On-chip Photonic Systems Research Articles

On-Chip Circulator-Free Chirped Spiral Multimode Waveguide Grating for Dispersion Management

Waveguide gratings are one of the key optical dispersion handling devices and are highly desired for many on-chip photonic systems. However, most of the previously reported waveguide gratings require optical circulators to separate input and reflected signals. Here, a circulator-free chirped spiral multimode waveguide grating (SMWG) composed of an asymmetric chirped multimode waveguide grating and a mode (de)multiplexer is first proposed and demonstrated for dispersion management on a silicon-on-insulator (SOI) platform. The chirped SMWG reflects the TE0 mode into the TE1 mode so that input and reflected signals can be separated with the assistance of a mode (de)multiplexer instead of a circulator. The spiral waveguide with a continuously varying radius enables a large amount of dispersion within a compact footprint. Group delay ripples are suppressed by superposition modulation on lateral corrugations. The fabricated 5 mm chirped SMWG realizes a maximum group delay of 150 ps and dispersion of 6.49 ps/nm within a bandwidth of 22 nm. The proposed circulator-free chirped SMWG will find many applications such as optical communication, microwave photonics, and pulse compression.

Efficient Meta-couplers Squeezing Propagating Light into On-Chip Subwavelength Devices in a Controllable Way

On-chip photonic systems play crucial roles in nanoscience and nanoapplications, but coupling external light to these subwavelength devices is challenging due to a large mode mismatch. Here, we establish a new scheme for realizing highly miniaturized couplers for efficiently exciting on-chip photonic devices in a controllable way. Relying on both resonant and Pancharatnam-Berry mechanisms, our meta-device can couple circularly polarized light to a surface plasmon, which is then focused into a spot placed with a target on-chip device. We experimentally demonstrate two meta-couplers. The first can excite an on-chip waveguide (with a 0.1λ × 0.2λ cross section) with an absolute efficiency of 51%, while the second can achieve incident spin-selective excitation of a dual-waveguide system. Background-free excitation of a gap-plasmon nanocavity with the local field enhanced by >1000 times is numerically demonstrated. Such a scheme connects efficiently propagating light in free space and localized fields in on-chip devices, being highly favored in many integration-optics applications.

Experimental demonstration of inverse-designed silicon integrated photonic power splitters

Abstract The on-chip optical power splitter is a common and important device in photonic integrated circuits (PICs). To achieve a low insertion loss and high uniformity while splitting the guided light, multi-mode interferometer-based structures utilizing a self-imaging principle are widely used mainly in the form of a 1 × 2 configuration. Recently, an inverse design method for nanophotonic devices has emerged to overcome the limited capability of the conventional design methods and make it possible to explore the vast number of design parameters. Because of the non-intuitive shape of inverse-designed structures, they allow us to discover interesting and complex optical responses which are almost impossible to find with conventional design methods. Here, we report two kinds of inverse-designed 1 × 4 optical power splitters composed of silicon bars of different lengths, which are fabricated with a standard CMOS-compatible process. The particle swarm optimization method was used to minimize the insertion loss and divide the power evenly into each output port with finite-difference time-domain method simulation. The first optical power splitter has a compact size of 8.14 × 12 μm2 and the second optical power splitter has an even more compact size of 6.0 × 7.2 μm2. With the inverse designed structures, we fabricated the chip with a CMOS-compatible fabrication process. Experimental verification of the structures is provided and good agreement with the numerical results is obtained. The first 1 × 4 optical power splitter has a low insertion loss of less than 0.76 dB and uniformity of less than 0.84 dB, and the second more compact optical power splitter has a low insertion loss of less than 1.08 dB and uniformity of less than 0.81 dB. As the complexity of on-chip photonic systems has steadily increased, the inverse design of photonic structures holds great potential to be an essential part of advanced design tools.

Application of the TDFA window in true optical time delay systems.

Recent advances in silicon photonic components operating in the thulium-doped fiber amplifier (TDFA) wavelength regime around 2-µm have shown that these wavelengths hold great promise for on-chip photonic systems. Here we present our work on characterizing a Mach-Zehnder interferometer coupled silicon photonic ring resonator operating in the TDFA window for optical time delay applications. We describe the optical transmission and variable time delay properties of the resonator, including a detailed characterization and comparison of the directional coupler and Mach-Zehnder interferometer base components at both 1930 and 1550 nm wavelengths. The results show tuning of a ring from a 190-ps peak time delay at a resonant extinction ratio of 5.1-dB to a 560-ps peak time delay at an extinction ratio of 11.0-dB, in good agreement with optical models of the device. These results demonstrate significant promise towards the future application of TDFA band devices in optical time delay systems.

Broadband tunable electro-optic switch/power divider as potential building blocks in integrated lithium niobate photonics.

We demonstrate an electro-optic (EO) switch or in general, an EO controllable power divider based on a periodically poled lithium niobate (PPLN) polarization mode converter (PMC) and a five-waveguide adiabatic coupler integrated on a Ti:LN photonic circuit chip. In this integrated photonic circuit (IPC) device, the PPLN works as an EO controllable polarization rotator (and therefore a PMC), while the adiabatic coupler functions as a broadband polarization beam splitter (PBS). The 1-cm long PPLN EO PMC of the IPC device is characterized to have a half-wave (or switching) voltage of Vπ∼20 V and a conversion bandwidth of ∼2.6 nm. The splitting ratios of the adiabatic coupler PBS in the IPC device are >99% for both polarization modes over a broad spectral range from 1500-1640 nm. The EO mode of the implemented IPC device is activated when the PPLN EO PMC section is driven by an external voltage; the characterized EO switching/power division behavior of the device is in good agreement with the theoretical fit. The tunability of the EO IPC device in the 100-nm experimental spectral range is also demonstrated via the temperature tuning. The featured broad tunability and high integrability of the EO device presented in this study facilitates it to be an advantageous building block for realizing an on-chip photonic system.

Non-Abelian braiding on photonic chips

Non-Abelian braiding has attracted significant attention because of its pivotal role in describing the exchange behaviors of anyons--a candidate for realizing quantum logics. The input and outcome of non-Abelian braiding are connected by a unitary matrix which can also physically emerge as a geometric-phase matrix in classical systems. Hence it is predicted that non-Abelian braiding should have analogues in photonics, but a feasible platform and the experimental realization remain out of reach. Here, we propose and experimentally realize an on-chip photonic system that achieves the non-Abelian braiding of up to five photonic modes. The braiding is realized by controlling the multi-mode geometric-phase matrix in judiciously designed photonic waveguide arrays. The quintessential effect of braiding--sequence-dependent swapping of photon dwell sites is observed in both classical-light and single-photon experiments. Our photonic chips are a versatile and expandable platform for studying non-Abelian physics, and we expect the results to motivate next-gen non-Abelian photonic devices.

General Rules Governing the Dynamical Encircling of an Arbitrary Number of Exceptional Points.

Dynamically encircling an exceptional point in non-Hermitian systems has drawn great attention recently, since a nonadiabatic transition process can occur and lead to intriguing phenomena and applications such as the asymmetric switching of modes. While all previous experiments have been restricted to two-state systems, the dynamics in multistate systems where more complex topology can be formed by exceptional points, is still unknown and associated experiments remain elusive. Here, we propose an on-chip photonic system in which an arbitrary number of exceptional points can be encircled dynamically. We reveal in experiment a robust state-switching rule for multistate systems, and extend it to an infinite-period system in which an exceptional line is encircled with outcomes being located at the Brillouin-zone boundary. The proposed versatile platform is expected to reveal more physics related to multiple exceptional points and exceptional lines, and give rise to applications in multistate non-Hermitian systems.

Fast thermo-optical modulators with doped-silicon heaters operating at 2 μm.

The 2-μm-waveband has been recognized as a potential telecommunication window for next-generation low-loss, low-latency optical communication. Thermo-optic (TO) modulators and switches, which are essential building blocks in a large-scale integrated photonic circuit, and their performances directly affect the energy consumption and reconfiguration time of an on-chip photonic system. Previous TO modulation based on metallic heaters at 2-μm-waveband suffer from slow response time and high power consumption. In this paper, high-performance thermo-optical Mach-Zehnder interferometer and ring resonator modulators operating at 2-μm-waveband were demonstrated. By embedding a doped silicon (p++-p-p++) junction into the waveguide, our devices reached a record modulation efficiency of 0.17 nm/mW for Mach-Zehnder interferometer based modulator and its rise/fall time was 3.49 μs/3.46 μs which has been the fastest response time reported in a 2-μm-waveband TO devices so far. And a lowest Pπ power of 3.33 mW among reported 2-μm TO devices was achieved for a ring resonator-based modulator.

Guided Mid‐IR and Near‐IR Light within a Hybrid Hyperbolic‐Material/Silicon Waveguide Heterostructure

Silicon waveguides have enabled large-scale manipulation and processing of near-infrared optical signals on chip. Yet, expanding the bandwidth of guided waves to other frequencies will further increase the functionality of silicon as a photonics platform. Frequency multiplexing by integrating additional architectures is one approach to the problem, but this is challenging to design and integrate within the existing form factor due to scaling with the free-space wavelength. This paper demonstrates that a hexagonal boron nitride (hBN)/silicon hybrid waveguide can simultaneously enable dual-band operation at both mid-infrared (6.5-7.0µm) and telecom (1.55µm) frequencies, respectively. The device is realized via the lithography-free transfer of hBN onto a silicon waveguide, maintaining near-infrared operation. In addition, mid-infrared waveguiding of the hyperbolic phonon polaritons (HPhPs) supported in hBN is induced by the index contrast between the silicon waveguide and the surrounding air underneath the hBN, thereby eliminating the need for deleterious etching of the hyperbolic medium. The behavior of HPhP waveguiding in both straight and curved trajectories is validated within an analytical waveguide theoretical framework. This exemplifies a generalizable approach based on integrating hyperbolic media with silicon photonics for realizing frequency multiplexing in on-chip photonic systems.

On-chip metalenses based on one-dimensional gradient trench in the broadband visible.

Metasurfaces are composed of flat, ultrathin subwavelength nanoantennas with strong capability in manipulating light propagation by modulations on its phase, amplitude, and polarization. For instance, the invention of two-dimensional (2D) metalenses has enabled light focusing and imaging in three-dimensional (3D) free space with miniaturized thickness and device size at a planar surface. However, such inherent form of 2D arrays and focusing functionality at 3D optical free-space limits the degree of freedom for light propagation and manipulation along a 2D planar surface and eventually the possibility of on-chip photonic system integration. Here, we theoretically study and demonstrate a new type of planar on-chip metalens, which enables light focusing and strong localization at a 2D surface. The planar on-chip architecture design is based on the one-dimensional (1D) length or width gradient trench metalens (GTM), which could yield the elaborately engineered phase shift for propagating light within the on-chip waveguide at the visible wavelength of 500 nm. By generating 1D phase arrangement at the nanoscale, a miniature on-chip metalens with ∼3×0.5µm dimension could achieve light focusing on a 2D waveguide surface with the flexibility to design scalable focal lengths and ultra-high numerical aperture of up to ∼0.99. Additionally, GTM metalens designs could also exhibit overlapped high depth-of-focus, which consequently could behave as achromatic-like lensing at the selected focal plane. Furthermore, we manifest that the focusing functionality can also be subject to dynamically tuning and switching on-and-off with TE/TM polarization change or waveguide index alteration. We believe this new form of on-chip 1D metalens holds potential applications including on-chip light manipulation functionality of focusing and diverging, optical on-chip sensing, next-generation on-chip optical communication, signal processing as well as imaging devices, etc.

Characterization, Selection, and Microassembly of Nanowire Laser Systems.

Semiconductor nanowire (NW) lasers are a promising technology for the realization of coherent optical sources with ultrasmall footprint. To fully realize their potential in on-chip photonic systems, scalable methods are required for dealing with large populations of inhomogeneous devices that are typically randomly distributed on host substrates. In this work two complementary, high-throughput techniques are combined: the characterization of nanowire laser populations using automated optical microscopy, and a high-accuracy transfer-printing process with automatic device spatial registration and transfer. Here, a population of NW lasers is characterized, binned by threshold energy density, and subsequently printed in arrays onto a secondary substrate. Statistical analysis of the transferred and control devices shows that the transfer process does not incur measurable laser damage, and the threshold binning can be maintained. Analysis on the threshold and mode spectra of the device populations proves the potential for using NW lasers for integrated systems fabrication.

Coupling-enhanced dual ITO layer electro-absorption modulator in silicon photonics

Abstract Electro-optic signal modulation provides a key functionality in modern technology and information networks. Photonic integration has not only enabled miniaturizing photonic components, but also provided performance improvements due to co-design addressing both electrical and optical device rules. The millimeter to centimeter footprint of many foundry-ready electro-optic modulators, however, limits density scaling of on-chip photonic systems. To address these limitations, here we experimentally demonstrate a coupling-enhanced electro-absorption modulator by heterogeneously integrating a novel dual-gated indium-tin-oxide phase-shifting tunable absorber placed at a silicon directional coupler region. This concept allows utilizing the normally parasitic Kramers-Kronig relations here in an synergistic way resulting in a strong modulation depth to insertion loss ratio of about 1. Our experimental modulator shows a 2 dB extinction ratio for a just 4 μm short device at 4 V bias. Since no optical resonances are deployed, this device shows spectrally broadband operation as demonstrated here across the entire C-band. In conclusion, we demonstrate a modulator utilizing strong index change from both real and imaginary parts of active material enabling compact and high-performing modulators using semiconductor near-foundry materials.

Low-loss fiber-to-chip couplers with ultrawide optical bandwidth

Providing efficient access from optical fibers to on-chip photonic systems is a key challenge for integrated optics. In general, current solutions allow either narrowband out-of-plane-coupling to a large number of devices or broadband edge-coupling to a limited number of devices. Here we present a hybrid approach using 3D direct laser writing, merging the advantages of both concepts and enabling broadband and low-loss coupling to waveguide devices from the top. In the telecom wavelength regime, we demonstrate a coupling loss of less than −1.8 dB between 1480 nm and 1620 nm. In the wavelength range between 730 nm and 1700 nm, we achieve coupling efficiency well above −8 dB which is sufficient for a range of broadband applications spanning more than an octave. The 3D couplers allow relaxed mechanical alignment with respect to optical fibers, with −1 dB alignment tolerance of about 5 µm in x- and y-directions and −1 dB alignment tolerance in the z-direction of 34 µm. Using automatized alignment, many such couplers can be connected to integrated photonic circuits for rapid prototyping and hybrid integration.

Second harmonic generation in metal-LiNbO3-metal and LiNbO3 hybrid-plasmonic waveguides.

Nanoplasmonic waveguides based on lithium niobate (LN) are shown to provide the light-matter interaction required for next-generation developments in nonlinear frequency-conversion nanostructures. Here, we numerically investigate second harmonic generation of a 1550 nm, 100 fs pulse in metal-LN-metal (MLNM) nanoplasmonic and LN hybrid-plasmonic (LNHP) waveguides. In comparison to a photonic LN waveguide, a 2.1 µm-long LNHP waveguide exhibits a conversion efficiency improvement of 11 times, whereas a 20 µm-long MLNM nanoplasmonic waveguide is shown to have a conversion efficiency of 1.1 × 10-4. The MLNM nanoplasmonic and LNHP waveguides have the potential to operate as sources of optical radiation for on-chip photonic systems.

Accelerating light with metasurfaces

It has been recently shown that specially engineered light beams have the remarkable ability to propagate along curved trajectories in vacuum. Current methods for generating accelerating beams use phase modulators and lenses, leading to length scales on the order of tens of centimeters or larger. This poses constraints and severely limits their applicability inside materials. Here, we accelerate light inside glass using a metasurface consisting of plasmonic nanoantennas. Highly bending beams with radii of curvature on the order of a hundred micrometers were generated, and the imaged intensities agree well with theory. Our approach for generating accelerating beams allows for their integration into on-chip photonic systems.

Semipolar InGaN quantum-well laser diode with integrated amplifier for visible light communications.

GaN-based semiconductor optical amplifier (SOA) and its integration with laser diode (LD) is an essential building block yet to be demonstrated for III-nitride photonic integrated circuits (PICs) at visible wavelength. This paper presents the InGaN/GaN quantum well (QW) based dual-section LD consisting of integrated amplifier and laser gain regions fabricated on a semipolar GaN substrate. The threshold current in the laser gain region was favorably reduced from 229mA to 135mA at SOA driving voltages, VSOA, of 0V and 6.25V, respectively. The amplification effect was measured based on a large gain of 5.7 dB at VSOA = 6.25V from the increased optical output power of 8.2 mW to 30.5 mW. Such integrated amplifier can be modulated to achieve Gbps data communication using on-off keying technique. The monolithically integrated amplifier-LD paves the way towards the III-nitride on-chip photonic system, providing a compact, low-cost, and multi-functional solution for applications such as smart lighting and visible light communications.

Semipolar III–nitride quantum well waveguide photodetector integrated with laser diode for on-chip photonic system

A high-performance waveguide photodetector (WPD) integrated with a laser diode (LD) sharing the single InGaN/GaN quantum well active region is demonstrated on a semipolar GaN substrate. The photocurrent of the integrated WPD is effectively tuned by the emitted optical power from the LD. The responsivity ranges from 0.018 to 0.051 A/W with increasing reverse bias from 0 to 10 V. The WPD shows a large 3 dB modulation bandwidth of 230 MHz. The integrated device, being used for power monitoring and on-chip communication, paves the way towards the eventual realization of a III–nitride on-chip photonic system.

On-chip photonic system using suspended p-n junction InGaN/GaN multiple quantum wells device and multiple waveguides

We propose, fabricate, and characterize the on-chip integration of suspended p-n junction InGaN/GaN multiple quantum wells (MQWs) device and multiple waveguides on the same GaN-on-silicon platform. The integrated devices are fabricated via a wafer-level process and exhibit selectable functionalities for diverse applications. As the suspended p-n junction InGaN/GaN MQWs device operates under a light emitting diode (LED) mode, part of the light emission is confined and guided by the suspended waveguides. The in-plane propagation along the suspended waveguides is measured by a micro-transmittance setup. The on-chip data transmission is demonstrated for the proof-of-concept photonic integration. As the suspended p-n junction InGaN/GaN MQWs device operates under photodiode mode, the light is illuminated on the suspended waveguides with the aid of the micro-transmittance setup and, thus, coupled into the suspended waveguides. The guided light is finally sensed by the photodiode, and the induced photocurrent trace shows a distinct on/off switching performance. These experimental results indicate that the on-chip photonic integration is promising for the development of sophisticated integrated photonic circuits in the visible wavelength region.

Hybrid integration of Fabry–Pérot cavity with waveguide photodetector on silicon substrate

This paper demonstrated a novel device which was fabricated by the integration of a Fabry–Pérot cavity with waveguide photodetectors on silicon substrate. The design of this device integrates two functions on a single semiconductor chip, which is a current trend to make the compact on-chip photonic system. Due to the introduction of waveguide photodetectors, the tradeoff between efficiency and bandwidth could be improved and these two important factors could be specified almost independently. The Fabry–Pérot cavity was grown by metal–organic chemical vapor deposition (MOCVD), which was used for wavelength selectivity. The waveguide photodetector fabricated by the wet etching technique was used for light detection. The peak wavelength of the hybrid integrated device was at 1538nm. The full-width at half-maximum was about 0.3nm, and a dark current of 3.2nA was achieved at the reverse bias of 3.0V.

Integrated photonic systems based on transformation optics enabled gradient index devices

Integrated photonics is expected to play an increasingly important role in optical communications, imaging, computing and sensing with the promise for significant reduction in the cost and weight of these systems. Future advancement of this technology is critically dependent on an ability to develop compact and reliable optical components and facilitate their integration on a common substrate. Here we reveal, with the utility of the emerging transformation optics technique, that functional components composed of planar gradient index materials can be designed and readily integrated into photonic circuits. The unprecedented design flexibility of transformation optics allows for the creation of a number of novel devices, such as a light source collimator, waveguide adapters and a waveguide crossing, which have broad applications in integrated photonic chips and are compatible with current fabrication technology. Using the finite-difference time-domain method, we perform full-wave numerical simulations to demonstrate their superior optical performance and efficient integration with other components in an on-chip photonic system. These components only require spatially-varying dielectric materials with no magnetic properties, facilitating low-loss, broadband operation in an integrated photonic environment.