This PDF file contains the front matter associated with SPIE Proceedings Volume 10904, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

Silicon is a widely used material in modern microelectronics and photonics. Extremely low optical losses in near and mid-IR wavelengths made it a promising material for whispering gallery mode (WGM) optical micro-resonators. But, till now its potential was not fully utilized because of the best-obtained quality factor - about 2 x 10⁷ - remained orders of magnitude below the material absorption limit. In this work we experimentally demonstrated a quality factor above 10⁹ for the WGM in millimetres size crystalline silicon resonators. Materials with different residual conductivity were compared. Application of original semi-spherical silicon coupler allowed to obtain up to 35% resonance peaks contrast.

Whispering-gallery resonators (WGRs) made of lithium niobate are very attractive for nonlinear-optical frequency conversion due to their small mode volumes and high Q-factors. To achieve phase matching, methods like birefringent phase matching and quasi phase matching (QPM) have been employed in millimeter-sized bulk WGRs. Among these, the latter provides ultimate flexibility in terms of wavelengths and polarization of the interacting waves. Integrated on-chip WGRs are in particular very appealing due to the possibility of building photonic circuits and the usage of highly-parallel and thus scalable semiconductor manufacturing techniques. Integrated WGRs are, however, fabricated on thin-film substrates. This leads to one major drawback: QPM is hard to achieve, since it is difficult to realize periodically-poled thin films by field-assisted domain inversion. We report on a method to resolve this issue. First, we do domain engineering in bulk material. Next, we bond this sample on a quartz substrate by direct wafer bonding and finally we polish the lithium niobate to a 2-μm-thick film. By lithography and reactive-ion etching we structure waveguide rings with 200 μm diameter into the thin film. Subsequent polishing of the waveguide sidewalls decreases surface-scattering losses and enables on-chip WGRs with quality factors exceeding one million. This allowed us to demonstrate for the first time quasi-phase matched second-harmonic generation in integrated WGRs, pumped by light with 1550 nm wavelength, obtaining a normalized conversion efficiency of 0.9 ‰/mW. Being now able to deploy type-0 and type-ii phase matching opens entirely new possibilities for frequency conversion with on-chip WGRs.

Dynamically reconfigurable photonic circuits are expected to have a rich variety of applications, enabling high-bandwidth optical interconnects and memories in next generation computer architectures, chip-based quantum networks, and on-chip coherent radar and microwave communication systems. Widely tuneable high quality microcavities are a key component for such circuits. Their passive response allows controllable optical phase shifts, memories and add-drop filters which together provide the reconfigurability of the circuit; their strong optical confinement enhances light-matter interactions and thereby enables components such as lasers, sensors, optical frequency combs, and quantum processors. However, wide tuneability is only currently practical on millimetre-scale device footprints. Here we overcome this barrier by developing an on-chip high quality microcavity with resonances that can be electrically tuned across a full free spectral range (FSR). FSR tuning allows resonance with any source or emitter, or between any number of networked microcavities. We achieve it by integrating nanoelectronic actuation with strong optomechanical interactions provided by a double-disk microcavity that create a highly strain-dependent effective refractive index. This allows low voltages and sub-nanowatt power consumption. We demonstrate a basic reconfigurable photonic network, bringing the microcavity into resonance with an arbitrary mode of a microtoroidal optical cavity across a telecommunications fiber link. Our results have applications beyond photonic circuits, including widely tuneable integrated lasers, cavity quantum electrodynamics, reconfigurable optical filters for telecommunications and astronomy, and on-chip sensor networks.

As a result of their ability to amplify input light, ultra-high quality factor (Q) whispering gallery mode optical resonators fabricated from silica have demonstrated extremely low threshold Raman lasing behavior. However, the efficiency of the lasing has been poor due to the intrinsic low Raman gain of silica (~5%). By grafting oriented monolayers of highly nonlinear organic small molecules to the surface of conventional silica whispering gallery mode optical resonators, we demonstrate a new strategy for fabricating Raman lasers. The laser efficiency is improved from 4% to over 40%. Density functional theory is performed to understand the mechanism giving rise to the improvement. This chemistry-based approach could be applied to nearly any whispering gallery mode cavity geometry to improve performance, providing a universal strategy for device performance improvement.

Optical trapping and propulsion has been demonstrated previously with microsphere and microring whispering gallery (WG) resonators. The WG mode forms an optical carousel for the trapped particles where the particles propagate in the evanescent field along the circumference of the resonator. Here we extend this study with the use of quasi-droplet WG resonators, and demonstrate the creation of an optical sling shot effect due to the non-uniform scattering of the light field and the unique mode profile of the quasi-droplet WG mode. The sling shot effect is the result of an orbital position dependent scattering force and results in the particle having higher acceleration at certain points in its orbit. This opto-fluidic opto-mechanics system forms a feedback loop that leads to a self-sustained oscillation of the cavity mode frequency and modulation of the transmitted optical power. At the same time, the particle speed can reach very high velocities due to the large overlap of the quasi-droplet WG mode with the particle leading to very efficient propulsion. We also demonstrate for the first time optical trapping using counter propagating WG modes.

My lab is exploring a new class of optical sensors that combine dielectric high Q micro resonators with plasmon resonances in metallic nano structures to achieve extraordinary detection sensitivity at the nanoscale.

The challenge of guiding or deflecting a laser beam in a disturbed medium involves careful consideration of various nonlinear interactions of light with air and their transient response. A light filament provides a high intensity region in space that maintain its shape for propagation lengths longer than the Rayleigh range. The possibility to use this light channel for coupling optical, teraherz and radio frequency had been spurring researches in this field. We present the contribution of transient nonlinear response of the medium on light propagation. The nonlinear interaction of ultrashort pulses with air leaves a spatial distribution of ionized medium that evolves into an equilibrium plasma. The light pulse has left the medium it ionized before the plasma has reached equilibrium. Therefore, the reaction of the medium onto the filament involves a transient evolution of the ensemble of ions and electrons that follow deterministic trajectories under the influence of the light field. Other nonlinear interactions such as molecular alignment also have a time dependent response. The relative influence of the plasma and molecular motion is modified by focusing conditions. The standard focusing of light into air combines the nonlinear Kerr focusing with the desired linear focusing of the optics. A focusing through a sharp pressure gradient made by an aerodynamic window enhances the pointing stability of the laser beam. The effect of transient response of the medium driven by nonlinear interaction of ultrashort pulse on the guided beam will be discussed.

The quantum properties of optical frequency combs have been the focus of several research works in recent years. Investigating the quantum correlations between the spectral components of the combs is of fundamental interest because it allows for a better understanding of light-matter interactions, but also of technological interest as it wold permits the implementation of quantum communication networks. In this communication, we present some of our latest advances in this field.

Optical frequency combs are a key technology for optical precision measurements. So far, most frequency combs operate in the near-infrared regime (NIR). Many applications, however, require combs in the ultraviolet (UV), visible (VIS) or mid-infrared (MIR) spectral ranges. This can be achieved by making use of nonlinear-optical processes. In this contribution, we demonstrate the efficient conversion of frequency combs with a repetition rate of 21 GHz to UV, VIS and MIR wavelengths in a synchronously driven high-Q microresonator with second-order optical nonlinearity. This opens up a new path for applications including, but not limited to, molecular sensing and quantum optics.

Chip-scale mode-locked dissipative Kerr solitons have been realized on various materials platforms, making it possible to achieve a miniature, highly coherent frequency comb source with high repetition rates. Aluminum nitride (AlN), an appealing nonlinear optical material having both Kerr (𝜒3) and Pockels (𝜒2) effects, has immerse potential for comb self-referencing without the need for external harmonic generators. Here we demonstrate deterministic Kerr cavity soliton generation in crystalline AlN microring resonators. By utilizing phase matched waveguides, we further show the extension of a Kerr comb to the UV band.

We demonstrate a frequency comb generation via a quadratic nonlinear optical waveguide in a resonator as a monolithic device. In the experiment, we constructed the device by a periodically-poled lithium niobate waveguide with high reflective coatings for 1.5-μm light at both ends of the waveguide. When we set a pump frequency at 1540 nm to on-resonance, we observed a wide spectral broadening up to 60 nm around the pump frequency due to cascaded processes of second harmonic generation and optical parametric oscillation. A narrow radio frequency beat spectrum of the frequency comb appeared at 3.5 GHz which corresponds to the free spectral range of the resonator. Furthermore, when we use two pump lights, we succeeded in a control of the mode spacing of the frequency comb by adjusting the frequency difference of the two pump light.

Microresonator solitons orbit around a closed waveguide path and produce a repetitive output pulse stream at a rate set by the roundtrip time. Here, counter-propagating solitons that simultaneously orbit in an opposing sense (clockwise/counter-clockwise) are studied. Despite sharing the same spatial mode family, their roundtrip times can be precisely and independently controlled. Furthermore, a state is possible in which their relative optical phase and repetition rates are locked. This state allows a single resonator to produce dual-soliton frequency-comb streams with different repetition rates, but with a high relative coherence that is useful in spectroscopy and ranging.

Optical-frequency combs are versatile tools for measuring time, identifying chemicals, and generating quantum states. A new direction is to produce frequency combs through intriguing nonlinear behaviors of light in Kerr microresonators. Experiments with whispering-gallery-mode and waveguide ring configurations have been highly productive, exploring the formation, properties, and uses of soliton pulses that are the nonlinear eigenstate of the resonator. The soliton’s spectrum is a comb with a repetition frequency given by the free-spectral range of the resonator. Fabry-Perot (FP) cavities with sufficient Kerr nonlinearity also support soliton pulses. I will discuss experiments that probe soliton frequency combs and ultraprecise measurements with FP cavities based on bulk fused silica, optical fiber, and nanofabricated photonic crystal reflectors.

Recent works on Kerr frequency combs demonstrated the possibility to simultaneously generate multiple soliton states with different group velocities in a single microresonator, which could be beneficial for many applications utilizing dual-comb systems. In this work we demonstrate a dual-comb configuration in a single crystalline microresonator by monochromatically pumping counter-propagating solitons in the same spatial mode with equal powers. Moreover, we demonstrate experimentally and through simulations the key role of Cherenkov radiation interferences on the repetition rate of a multi-soliton state. This result not only shines new light on the impact of dispersive waves on dissipative Kerr solitons but also introduces a novel approach to develop coherent dual-comb spectrometer based on microcombs.

We introduce a technique capable to produce and control stabilized single-frequency emission with a sub-kHz linewidth and independently soliton comb generation from a multi–frequency regular Fabry-Perot laser diode selfinjection locked to a high-Q optical microresonator. We also observed novel regimes of controllable single, dual, and multiple-frequency generation that may be useful for the creation of narrow-linewidth lasers required for the spectroscopy, LIDARs, and telecommunications. For analysis of the considered effects original theoretical models taking into account self-injection locking effect, mode competition and Bogatov asymmetric mode interaction were developed and numerical modeling was performed.

Kerr comb generation in a coupled mode system is of interest, because it supports dual-comb generation and can initialize modulation instability gain even in a normal dispersion cavity. This talk will describe how linear and nonlinear mode coupling affect Kerr comb generation. In particular, we model and study soliton trapping in a coupled mode system, and also discuss in detail how the coupling strength affects the generation of a Kerr comb in a normal dispersion system.

Nonlinear wave mixing in optical microresonators new perospects for compact optical frequency combs with many promising applications. We demonstrate simultaneous generation of multiple frequency combs from a single optical microresonator and a single continuous-wave laser. Similar to space-division multiplexing, we generate several dissipative Kerr soliton states – circulating solitonic pulses driven by a continuous-wave laser – in different spatial (or polarization) modes of a MgF₂ microresonator. Up to three distinct combs are produced simultaneously, featuring excellent mutual coherence and substantial repetition rate differences, useful for fast acquisition and efficient rejection of soliton intermodulation products. This method could enable the deployment of dual- and triple-comb-based methods to applications where they remained impractical with current technology.

A Kerr cavity soliton stabilization technique based on excitation by two continuous-wave pumps is proposed and theoretically and numerically investigated. Slow scanning of the second pump frequency near a comb harmonic of the soliton excited by the main pump locks the auxiliary pump to the soliton within a locking range. With the pumps locked to two frequency references, comb degrees of freedom will be stabilized. Evidence of stabilization and control of comb repetition rate using this technique is presented and the connection of duallypumped microcombs to "tiime crystals" is highlighted. The proposed approach extends to form a universal framework expounding soliton crystallization in microresonators supporting dispersive waves emitted by higherorder dispersion or mode anti-crossing. It can also obviate the exacting demand of octave-spanning combs for self-referencing.

Whispering gallery mode resonators (WGMR) have attracted a great interest in the last decade. WGMR have been fabricated in different geometries, solid and hollow, spherical, toroidal, and bottled shaped. Hollow spherical WGMR or microbubble resonators (MBR) are the last arrived in the family of resonators. The approach used for their fabrication is based on surface tension driven plastic deformation on a pressurized capillary, similar to glassblowing. Using such technique we are able to fabricate large surface area and thin spherical shells with high quality factor (Q). MBR are efficient phoxonic cavities that can sustain both optical photons and acoustic phonons. It has been demonstrated that MBR can be used to study Turing comb patterns (Kerr modulation) and Stimulated Brillouin Scattering (SBS). Radiation pressure is another mechanism that also leads to excitation of acoustic phonons with lower frequencies, in the range of hundreds of kHz to tens of MHz in the case of silica MBR. The frequency of such oscillations occurs very close to the mechanical eigenfrequencies of the cavity. We have studied the temporal behavior of the cavity, the coexistence and the suppression of the oscillation while generating Turing comb patterns. The observed phenomenology can be explained by the geometrical characteristics of a MBR. MBRs are spheroidal WGM resonators with quite dense spectral characteristics. The total dispersion of MBR is anomalous and large, as expected for very large MBR. Thus, Kerr comb formation is allowed for all MBR used in this work.

The generation of on-chip optical frequency combs in silicon-based charge injection modulators through strong phase modulation has been investigated as a prospective source for on-chip wavelength division multiplexing (WDM). Being CMOS-fabrication compatible, these modulators are easily integrated with other silicon photonic components. Resonant structure based implementations using PN-doped silicon micro-ring modulators have been used to generate frequency combs. However, in contrast to linear modulators, the use of such resonant structures leads to resonance shifts resulting from microwave induced thermal effects as well as causing a degradation in the quality of the modulator resonance. In this work, we report and investigate interesting effects observed as the optical and microwave power driving the comb generator is changed. We utilize a comb generator operating at a repetition rate of 10GHz and driven by optical powers in the range of 3mW to 30mW and microwave power in the range of 6dBm to 25dBm. We observe novel effects wherein the skewness of the comb envelope, the number of comb lines generated varies in a non-monotonic function with changes in optical power and DC bias. Abrupt transitions are reported from a broadband comb to no comb generation conditions with slight changes to drive RF powers. We attribute the variations to an interplay between the thermo-optic effects in the ring and its impact in moving the resonance of the ring with changing power.

Ultrahigh quality factor microresonators have extremely long photon lifetimes, enabling high circulating power. As a result of the amplification of the input optical power, these devices are able to excite various nonlinear optical phenomena, such as four-wave mixing (FWM) and stimulated Raman scattering (SRS). Previously, FWM and their cascaded peaks enabled frequency comb generation in silica toroidal resonators. However, high input power (> 60 mW) is required to generate broad frequency combs (>500 nm span) due to the intrinsic material properties of silica. In this present work, we modify the material properties of silica by coating a silica toroidal cavity with a thin film of Zirconium (Zr) doped solgel. This thin layer substantially improves the performance of Raman-Kerr frequency comb generation in hybrid microcavities. A series of concentrations of Zr-doped solgel are synthesized, and the effects of the Zr dopants are characterized with both theoretical calculations and experimental measurements. Doping Zr into the silica matrix enables the Zr-doped devices to have a lower dispersion than a bare silica device, enabling the frequency comb span to increase. Additionally, Zr dopants increase the efficiency of the SRS process. As Zr concentrations increase, Stokes as well as anti-Stokes Raman scattering and their cascaded FWM peaks start contributing to the formation of the frequency comb, generating Raman-Kerr frequency combs. Consequently, Zr doping enables large frequency comb spans with significantly reduced input power.

Numerical studies on Kerr frequency comb generation with vertically-coupled whispering-gallery-mode (WGM) Si₃N₄ resonators are presented. These resonators include a frequency-dependent access coupler and are characterized by a free spectral range (FSR) of 220 GHz. We present numerical simulations based on the Ikeda map that allows implementation of complex-valued frequency-dependent and non-reciprocal access coupler transfer matrix in the simulation of Kerr comb in the cavities modelled by Arlotti et al. We use a Runge-Kutta 4 Interaction picture (RK4IP) method with adaptive step-size control as developed by Balac et al. to circumvent the numerical burden added by this modelling approach and successfully simulate Kerr comb generation using an approach that accurately models any optical cavity that can be considered as spatially one-dimensional regardless of its quality factor, finesse or dispersive properties which comes in useful in this study when access coupling properties degrade the resonator quality factor.

Self-rolled-up microtubes have been developed over the past decade as optical ring resonators, which exhibit unique merits such as directional emission, perfect emitter-field overlap, controlled transferability and flexible structural design, and find various applications in coherent light sources, micro-sensing, nanomechanics and topological devices. Nevertheless, the reported rolled-up microtube resonators so far are based on either passive material or active media with relatively small exciton binding energy and oscillator strength, making it less favorable for strong light-matter interaction at room temperature. Although wide bandgap materials, such as GaN, ZnO and organics, exhibit large exciton binding energy and oscillator strength, it is very challenging to make such materials into 3D rolled-up structures. In this work, we overcome the technical difficulties and fabricat GaN-based self-rolled-up microtubes from coherently strained AlGaN/GaN bilayer around 50nm thick with an embedded InGaN quantum well grown by MOCVD. Such ultrathin-walled microtubes can then be easily shifted onto an etched trench to become freestanding, with a typical Q-factor measured to be over 800. Both the on-substrate and freestanding microtubes exhibit lasing at room temperature, which have not been observed on ultrathin-walled microtubes based on other materials, highlighting the outstanding optical properties of the active media. Based on the same method, we also fabricated self-bent-up microdisks which support 3-dimensional WGM with a Q-factor of ~1300 and single mode lasing at room temperature, which is not achieved by conventional microdisks with the same size and material. Such rolled-up devices provide the degree of freedom of the WGM photons in the vertical dimension and is promising for applications in multifunction on-chip devices.

Optofluidic laser, which incorporates optical cavities and luminescent probes in fluidic environments, has become a powerful platform for biosensing and medical diagnosis. To date, fluorescent dyes and proteins have been widely utilized as gain materials for biological analysis due to their good biocompatibility, but the limited photostability restricts their reliability and sensitivity. Here, we bridge this gap by demonstrating an optofluidic microlaser using the biocompatible conjugated polymer. Assisted by the ultrahigh-Q whispering gallery microcavity, the optofluidic laser is achieved with an ultra-low threshold down to 7.8 μJ/cm^2. More importantly, this conjugated polymer exhibits a significant enhancement in the lasing stability compared with a typical laser dye (Nile red). In the experiment, after 20 minutes of illumination with the excitation intensity of 23.2 MW/cm^2, the lasing intensity of the conjugated polymer experiences a decrease of less than 10%, while the lasing feature of Nile red completely disappears. Additionally, by mechanically stretching the resonator, the lasing frequency can be fine-tuned with the range of about 2 nm, which exceeds a free spectral range of the resonator.

Three-dimensional (3D) fabrication by direct laser writing opens new perspectives in resonator design and laser applications. We fabricated various shapes and sizes of microlasers of nanoscale quality: pyramids, tetrahedra, and 3D plano-concave cavities. The lasing thresholds depend on the resonator shapes and are lower than those of equivalent two-dimensional microlasers. The experimental spectra and the directions of emission may be predicted via a semiclassical analysis using periodic orbits, in good agreement with experiments. The screw angle is a key quantity to characterize a periodic orbit in 3D resonators. In this proceeding, we present different methods to calculate the screw angle in polyhedra, based on the example of the folded diamond periodic orbit in the square pyramid. Preliminary experimental results on the regular tetrahedron are also reported.

A crystalline microresonator with overall thermal sensitivity of the optical spectrum approaching zero is designed and demonstrated experimentally. The resonator is made by integrating a calcium fluoride layer forming an optical whispering gallery mode resonator with ceramic compensation layers. The ceramics is characterized with negative thermal expansion coefficient in a limited temperature range. The thermally compensated resonator has a potential application for laser frequency stabilization. We demonstrate a self-injection locked laser characterized with Allan Deviation on the order of 10^-12 at 1s integration time and study the factors limiting its stability.

Development of quantum information processing requires realization of solid state structures able to manipulate light or matter quantum bits. One of the promising candidates for been active elements of such solid-state platform are color centers in diamond. The most famous nitrogen-vacancy color center has number of attractive features and found a lot of applications in sensing and imaging. Still, it has number of considerable disadvantages, among which it sensitivity to the surface damages and thus its incompatibility with nanostructures. On another side implementation of nano- and micro- structures enabled considerable progress in manipulation of light quanta. In particular photonic crystal cavities allowed to realize strong coupling of cavity and spin system. This led to demonstration of efficient light collection and realization of simple quantum gates with artificial or real atoms. Novel color centers such as silicon-vacancy or germanium-vacancy color center due to inversion symmetry of the electron structure are not sensitive to the surface damages and presence of surface nearby. Thus, those are perfect candidates for been combined with photonic crystal structures. Novel technologies enabled growing of the nanodiamonds of ultra-small size having well-defined color center inside. Along with techniques to position those precisely on the nano- and micro structures these achievements opened opportunity to integrate high-fines photonic-crystal cavities with the germanium-vacancy containing nanocrystals thus forming fully solid-state platform for quantum manipulation of light. In my talk I will describe our progress towards realization of this ambitious goal.

Photon-mediated interactions between quantum systems are essential for realizing quantum networks and scalable quantum information processing. We demonstrate such interactions between pairs of silicon-vacancy (SiV) color centers strongly coupled to a diamond nanophotonic cavity. When the optical transitions of the two color centers are tuned into resonance, the coupling to the common cavity mode results in a coherent interaction between them, leading to spectrally-resolved superradiant and subradiant states. We use the electronic spin degrees of freedom of the SiV centers to control these optically-mediated interactions. Our experiments pave the way for implementation of cavity-mediated quantum gates between spin qubits and for realization of scalable quantum network nodes.

An optical resonator like a fiber ring (FR) or a whispering gallery mode (WGM) resonator with two couplers along its loop is referred to be in the add-drop configuration, in analogy with the add-drop multiplexer in telecom networks. Both for practical applications as well as in several fundamental studies involving high-Q resonators, this configuration is of great interest and the assessment of the intrinsic properties of the resonator and of its interaction with the coupling systems is extremely important. We developed an original method able to fully characterize high-Q resonators in an add-drop configuration. The method is based on the study of the two cavity ringdown (CRD) signals, which are produced at the transmission and drop ports by wavelength sweeping a resonance in a time interval comparable with the photon cavity lifetime. All the resonator parameters can be assessed with a single set of simultaneous measurements. We implemented the model describing the two CRD profiles from which a best fit process of the measured profiles allows deducing the key parameters. We successfully validated the model with an experiment based on a FR resonator of known characteristics. Finally, we fully characterized a high-Q, home-made, MgF₂ WGM disk resonator in the add-drop configuration, assessing its intrinsic and coupling parameters.

The paper illustrates both review and original simulation results obtained via the modelling of different set-ups based on optical microresonators for applications in optical sensing, lasing and spectroscopy. Passive microbubbles and microspheres coupled via long period fiber gratings (LPGs) and tapered fibers are designed and/or constructed for sensing of biological fluids in the near infrared (NIR) wavelength range. Rare earth doped chalcogenide glass integrated microdisks are designed for active sensing in the medium infrared (MIR) wavelength range. A home-made numerical code modelling the optical coupling and the active behavior via rate equations of ion population is employed for a realistic design, by taking into account the most important active phenomena in rare earths, such as the absorption rates, the stimulated emission rates, the amplified spontaneous emission, the lifetime and branching ratios, the ion-ion energy transfers and the excited state absorption. Optical coupling is obtained by employing ridge waveguides, for micro-disks, and tapered fibers, for microspheres and microbubbles. Different dopant rare earths as Erbium (Er³⁺) and Praseodymium (Pr³⁺) are considered.

Non-reciprocal devices, such as circulators and isolators, are indispensable components in classical and quantum information processing in an integrated photonic circuit. Aside from those applications, the non-reciprocal phase shift is of fundamental interest for exploring exotic topological photonics, such as the realization of chiral edge states and topological protection. However, incorporating low optical-loss magnetic materials into a photonic chip is technically challenging. In this study, the non-reciprocal transmission in the high-Q whispering gallery modes microresonator is experimentally demonstrated. The underlying mechanism of the non-reciprocity demonstrated in this study is actually universal and can be generalized to any traveling wave resonators with a mechanical oscillator, such as the integrated disk-type microresonator coupled with a nanobeam. Considering that higher cooperativity and cascading of the optical devices have been reported in a photonic integrated chip, non-reciprocity in such an microresonator has applications for reconfigurable isolator, circulator and directional amplifier, which will play important roles in a hybrid quantum Internet.

Microcavities are fundamental building blocks for on-chip integrated photonic circuits. Till now, most of the microdisks (microrings) are coupled to the waveguides via the evanescent coupling, which is strongly restricted by the nanosized gap distance (between waveguide and cavity) and narrow bandwidth to realize the phase matching condition. In this talk, we will present our recent results on a brand new coupling mechanisms. Based on the time reversal process of laser, we show that the light can be efficientyl coupled into a silicon microdisk by simply connecting a waveguide onto it. The coupling efficiency and quality (Q) factors are ~ 60% and 7*10^5, respectively. As the new mechanism doesn't rely on the phase matching considtion, the high efficiency end-fire injection can be realized within a large range of waveguide width (> 500 nm), tilt angle, and spectral range, sigificnatly reducing the costs and expanding the potential applications of potential photonic devices.

Recent advances in optical materials have enabled the development of a wide range of integrated photonic devices from high speed modulators to frequency combs. With low optical loss over a wide wavelength range and environmental stability in ambient environments for several weeks, silicon oxynitride (SiO_xN_y) shows potential in many of these applications. However, unlike many classic optical materials, the thermo-optic response (dn/dT) in both the visible and near-IR is poorly characterized, limiting researcher’s ability to accurately model device performance. Here, we leverage the intrinsic thermal response of resonant cavities to measure the dn/dT of SiO_xN_y with a 12.7:1 and 4:1 oxygen to nitrogen ratio based on EDX measurements. The thermo-optic coefficient is measured in the visible and near-IR and compared with SiO₂. The refractive indices of the silicon oxynitride films were also measured using spectroscopic ellipsometry. Based on an analysis of the O:N ratio and a comparison with both SiO₂ and Si₃N₄, an expression for the dependence of the dn/dT on the stoichiometric ratio is developed.

Light can couple between TE and TM whispering-gallery modes (WGMs) of a microresonator; the effect is easily observable when those modes are frequency-degenerate, and can result in coupled-mode induced transparency (CMIT). Fitting experimental observations of CMIT with a numerical model in which the cross-polarization coupling strength is a free parameter shows that the coupling strength is typically 10^-8 – 10^-7 per round trip. It is shown here that polarization rotation of this magnitude can result from optical spin-orbit interaction through the asymmetry of a WGM. Using the eikonal approximation to describe a WGM, along with asymmetry of the microresonator about its equator, the maximum possible polarization rotation per round trip can be calculated. Then accounting for spatial overlap and phase mismatch of the coresonant WGMs, using coupled-mode theory, gives coupling strengths in agreement with experiment.

Adiabatically tapered fibers are often used to excite whispering gallery modes (WGMs) of microresonators used as chemical sensors. Recently it was demonstrated that using a non-adiabatic tapered fiber can enhance refractive index sensing. The incoming light is distributed between fundamental and higher-order fiber modes, whereas only the fundamental mode is detected because the uptaper is adiabatic. The interference effect between these fiber modes when exciting a WGM leads to the sensitivity enhancement. We have shown theoretically that even greater enhancement is possible for absorption sensing. For a given WGM, the predicted enhancement can be calculated by measuring the throughput power when the two fiber modes are in and out of phase at the input. Enhancement can be confirmed by sending the light in the reverse direction through the asymmetrically tapered fiber so that only one fiber mode is incident on the microresonator. Using a carefully designed asymmetrically tapered fiber, we have demonstrated this enhancement in experiments using a hollow bottle resonator (HBR) with an internal analyte. Absorption in the analyte causes a change in the WGM throughput fractional dip depth; these changes were studied with varying analyte concentration for forward and reverse propagation to evaluate the absorption sensitivity. For both liquid and gaseous analytes, our measured sensitivity enhancements are not inconsistent with the predicted enhancements of at least a factor of 100.

The remarkable temporal properties of ultra-short pulsed lasers in combination with novel beam shaping concepts enable the development of completely new material processing strategies. We demonstrate the benefit of employing focus distributions being tailored in all three spatial dimensions. As example advanced Bessel-like beam profiles, 3D-beam splitting concepts and flat-top focus distributions are used to achieve high-quality and efficient results for cutting, welding and drilling applications. Spatial and temporal in situ diagnostics is employed to analyze light-matter interaction and, in combination with flexible digital-holographic beam shaping techniques, to find the optimal beam shape for the respective laser application.

The manipulation of a laser’s transverse profile is of great interest for many applications. The most common and simple approach to beam shaping is by the use of optical phase masks. Conventional phase masks fabricated by surface profiling using spatially selective etching or deposition are easily damaged and limit high energy applications. We have shown in past work, that it is possible to create different phase profiles in the volume of photo-thermorefractive (PTR) glass, which unlike conventional phase mask materials has a high damage threshold that can withstand high peak and average powers. Here we present an approach for mode conversion of a high power fiber laser system using two different types of phase masks fabricated by encoding phase profiles into volume Bragg gratings. These holographic phase masks (HPMs) can successfully introduce wavefront change and achieve high diffraction efficiency (based on wavelength and the grating strength) for a broad range of wavelengths by tuning the element to the gratings Bragg condition. The first element had a grayscale vortex phase profile for HG₁ conversion. The second had a binary four-sector phase profile, and was capable to perform fundamental mode conversion to a TEM₀₁, TEM₁₀, or TEM₁₁. Mode conversion efficiency and thermal stress on each type of phase element were investigated using a 150 W of continuous wave power with a TEM₀₀ profile Yb:fiber laser.

This article discusses the use of bimorph adaptive mirrors to improve the focusing of laser radiation. The criterion of focusing efficiency is the fraction of the energy of the laser radiation passing through the pinhole located in the focal plane of the focusing lens.

In simultaneous spatial and temporal focusing (SSTF) a wide bandwidth pulse with transverse spatial chirp is focused, resulting in a pulse that is temporally compressed only near the focal plane. The pulse also has a pulse front tilt angle that depends on the amount of initial transverse chirp. In this work, we explore computationally and experimentally the properties of SSTF vortex and vector beams. To analyze the beam propagation, we build on the concept that a spatially chirped beam is a superposition of Gaussian beams with a position or angle that depends on frequency. We extend this to superpositions of Hermite-Gauss high-order modes to describe the singular beams. At focus, the beams of the ultrashort pulses are tilted versions of the familiar doughnut beams. Away from focus, however, where the spectral components do not fully overlap, we find that vortex and vector beams result in strikingly different mapping of the singularity mapping in the spatio-temporal domain. The use of higher-order modes increases the focal spot size without reducing the already short SSTF depth of focus. Experimentally, we use spiral phase plates to produce vortex beams and a linear to radial polarization converter for the vector beams. The vector beam is not distorted by the polarization-insensitive transmission gratings. The spatial chirp compressor is improved over our previous work to vary the chirp positive and negative. The sensitivity of the singular beam focus to grating misalignment can actually be used to optimize the compressor alignment.

We present our latest research results on intensity distribution transformation from Gaussian to a flattop and doughnut. The theoretical calculations and experimental results of the efficiency of different types of deformable mirrors are given. During the experiments the wavefront was measured with Shack-Hartmann sensor and then modified with bimorph deformable mirror to reach the desired intensity distribution in the far-field. Then the bimorph mirror was substituted with the stacked-actuators deformable mirror to confirm the simulations.

Nowadays, diffractive optical elements are used for a variety of applications because of their high design flexibility, compact size, and mass productivity. At the same time, they require having high and complex optical functionalities such as a large number of diffraction orders and a wide diffraction angle, which is beyond the limits of scalar paraxial diffraction domain. We propose a stable and fast gradient-based optimization algorithm based on step-transition perturbation approach applied to design binary diffractive elements with small and many features for being performed in a large number of diffraction orders and wide diffraction angles. Using our optimization, we obtained high-performance elements than using optimization based on purely scalar theory. In addition, it needs much less calculation time than parametric optimization based on rigorous diffraction theory. Upon verification with the experimental results, we observed that our gradient-based optimization method is valid for 1-by-117 fan-out grating with some small features (on the order of the illumination wavelength) and about 22° full pattern diffraction angle.

Liquid crystal on silicon phase-only spatial light modulators are widely used for the generation of multi-spot patterns. The phase distribution in the modulator plane, corresponding to the target multi-spot intensity distribution in the focal plane, is calculated by means of the so-called phase retrieval algorithms. Due to deviations of the real optical setup from the ideal model, these algorithms often do not achieve the desired power distribution accuracy within the multi-spot patterns. In this study, we present a novel method for generating high quality multi-spot patterns even in the presence of optical system disturbances. The standard Iterative Fourier Transform Algorithm is extended by means of machine learning methods combined with an open camera feedback loop. The machine learning algorithm is used to predict the mapping function between the desired and the measured multi-spot beam profiles. The problem of generation of multispot patterns is divided into three complexity levels. Due to distinct parameter structures, each of the complexity levels requires differing solution approaches, particularly differing machine learning algorithms. This relation is discussed in detail eventually providing a solution for the simplest case of beam splitter pattern generation. Solutions for more complex problems are also suggested. The approach is validated, whereby one machine learning method is successfully implemented and tested experimentally.

Passive phase adjustment via Kramers-Kronig (K-K) phase in coherent beam combining is reviewed and assessed. While recent experiments and modeling show that K-K does not improve the average output power when it is the only passive phasing mechanism in a resonator, K-K does improve the combining efficiency in the presence of other effects such as wavelength tuning, Kerr nonlinearity, and regenerative phase. For a two-laser array, an improvement in output power can be achieved by K-K alone if the constant of proportionality α between the gain and the K-K phase is very large (e.g., α = 100 rad/Neper), but this is unlikely to be practical in available materials, and does not appear to scale with array size.

In this paper, higher-order modes were generated using a diode-pumped solid-state digital laser operating at 532 nm (visible) wavelength. We inserted a non-linear crystal (potassium titanyl phosphate, KTiOPO₄ or KTP) inside the laser cavity operating at 1064 nm (Near-IR). The KTP was pumped using higher-order laser modes generated by the 1064 nm solid-state digital laser. We generated Laguerre-Gaussian laser modes and Hermite-Gaussian laser modes inside a cavity. The laser modes were characterised by analysing the intensity distribution.

CO2 lasers as well as sealed CO2 lasers are well known and established in the industrial market. Due to the wavelength there is still a need for this type of lasers. New applications are demanding regarding pulse frequencies up to 100 kHz and more. Especially the printing and the automotive industry are markets which push innovations for these lasers. A novel design of combined rf amplifiers for high speed pulsing and gated pulsing for high pwm (pulse width modulation) in combination with suitable resonators is presented. The transient behavior of the rf signal during ignition brings todays semiconductors to their limits. Solutions for a reliable design are presented. The resulting super pulsed and gated pulsed CO2 laser beam is promising for state of the art applications. CLSM and SEM evaluation shows the advantages of pulsed laser operation with optimized laser resonators.

Ghost images at an image plane in a laser imaging system where a pixelated detector of a Charged Coupled Device (CCD) or Complementary Metal-Oxide-Semiconductor (CMOS) digital camera, or other digital imaging system, are caused by reflections of light from the optical media of the imaging system, wherein the optical media have non-zero reflection and transmission coefficients. The non-zero reflection and transmission of light is a result of the difference in the refractive index on either side of the interface. At the interface, a portion of the incident light from a laser beam is transmitted at the surface of the optical media, while another portion of the incident light is reflected. The reflected light then propagates back to another optical media surface, which is reflected again and eventually propagates to the image plane, resulting in the pixelated detector sensing a ghost image. The ghost image is undesirable because it negatively affects the ability of the pixelated detector to accurately measure the laser beam or distort an image. While antireflection coatings for the optical media are employed for reducing the ghost images, even the best anti-reflection coatings are not effective in mitigating the ghost reflections in applications utilizing pixelated detectors that are extremely sensitive to the ghost images or when the wavelength band is broader then about 25 - 50 nm. We show a novel method to mitigate a second surface Fresnel reflection by more than a factor of 10 better than existing antireflection coatings.

The cross polarization coupling (CPC) between orthogonally polarized modes in a single whispering-gallery microresonator can lead to electromagnetically induced transparency (EIT) like effects. Depending on the CPC strength, coupled mode induced transparency (CMIT), coupled mode induced attenuation (CMIA), or Autler-Townes splitting (ATS) can be observed. Previously, the values of CPC strength were found by fitting the experimental throughput spectra to a steady-state model. However, our dynamical analysis suggests an independent way of estimating the CPC strength by sinusoidal modulation at the input. From experimentally determined parameters, we first find one estimate of the CPC strength by model fitting as before. From the modulation frequency that gives the minimum throughput amplitude on resonance, we find another estimate of the CPC strength. Our preliminary experimental results show that the two values agree quite well, which means we have an independent way of finding the CPC strength.

We present theoretical framework to describe Brillouin lasing in microcavities in the case of a significant mismatch between the Brillouin shift and the cavity intermode spacing. We show that despite an increase of the lasing threshold a significant increase of the Brillouin power in comparison with the resonance case is achievable. A necessary condition for this effect is the optimal value of the pump frequency detuning from the cavity mode frequency. An increase of the Brillouin threshold is accompanied by narrowing of the spectrum range where the Brillouin signal could be generated in non-resonant case. Besides, with the optimal pump frequency detuning the Brillouin signal noise level is reduced. Analytical results are in quantitative agreement with the results of numerical simulations.

A tapered hollow annular core fiber (HACF) coupler for excitation of whispering-gallery modes (WGMs) of an embedded microsphere resonator is proposed and demonstrated. Using HACF as the coupling medium can avoid the complicated fabrication process such as chemical corrosion and improve the robustness and stability of the device. The coupling efficiency from the SMF to HACF is enhanced after tapering the joint section to excite various WGMs. In both theory and experiment, we observe symmetrical Lorentzian and asymmetric Fano line shapes by varying the microsphere resonator size and location. Finally, the temperature sensing sensitivity and stability of the device are tested. The sensitivity of the device reaches 10.8 pm/°C. This novel tapered HACF based microsphere resonator is expected to promote the environmental adaptability in the practical application.

Monitoring of multiple physical parameters, such as humidity, temperature, strain, concentrations of certain chemicals or gases in various environments is of great importance in many industrial applications both for minimizing adverse effects on human health as well as for maintaining production levels and quality of products. In this paper we demonstrate two different approaches to the design of multi-parametric sensors using coupled whispering gallery mode (WGM) optical fiber micro-resonators. In the first approach, a small array of micro-resonators is coupled to a single fiber taper, while in the second approach each of the micro-resonators within an array is coupled to a different tapered fiber section fabricated along the same fiber length. Simultaneous measurement of relative humidity and ammonia concentration in air is demonstrated with an array of two microspheres with different functional coatings coupled to a single fiber taper. Sensitivity to ammonia of 19.07 pm/ppm ammonia molecules and sensitivity to relative humidity of 1.07 pm/% RH have been demonstrated experimentally. In the second approach, an inline cascade of two cylindrical micro-resonators fabricated by coupling to multiple tapered sections along a single optical fiber is demonstrated for measurement of strain and temperature simultaneously. A strain sensitivity of 1.4 pm/με and temperature sensitivity of 330 pm/ºC have been demonstrated experimentally. Both the proposed sensing systems have the potential for increase of the number of microresonators within an array for sensing of a larger number of parameters allowing for reduction of the overall cost of sensing system.

We have demonstrated the highest conical refraction (CR) laser output power to date by placing a CR crystal inside of a diode-pumped Nd:YVO laser cavity. The CR crystal did not have a significant influence on laser output power as well as efficiency. The CR laser produced the maximum output power of 3.68 W with the slope efficiency of 42 % and opticalto- optical efficiency of 34 %. Therefore, this approach could be an attractive pathway for further power scaling of the CR lasers.

A conical refraction (CR) laser based on a separate gain medium (Nd:YVO₄) and an intracavity CR element (KGW) was demonstrated. The decoupling of the gain and CR media enabled the laser to produce a well-behaved CR laser beam with excellent quality, while reducing the complexity of the pumping scheme. The proposed laser setup has the potential for power scaling using the efficient diode pumping approach and the properties of the generated CR beam are independent from the laser gain medium.