arXiv subjects

Kartik Srinivasan

Publications and source records attributed to Kartik Srinivasan.

At least 37 records · Page 2Linked to original sources

arXiv2025

Integrated broadband optical isolator via dynamic rotating destructive interference

Photonic integrated circuits route and shape light on a chip, but back-reflections feed back into coherent on-chip lasers, destabilizing operation and corrupting signals. Robust operation requires an integrated optical isolator that strongly suppresses backward propagation while maintaining low-loss, broadband forward transmission. However, prior on-chip isolators rely on magneto-optic materials or resonance-based filters, which respectively demand non-standard processes or inherently constrain bandwidth. Here, we propose and experimentally demonstrate a traveling-wave optical isolator without magnetic materials or resonant elements. By driving four parallel optical channels with periodic RF waves, we realize dynamic rotating destructive interference that continuously cancels backward-propagating light while leaving forward-propagating light unaffected. We achieve about 30 dB isolation at a wavelength of 789.7 nm and maintain over 24 dB isolation across an approximately 30 nm bandwidth (770 nm to 800 nm), including >20 dB isolation for two simultaneous lasers within an approximately 10 nm wavelength window. This wavelength span covers key alkali atomic transitions, enabling strong suppression of feedback-induced frequency noise and laser instability in atomic spectroscopy, laser cooling, and locking applications. We demonstrate a practical, broadband on-chip isolator applicable from the visible to the near-infrared, which is a crucial step toward fully integrated photonic platforms.

physics.optics↗

arXiv2025

On-chip multi-timescale spatiotemporal optical synchronization

Mode-locking mechanisms are key resources in nonlinear optical phenomena, such as micro-ring solitonic states, and have transformed metrology, precision spectroscopy, and optical communication. However, despite significant efforts, mode-locking has not been demonstrated in the independently tunable multi-timescale regime. Here, we vastly expand the nonlinear mode-locking toolbox into multi-timescale synchronization on a chip. We use topological photonics to engineer a 2D lattice of hundreds of coupled silicon nitride ring resonators capable of hosting nested mode-locked states with a fast (near 1 THz) single-ring and a slow (near 3 GHz) topological super-ring timescales. We demonstrate signatures of multi-timescale mode-locking including quadratic distribution of the pump noise with the two-time azimuthal mode dimensions, as expected by mode-locking theory. Our observations are further corroborated by direct signatures of the near-transform-limit repetition beats and the formation of the temporal pattern on the slow timescale. Moreover, we show that these exotic properties of edge-confined mode-locked states are in sharp contrast to bulk and single-ring counterparts and establish a clear pathway for their identification. Our unprecedented demonstration of mode-locking in topological combs unlocks the implementation of lattice-scale synchronization and independently tunable multi-timescale mode-locking phenomena, also the exploration of the fundamental nonlinearity-topology interplay on a chip.

physics.optics↗

arXiv2025

Versatile Optical Frequency Division with Kerr-induced Synchronization at Tunable Microcomb Synthetic Dispersive Waves

Kerr-induced synchronization (KIS) provides a new key tool for the control and stabilization of the repetition rate of a cavity soliton frequency comb. It enables direct external control of a given comb tooth of a dissipative Kerr soliton (DKS) thanks to its capture by an injected reference laser. Efficient KIS requires its coupling energy to be sufficiently large, and hence both the comb tooth and intracavity reference power must be optimized, which can be achieved through higher-order dispersion that enables phase-matched dispersive waves (DWs), where comb teeth are on resonance. However, such a design is highly restrictive, preventing arbitrary use of reference wavelengths away from the DW(s). In particular, for large spectral separations from the main pump the cavity dispersion yields large detuning between comb teeth and their respective cavity resonances, thereby decreasing the coupling energy and rendering KIS to be highly inefficient or practically impossible. Here, we demonstrate an alternative KIS method where efficient synchronization can be tailored at arbitrary modes as needed. Using a multi-color DKS created from multi-pumping a microresonator, a synthetic DW at the second-color wavepacket can be selectively created where otherwise dispersion is far too large for KIS to be experimentally feasible. Since a unique group velocity for both colors exists thanks to cross-phase modulation, the repetition rate disciplining of the secondary color wavepacket through its KIS automatically translates into the DKS microcomb control. We first investigate this color-KIS phenomenon theoretically, and then experimentally demonstrate its control and tuning of the soliton microcomb repetition rate. As a consequence, we demonstrate optical frequency division that is uncoupled from the main pump that generates the DKS.

physics.optics↗

arXiv2025

All-Optical Noise Quenching of An Integrated Frequency Comb

Integrated frequency combs promise transformation of lab-based metrology into disruptive real-world applications. These microcombs are, however, sensitive to stochastic thermal fluctuations of the integrated cavity refractive index, with its impact becoming more significant as the cavity size becomes smaller. This tradeoff between microcomb noise performance and footprint stands as a prominent obstacle to realizing applications beyond a controlled lab environment. Here, we demonstrate that small footprint and low noise become compatible through the all-optical Kerr-induced synchronization (KIS) method. Our study unveils that the phase-locking nature of the synchronization between the cavity soliton and the injected reference pump laser enables the microcomb to no longer be limited by internal noise sources. Instead, the microcomb noise is mostly limited by external sources, namely, the frequency noise of the two pumps that doubly pin the microcomb. First, we theoretically and experimentally show that the individual comb tooth linewidths of an octave-spanning microcomb remain within the same order-of-magnitude as the pump lasers, contrary to the single-pumped case that exhibits a more than two order-of-magnitude increase from the pump to the comb edge. Second, we theoretically show that intrinsic noise sources such as thermorefractive noise in KIS are quenched at the cavity decay rate, greatly decreasing its impact. Experimentally, we show that even with free-running lasers, the KIS microcomb can exhibit better repetition rate noise performance than the predicted thermorefractive noise limitation in absence of KIS.

physics.optics↗

arXiv2025

Multimode Nanobeam Photonic Crystal Cavities for Purcell Enhanced Quantum Dot Emission

Epitaxial III-V semiconductor quantum dots in nanopthonic structures are promising candidates for implementing on-demand indistinguishable single-photon emission in integrated quantum photonic circuits. Quantum dot proximity to the etched sidewalls of hosting nanophotonic structures, however, has been shown to induce linewidth broadening of excitonic transitions, which limits emitted single-photon indistinguishability. Here, we design and demonstrate GaAs photonic crystal nanobeam cavities that maximize quantum dot distances to etched sidewalls beyond an empirically determined minimum that curtails spectral broadening. Although such geometric constraint necessarily leads to multimode propagation in nanobeams, which significantly complicates high quality factor cavity design, we achieve resonances with quality factors $Q\approx10^3$, which offer the potential for achieving Purcell radiative rate enhancements $F_p\approx100$.

physics.optics↗

arXiv2025

300 mm Wafer-Scale SiN Platform for Broadband Soliton Microcombs Compatible with Alkali Atomic References

Chip-integrated optical frequency combs (OFCs) based on Kerr nonlinear resonators are of great significance given their scalability and wide range of applications. Broadband on-chip OFCs reaching visible wavelengths are especially valuable as they address atomic clock transitions that play an important role in position, navigation, and timing infrastructure. Silicon nitride (SiN) deposited via low pressure chemical vapor deposition (LPCVD) is the usual platform for the fabrication of chip-integrated OFCs, and such fabrication is now standard at wafer sizes up to 200 mm. However, the LPCVD high temperature and film stress poses challenges in scaling to larger wafers and integration with electronic and photonic devices. Here, we report the linear performance and broadband frequency comb generation from microring resonators fabricated on 300 mm wafers at AIM Photonics, using a lower temperature, lower stress plasma enhanced chemical vapor deposition process that is suitable for thick ($\approx$ 700 nm) SiN films and compatible with electronic and photonic integration. The platform exhibits consistent insertion loss, high intrinsic quality factor, and thickness variation of $\pm$2 % across the whole 300 mm wafer. We demonstrate broadband soliton microcomb generation with a lithographically tunable dispersion profile extending to wavelengths relevant to common alkali atom transitions. These results are a step towards mass-manufacturable devices that integrate OFCs with electronic and active photonic components, enabling advanced applications including optical clocks, LiDAR, and beyond.

physics.optics↗

arXiv2025

Multi-timescale frequency-phase matching for high-yield nonlinear photonics

Integrated nonlinear photonic technologies, even with state-of-the-art fabrication with only a few nanometer geometry variations, face significant challenges in achieving wafer-scale yield of functional devices. A core limitation lies in the fundamental constraints of energy and momentum conservation laws. Imposed by these laws, nonlinear processes are subject to stringent frequency and phase matching (FPM) conditions that cannot be satisfied across a full wafer without requiring a combination of precise device design and active tuning. Motivated by recent theoretical and experimental advances in integrated multi-timescale nonlinear systems, we revisit this long-standing limitation and introduce a fundamentally relaxed and passive framework: nested frequency-phase matching. As a prototypical implementation, we investigate on-chip multi-harmonic generation in a two-timescale lattice of commercially available silicon nitride (SiN) coupled ring resonators, which we directly compare with conventional single-timescale counterparts. We observe distinct and striking spatial and spectral signatures of nesting-enabled relaxation of FPM. Specifically, for the first time, we observe simultaneous fundamental, second, third, and fourth harmonic generation, remarkable 100 percent multi-functional device yield across the wafer, and ultra-broad harmonic bandwidths. Crucially, these advances are achieved without constrained geometries or active tuning, establishing a scalable foundation for nonlinear optics with broad implications for integrated frequency conversion and synchronization, self-referencing, metrology, squeezed light, and nonlinear optical computing.

physics.optics↗

arXiv2025

All-Optical Azimuthal Trapping of Dissipative Kerr Multi-Solitons for Relative Noise Suppression

Temporal cavity solitons, or dissipative Kerr solitons (DKS) in integrated microresonators, are essential for deployable metrology technologies. Such applications favor the lowest noise state, typically the single-DKS state where one soliton is in the resonator. Other multi-DKS states can also be reached, offering better conversion efficiency and thermal stability, potentially simplifying DKS-based technologies. Yet they exhibit more noise due to relative soliton jitter, and are usually not compatible with targeted applications. We demonstrate that Kerr-induced synchronization, an all-optical trapping technique, can azimuthally pin the multi-DKS state to a common reference field. This method ensures repetition rate noise independent of the number of solitons, making a multi-DKS state indistinguishable from a single-DKS state in that regard, akin to trapped-soliton molecule behavior. Supported by theoretical analysis and experimental demonstration in an integrated microresonator, this approach provides metrological capacity regardless of the number of cavity solitons, benefiting numerous DKS-based metrology applications.

physics.optics↗

arXiv2025

Full spectral response of grating-induced loss in photonic crystal microrings

Photonic crystal microrings (PhCRs) have emerged as powerful and versatile platforms for integrated nonlinear photonics, offering precise control over frequency and phase matching while maintaining high optical quality factors. Through grating-mediated mode coupling, PhCRs enable advanced dispersion engineering, which is critical for wideband nonlinear processes such as optical parametric oscillation, Kerr frequency comb generation, and dual-pump spontaneous and Bragg scattering four-wave mixing. Beyond dispersion control, PhCRs also facilitate the manipulation of orbital angular momentum (OAM) emission, a key functionality for encoding high-dimensional quantum states in emerging quantum photonic platforms. Despite these advances, the broadband spectral behavior of grating-induced losses in PhCRs remains largely unexplored, with most studies focusing on grating periods near the modal wavelength or its half. Such losses can significantly impact broadband nonlinear processes, where excess loss at unintended wavelengths can degrade device performance. In this work, we experimentally characterize grating-induced losses in PhCRs and reveal their full spectral response as a function of the ratio between modal wavelength and grating period. We identify distinct loss channels arising from either radiation or mode conversion, including a broad excess-loss region attributed to vertical out-coupling into OAM-carrying states. These observations are supported by three-dimensional finite-difference time-domain simulations and further analyzed through OAM radiation angle and phase-mismatch analysis. The resulting broadband loss spectrum highlights critical design trade-offs and provides practical guidelines for optimizing PhCR-based devices for nonlinear photonic applications involving widely separated frequencies.

physics.optics↗

arXiv2025

Parasitic loss in microring-waveguide coupling and its impact on wideband nonlinear photonics

Microring resonators enable the enhancement of nonlinear frequency mixing processes, generating output fields at frequencies that widely differ from the inputs, in some cases by more than an octave. The efficiency of such devices depends on effective in- and out-coupling between access waveguides and the microrings at these widely separated frequencies. One successful approach is to separate the coupling task across multiple waveguides, with a cutoff waveguide (a waveguide that does not support guided modes above a certain wavelength) being judiciously used to prevent unwanted excessive overcoupling at low frequencies. Here, we examine how such a cutoff waveguide can still induce parasitic loss in the coupling region of a microring resonator, thereby impacting nonlinear device performance. We verified this parasitic loss channel through both experiment and simulation, showing that a waveguide optimized for 532 nm (visible) and 780 nm (near-infrared), while nominally cut off at 1550 nm, can still introduce significant parasitic loss at telecom wavelengths. This is studied in the context of visible-telecom optical parametric oscillation, where the excess parasitic loss can be strong enough to prevent threshold from being reached. Our finding elucidates a major challenge for wideband integrated nonlinear photonics processes when efficient coupling of widely-separated frequencies is needed.

physics.optics↗

arXiv2025

Broadband Visible Wavelength Microcomb Generation In Silicon Nitride Microrings Through Air-Clad Dispersion Engineering

The development of broadband microresonator frequency combs at visible wavelengths is pivotal for the advancement of compact and fieldable optical atomic clocks and spectroscopy systems. Yet, their realization necessitates resonators with anomalous dispersion, an arduous task due to the prevailing normal dispersion regime of materials within the visible spectrum. In this work, we evince that silicon nitride microring resonators with air cladding on top and sides -- a deviation from the frequently employed silica-embedded resonators -- allows for the direct generation of broadband microcombs in the visible range. We experimentally demonstrate combs pumped at 1060~nm (283~THz) that reach wavelengths as short as 680~nm (440 THz), and combs pumped at 780~nm (384 THz) that reach wavelengths as short as 630 nm (475 THz). We further show through simulations that microcombs extending to wavelengths as low as 461 nm (650 THz) should be accessible in this platform.

physics.optics↗

arXiv2025

On-Chip Parametric Synchronization of a Dissipative Kerr Soliton Microcomb

Synchronization of oscillators is ubiquitous in nature. Often, the synchronized oscillators couple directly, yet in some cases synchronization can arise from their parametric interactions. Here, we theoretically predict and experimentally demonstrate the parametric synchronization of a dissipative Kerr soliton frequency comb. We specifically show that the parametric interaction between the soliton and two auxiliary lasers permits the entrainment of the frequency comb repetition rate. Besides representing the first prediction and demonstration of parametric synchronization of soliton frequency combs, our scheme offers significant flexibility for all-optical metrological-scale stabilization of the comb.

physics.optics↗

arXiv2025

Highly squeezed nanophotonic quantum microcombs with broadband frequency tunability

Squeezed light offers genuine quantum advantage in enhanced sensing and quantum computation; yet the level of squeezing or quantum noise reduction generated from nanophotonic chips has been limited. In addition to strong quantum noise reduction, key desiderata for such a nanophotonic squeezer include frequency agility or tunability over a broad frequency range, and simultaneous operation in many distinct, well-defined quantum modes (qumodes). Here we present a strongly overcoupled silicon nitride squeezer based on a below-threshold optical parametric amplifier (OPA) that produces directly detected squeezing of 5.6 dB $\pm$ 0.2 dB, surpassing previous demonstrations in both continuous-wave and pulsed regimes. We introduce a seed-assisted detection technique into such nanophotonic squeezers that reveals a quantum frequency comb (QFC) of 16 qumodes, with a separation of 11~THz between the furthest qumode pair, while maintaining a strong squeezing. Additionally, we report spectral tuning of a qumode comb pair over one free-spectral range of the OPA, thus bridging the spacing between the discrete modes of the QFC. Our results significantly advance both the generation and detection of nanophotonic squeezed light in a broadband and multimode platform, establishing a scalable, chip-integrated path for compact quantum sensors and continuous-variable quantum information processing systems.

physics.optics↗

arXiv2025

Laser injection locking and nanophotonic spectral translation of electro-optic frequency combs

High-resolution electro-optic frequency combs (EO combs) consisting of thousands to millions of comb teeth across a bandwidth between 1 GHz to 500 GHz are powerful tools for atomic, molecular, and cavity-based spectroscopy, including in the context of deployable quantum sensors. However, achieving sufficiently high signal-to-noise ratio (SNR) EO combs for use across the broad range of wavelengths required in the aforementioned applications is hindered by the corresponding unavailability of relevant components such as narrow-linewidth lasers, electro-optic phase modulators with adequate optical power handling, and low-noise optical amplifiers. Here, we address the latter two points by showing that optical injection locking of commercial Fabry-Perot (FP) laser diodes can help enable high SNR EO combs. We injection lock the FP laser diode to more than 10^6 comb teeth at injected comb powers as low as 1 nW and produce a high SNR replica of the EO comb. In comparison to a commercial semiconductor optical amplifier, injection locking achieves approximately 100x greater SNR for the same input power (when <1 microwatt) and equal SNR for > 35x lower input power. Such low-power injection locking is of particular relevance in conjunction with nanophotonic spectral translation, which extends the range of wavelengths available for EO combs. We show that the usable wavelength range of an EO comb produced by photo-induced second harmonic generation of an EO comb in a silicon nitride resonator is significantly increased when combined with optical injection locking. Our results demonstrate that optical injection locking provides a versatile and high-performance approach to addressing many different scenarios in which EO comb SNR would be otherwise limited.

physics.optics↗

arXiv2024

Multi-color solitons and frequency combs in microresonators

Multi-color solitons that are parametrically created in dual-pumped microresonators generate interleaved frequency combs that can be used to obtain combs at new frequencies and when synchronized can be used for low-noise microwave generation and potentially as an element in a chip-scale clockwork. Here, we first derive three-wave equations that describe multi-color solitons that appear in microresonators with a nearly quartic dispersion profile. These solitons are characterized by a single angular group velocity and multiple angular phase velocities. We then use these equations to explain the interleaved frequency combs that are observed at the output of the microresonator. Finally, we used these equations to describe the experimentally-observed soliton-OPO effect. In this effect, the pump frequency comb interacts nonlinearly with a signal frequency comb to create an idler frequency comb in a new frequency range, analogous to an optical parametric oscillation (OPO) process. We determine the conditions under which we expect this effect to occur. We anticipate that the three-wave equations and their extensions will be of use in designing new frequency comb systems and determining their stability and noise performance.

physics.optics↗

arXiv2024

Terahertz Voltage-controlled Oscillator from a Kerr-Induced Synchronized Soliton Microcomb

The generation of controlled and arbitrarily tunable terahertz radiation, essential for many applications, has proven challenging due to the complexity of experimental setups and fabrication techniques. We introduce a new strategy involving control over a terahertz repetition rate integrated frequency comb, using Kerr-induced synchronization, that results in a terahertz-voltage-controlled oscillator. By modulating the reference laser, we can transfer any microwave waveform onto the microcomb repetition rate via a linear transfer function based on optical frequency division. The resulting frequency comb with a terahertz carrier can be created using integrated components, with a bandwidth constrained only by the synchronization bandwidth and high coherence resulting from the low-noise soliton microcomb in the Kerr-induced synchronized state.

physics.optics↗

arXiv2024

On-chip Kerr parametric oscillation with integrated heating for enhanced frequency tuning and control

Nonlinear microresonators can convert light from chip-integrated sources into new wavelengths within the visible and near-infrared spectrum. For most applications, such as the interrogation of quantum systems with specific transition wavelengths, tuning the frequency of converted light is critical. Nonetheless, demonstrations of wavelength conversion have mostly overlooked this metric. Here, we apply efficient integrated heaters to tune the idler frequency produced by Kerr optical parametric oscillation in a silicon-nitride microring across a continuous 1.5 terahertz range. Finally, we suppress idler frequency noise between DC and 5 kHz by several orders of magnitude using feedback to the heater drive.

physics.optics↗

arXiv2024

Low-power, agile electro-optic frequency comb spectrometer for integrated sensors

Sensing platforms based upon photonic integrated circuits have shown considerable promise; however, they require corresponding advancements in integrated optical readout technologies. Here, we present an on-chip spectrometer that leverages an integrated thin-film lithium niobate modulator to produce a frequency-agile electro-optic frequency comb for interrogating chip-scale temperature and acceleration sensors. The chirped comb process allows for ultralow radiofrequency drive voltages, which are as much as seven orders of magnitude less than the lowest found in the literature and are generated using a chip-scale, microcontroller-driven direct digital synthesizer. The on-chip comb spectrometer is able to simultaneously interrogate both an on-chip temperature sensor and an off-chip, microfabricated optomechanical accelerometer with cutting-edge sensitivities of $\approx 5\ μ \mathrm{K} \cdot \mathrm{Hz}^{-1/2}$ and $\approx 130\ μ\mathrm{m} \cdot \mathrm{s}^{-2} \cdot \mathrm{Hz}^{-1/2}$, respectively. This platform is compatible with a broad range of existing photonic integrated circuit technologies, where its combination of frequency agility and ultralow radiofrequency power requirements are expected to have applications in fields such as quantum science and optical computing.

physics.optics↗