arXiv subjects

Guo-Qiang Lo

Publications and source records attributed to Guo-Qiang Lo.

At least 19 recordsLinked to original sources

arXiv2025

Hybrid Integration of InGaN Lasers in a Foundry-Fabricated Visible-Light Photonics Platform

Visible-spectrum photonic integrated circuits (PICs) present compact and scalable solutions for emerging technologies including quantum computing, biosensing, and virtual/augmented reality. Realizing their full potential requires the development of scalable visible-light-source integration methods compatible with high-volume manufacturing and capable of delivering high optical coupling efficiencies. Here, we demonstrate passive-alignment flip-chip bonding of 450-nm InGaN laser diodes onto a foundry-fabricated visible-light silicon (Si) photonics platform with silicon nitride (SiN) waveguides, thermo-optic (TO) devices, and photodetectors. Hybrid laser integration is realized using a sub-micron-precision die bonder equipped with a vision alignment system and a heatable pickup tool, allowing independent placement of multiple lasers onto a single Si chip. Co-design of the lasers and Si photonics, with lithographically defined alignment marks and mechanical stoppers, enables precise postbonding alignment. Efficient optical coupling between lasers and the SiN waveguides is demonstrated, with a minimum measured coupling loss of 1.1 dB. We achieve a maximum on-chip optical power of 60.7 mW and an on-chip wall-plug efficiency of 7.8%, the highest reported for hybrid-integrated visible-spectrum lasers, to our knowledge. An active PIC is also shown, integrating a bonded laser, an on-chip photodetector for power monitoring, and a thermo-optic switch for optical routing and variable attenuation. Overall, this work highlights passive-alignment flip-chip bonding as a practical, high-performance approach for integrating lasers onto visible-spectrum PICs. We envision that continued refinement of this technique within our photonics platform will support increasingly complex PICs with integrated lasers spanning the visible spectrum.

physics.optics↗

arXiv2025

Broadband Waveguide-Coupled Photodetectors in a Submicrometer-Wavelength Silicon Photonics Platform

Advances in silicon (Si) photonics at submicrometer wavelengths are unlocking new opportunities to realize miniaturized, scalable optical systems for biophotonics, quantum information, imaging, spectroscopy, and displays. Addressing this array of applications with a single integrated photonics technology requires the development of high-performance active components compatible with both visible and near-infrared light. Here, we report waveguide-coupled photodetectors monolithically integrated in a foundry-fabricated, short-wavelength, Si photonics platform. We demonstrate two detector variants that collectively cover a continuous wavelength span of $\lambda =$ 400 - 955 nm. The devices exhibited external quantum efficiencies exceeding 60% and 12% over 400 - 748 nm and 749 - 955 nm wavelength ranges, respectively. Measured dark currents were $<$ 2 pA at a 2 V reverse bias. High-speed measurements at $\lambda =$ 785 nm demonstrated optoelectronic bandwidths up to 18 GHz. Avalanche operation was characterized, yielding a gain-bandwidth product of 374 GHz.

physics.optics↗

arXiv2025

Thermally Induced Refractive Index Trimming of Visible-Light Silicon Nitride Waveguides Using Suspended Heaters

We demonstrate refractive index trimming of visible-light silicon nitride (SiN) waveguides using suspended heater structures. The thermal isolation of the suspended heaters enabled a semi-uniform temperature distribution with estimated temperatures of $\sim$350{\deg}C in the waveguides without reaching potentially damaging temperatures in the titanium nitride resistive heaters. The thermal isolation also enabled trimming temperatures to be reached with a moderate power dissipation of 30 to 40 mW. At a wavelength of 561 nm, modal effective index changes up to $-8.3 \times 10^{-3}$ were observed following thermal trimming, and the index changes were stable over an observation period of 97 days. The devices were fabricated as part of our visible-light integrated photonics platform on 200-mm diameter silicon wafers. The suspended heaters also functioned as efficient thermo-optic phase shifters with power dissipation for a $\pi$ phase shift of about $1.2-1.8$ mW. The trimming method was applied to set the bias points of thermo-optic Mach-Zehnder interferometer switches to reduce the bias power of five devices from $0.29-2.32$ mW to $0.1-0.16$ mW. Thermal trimming at a wavelength of 445 nm was also demonstrated. Through material analysis before and after thermal treatment, we hypothesize that index trimming of the silica (SiO$_2$) waveguide cladding may be a potential underlying mechanism. Additionally, via extrapolations of the measured trimming data, we estimate the thermal aging behavior of the SiN waveguides in the suspended heaters at lower (125 - 250{\deg}C) operating temperatures.

physics.optics↗

arXiv2024

Variational learning of integrated quantum photonic circuits

Integrated photonic circuits play a crucial role in implementing quantum information processing in the noisy intermediate-scale quantum (NISQ) era. Variational learning is a promising avenue that leverages classical optimization techniques to enhance quantum advantages on NISQ devices. However, most variational algorithms are circuit-model-based and encounter challenges when implemented on integrated photonic circuits, because they involve explicit decomposition of large quantum circuits into sequences of basic entangled gates, leading to an exponential decay of success probability due to the non-deterministic nature of photonic entangling gates. Here, we present a variational learning approach for designing quantum photonic circuits, which directly incorporates post-selection and elementary photonic elements into the training process. The complicated circuit is treated as a single nonlinear logical operator, and a unified design is discovered for it through variational learning. Engineering an integrated photonic chip with automated control, we adjust and optimize the internal parameters of the chip in real time for task-specific cost functions. We utilize a simple case of designing photonic circuits for a single ancilla CNOT gate with improved success rate to illustrate how our proposed approach works, and then apply the approach in the first demonstration of quantum stochastic simulation using integrated photonics.

quant-ph↗

arXiv2024

Room-temperature waveguide-coupled silicon single-photon avalanche diodes

Single photon detection is important for a wide range of low-light applications, including quantum information processing, spectroscopy, and light detection and ranging (LiDAR). A key challenge in these applications has been to integrate single-photon detection capability into photonic circuits for the realization of complex photonic microsystems. Short-wavelength ($λ$ < 1.1 $μ$m) integrated photonics platforms that use silicon (Si) as photodetectors offer the opportunity to achieve single-photon avalanche diodes (SPADs) that operate at or near room temperature. Here, we report the first waveguide-coupled Si SPAD. The device is monolithically integrated in a Si photonic platform and operates in the visible spectrum. The device exhibited a single photon detection efficiency of > 6% for wavelengths of 488 nm and 532 nm with an excess voltage less than 20% of the breakdown voltage. The dark count rate was below 100 kHz at room temperature, with the possibility of improving by approximately 35% by reducing the temperature to -5$^{\circ}$C.

physics.optics↗

arXiv2024

Implantable Photonic Neural Probes with Out-of-Plane Focusing Grating Emitters

We have designed, fabricated, and characterized implantable silicon neural probes with nanophotonic grating emitters that focus the emitted light at a specified distance above the surface of the probe for spatially precise optogenetic targeting of neurons. Using the holographic principle, we designed gratings for wavelengths of 488 and 594 nm, targeting the excitation spectra of the optogenetic actuators Channelrhodopsin-2 and Chrimson, respectively. The measured optical emission pattern of these emitters in non-scattering medium and tissue matched well with simulations. To our knowledge, this is the first report of focused spots with the size scale of a neuron soma in brain tissue formed from implantable neural probes.

physics.optics↗

arXiv2023

Rigorous noise reduction with quantum autoencoders

Reducing noise in quantum systems is a major challenge towards the application of quantum technologies. Here, we propose and demonstrate a scheme to reduce noise using a quantum autoencoder with rigorous performance guarantees. The quantum autoencoder learns to compresses noisy quantum states into a latent subspace and removes noise via projective measurements. We find various noise models where we can perfectly reconstruct the original state even for high noise levels. We apply the autoencoder to cool thermal states to the ground state and reduce the cost of magic state distillation by several orders of magnitude. Our autoencoder can be implemented using only unitary transformations without ancillas, making it immediately compatible with the state of the art. We experimentally demonstrate our methods to reduce noise in a photonic integrated circuit. Our results can be directly applied to make quantum technologies more robust to noise.

quant-ph↗

arXiv2023

Efficient option pricing with unary-based photonic computing chip and generative adversarial learning

In the modern financial industry system, the structure of products has become more and more complex, and the bottleneck constraint of classical computing power has already restricted the development of the financial industry. Here, we present a photonic chip that implements the unary approach to European option pricing, in combination with the quantum amplitude estimation algorithm, to achieve a quadratic speedup compared to classical Monte Carlo methods. The circuit consists of three modules: a module loading the distribution of asset prices, a module computing the expected payoff, and a module performing the quantum amplitude estimation algorithm to introduce speed-ups. In the distribution module, a generative adversarial network is embedded for efficient learning and loading of asset distributions, which precisely capture the market trends. This work is a step forward in the development of specialized photonic processors for applications in finance, with the potential to improve the efficiency and quality of financial services.

quant-ph↗

arXiv2023

Implantable Photonic Neural Probes with 3D-Printed Microfluidics and Applications to Uncaging

Advances in chip-scale photonic-electronic integration are enabling a new generation of foundry-manufacturable implantable silicon neural probes incorporating nanophotonic waveguides and microelectrodes for optogenetic stimulation and electrophysiological recording in neuroscience research. Further extending neural probe functionalities with integrated microfluidics is a direct approach to achieve neurochemical injection and sampling capabilities. In this work, we use two-photon polymerization 3D printing to integrate microfluidic channels onto photonic neural probes, which include silicon nitride nanophotonic waveguides and grating emitters. The customizability of 3D printing enables a unique geometry of microfluidics that conforms to the shape of each neural probe, enabling integration of microfluidics with a variety of existing neural probes while avoiding the complexities of monolithic microfluidics integration. We demonstrate the photonic and fluidic functionalities of the neural probes via fluorescein injection in agarose gel and photoloysis of caged fluorescein in solution and in flxed brain tissue.

physics.optics↗

arXiv2023

Polarization-diverse soliton transitions and deterministic switching dynamics in strongly-coupled and self-stabilized microresonator frequency combs

Dissipative Kerr soliton microcombs in microresonators has enabled fundamental advances in chip scale precision metrology, communication, spectroscopy, and parallel signal processing. Here we demonstrate polarization diverse soliton transitions and deterministic switching dynamics of a self stabilized microcomb in a strongly coupled dispersion-managed microresonator driven with a single pump laser. The switching dynamics are induced by the differential thermorefractivity between coupled transverse magnetic and transverse electric supermodes during the forward backward pump detunings. The achieved large soliton existence range and deterministic transitions benefit from the switching dynamics, leading to the cross polarized soliton microcomb formation when driven in the transverse magnetic supermode of the single resonator. Resultantly the pump laser always exists at the effective blue detuning of the transverse magnetic resonance, fundamentally mitigating the thermal destabilization barrier and improving accessibility of the soliton formation regime. Subsequently and secondly, we demonstrate two distinct polarization diverse soliton formation routes arising from chaotic or periodically modulated waveforms via pump power selection. The generated self stabilized supermode microcomb features an extraordinarily large soliton existence range, a variety of soliton state transitions with well defined pump laser tuning, high pump microcomb conversion efficiency, and low repetition rate phase noise.

physics.optics↗

arXiv2022

Microcantilever-integrated photonic circuits for broadband laser beam scanning

Laser beam scanning is central to many applications, including displays, microscopy, three-dimensional mapping, and quantum information. Reducing the scanners to microchip form factors has spurred the development of very-large-scale photonic integrated circuits of optical phased arrays and focal plane switched arrays. An outstanding challenge remains to simultaneously achieve a compact footprint, broad wavelength operation, and low power consumption. Here, we introduce a laser beam scanner that meets these requirements. Using microcantilevers embedded with silicon nitride nanophotonic circuitry, we demonstrate broadband, one- and two-dimensional steering of light with wavelengths from 410 nm to 700 nm. The microcantilevers have ultracompact ~0.1 mm$^2$ areas, consume ~31 to 46 mW of power, are simple to control, and emit a single light beam. The microcantilevers are monolithically integrated in an active photonic platform on 200-mm silicon wafers. The microcantilever-integrated photonic circuits miniaturize and simplify light projectors to enable versatile, power-efficient, and broadband laser scanner microchips.

physics.optics↗

arXiv2021

Power-Efficient Silicon Nitride Thermo-Optic Phase Shifters for Visible Light

We demonstrate power-efficient, thermo-optic, silicon nitride waveguide phase shifters for blue, green, and yellow wavelengths. The phase shifters operated with low power consumption due to a suspended structure and multi-pass waveguide design. The devices were fabricated on 200-mm silicon wafers using deep ultraviolet lithography as part of an active visible-light integrated photonics platform. The measured power consumption to achieve a $π$ phase shift (averaged over multiple devices) was 0.78, 0.93, 1.09, and 1.20 mW at wavelengths of 445, 488, 532, and 561 nm, respectively. The phase shifters were integrated into Mach-Zehnder interferometer switches, and $10- 90$\% rise(fall) times of about 570(590) $μ$s were measured.

physics.optics↗

arXiv2016

Silicon Photonics WDM Transceiver with SOA and Semiconductor Mode-Locked Laser

We demonstrate a complete Silicon Photonics WDM link relying on a single section semiconductor mode-locked laser and a single SOA to support up to 12 multiplexed channels with a bit error rate of 1e-12 at serial data rates of 14 Gbps without channel pre-emphasis, equalization or forward error correction. Individual channels reach error free operation at 25 Gbps and multi-channel operation at 25 Gbps is shown to be compatible with standard 7% overhead hard decision forward error correction. Silicon Photonics transmitter and receiver chips are hybridly integrated with driver and receiver electronics. A detailed link model is derived and verified. Particular emphasis is placed on accurate system level modeling of laser RIN, SOA amplified spontaneous emission noise and receiver noise. The impact of the electrical receiver bandwidth and non-Gaussian statistics on level dependent amplified spontaneous emission noise are investigated in detail. The channel count scalability as limited by SOA saturation is further analyzed taking cross gain modulation and four wave mixing into account. While semiconductor mode-locked lasers have been identified as a potential light source for low cost Datacom WDM transceivers for some time, this is, to the best of our knowledge, the first comprehensive investigation of the overall link budget in a Silicon Photonics implementation showing this technology to be a credible contender for low latency datacenter interconnects.

physics.optics↗

arXiv2014

Radio frequency regenerative oscillations in monolithic high-Q/V heterostructured photonic crystal cavities

We report temporal and spectral domain observation of regenerative oscillation in monolithic silicon heterostructured photonic crystals cavities with high quality factor to mode volume ratios (Q/V). The results are interpreted by nonlinear coupled mode theory (CMT) tracking the dynamics of photon, free carrier population and temperature variations. We experimentally demonstrate effective tuning of the radio frequency (RF) tones by laser-cavity detuning and laser power levels, confirmed by the CMT simulations with sensitive input parameters.

physics.optics↗

arXiv2012

Near-infrared Hong-Ou-Mandel interference on a silicon quantum photonic circuit

Near-infrared Hong-Ou-Mandel quantum interference is observed in silicon nanophotonic directional couplers with raw visibilities on-chip at 90.5%. Spectrally-bright 1557-nm two-photon states are generated in a periodically-poled KTiOPO4 waveguide chip, serving as the entangled photon source and pumped with a self-injection locked laser, for the photon statistical measurements. Efficient four-port coupling in the communications C-band and in the high-index-contrast silicon photonics platform is demonstrated, with matching theoretical predictions of the quantum interference visibility. Constituents for the residual quantum visibility imperfection are examined, supported with theoretical analysis of the sequentially-triggered multipair biphoton contribution and techniques for visibility compensation, towards scalable high-bitrate quantum information processing and communications.

physics.optics↗

arXiv2012

Regenerative oscillation and four-wave mixing in graphene optoelectronics

The unique linear and massless band structure of graphene, in a purely two-dimensional Dirac fermionic structure, have led to intense research spanning from condensed matter physics to nanoscale device applications covering the electrical, thermal, mechanical and optical domains. Here we report three consecutive first-observations in graphene-silicon hybrid optoelectronic devices: (1) ultralow power resonant optical bistability; (2) self-induced regenerative oscillations; and (3) coherent four-wave mixing, all at a few femtojoule cavity recirculating energies. These observations, in comparison with control measurements with solely monolithic silicon cavities, are enabled only by the dramatically-large and chi(3) nonlinearities in graphene and the large Q/V ratios in wavelength-localized photonic crystal cavities. These results demonstrate the feasibility and versatility of hybrid two-dimensional graphene-silicon nanophotonic devices for next-generation chip-scale ultrafast optical communications, radio-frequency optoelectronics, and all-optical signal processing.

physics.optics↗

arXiv2012

A 25 Gb/s Silicon Photonics Platform

Silicon has attracted attention as an inexpensive and scalable material system for photonic-electronic, system-on-chip development. For this, a platform with both photodetectors and modulators working at high speeds, with excellent cross-wafer uniformity, is needed. We demonstrate an optical-lithography, wafer-scale photonics platform with 25 Gb/s operation. We also demonstrate modulation with an ultra-low drive voltage of 1 Vpp at 25 Gb/s. We demonstrate attractive cross-wafer uniformity, and provide detailed information about the device geometry. Our platform is available to the community as part of a photonics shuttle service.

physics.optics↗

arXiv2011

Selective tuning of high-Q silicon photonic crystal nanocavities via laser-assisted local oxidation

We examine the cavity resonance tuning of high-Q silicon photonic crystal heterostructures by localized laser-assisted thermal oxidation using a 532 nm continuous wave laser focused to a 2.5 mm radius spot-size. The total shift is consistent with the parabolic rate law. A tuning range of up to 8.7 nm is achieved with ~ 30 mW laser powers. Over this tuning range, the cavity Q decreases from 3.2\times10^5 to 1.2\times10^5. Numerical simulations model the temperature distributions in the silicon photonic crystal membrane and the cavity resonance shift from oxidation.

physics.optics↗