride (CaF₂) are investigated. The dynamical stability of 1T-CaF₂ is confirmed by the phonon dispersions. Raman active vibrational modes of 1T-CaF₂ enable its characterization via Raman spectroscopy. In addition, the calculated electronic properties of 1T-CaF₂ confirmed insulating behavior with an indirect wide band gap which is larger than that of a well-known single-layered insulator, h-BN. Moreover, one-dimensional nanoribbons of CaF₂ are investigated for two main edge orientations, namely zigzag and armchair, and it is revealed that both structures maintain the 1T nature of CaF₂ without any structural edge reconstructions. Electronically, both types of CaF₂ nanoribbons display robust insulating behavior with respect to the nanoribbon width. The results show that both the 2D and 1D forms of 1T-CaF₂ show potential in nanoelectronics as an alternative to the widely-used insulator h-BN with its similar properties and wider electronic band gap.

Approaching the transit time limit for high-precision spectroscopy on metastable CO around 6 μm

As molecular spectroscopy makes its comeback to the limelight of fundamental sciences, scientists need ever better coherent light sources and diagnostic methods. Of particular importance for molecular spectroscopy is the mid infrared spectral region, where strong and narrow ro-vibrational excitations have their fundamental transition frequencies. Unfortunately, much technology in some portions of this spectral region is still rather pioneering. Here we present a high-resolution spectroscopy experiment, based on a molecular beam setup, which pushes the measured linewidth close to the transit time limit, on the order of 100 kHz. Moreover, we discuss the issue of frequency-noise characterization and the linewidth measurement of the ultrastable infrared laser used in the experiment.

Octave-wide phase-matched four-wave mixing in dispersion-engineered crystalline microresonators

In this Letter, we report phase-matched four-wave mixing separated by over one octave in a dispersion-engineered crystalline microresonator. Experimental and numerical results presented here confirm that primary sidebands were generated with a frequency shift up to 140 THz, and that secondary sidebands formed a localized comb structure, known as a clustered comb in the vicinity of the primary sidebands. A theoretical analysis of the phase-matching condition validated our experimental observations, and our results agree well with numerical simulations. These results offer the potential to realize a frequency-tunable comb cluster generator operating from 1 μm to mid-infrared wavelengths with a single and compact device.

Self-injection locking of a laser diode to a high-Q silicon WGM microresonator

The key properties of the self-injection locking regime of a laser diode to a high-Q microresonator with whispering gallery mode made of crystalline silicon are considered. It has been experimentally demonstrated the possibility of the self-injection locking using cavity made of crystalline silicon. This result opens up new possibilities for creating narrow-band highly stable laser sources in midIR, over 2.3 microns, on a new hardware base.

Spectrum collapse, narrow linewidth, and Bogatov effect in diode lasers locked to high-Q optical microresonators

We present a novel method allowing high-power single-frequency emission with sub-kHz linewidth from a compact multi-frequency diode laser locked to high-Q optical microresonator. Using high-Q MgF₂microresonator and multi-frequency diode laser operating at 1535 nm with the output power of 100 mW and an emission spectrum consisting of approximately 50 lines with MHz linewidth, we observed a spectrum collapse to a single line or several lines with a sub-kHz linewidth and output power power of 50 mW. The Bogatov effect predicted more than 30 years ago was observed and studied in the spectrum of the locked laser. For analysis of the considered effect, original theoretical model taking into account self-injection locking effect, mode competition and Bogatov asymmetric mode interaction was developed and numerical modeling was performed. All numerical results are in a good agreement with our experimental data. Accurate analytical estimations for the parameters critical for the considered effect were obtained. The proposed method may be applied for different types of diode lasers operating in different spectral ranges.

Long-term measurement of high Q optical resonators based on optical vector network analysis with Pound Drever Hall technique

An optical vector network analysis (OVNA) based on double sidebands (DSB) modulation and the Pound Drever Hall (PDH) technique is proposed and demonstrated. The frequency responses measurement of the high Q optical device with high stability are achieved by transmitting the DSB modulation signals through the device. The high stability can be realized by using the PDH feedback loop. Compared with the conventional DSB-based OVNA, the proposed scheme with only one step measurement avoids the complex post-processing. Moreover, the long-term measurement with high stability can also be realized. A proof-of-concept experiment is carried out, which achieves the magnitude and phase responses of the Fabry-Perot interferometer, and there is no frequency response shift even though the test time is up to 90 minutes. The proposed method is simple and stable, which can be potentially applied in characterization and fabrication of high Q optical devices.

Phase-stabilized 100 mW frequency comb near 10 μm

Long-wavelength mid-infrared (MIR) frequency combs with high power and flexible tunability are highly desired for molecular spectroscopy, including investigation of large molecules such as C60. We present a high power, phase-stabilized frequency comb near 10 μm, generated by a synchronously pumped, singly resonant optical parametric oscillator (OPO) based on AgGaSe2. The OPO can be continuously tuned from 8.4 to 9.5 μm, with a maximum average idler power of 100 mW at the center wavelength of 8.5 μm. Both the repetition rate (f rep) and the carrier-envelope offset frequency (f ceo) of the idler wave are phase-locked to microwave signals referenced to a Cs clock. We describe the detailed design and construction of the frequency comb, and discuss potential applications for precise and sensitive direct frequency comb spectroscopy.

Highly efficient coupling of crystalline microresonators to integrated photonic waveguides

Crystalline optical whispering gallery mode resonators made from alkaline earth fluorides can achieve exceptionally large optical finesse, and are used in a variety of applications, from frequency stabilization and narrow linewidth lasers, to low-noise microwave generation or soliton Kerr frequency combs. Here we demonstrate an efficient coupling method to resonators of these materials, which employs photonic integrated waveguides on a chip based on silicon nitride. By converting a mode from silicon nitride to a free-hanging silica waveguide on a silicon chip, coupling to a crystalline resonator is achieved with a high extinction, while preserving a quality factor exceeding 200 million. This compact, heterogeneous integration of ultra-high Q-factor crystalline resonators with photonic waveguides provides a proof of concept for wafer scale integration and robust, compact packaging for a wide range of applications.

Optical Microresonators for Sensing and Transduction: A Materials Perspective

Optical microresonators confine light to a particular microscale trajectory, are exquisitely sensitive to their microenvironment, and offer convenient readout of their optical properties. Taken together, this is an immensely attractive combination that makes optical microresonators highly effective as sensors and transducers. Meanwhile, advances in material science, fabrication techniques, and photonic sensing strategies endow optical microresonators with new functionalities, unique transduction mechanisms, and in some cases, unparalleled sensitivities. In this progress report, the operating principles of these sensors are reviewed, and different methods of signal transduction are evaluated. Examples are shown of how choice of materials must be suited to the analyte, and how innovations in fabrication and sensing are coupled together in a mutually reinforcing cycle. A tremendously broad range of capabilities of microresonator sensors is described, from electric and magnetic field sensing to mechanical sensing, from single-molecule detection to imaging and spectroscopy, from operation at high vacuum to in live cells. Emerging sensing capabilities are highlighted and put into context in the field. Future directions are imagined, where the diverse capabilities laid out are combined and advances in scalability and integration are implemented, leading to the creation of a sensor unparalleled in sensitivity and information content.