Optical Quality Factor Research Articles

Surface morphology and optical properties of thin films of plasma polymerized methyl acrylate

Thermally stable plasma polymerized methyl acrylate (PPMA) thin films are fabricated onto spotless glass substrates by AC power plasma polymerization technique. Field emission scanning electron micrographs of these films exhibit clusters/agglomerations on the surface. Energy dispersive x-ray analysis confirms the existence of 52.38, 52.63 and 51.55% of carbon and 47.62, 47.31 and 46.45% of oxygen, respectively in 170, 230 and 290 nm thick PPMA films. The molecular vibrations of various natures observed by attenuated total reflectance Fourier transform infrared spectroscopy of methyl acrylate and PPMA represent that the percentage of hydrophilic groups is smaller than the hydrophobic groups. Thermogravimetric analyses demonstrate that the major weight loss occurs in the temperature range from 500 to 850 K. Ultraviolet–visible spectroscopy demonstrates that the allowed direct energy band gap varies from 3.77 to 3.83 eV, whereas the indirect energy band gap alters from 3.40 to 3.48 eV with film thicknesses from 120 to 290 nm. Other optical parameters such as Urbach energy, refractive index, optical extinction coefficient, dielectric constants, quality factor, and skin depth have also been investigated for the PPMA thin films to elucidate their suitability in optoelectronic devices.

Neon lasing (λ = 540.1 nm) in a dense optical cavity pumped by a pulsed inductive cylindrical discharge

The experimental results of the lasing characteristics dependence of the inductive neon laser (λ = 540.1 nm) on the optical cavity quality factor are presented. In the described experiments, the laser was operated in the amplified spontaneous emission mode. A frontal-substrate resonator (made according to the type of an open Fabry–Perot interferometer) was used, as well as a dense resonator with a reflection coefficient of the front mirror of 82%. Replacing the front substrate with an output mirror resulted in an increase in the lasing energy and optical pulse duration from 8.5 μJ to 41 μJ and from 10.0 ± 0.1 ns to 14.8 ± 0.1 ns, respectively. It is shown that the use of a dense resonator allows us to change the shape of the laser beam in the far field from annular to circular and to obtain intensity distribution close to uniform.

Fractional Optical Angular Momentum and Multi-Defect-Mediated Mode Renormalization and Orientation Control in Photonic Crystal Microring Resonators.

Whispering gallery modes (WGMs) in circularly symmetric optical microresonators exhibit integer quantized angular momentum numbers due to the boundary condition imposed by the geometry. Here, we show that incorporating a photonic crystal pattern in an integrated microring can result in WGMs with fractional optical angular momentum. By choosing the photonic crystal periodicity to open a photonic band gap with a band-edge momentum lying between that of two WGMs of the unperturbed ring, we observe hybridized WGMs with half-integer quantized angular momentum numbers (m∈Z+1/2). Moreover, we show that these modes with fractional angular momenta exhibit high optical quality factors with good cavity-waveguide coupling and an order of magnitude reduced group velocity. Additionally, by introducing multiple artificial defects, multiple modes can be localized to small volumes within the ring, while the relative orientation of the delocalized band-edge states can be well controlled. Our Letter unveils the renormalization of WGMs by the photonic crystal, demonstrating novel fractional angular momentum states and nontrivial multimode orientation control arising from continuous rotational symmetry breaking. The findings are expected to be useful for sensing and metrology, nonlinear optics, and cavity quantum electrodynamics.

A Novel Buckle‐Free Large Rib Microdisk with Submicrometer Thickness

Thin large microdisks, that are key for dense spectral microcomb generation at visible to UV wavelengths, face challenges in fabrication. One of the most difficult issues is the buckling effect that significantly reduces the cavity optical quality factor. This work introduces a novel rib disk structure that significantly mitigates the buckling effects. Using this approach, we obtained millimeter size buckle-free microdisks with sub-micron thickness and high optical quality factor exceeding $10^7$.

Ultrasound sensing at thermomechanical limits with optomechanical buckled-dome microcavities.

We describe the use of monolithic, buckled-dome cavities as ultrasound sensors. Patterned delamination within a compressively stressed thin film stack produces high-finesse plano-concave optical resonators with sealed and empty cavity regions. The buckled mirror also functions as a flexible membrane, highly responsive to changes in external pressure. Owing to their efficient opto-acousto-mechanical coupling, thermal-displacement-noise limited sensitivity is achieved at low optical interrogation powers and for modest optical (Q ∼ 103) and mechanical (Q ∼ 102) quality factors. We predict and verify broadband (up to ∼ 5 MHz), air-coupled ultrasound detection with noise-equivalent pressure (NEP) as low as ∼ 30-100 µPa/Hz1/2. This corresponds to an ultrasonic force sensitivity ∼ 2 × 10-13 N/Hz1/2 and enables the detection of MHz-range signals propagated over distances as large as ∼ 20 cm in air. In water, thermal-noise-limited sensitivity is demonstrated over a wide frequency range (up to ∼ 30 MHz), with NEP as low as ∼ 100-800 µPa/Hz1/2. These cavities exhibit a nearly omnidirectional response, while being ∼ 3-4 orders of magnitude more sensitive than piezoelectric devices of similar size. Easily realized as large arrays and naturally suited to direct coupling by free-space beams or optical fibers, they offer significant practical advantages over competing optical devices, and thus could be of interest for several emerging applications in medical and industrial ultrasound imaging.

Kerr optical parametric oscillation in a photonic crystal microring for accessing the infrared.

Continuous wave optical parametric oscillation (OPO) provides a flexible approach for accessing mid-infrared wavelengths between 2 µm and 5 µm, but operation at these wavelengths has not yet been integrated into silicon nanophotonics. Typically, a Kerr OPO uses a single transverse mode family for pump, signal, and idler modes, and relies on a delicate balance to achieve normal (but close-to-zero) dispersion near the pump and the requisite higher-order dispersion needed for phase- and frequency-matching. Within integrated photonics platforms, this approach results in two major problems. First, the dispersion is very sensitive to geometry, so that small fabrication errors can have a large impact. Second, the device is susceptible to competing nonlinear processes near the pump. In this Letter, we propose a flexible solution to infrared OPO that addresses these two problems by using a silicon nitride photonic crystal microring (PhCR). The frequency shifts created by the PhCR bandgap enable OPO that would otherwise be forbidden. We report an intrinsic optical quality factor up to (1.2 ± 0.1)×106 in the 2-µm band, and use a PhC ring to demonstrated an OPO with a threshold dropped power in the cavity of (90 ± 20) mW, with the pump wavelength at 1998 nm, and the signal and idler wavelengths at 1937 nm and 2063 nm, respectively. We further discuss how to extend the OPO spectral coverage in the mid-infrared. These results establish the PhCR OPO as a promising route for integrated laser sources in the infrared.

Electro-optic tuning of a single-frequency ultranarrow linewidth microdisk laser

Single-frequency ultranarrow linewidth on-chip microlasers with a fast wavelength tunability play a game-changing role in a broad spectrum of applications ranging from coherent communication, light detection and ranging, to metrology and sensing. Design and fabrication of such light sources remain a challenge due to the difficulties in making a laser cavity that has an ultrahigh optical quality (Q) factor and supports only a single lasing frequency simultaneously. Here, we demonstrate a unique single-frequency ultranarrow linewidth lasing mechanism on an erbium ion-doped lithium niobate (LN) microdisk through simultaneous excitation of high-Q polygon modes at both pump and laser wavelengths. As the polygon modes are sparse within the optical gain bandwidth compared with the whispering gallery mode counterpart, while their Q factors (above 10 million) are even higher due to the significantly reduced scattering on their propagation paths, single-frequency lasing with a linewidth as narrow as 322 Hz is observed. The measured linewidth is three orders of magnitude narrower than the previous record in on-chip LN microlasers. Finally, enabled by the strong linear electro-optic effect of LN, real-time electro-optical tuning of the microlaser with a high tuning efficiency of ∼50 pm / 100 V is demonstrated.

Ultrasensitive nanoscale optomechanical electrometer using photonic crystal cavities

Abstract High-precision detection of electric charge is critical for physical, chemical, and biological measurements. Nanophotonic optomechanical system confines the optical field at the nanoscale and enables a strong interaction between optical cavity and mechanical resonator. Its high optical quality factor cavity and strong optomechanical coupling are promising for precision sensing applications. Here an integrated optomechanical electrometer is proposed for the electric charge sensing using a zipper cavity with a suspended photonic crystal nanobeam (PCN) acting as a movable mechanical resonator. As the electrostatic force arising from the electric voltage to be measured interacts with the mechanical motion of the movable PCN and modulates its resonance through electrostatic stiffening effect, optomechanical coupling transduces the mechanical motion to the optical field with enhanced sensitivity. The resonance shift of the mechanical resonator can be monitored to detect the electric voltage with a sensitivity of 0.007 Hz / m V 2 $\mathrm{Hz}/\mathrm{m}{\mathrm{V}}^{2}$ . Moreover, the sensing performance can be further enhanced with the operation of the optomechanical electrometer in the self-sustained oscillation above threshold power. Owing to the narrow-linewidth of detector radio frequency (RF) spectrum with a large peak-to-noise floor ratio (up to 73.5 dB), the enhanced electrical sensitivity of 0.014 Hz / m V 2 $\mathrm{Hz}/\mathrm{m}{\mathrm{V}}^{2}$ is achieved with a high resolution of 1.37 m V 2 H z − 1 / 2 $1.37\,\mathrm{m}{\mathrm{V}}^{2}\mathrm{H}{\mathrm{z}}^{-1/2}$ . A theoretical minimal detectable electrostatic charge is calculated as 1.33 × 10 − 2 eH z − 1 / 2 $1.33{\times}{10}^{-2}\,\mathrm{eH}{\mathrm{z}}^{-1/2}$ by converting the measured electric voltage versus RF shift to an approximatively linear relationship. This on-chip optomechanical electrometry scheme provides a powerful solution to the ultrasensitive determination of charged nanoparticles in biological and chemical applications.

Atomically Smooth Single-Crystalline Platform for Low-Loss Plasmonic Nanocavities.

Nanoparticle-on-mirror plasmonic nanocavities, capable of extreme optical confinement and enhancement, have triggered state-of-the-art progress in nanophotonics and development of applications in enhanced spectroscopies. However, the optical quality factor and thus performance of these nanoconstructs are undermined by the granular polycrystalline metal films (especially when they are optically thin) used as a mirror. Here, we report an atomically smooth single-crystalline platform for low-loss nanocavities using chemically synthesized gold microflakes as a mirror. Nanocavities constructed using gold nanorods on such microflakes exhibit a rich structure of plasmonic modes, which are highly sensitive to the thickness of optically thin (down to ∼15 nm) microflakes. The microflakes endow nanocavities with significantly improved quality factor (∼2 times) and scattering intensity (∼3 times) compared with their counterparts based on deposited films. The developed low-loss nanocavities further allow for the integration with a mature platform of fiber optics, opening opportunities for realizing nanocavity-based miniaturized photonic devices for practical applications.

Demonstration of Hybrid High-Q Hexagonal Boron Nitride Microresonators

Hexagonal boron nitride (hBN) is a wide bandgap van der Waals material that is emerging as a powerful platform for quantum optics and nanophotonics. In this work, we demonstrate whispering gallery mode silica microresonators hybridized with thin layers of hBN that exhibit high intrinsic optical quality factor >7 × 105 and a linear absorption coefficient of 9.5 cm–1. Measurements of the effect of hBN thickness on optical Q and comparison with a theoretical model allows the linear optical absorption coefficient of the hBN films to be estimated. These high-Q devices will be useful for applications in quantum and nonlinear optics, and their hybridized geometry provides a sensitive platform for evaluating losses in hBN and other 2D materials.

Fabrication of ultra-high Q silica microdisk using chemo-mechanical polishing

Here, we demonstrate that adding a chemo-mechanical polishing procedure to conventional photolithography, a silica microdisk with ultra-high quality factors (>108) can be fabricated. By comparing the intrinsic optical quality factor (Q) measured at 1550, 970, and 635 nm wavelengths, we observe that due to the significantly reduced surface roughness and the hydrophilic nature of the disk surface, at 1550 nm wavelength, the water molecule absorption at the cavity surface supersedes Rayleigh scattering as the dominant factor for Q degradation.

Tantala Kerr nonlinear integrated photonics

Integrated photonics plays a central role in modern science and technology, enabling experiments from nonlinear science to quantum information, ultraprecise measurements and sensing, and advanced applications such as data communication and signal processing. Optical materials with favorable properties are essential for nanofabrication of integrated photonics devices. Here we describe a material for integrated nonlinear photonics, tantalum pentoxide ( T a 2 O 5 , hereafter tantala), which offers low intrinsic material stress, low optical loss, and efficient access to Kerr nonlinear processes. We utilize > 800 n m thick tantala films deposited via ion beam sputtering on oxidized silicon wafers. The tantala films contain a low residual tensile stress of 38 MPa, and they offer a Kerr index of n 2 = 6.2 ± 2.3 × 10 − 19 m 2 / W , which is approximately a factor of 3 higher than silicon nitride. We fabricate integrated nonlinear resonators and waveguides without the cracking challenges that are prevalent in stoichiometric silicon nitride. The tantala resonators feature an optical quality factor up to 3.8 × 10 6 , which enables us to generate ultrabroad bandwidth Kerr soliton frequency combs with low threshold power. Moreover, we characterize tantala waveguides by supercontinuum generation from low-energy, ultrafast seed pulses. These demonstrations characterize future application directions with tantala integrated nonlinear photonics.

Resonators made of a disk and a movable continuous-membrane

Microcavities are used for resonantly enhanced interactions of light with matter or particles. Usually, the resonator’s sensitivity drops down with every particle attached to its interface due to the inherent scattering losses and the corresponding degradation of the optical quality factor. Here, we demonstrate, for the first time, a hybrid resonator made of a dielectric disk and a continuous membrane. The membrane is evanescently coupled to the disk while both membrane and disk are mechanically separated. Therefore, the optical mode is co-hosted by the disk and the membrane, while we use a nanopositioning system to control the disk motion. We experimentally demonstrate that spreading scatterers on the membrane and then moving the membrane parallel to the disk brings different scatterers into and out of the optical-mode region. We also show that the membrane’s motion toward the disk results in a 35 GHz drift in the optical resonance frequency. The membrane is continuous in two dimensions and can move a practically unlimited distance in these directions. Furthermore, the membrane can move from a state where it touches the disk to an unlimited distance from the disk. Our continuum-coupled resonator might impact sustainable sensors where the perpetual motion of analytes into and out of the optical-mode region is needed. Additionally, the membrane can carry quantum dots or point defects such as nitrogen-vacancy centers to overlap with the optical mode in a controllable manner. As for non-parallel motion, the membrane’s flexibility and its ability to drift resonance frequency might help in detecting weak forces.

Frequency-multiplexed hybrid optical entangled source based on the Pockels effect

In the recent years important experimental advances in resonant electro-optic modulators as high-efficiency sources for coherent frequency combs and as devices for quantum information transfer have been realized, where strong optical and microwave mode coupling were achieved. These features suggest electro-optic-based devices as candidates for entangled optical frequency comb sources. In the present work, I study the generation of entangled optical frequency combs in millimeter-sized resonant electro-optic modulators. These devices profit from the experimentally proven advantages such as nearly constant optical free spectral ranges over several gigahertz, and high optical and microwave quality factors. The generation of frequency multiplexed quantum channels with spectral bandwidth in the MHz range for conservative parameter values paves the way towards novel uses in long-distance hybrid quantum networks, quantum key distribution, enhanced optical metrology, and quantum computing.

Magnetic field sensing performance of centimeter-scale resonator with optimized structure

Applications of magnetometers are affected mainly by their sensitivities and detection bandwidths. Till now, the applications of the centimeter-scale optomechanical magnetometer have been still limited by those two factors. In order to improve its sensing performance in a low frequency regime of the alternating current (AC) magnetic field sensor based on centimeter-scale whispering gallery mode resonator, we design a new centimeter-scale crystalline whispering gallery mode resonator which has different relative distributions of the magnetostrictive material (Terfenol-D) and the optical material (CaF<sub>2</sub>) from the unoptimized centimeter-scale whispering gallery mode resonator. Experimental results show that this new resonator is able to detect the AC magnetic field ranging from 6 Hz to 1 MHz, and a peak sensitivity of 530 pT·Hz<sup>–1/2</sup> at 123.8 kHz is achieved without DC bias field in a magnetically unshielded non-cryogenic environment. On condition that the optical quality factor is at the same level of 10<sup>8</sup> and there is no DC bias magnetic field, the best sensitivity of the optimized resonator is 11 times higher than that of the unoptimized resonator, and the corresponding detection frequency band is expanded by 1.67 times, switching from the frequency band of 10 Hz–600 kHz to 6 Hz–1 MHz. Besides, the device only needs 100 μW light intensity to operate, which offers us a low optical power consumption magnetometer. Within the detection frequency band, the proposed magnetometer can detect both a single frequency alternating magnetic field signal and an alternating magnetic field signal covering a certain frequency range. It can detect 50 or 60 Hz alternating magnetic field signal generated by current in the wire so that the working status of the power system can be monitored. If the sensing performance is further improved, it may be able to detect the magnetic field signal at frequency in a range of 1 kHz–10 MHz generated by the partial discharge current and the extremely low frequency human body magnetic field signal located in a frequency band of [10 mHz–1 kHz]. Further improvement in sensing performance is possible through optimizing the system noise and the magnetic field response capability of the device, which might allow the device to possess the applications in the fields of power system fault monitoring and medical diagnosis.

Cavity electro-optics in thin-film lithium niobate for efficient microwave-to-optical transduction

Linking superconducting quantum devices to optical fibers via microwave-optical quantum transducers may enable large-scale quantum networks. For this application, transducers based on the Pockels electro-optic (EO) effect are promising for their direct conversion mechanism, high bandwidth, and potential for low-noise operation. However, previously demonstrated EO transducers require large optical pump power to overcome weak EO coupling and reach high efficiency. Here, we create an EO transducer in thin-film lithium niobate, a platform that provides low optical loss and strong EO coupling. We demonstrate on-chip transduction efficiencies of up to and of optical pump power. The transduction efficiency can be improved by further reducing the microwave resonator’s piezoelectric coupling to acoustic modes, increasing the optical resonator quality factor to previously demonstrated levels, and changing the electrode geometry for enhanced EO coupling. We expect that with further development, EO transducers in thin-film lithium niobate can achieve near-unity efficiency with low optical pump power.

Alumina coating for dispersion management in ultra-high Q microresonators

Silica optical microspheres often exhibit ultra-high quality factors, yet their group velocity dispersion, which is crucial for nonlinear optics applications, can only be coarsely tuned. We experimentally demonstrate that group-velocity dispersion of a silica microsphere can be engineered by coating it with conformal nanometric layers of alumina yet preserving its ultra-high optical quality factors (∼107) at telecom wavelengths. Using the atomic layer deposition technique for the dielectric coating, which ensures nm-level thickness control, we not only achieve a fine dispersion tailoring but also maintain a low surface roughness and material absorption to ensure a low optical loss. Numerical simulations supporting our experimental results show that the alumina layer thickness is a promising technique for precise tuning of group-velocity dispersion. As an application, we demonstrate the generation of Kerr optical frequency combs, showing that the alumina coatings can also sustain the high optical intensities necessary for nonlinear optical phenomena.

Polygon Coherent Modes in a Weakly Perturbed Whispering Gallery Microresonator for Efficient Second Harmonic, Optomechanical, and Frequency Comb Generations.

We observe high optical quality factor (Q) polygonal and star coherent optical modes in a lithium niobate microdisk. In contrast to the previous polygon modes achieved by deformed microcavities at lower mechanical and optical Q, we adopt weak perturbation from a tapered fiber for the polygon mode formation. The resulting high intracavity optical power of the polygon modes triggers second harmonic generation at high efficiency. With the combined advantages of a high mechanical Q cavity, we observe optomechanical oscillation in polygon modes for the first time. Finally, we observe frequency microcomb generation from the polygon modes with an ultrastable taper-on-disk coupling mechanism.

Properties of nanocrystalline silicon probed by optomechanics

Abstract Nanocrystalline materials exhibit properties that can differ substantially from those of their single crystal counterparts. As such, they provide ways to enhance and optimize their functionality for devices and applications. Here, we report on the optical, mechanical and thermal properties of nanocrystalline silicon probed by means of optomechanical nanobeams to extract information of the dynamics of optical absorption, mechanical losses, heat generation and dissipation. The optomechanical nanobeams are fabricated using nanocrystalline films prepared by annealing amorphous silicon layers at different temperatures. The resulting crystallite sizes and the stress in the films can be controlled by the annealing temperature and time and, consequently, the properties of the films can be tuned relatively freely, as demonstrated here by means of electron microscopy and Raman scattering. We show that the nanocrystallite size and the volume fraction of the grain boundaries play a key role in the dissipation rates through nonlinear optical and thermal processes. Promising optical (13,000) and mechanical (1700) quality factors were found in the optomechanical cavity realized in the nanocrystalline Si resulting from annealing at 950°C. The enhanced absorption and recombination rates via the intragap states and the reduced thermal conductivity boost the potential to exploit these nonlinear effects in applications including Nanoelectromechanical systems (NEMS), phonon lasing and chaos-based devices.

A universal frequency engineering tool for microcavity nonlinear optics: multiple selective mode splitting of whispering-gallery resonances.

Whispering-gallery microcavities have been used to realize a variety of efficient parametric nonlinear optical processes through the enhanced light-matter interaction brought about by supporting multiple high quality factor and small modal volume resonances. Critical to such studies is the ability to control the relative frequencies of the cavity modes, so that frequency matching is achieved to satisfy energy conservation. Typically this is done by tailoring the resonator cross-section. Doing so modifies the frequencies of all of the cavity modes, that is, the global dispersion profile, which may be undesired, for example, in introducing competing nonlinear processes. Here, we demonstrate a frequency engineering tool, termed multiple selective mode splitting (MSMS), that is independent of the global dispersion and instead allows targeted and independent control of the frequencies of multiple cavity modes. In particular, we show controllable frequency shifts up to 0.8 nm, independent control of the splitting of up to five cavity modes with optical quality factors ≳ 105, and strongly suppressed frequency shifts for untargeted modes. The MSMS technique can be broadly applied to a wide variety of nonlinear optical processes across different material platforms, and can be used to both selectively enhance processes of interest and suppress competing unwanted processes.