Si Photonic Integrated Circuits Research Articles

Minimal-gain-printed silicon nanolaser.

While there have been notable advancements in Si-based optical integration, achieving compact and efficient continuous-wave (CW) III-V semiconductor nanolasers on Si at room temperature remains a substantial challenge. This study presents an innovative approach: the on-demand minimal-gain-printed Si nanolaser. By using a carefully designed minimal III-V optical gain structure and a precise on-demand gain-printing technique, we achieve lasing operation with superior spectral stability under pulsed conditions and observe a strong signature of CW operation at room temperature. These achievements are attributed to addressing both fundamental and technological issues, including carrier diffusion, absorption loss, and inefficient thermal dissipation, through minimal-gain printing in the nanolaser. Moreover, our demonstration of the laser-on-waveguide structure emphasizes the integration benefits of this on-demand gain-printed Si nanolaser, highlighting its potential significance in the fields of Si photonics and photonic integrated circuits.

OFDR analysis of Si photonics FMCW LiDAR chip.

We experimentally analyzed the internal reflection and loss of each component in a Si photonics frequency-modulated continuous-wave light detection and ranging (FMCW LiDAR) device using optical frequency domain reflectometry (OFDR) with a spatial resolution of better than 2.5 µm. Sweeping the incident laser wavelength by 120 nm, the reflections and losses of wire waveguides, widened waveguides, and optical switches on the chip were individually revealed. The slow-light grating (SLG) beam scanner, which has a limited working wavelength range, was evaluated with a spatial resolution of >10 µm by narrowing the wavelength sweep range. Consequently, a strong reflection was observed at the transition between the wire waveguide and the SLG, which can be a noise source in the FMCW LiDAR. Additionally, this study showed that the OFDR can be an important analysis tool for Si photonics integrated circuits. To our knowledge, this is the first demonstration, showing that the OFDR can be an important analysis tool for Si photonic integrated circuits.

Micro-Transfer Printing for Heterogeneous Si Photonic Integrated Circuits

Silicon photonics (SiPh) is a disruptive technology in the field of integrated photonics and has experienced rapid development over the past two decades. Various high-performance Si and Ge/Si-based components have been developed on this platform that allow for complex photonic integrated circuits (PICs) with small footprint. These PICs have found use in a wide range of applications. Nevertheless, some non-native functions are still desired, despite the versatility of Si, to improve the overall performance of Si PICs and at the same time cut the cost of the eventual Si photonic system-on-chip. Heterogeneous integration is verified as an effective solution to address this issue, e.g. through die-wafer-bonding and flip-chip. In this paper, we discuss another technology, micro-transfer printing, for the integration of non-native material films/opto-electronic components on SiPh-based platforms. This technology allows for efficient use of non-native materials and enables the (co-)integration of a wide range of materials/devices on wafer scale in a massively parallel way. In this paper we review some of the recent developments in the integration of non-native optical functions on Si photonic platforms using micro-transfer printing.

Light Modulation in Silicon Photonics by PZT Actuated Acoustic Waves.

Tailoring the interaction between light and sound has opened new possibilities in photonic integrated circuits (PICs) that range from achieving quantum control of light to high-speed information processing. However, the actuation of sound waves in Si PICs usually requires integration of a piezoelectric thin film. Lead zirconate titanate (PZT) is a promising material due to its strong piezoelectric and electromechanical coupling coefficient. Unfortunately, the traditional methods to grow PZT on silicon are detrimental for photonic applications due to the presence of an optical lossy intermediate layer. In this work, we report integration of a high quality PZT thin film on a silicon-on-insulator (SOI) photonic chip using an optically transparent buffer layer. We demonstrate acousto-optic modulation in silicon waveguides with the PZT actuated acoustic waves. We fabricate interdigital transducers (IDTs) on the PZT film with a contact photolithography and electron-beam lithography to generate the acoustic waves in MHz and GHz ranges, respectively. We obtain a V π L ∼ 3.35 V·cm at 576 MHz from a 350 nm thick gold (Au) IDT with 20 finger-pairs. After taking the effect of mass-loading and grating reflection into account, we measured a V π L ∼ 3.60 V·cm at 2 GHz from a 100 nm thick aluminum (Al) IDT consisting of only four finger-pairs. Thus, without patterning the PZT film nor suspending the device, we obtained figures-of-merit comparable to state-of-the-art modulators based on SOI, making it a promising candidate for a broadband and efficient acousto-optic modulator for future integration.

Single-Mode Photonic Crystal Nanobeam Lasers Monolithically Grown on Si for Dense Integration

Ultra-compact III-V nanolasers monolithically integrated on Si with ultra-low energy consumption and small modal volume have been emerged as one of the most promising candidates to achieve Si on-chip light sources. However, the significant material dissimilarities between III-V and Si fundamentally limit the performance of Si-based III-V nanolasers. In this work, we report 1.3 μm InAs/GaAs quantum-dot photonic-crystal (PhC) nanobeam lasers directly grown on complementary metal-oxide-semiconductor compatible on-axis Si (001) substrates. The continuous-wave optically pumped PhC nanobeam lasers exhibited a single-mode operation, with an ultra-low lasing threshold of ∼ 0.8 μW at room temperature. In addition, a nanoscale physical volume of ∼ 8 × 0.53 × 0.36 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> (∼ 25 (λ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> ) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ) was realized through a small number of air-holes in PhC nanobeam laser. The promising characteristics of the PhC nanobeam lasers with small footprint and ultra-low energy consumption show their advanced potential towards densely integrated Si photonic integrated circuits.

Ultrafast and energy-efficient all-optical switching with graphene-loaded deep-subwavelength plasmonic waveguides

All-optical switches have attracted attention because they can potentially overcome the speed limitation of electric switches. However, ultrafast, energy-efficient all-optical switches have been challenging to realize due to the intrinsically small optical nonlinearity in existing materials. As a solution, we propose graphene-loaded deep-subwavelength plasmonic waveguides (30 nm x 20 nm). Thanks to extreme light confinement, we have significantly enhanced optical nonlinear absorption in graphene, and achieved ultrafast all-optical switching with a switching energy of 35 fJ and a switching time of 260 fs. The switching energy is four orders of magnitudes smaller than that in previous graphene-based devices and is the smallest value ever reported for any all-optical switch operating at a few picoseconds or less. This device can be efficiently connected to conventional Si waveguides and employed in Si photonic integrated circuits. We believe that this graphene-based device will pave the way towards on-chip ultrafast and energy-efficient photonic processing.

III-V-on-Si photonic integrated circuits realized using micro-transfer-printing

Silicon photonics (SiPh) enables compact photonic integrated circuits (PICs), showing superior performance for a wide variety of applications. Various optical functions have been demonstrated on this platform that allows for complex and powerful PICs. Nevertheless, laser source integration technologies are not yet as mature, hampering the further cost reduction of the eventual Si photonic systems-on-chip and impeding the expansion of this platform to a broader range of applications. Here, we discuss a promising technology, micro-transfer-printing (μTP), for the realization of III-V-on-Si PICs. By employing a polydimethylsiloxane elastomeric stamp, the integration of III-V devices can be realized in a massively parallel manner on a wafer without substantial modifications to the SiPh process flow, leading to a significant cost reduction of the resulting III-V-on-Si PICs. This paper summarizes some of the recent developments in the use of μTP technology for realizing the integration of III-V photodiodes and lasers on Si PICs.

Ultra-power-efficient 2 × 2 Si Mach-Zehnder interferometer optical switch based on III-V/Si hybrid MOS phase shifter.

We have demonstrated an ultra-power-efficient 2 × 2 Si Mach-Zehnder interferometer optical switch with III-V/Si hybrid metal-oxide-semiconductor (MOS) phase shifters. The efficient low-loss phase modulation enables low-crosstalk and broadband switching in conjunction with multimode interference couplers consisting of tapered input and output ports. Owing to the negligible gate leakage current in the hybrid MOS capacitor, the power consumption required for switching is 0.18 nW, approximately 107 times smaller than that of a Si thermo-optic phase shifter. We also demonstrated a switching time of less than 20 ns. The III-V/Si hybrid MOS phase shifter is promising for fabricating large-scale Si photonic integrated circuits that require efficient, low-loss, and high-speed optical phase control.

Annealing-free Si3N4 frequency combs for monolithic integration with Si photonics

Silicon-nitride-on-insulator (SiNOI) is an attractive platform for optical frequency comb generation in the telecommunication band because of the low two-photon absorption and free carrier induced nonlinear loss when compared with crystalline silicon. However, high-temperature annealing that has been used so far for demonstrating Si3N4-based frequency combs made co-integration with silicon-based optoelectronics elusive, thus reducing dramatically its effective complementary metal oxide semiconductor (CMOS) compatibility. We report here on the fabrication and testing of annealing-free SiNOI nonlinear photonic circuits. In particular, we have developed a process to fabricate low-loss, annealing-free, and crack-free Si3N4 740-nm-thick films for Kerr-based nonlinear photonics featuring a full process compatibility with front-end silicon photonics. Experimental evidence shows that micro-resonators using such annealing-free silicon nitride films are capable of generating a frequency comb spanning 1300–2100 nm via optical parametrical oscillation based on four-wave mixing. This work constitutes a decisive step toward time-stable power-efficient Kerr-based broadband sources featuring full process compatibility with Si photonic integrated circuits on CMOS lines.

Crack-Free Silicon-Nitride-on-Insulator Nonlinear Circuits for Continuum Generation in the ${C}$ -Band

We report on the fabrication and testing of silicon-nitride-on-insulator nonlinear photonic circuits for complementary metal–oxide–semiconductor (CMOS) compatible monolithic co-integration with silicon-based optoelectronics. In particular, a process has been developed to fabricate low-loss crack-free Si3N4 730-nm-thick films for Kerr-based nonlinear functions featuring full process compatibility with existing silicon photonics and front-end Si optoelectronics. Experimental evidence shows that 2.1-cm-long nanowires based on such crack-free silicon nitride films are capable of generating a frequency continuum spanning 1515–1575 nm via self-phase modulation. This work paves the way to time-stable power-efficient Kerr-based broadband sources featuring full process compatibility with Si photonic integrated circuits on CMOS lines.

Germanium Quantum Well QCSE Waveguide Modulator With Tapered Coupling in Distributed Modulator–Detector System

Optical interconnections (interconnects) have been proposed as solutions to the ever-increasing bandwidth requirements and energy consumption in communication systems. Among possible photonic modulation strategies, the Ge quantum well (QW) based quantum-confined Stark effect (QCSE) stands out, as its strong electro-absorption effect allows for potentially lower power consumption and smaller device sizes compared to other modulation mechanisms. Here, we experimentally demonstrate a thin buffer layer Ge QW QCSE waveguide modulator that evanescently couples to and from an Si waveguide through an adiabatic three-dimensional (3D) taper. Simulations confirm that this 3-D taper yields higher coupling efficiency and improved maintenance of the fundamental mode after coupling compared to a 2-D taper. We also demonstrate that this geometry could potentially work in an integrated modulator-detector system. The combination of thin SiGe epitaxy (i.e., the buffer and device layers) with Si waveguides paves the way to easier integration of Si photonic integrated circuits.

Wide-band polarization controller for Si photonic integrated circuits.

A circuit for the management of any arbitrary polarization state of light is demonstrated on an integrated silicon (Si) photonics platform. This circuit allows us to adapt any polarization into the standard fundamental TE mode of a Si waveguide and, conversely, to control the polarization and set it to any arbitrary polarization state. In addition, the integrated thermal tuning allows kilohertz speed which can be used to perform a polarization scrambler. The circuit was used in a WDM link and successfully used to adapt four channels into a standard Si photonic integrated circuit.

Porous Silicon Gradient Refractive Index Micro-Optics.

The emergence and growth of transformation optics over the past decade has revitalized interest in how a gradient refractive index (GRIN) can be used to control light propagation. Two-dimensional demonstrations with lithographically defined silicon (Si) have displayed the power of GRIN optics and also represent a promising opportunity for integrating compact optical elements within Si photonic integrated circuits. Here, we demonstrate the fabrication of three-dimensional Si-based GRIN micro-optics through the shape-defined formation of porous Si (PSi). Conventional microfabrication creates Si square microcolumns (SMCs) that can be electrochemically etched into PSi elements with nanoscale porosity along the shape-defined etching pathway, which imparts the geometry with structural birefringence. Free-space characterization of the transmitted intensity distribution through a homogeneously etched PSi SMC exhibits polarization splitting behavior resembling that of dielectric metasurfaces that require considerably more laborious fabrication. Coupled birefringence/GRIN effects are studied by way of PSi SMCs etched with a linear (increasing from edge to center) GRIN profile. The transmitted intensity distribution shows polarization-selective focusing behavior with one polarization focused to a diffraction-limited spot and the orthogonal polarization focused into two laterally displaced foci. Optical thickness-based analysis readily predicts the experimentally observed phenomena, which strongly match finite-element electromagnetic simulations.

Flip-chip integration of tilted VCSELs onto a silicon photonic integrated circuit.

In this article we describe a cost-effective approach for hybrid laser integration, in which vertical cavity surface emitting lasers (VCSELs) are passively-aligned and flip-chip bonded to a Si photonic integrated circuit (PIC), with a tilt-angle optimized for optical-insertion into standard grating-couplers. A tilt-angle of 10° is achieved by controlling the reflow of the solder ball deposition used for the electrical-contacting and mechanical-bonding of the VCSEL to the PIC. After flip-chip integration, the VCSEL-to-PIC insertion loss is -11.8 dB, indicating an excess coupling penalty of -5.9 dB, compared to Fibre-to-PIC coupling. Finite difference time domain simulations indicate that the penalty arises from the relatively poor match between the VCSEL mode and the grating-coupler.

UV/O3 assisted InP/Al2O3–Al2O3/Si low temperature die to wafer bonding

Direct bonding of InP dies to Si wafer at low temperature utilizing Al2O3 high-κ dielectric as the interfacial material for homogeneous bonding is reported. The bonding technique is assisted with a UV/Ozone exposure for surface activation and the activation time is optimized for the various intermediate layer thicknesses (5, 10, 20 nm). After the pre-bonding stage, annealing is carried out at 300 °C for 3 h. A bonding interface with minimal interfacial voids is reported for low intermediate layer thickness. The bonding interfaces are examined and a homogeneously bonded interface is shown in the IR images as well as in the FIB micrographs. Additionally a heat transfer simulation is also carried out and the InP/Al2O3–Al2O3/Si bonded structure is shown to closely match the thermal characteristics of a direct bonding approach with no intermediate layer. A high quality bonding interface is revealed along with improved heat dissipation characteristic for Al2O3 interface. Therefore, Al2O3 proves to be an advantageous candidate for its use in potential Si photonic integrated circuits application.

CMOS-Compatible Plasmonic Bragg Reflectors Based on Cu-Dielectric-Si Structures

Cu-dielectric-Si hybrid plasmonic waveguide (HPW)-based plasmonic Bragg reflectors (PBRs) are fabricated on an SoI platform using standard CMOS technology and characterized in the 1515-1615-nm wavelength range. Optical stop-bands are experimentally observed, depending on the grating size and the number of grating periods. PBRs with 20 periods exhibit ~ -30-dB transmission within the stop-band, ~ -10-dB transmission outside the stop-band (both are normalized by the corresponding 2- μm-long straight HPW), steep band edges of ~ 0.92 dB/nm, and small ripples in the transmission spectra beyond the band edges, in agreement with those predicted from finite-difference-time-domain simulations. These favorable performances, together with ease fabrication and CMOS compatibility, make the proposed HPW-based PBRs useful for dense Si photonic integrated circuits.

Advanced Ge-on-Si Telecommunication Receivers

Ge can be deposited on Si to provide a photodetector at wavelengths where optical fiber has the lowest loss. These Ge-on-Si photodetectors can be used in Si photonic integrated circuits to make high-speed receivers and power monitors. We discuss the technology and give device examples.

Integrated Optoelectronic Devices on Silicon

ABSTRACTSilicon (Si) has been the dominating material platform of microelectronics over half century. Continuous technological advances in circuit design and manufacturing enable complementary metal-oxide semiconductor (CMOS) chips with increasingly high integration complexity to be fabricated in an unprecedently scale and economical manner. Conventional Si-based planar lightwave circuits (PLCs) has benefited from advanced CMOS technology but only demonstrate passive functionalities in most circumstances due to poor light emission efficiency and weak major electro-optic effects (e.g., Pockels effect, the Kerr effect and the Franz–Keldysh effect) in Si. Recently, a new hybrid III-V-on-Si integration platform has been developed, aiming to bridge the gap between Si and III-V direct-bandgap materials for active Si photonic integrated circuit applications. Since then high-performance lasers, amplifiers, photodetectors and modulators, etc. have been demonstrated. Here we review the most recent progress on hybrid Si lasers and high-speed hybrid Si modulators. The former include distributed feedback (DFB) lasers showing over 10 mW output power and up to 85 oC continuous-wave (cw) operation, compact hybrid microring lasers with cw threshold less than 4 mA and over 3 mW output power, and 4-channel hybrid Si AWG lasers with channel space of 360 GHz. Recently fabricated traveling-wave electro-absorption modulators (EAMs) and Mach-Zehnder interferometer modulators (MZM) on this platform support 50 Gb/s and 40 Gb/s data transmission with over 10 dB extinction ratio, respectively.

Experimental Demonstration of Horizontal Nanoplasmonic Slot Waveguide-Ring Resonators With Submicrometer Radius

Ultracompact plasmonic waveguide-ring resonators (WRRs) based on horizontal Cu/SiO2/Si/SiO2/Cu nanoplasmonic slot waveguides are fabricated using standard Si complementary metal-oxide-semiconductor (CMOS) technology for operation at telecommunication wavelengths. Efficient coupling between the ring and bus waveguides is achieved through a small aperture in the metallic sidewalls of the waveguides. A WRR with a radius 0.91 μm (cavity mode volume of ~0.145 μm3 ) is characterized, exhibiting an insertion loss of ~2.4 dB, an extinction ratio of ~28 dB at the 1596-nm resonance (corresponding to the seventh cavity mode), a free-spectral range of ~143 nm, and a quality factor of ~638. The demonstrated WRR can be used as a building block for various functional nanoplasmonic devices to be seamlessly integrated in the existing Si photonic and electronic integrated circuits.

Low-loss plasmonic hybrid optical ridge waveguide on silicon-on-insulator substrate

To achieve low loss and complete confinement of light, we propose and study a plasmonic hybrid optical waveguide fabricated on an silicon-on-insulator substrate. Using controlled oxidation and etching processes, a deep submicrometer Si core is fabricated based on UV photolithography patterning. The as-fabricated waveguide demonstrates a very low propagation loss of 1.6 dB/mm at 1550 nm. The numerical study discloses the hybrid characteristics of the mode. It is shown that the optical power of the waveguiding mode distributes more in the high-index Si-core region, and the metal cover further confines the light beyond the diffraction limit. The hybrid waveguide may be applied in compact, high-density Si photonic integrated circuits.