Grating integrated single mode microring laser

Coexistence of the Radial-Guided Mode and WGM in Azimuthal-Grating-Integrated Microring Lasers

Whispering-gallery mode (WGM) resonators, renowned for their high Q-factors and narrow line widths, are widely utilized in integrated photonics. Integrating diffraction gratings onto WGM cavities has gained significant attention because these gratings function as azimuthal refractive index modulators, enabling single-mode WGM emissions and supporting beams with orbital angular momentum (OAM). The introduction of curved grating structures facilitates guided mode resonances by coupling high-order diffracted waves with leaking modes from the waveguide. These gratings act as wavelength-selective mirrors and support concentric circular radial-guided modes. This study investigates the coexistence and interaction between OAM-carrying WGMs and radial-guided modes with Bessel beam characteristics in an active cladding grating-integrated microring laser. These phenomena are examined through both three-dimensional simulations and experiments. The active layer enhances the radial-guided modes at the microring's center, where cylindrical waves from the active cladding produce strong guided mode resonance at specific wavelengths corresponding to radial modes. Additionally, general WGMs are formed and confined within the microring. The effects of grating depth and microring size on radial-guided mode resonance are evaluated through two-dimensional simulations and experiments. These insights pave the way for integrating functional lasers into photonic circuits and advancing technologies for topological optical vortex emission and manipulation.

Band flipping and bandgap closing in a photonic crystal ring and its applications

The size of the bandgap in a photonic crystal ring is typically intuitively considered to monotonically grow as the modulation amplitude of the grating increases, causing increasingly large frequency splittings between the "dielectric" and "air" bands. In contrast, here we report that as the modulation amplitude in a photonic crystal ring increases, the bandgap does not simply increase monotonically. Instead, after the initial increase, the bandgap closes and then reopens again with the two bands flipped in energy. The air and dielectric band edges are degenerate at the bandgap closing point. We demonstrate this behavior experimentally in silicon nitride photonic crystal microrings, where we show that the bandgap is closed to within the linewidth of the optical cavity mode, whose intrinsic quality factor remains unperturbed with a value ≈ 1×106. Moreover, through finite-element simulations, we show that such bandgap closing and band flipping phenomena exist in a variety of photonic crystal rings with varying unit cell geometries and cladding layers. At the bandgap closing point, the two standing wave modes with a degenerate frequency are particularly promising for single-frequency lasing applications. Along this line, we propose a compact self-injection locking scheme that integrates many core functionalities in one photonic crystal ring. Additionally, the single-frequency lasing might be applicable to distributed-feedback (DFB) lasers to increase their manufacturing yield.

Double slot micro ring resonators with inner wall angular gratings as ultra-sensitive biochemical sensors

We simulate and demonstrate experimentally an inner-wall grating double slot micro ring resonator (IG-DSMRR) with a center slot ring radius of only 6.72 µm based on the silicon-on-insulator platform. This novel photonic-integrated sensor for optical label-free biochemical analysis boosts the measured refractive index (RI) sensitivity in glucose solutions to 563 nm/RIU with the limit of detection value being 3.7 × 10^-6 RIU (refractive index units). The concentration sensitivity for sodium chloride solutions can reach 981 pm/%, with a minimum concentration detection limit of 0.02%. Using the combination of DSMRR and IG, the detection range is enlarged significantly to 72.62 nm, three times the free spectral range of conventional slot micro ring resonators. The measured Q-factor is 1.6 × 10⁴, and the straight strip and double slot waveguide transmission losses are 0.9 dB/cm and 20.2 dB/cm, respectively. This IG-DSMRR combines the advantages of a micro ring resonator, slot waveguide, and angular grating and is highly desirable for biochemical sensing in liquids and gases offering an ultra-high sensitivity and ultra-large measurement range. This is the first report of a fabricated and measured double-slot micro ring resonator with an inner sidewall grating structure.

Highly-twisted states of light from a high quality factor photonic crystal ring

Twisted light with orbital angular momentum (OAM) has been extensively studied for applications in quantum and classical communications, microscopy, and optical micromanipulation. Ejecting high angular momentum states of a whispering gallery mode (WGM) microresonator through a grating-assisted mechanism provides a scalable, chip-integrated solution for OAM generation. However, demonstrated OAM microresonators have exhibited a much lower quality factor (Q) than conventional WGM resonators (by >100×), and an understanding of the limits on Q has been lacking. This is crucial given the importance of Q in enhancing light-matter interactions. Moreover, though high-OAM states are often desirable, the limits on what is achievable in a microresonator are not well understood. Here, we provide insight on these two questions, through understanding OAM from the perspective of mode coupling in a photonic crystal ring and linking it to coherent backscattering between counter-propagating WGMs. In addition to demonstrating high-Q (10⁵ to 10⁶), a high estimated upper bound on OAM ejection efficiency (up to 90%), and high-OAM number (up to l = 60), our empirical model is supported by experiments and provides a quantitative explanation for the behavior of Q and the upper bound of OAM ejection efficiency with l. The state-of-the-art performance and understanding of microresonator OAM generation opens opportunities for OAM applications using chip-integrated technologies.

Directly modulated parity-time symmetric single-mode Fabry-Perot laser

Effective manipulation of resonant mode, output power and modulation bandwidth of lasers are all of vital importance for practical application scenarios such as communication systems. We show that by breaking the parity-time (PT) symmetry, single mode operation lasing can be realized in an intrinsic multiple mode Fabry-Perot (FP) resonator. Two identical FP resonators are employed to establish a symmetric system and high output power can be achieved with lower fabrication difficulty and intracavity losses compared with ring resonators. The small-signal response and direct modulation of the PT-symmetric FP laser have also been demonstrated with electrical pumping. Our work opens new avenues for mode selection of high-performance FP lasers and provides a cost-effective candidate for practical applications such as communication systems.

Rod and slit photonic crystal microrings for on-chip cavity quantum electrodynamics

Micro-/nanocavities that combine high quality factor (Q) and small mode volume (V) have been used to enhance light-matter interactions for cavity quantum electrodynamics (cQED). Whispering gallery mode (WGM) geometries such as microdisks and microrings support high-Q and are design- and fabrication-friendly, but V is often limited to tens of cubic wavelengths to avoid WGM radiation. The stronger modal confinement provided by either one-dimensional or two-dimensional photonic crystal defect geometries can yield sub-cubic-wavelength V, yet the requirements on precise design and dimensional control are typically much more stringent to ensure high-Q. Given their complementary features, there has been sustained interest in geometries that combine the advantages of WGM and photonic crystal cavities. Recently, a "microgear" photonic crystal ring (MPhCR) has shown promise in enabling additional defect localization ( > 10× reduction of V) of a WGM, while maintaining high-Q ( ≈ 1 0 6 ) and other WGM characteristics in ease of coupling and design. However, the unit cell geometry used is unlike traditional PhC cavities, and etched surfaces may be too close to embedded quantum nodes (quantum dots, atomic defect spins, etc.) for cQED applications. Here, we report two novel PhCR designs with "rod" and "slit" unit cells, whose geometries are more traditional and suitable for solid-state cQED. Both rod and slit PhCRs have high-Q ( > 1 0 6 ) with WGM coupling properties preserved. A further ≈10× reduction of V by defect localization is observed in rod PhCRs. Moreover, both fundamental and 2nd-order PhC modes co-exist in slit PhCRs with high Qs and good coupling. Our work showcases that high-Q/V PhCRs are in general straightforward to design and fabricate and are a promising platform to explore for cQED.

Multi-mode microcavity frequency engineering through a shifted grating in a photonic crystal ring

Frequency engineering of whispering-gallery resonances is essential in microcavity nonlinear optics. The key is to control the frequencies of the cavity modes involved in the underlying nonlinear optical process to satisfy its energy conservation criterion. Compared to the conventional method that tailors dispersion by cross-sectional geometry, thereby impacting all cavity mode frequencies, grating-assisted microring cavities, often termed as photonic crystal microrings, provide more enabling capabilities through mode-selective frequency control. For example, a simple single period grating added to a microring has been used for single frequency engineering in Kerr optical parametric oscillation (OPO) and frequency combs. Recently, this approach has been extended to multi-frequency engineering by using multi-period grating functions, but at the cost of increasingly complex grating profiles that require challenging fabrication. Here, we demonstrate a simple approach, which we term as shifted grating multiple mode splitting (SGMMS), where spatial displacement of a single period grating imprinted on the inner boundary of the microring creates a rotational asymmetry that frequency splits multiple adjacent cavity modes. This approach is easy to implement and presents no additional fabrication challenges compared to an unshifted grating, and yet is very powerful in providing multi-frequency engineering functionality for nonlinear optics. We showcase an example where SGMMS enables OPO across a wide range of pump wavelengths in a normal-dispersion device that otherwise would not support OPO.

Dynamic manipulation of WGM lasing by tailoring the coupling strength

Miniaturized lasing with dynamic manipulation is critical to the performance of compact and versatile photonic devices. However, it is still a challenge to manipulate the whispering gallery mode lasing modes dynamically. Here, we design the quasi-three-dimensional coupled cavity by a micromanipulation technique. The coupled cavity consists of two intersection polymer microfibers. The mode selection mechanism is demonstrated experimentally and theoretically in the coupled microfiber cavity. Dynamic manipulation from multiple modes to single-mode lasing is achieved by controlling the coupling strength, which can be quantitatively controlled by changing the coupling angle or the coupling distance. Our work provides a flexible alternative for the lasing mode modulation in the on-chip photonic integration.

Single-mode lasing in an AlGaInAs/InP dual-port square microresonator

Mode selection is crucial to achieving stable single-mode lasing in microlasers. Here, we demonstrate experimentally a dual-port square microresonator for single-mode lasing with a side-mode-suppression ratio (SMSR) exceeding 40 dB. By connecting waveguides at two opposite vertices, the quality factor for the antisymmetric mode (ASM) is much higher than that of the symmetric mode (SM), enabling single-mode lasing. Furthermore, far-field interference patterns similar to Young's two-slit interference are observed. This microlaser is capable of providing two optical sources simultaneously for optical signal processing in high-density integrated photonic circuits.

Free-space-coupled wavelength-scale disk resonators

Optical microresonators with low quality factor ( Q ) can be efficiently excited by and scatter freely propagating optical waves, but those with high Q typically cannot. Here, we present a universal model for resonators interacting with freely propagating waves and show that the stored energy of a resonator excited by a plane wave is proportional to the product of its Q and directivity. Guided by this result, we devise a microdisk with periodic protrusions in its circumference that couples efficiently to normally incident plane waves. We experimentally demonstrate several microdisk designs, including one with a radius of 0.75λ 0 and Q of 15,000. Our observation of thermally-induced bistability in this resonator at input powers as low as 0.7 mW confirms strong excitation. Their small footprints and mode volumes and the simplicity of their excitation and fabrication make wavelength-scale, free-space-coupled microdisks attractive for sensing, enhancing emission and nonlinearity, and as micro-laser cavities.

Controlled single-mode emission in quantum dot micro-lasers

In this paper, we demonstrate an efficient and easy fabrication method for whispering-gallery-mode (WGM) manipulation and report the first electrically driven single-mode quantum dot micro-ring (QDMR) lasers. Using self-assembled InAs/InAlGaAs QD active layers with deeply etched azimuthal gratings, continuous-wave (CW) lasing with controllable single-mode emission wavelengths covering 1300 nm to 1370 nm has been achieved. A record-high side-mode-suppression-ratio (SMSR) value of 49 dB is obtained. These QDMR lasers exhibit excellent single-mode lasing stabilities over the current and temperature tuning range with a thermal tunability of 0.092 nm/°C. The concept is applicable to other wavelength bands depending on the gain spectrum, demonstrating a feasible solution in realizing energy-efficient and densely integrated photonic building blocks.

Active mode selection by defects in lithium niobate on insulator microdisks

Whispering gallery mode (WGM) optical microcavities are important building blocks in photonic integrated circuits. Operation of such cavities on specific lower- or higher- order transverse modes has much interest in application perspectives. Here, we demonstrate active mode selection by introducing defects in lithium niobate on insulator microdisks. A focused ion beam is applied to precisely inscribe nano slits into the perimeter of the microdisk. The transmission spectra can be significantly thinned out without severe quality factor degradation. Either fundamental or high-order transverse WGMs can be retained by properly designing the size and location of the defects. The approach may have promising applications in single-mode lasing and nonlinear optics.

Concentric microcavities for cylindrical vector beam lasers

Cylindrical vector (CV) beams with polarization singularities have attracted intense research interest because of their important applications in optical trapping and manipulation, imaging, and high-speed optical communication. In this Letter, we propose a high-speed integrated device designed to emit fundamental CV beams, including both radially and azimuthally polarized beams. The device is composed of two grating-assisted concentric microcavities based on an InP platform. The microcavity with only a second-order grating shallowly etched on the top is optimized and used for improved azimuthally polarized CV beam emission. Another microcavity with both a triangular-shaped side grating and a rectangular-shaped top grating is employed for radially polarized CV beam lasing. The proposed devices can be further developed to be compatible with wavelength-division multiplexing and mode-division multiplexing techniques. They hold great potential in CV beam-based classical and quantum communication systems.

Single-mode surface-emitting microcylinder/microring laser assisted by a shallowly-etched top grating

Semiconductor lasers based on microcylinder/microring cavities supporting high-quality factor modes are promising candidates of optical sources for optical interconnect. However, their multi-mode lasing performance and non-directional emission characteristic restrict their applications. In this paper, a single mode surface-emitting laser at O-band based on a second-order grating shallowly etched on the top of the microcylinder/microring cavity is proposed and demonstrated. The second-order top grating cannot only scatter the whispering gallery modes vertically to form surface emission but also select a single mode to lase. The laser is electrically pumped and continuous wave operated in a wide temperature range with side-mode suppression-ratio larger than 40 dB. The upward surface-emitting optical power exceeds 1 mW. Except for O-band, the laser can be easily realized at other long-wavelength communication bands, such as C-band and L-band.

Polymer-Based Microring Resonator with the Multimode Interference Coupler Operating at Very-Near-Infrared Wavelengths

A microring resonator with the multimode interference coupler is fabricated on the polymer platform by using UV-based soft nanoimprint technique. A unique class of fluorinated polymer, perfluoropolyether (PFPE), is employed for the fabrication of the flexible soft mold. By optimizing the proportion between Ormocore and the thinner maT, the microring resonator is fabricated almost without residual layer. The fabricated device with a Q-factor up to 2.3 × 104 is demonstrated for very-near-infrared wavelengths, which shows high potential for sensing applications.

Analytical model for active racetrack resonators with intracavity reflections and its application in Fano resonance tailoring

We present an analytical model for estimating the spectral properties of an active racetrack resonator/waveguide system. Under reasonable approximations, we show that the transfer function can be approximated by a rational function, the coefficients of which are determined by the parameters of the structure. The model takes into account intracavity reflections that can provide additional degrees of freedom in the design. We identify conditions under which asymmetric transitions around a spectral peak can occur, which are characteristic of Fano-type resonances. The accuracy of our model is verified by rigorous transfer matrix numerical simulations. We also discuss how the model can be applied for tailoring the transfer function in order to obtain sharper transitions from the spectral peaks to the minima in order for the structure to be used for sensing applications.

Ultra-compact and low-threshold thulium microcavity laser monolithically integrated on silicon

We demonstrate an ultra-compact and low-threshold thulium microcavity laser that is monolithically integrated on a silicon chip. The integrated microlaser consists of an active thulium-doped aluminum oxide microcavity beside a passive silicon nitride bus waveguide, which enables on-chip pump-input and laser-output coupling. We observe lasing in the wavelength range of 1.8-1.9 μm under 1.6 μm resonant pumping and at varying waveguide-microcavity gap sizes. The microlaser exhibits a threshold as low as 773 μW (226 μW) and a slope efficiency as high as 24% (48%) with respect to the pump power coupled into the silicon nitride bus waveguide (microcavity). Its small footprint, minimal energy consumption, high efficiency, and silicon compatibility demonstrate that on-chip thulium lasers are promising light sources for silicon microphotonic systems.

Fusion of Renewable Ring Resonator Lasers and Ultrafast Laser Inscribed Photonic Waveguides

We demonstrated the monolithic integration of reusable and wavelength reconfigurable ring resonator lasers and waveguides of arbitrary shapes to out-couple and guide laser emission on the same fused-silica chip. The ring resonator hosts were patterned by a single-mask standard lithography, whereas the waveguides were inscribed in the proximity of the ring resonator by using 3-dimensional femtosecond laser inscription technology. Reusability of the integrated ring resonator - waveguide system was examined by depositing, removing, and re-depositing dye-doped SU-8 solid polymer, SU-8 liquid polymer, and liquid solvent (toluene). The wavelength reconfigurability was validated by employing Rhodamine 6G (R6G) and 3,3'-Diethyloxacarbocyanine iodide (CY3) as exemplary gain media. In all above cases, the waveguide was able to couple out and guide the laser emission. This work opens a door to reconfigurable active and passive photonic devices for on-chip coherent light sources, optical signal processing, and the investigation of new optical phenomena.

Dark-state lasers: mode management using exceptional points

By exploiting the inherent characteristics of dark-state resonators, we experimentally realize a single-frequency integrated microring laser system. This semiconductor laser can remain single-mode, even at high pump power levels, while allowing tunability over a wide spectral range. Our results demonstrate the potential of exceptional points as a versatile tool for mode selection in micro-cavity laser configurations.

Integrated label-free optical biochemical sensor with a large measurement range based on an angular grating-microring resonator

We propose and design a photonic-integrated optical biochemical sensor, which comprises a microring resonator and angular gratings in a silicon-on-insulator waveguide. With the combination of the angular gratings, the measurement range of the angular grating-microring resonator-based sensor significantly increases without the restriction of a free spectral range. Optimization of the several key structural parameters is investigated to achieve favorable transmission properties. A high-quality factor of more than 1.03×10⁵ can meet the requirements of high sensitivity and low detection limit. The simulation results on the biochemical bulk sensing show that a concentration sensitivity of more than 95.27 pm/% and detection limit of less than 0.329% can be obtained. A large measurement range of 50.2 nm is achieved by the combination of the angular gratings. The investigation on the combination of microring resonator and angular grating is a valuable exploration of the liquid and gas biomedical sensing for the ultra-large measurement range.

Tailoring the lasing modes in CH3NH3PbBr3 perovskite microplates via micro-manipulation

Laser emissions from perovskite microplates have been intensively studied recently.