Novel Light Source Integration Approaches for Silicon Photonics

Silicon does not emit light efficiently, therefore the integration of other light‐emitting materials is highly demanded for silicon photonic integrated circuits. A number of integration approaches have been extensively explored in the past decade. Here, the most recent progress in this field is reviewed, covering the integration approaches of III‐V‐to‐silicon bonding, transfer printing, epitaxial growth and the use of colloidal quantum dots. The basic approaches to create waveguide‐coupled on‐chip light sources for different application scenarios are discussed, both for silicon and silicon nitride based waveguides. A selection of recent representative device demonstrations is presented, including high speed DFB lasers, ultra‐dense comb lasers, short (850nm) and long (2.3μm) wavelength lasers, wide‐band LEDs, monolithic O‐band lasers and micro‐disk lasers operating in the visible. The challenges and opportunities of these approaches are discussed.     

石墨烯-硅基混合光子集成电路

近年来硅基光子学已经慢慢走向成熟,它被认为是未来取代电子集成电路,实现下一代更高性能的光子集成电路的关键技术.这得益于硅基光子器件与现代的互补金属氧化物半导体工艺相兼容,能够实现廉价的大规模集成.然而,由于受硅材料本身的光电特性所限,在硅基平台上实现高性能的有源器件仍然存在着巨大挑战.石墨烯-硅基混合光子集成电路的发展为解决这一问题提供了可行的方案.这得益于石墨烯作为一种兼具高载流子迁移率、高电光系数和宽带吸收等优点的二维光电材料,能够方便地与现有硅基器件相集成,并充分发挥自身的光电性能优势.本文结合我们课题组在该领域研究的一些最新成果,介绍了国际上在石墨烯-硅基混合光子集成电路上的一些重要研究进展,涵盖了光源、光波导、光调制器和光探测器四个重要组成部分.     

硅基光子集成研究进展

报道了国际上关于硅基光子集成的最新研究进展和本课题组在该领域的研究成果,包括对一些光收发模块、III-V族/硅基激光器等集成器件的结构改进和工艺的探索,展示了兼容互补金属氧化物半导体工艺的硅基光子集成在信息技术领域中的巨大前景.可以预见,硅基光子集成已成为硅光子学的主要研究内容,硅光子学及硅基光子集成的发展目标是趋向更高速率、更低功耗及更大集成密度.     

Nanosilicon photonics

Silicon photonics is no longer an emerging field of research and technology but a present reality with commercial products available on the market, where low‐dimensional silicon (nanosilicon or nano‐Si) can play a fundamental role. After a brief history of the field, the optical properties of silicon reduced to nanometric dimensions are introduced. The use of nano‐Si, in the form of Si nanocrystals, in the main building blocks of silicon photonics (waveguides, modulators, sources and detectors) is reviewed and discussed. Recent advances of nano‐Si devices such as waveguides, optical resonators (linear, rings, and disks) are treated. Emphasis is placed on the visible optical gain properties of nano‐Si and to the sensitization effect on Er ions to achieve infrared light amplification. The possibility of electrical injection in light‐emitting diodes is presented as well as the recent attempts to exploit nano‐Si for solar cells. In addition, nonlinear optical effects that will enable fast all‐optical switches are described.     

硅基混合表面等离子体纳米光波导及集成器件

总结并展望了硅基混合表面等离子体纳米光波导及集成器件方面的理论和实验研究工作。首先介绍了几种硅基混合表面等离子体纳米光波导结构,其尺寸可小至100 nm以下,而传播长度达100μm量级;其次介绍了基于硅基混合表面等离子体纳米光波导的功分器、偏振分束器和谐振器等集成器件,其尺寸为亚微米量级;最后探讨了硅基混合表面等离子体纳米光波导与硅纳米线光波导的耦合及对其进行增益补偿。     

Broadband 10 Gb/s operation of graphene electro‐absorption modulator on silicon

High performance integrated optical modulators are highly desired for future optical interconnects. The ultra‐high bandwidth and broadband operation potentially offered by graphene based electro‐absorption modulators has attracted a lot of attention in the photonics community recently. In this work, we theoretically evaluate the true potential of such modulators and illustrate this with experimental results for a silicon integrated graphene optical electro‐absorption modulator capable of broadband 10 Gb/s modulation speed. The measured results agree very well with theoretical predictions. A low insertion loss of 3.8 dB at 1580 nm and a low drive voltage of 2.5 V combined with broadband and athermal operation were obtained for a 50 μm‐length hybrid graphene‐Si device. The peak modulation efficiency of the device is 1.5 dB/V. This robust device is challenging best‐in‐class Si (Ge) modulators for future chip‐level optical interconnects.     

Polarization management for silicon photonic integrated circuits

Polarization management is very important for photonic integrated circuits (PICs) and their applications. Due to geometrical anisotropy and fabrication inaccuracies, the characteristics of the guided transverse‐electrical (TE) and transverse‐magnetic (TM) modes are generally different. Polarization‐dependent dispersion and polarization‐dependent loss are such manifestations in PICs. These issues become more severe in high index contrast structures such as nanophotonic waveguides made of silicon‐on‐insulator (SOI), which has been regarded as a good platform for optical interconnects because of the compatibility with CMOS processing. Recently, polarization division multiplexing (PDM) with coherent detection using silicon photonics has also attracted much attention. This trend further highlights the importance of polarization management in silicon PICs. The authors review their work on polarization management for silicon PICs using the polarization independence and polarization diversity methods. Polarization issues and solutions in PICs made of SOI nanowires and ridge waveguides are discussed.     

Semiconductor optical amplifiers at 2.0‐µm wavelength on silicon

A semiconductor optical amplifier at 2.0‐µm wavelength is reported. This device is heterogeneously integrated by directly bonding an InP‐based active region to a silicon substrate. It is therefore compatible with low‐cost and high‐volume fabrication infrastructures, and can be efficiently coupled to other active and passive devices in a photonic integrated circuit. On‐chip gain larger than 13 dB is demonstrated at 20 °C, with a 3‐dB bandwidth of ∼75 nm centered at 2.01 µm. No saturation of the gain is observed for an on‐chip input power up to 0 dBm, and on‐chip gain is observed for temperatures up to at least 50 °C. This technology paves the way to chip‐level applications for optical communication, industrial or medical monitoring, and non‐linear optics.

     

High efficiency,small size,and large bandwidth vertical interlayer waveguide coupler

Since the advent of three-dimensional photonic integrated circuits,the realization of efficient and compact optical interconnection between layers has become an important development direction.A vertical interlayer coupler between two silicon layers is presented in this paper.The coupling principle of the directional coupler is analyzed,and the traditional method of using a pair of vertically overlapping inverse taper structures is improved.For the coupling of two rectangular waveguide layers,a pair of nonlinear tapers with offset along the transmission direction is demonstrated.For the coupling of two ridge waveguide layers,a nonlinear taper in each layer is used to achieve high coupling efficiency.The simulation results show that the coupling efficiency of the two structures can reach more than 90%in a wavelength range from 1500 nm to 1650 nm.Moreover,the crosstalk is reduced to less than-50 d B by using multimode waveguides at intersections.The vertical interlayer coupler with a nonlinear taper is expected to realize the miniaturization and dense integration of photonic integrated chips.     

Plasmonic Metasurfaces Enabled Ultra‐Compact Broadband Waveguide TM‐Pass Polarizer

Silicon waveguide polarizers offer a simple yet robust approach to address the polarization‐dependent issue of silicon‐based optical components, and hence have found numerous applications in silicon photonics. However, the available silicon waveguide polarizers suffer from the issue of large device footprint, high insertion loss (IL), and/or fabrication complexities. Here, a silicon waveguide transverse magnetic (TM)‐pass polarizer is constructed by coating a silicon waveguide with an ultra‐thin plasmonic metasurface structure that is capable of guiding slow surface wave (SW) mode. The transverse electric (TE) waveguide mode can be converted into SW mode with the involvement of metasurfaces, and hence is intrinsically absorbed and forbidden to pass, while the TM waveguide mode can be well guided due to little influence. A typical metasurface polarizer with an ultra‐short length of 2.4 µm enables the IL of 28.16 dB for the TE mode, and that of 0.53 dB for the TM mode at 1550 nm. Multiple‐band TM‐pass polarizers can be obtained by cascading two or more different metasurface‐coated silicon waveguides along the propagation direction, and a dual‐band TM‐pass polarizer is demonstrated with the IL being of 19.21 and 29.09 dB for the TE mode at 1310 and 1550 nm, respectively.     

III‐V/silicon photonics for on‐chip and intra‐chip optical interconnects

In this paper III‐V on silicon‐on‐insulator (SOI) heterogeneous integration is reviewed for the realization of near infrared light sources on a silicon waveguide platform, suitable for inter‐chip and intra‐chip optical interconnects. Two bonding technologies are used to realize the III‐V/SOI integration: one based on molecular wafer bonding and the other based on DVS‐BCB adhesive wafer bonding. The realization of micro‐disk lasers, Fabry‐Perot lasers, DFB lasers, DBR lasers and mode‐locked lasers on the III‐V/SOI material platform is discussed.     

Highly Efficient Excitation of Surface Plasmons Using a Si Gable Tip

A compact and highly efficient technique to excite SPP mode at an Au/SiO₂ interface by using an engineered high index (silicon) gabled tip at the 1550 nm wavelength has been proposed. The optimized geometry of the Si tip enables a highly efficient excitation of the single interface SPP mode through near field interaction in an ultra‐compact setup. An experimental demonstration of the proposed scheme is also presented in the paper which converts 25.5% of the total input power to an SPP mode. With an improved fabrication, this efficiency can reach as high as 52%. The device is compact, facilitates on‐chip excitation of the SPP, its fabrication is compatible with the standard Si fabrication processes, and, as such it is expected to be very useful in the design of future integrated photonic circuits as well as integrated sensors. Also, this scheme can find applications in studying nonlinear characteristics of materials.     

All-dielectric bowtie waveguide with deep subwavelength mode confinement

Plasmonic waveguides and conventional dielectric waveguides have favorable characteristics in photonic integrated circuits. Typically, plasmonic waveguides can provide subwavelength mode confinement, as shown by their small mode area, whereas conventional dielectric waveguides guide light with low loss, as shown by their long propagation length. However, the simultaneous achievement of subwavelength mode confinement and low-loss propagation remains limited. In this paper, we propose a novel design of an alldielectric bowtie waveguide, which simultaneously exhibits both subwavelength mode confinement and theoretically lossless propagation. Contrary to traditional dielectric waveguides, where the guidance of light is based on total internal reflection, the principle of the all-dielectric bowtie waveguide is based on the combined use of the conservation of the normal component of the electric displacement and the tangential component of the electric field, such that it can achieve a mode area comparable to its plasmonic counterparts. The mode distribution in the all-dielectric bowtie waveguide can be precisely controlled by manipulating the geometric design. Our work shows that it is possible to achieve extreme light confinement by using dielectric instead of lossy metals.     

Low‐loss aluminium nitride thin film for mid‐infrared microphotonics

Mid‐infrared (mid‐IR) microphotonic devices including (i) straight/bent waveguides and (ii) Y‐junction beam splitters are developed on thin films of CMOS‐compatible sputter deposited aluminum nitride (AlN)‐on‐silicon. An optical loss of 0.83 dB/cm at λ = 2.5 µm is achieved. In addition, an efficient mid‐IR 50:50 beam splitter is demonstrated over 200 nm spectral bandwidth along with a <2% power difference between adjacent channels. With the inherent advantage of an ultra‐wide transparent window (ultraviolent to mid‐IR), our AlN mid‐IR platform can enable broadband optical networks on a chip.     

Low-Temperature Sputtered Ultralow-Loss Silicon Nitride for Hybrid Photonic Integration

Silicon-nitride-on-insulator (Si₃N₄) photonic circuits have seen tremendous advances in many applications, such as on-chip frequency combs, Lidar, telecommunications, and spectroscopy. So far, the best film quality has been achieved with low pressure chemical vapor deposition (LPCVD) and high-temperature annealing (1200°C). However, high processing temperatures pose challenges to the cointegration of Si₃N₄ with pre-processed silicon electronic and photonic devices, lithium niobate on insulator (LNOI), and Ge-on-Si photodiodes. This limits LPCVD as a front-end-of-line process. Here, ultralow-loss Si₃N₄ photonics based on room-temperature reactive sputtering is demonstrated. Propagation losses as low as 5.4 dB m⁻¹ after 400°C annealing and 3.5 dB m⁻¹ after 800°C annealing are achieved, enabling ring resonators with highest optical quality factors of > 10 million and an average quality factor of 7.5 million. To the best of the knowledge, these are the lowest propagation losses achieved with low temperature Si₃N₄. This ultralow loss enables the generation of microresonator soliton frequency combs with threshold powers of 1.1 mW. The introduced sputtering process offers full complementary metal oxide semiconductor (CMOS) compatibility with front-end silicon electronics and photonics. This could enable hybrid 3D integration of low loss waveguides with integrated lasers and lithium niobate on insulator.     

Hybrid vertical‐cavity laser with lateral emission into a silicon waveguide

We experimentally demonstrate an optically‐pumped III‐V/Si vertical‐cavity laser with lateral emission into a silicon waveguide. This on‐chip hybrid laser comprises a distributed Bragg reflector, a III‐V active layer, and a high‐contrast grating reflector, which simultaneously funnels light into the waveguide integrated with the laser. This laser has the advantages of long‐wavelength vertical‐cavity surface‐emitting lasers, such as low threshold and high side‐mode suppression ratio, while allowing integration with silicon photonic circuits, and is fabricated using CMOS compatible processes. It has the potential for ultrahigh‐speed operation beyond 100 Gbit/s and features a novel mechanism for transverse mode control.

     

基于自相位调制效应的硅基中红外全光二极管

信号单向导通器件是集成光学中一种重要的基本元件,而中红外波段在空间遥感,光谱分析等领域都有极其重要的应用.本文提出了一种由两个硅基微环谐振腔构成,基于自相位调制效应的硅基中红外全光二极管,并利用数值模拟的方法进行了分析.结果表明,在输入光强为0.5 mW到20 mW之间时,其非互易导通率可以大于20 dB,且正向透过时损耗小于10 dB.此外,本文还讨论了环形谐振腔中的线性吸收率,以及双稳态效应对结果的影响.     

Hybrid photonic circuit for multiplexed heralded single photons

A key resource for quantum optics experiments is an on‐demand source of single and multiple photon states at telecommunication wavelengths. This letter presents a heralded single photon source based on a hybrid technology approach, combining high efficiency periodically poled lithium niobate waveguides, low‐loss laser inscribed circuits, and fast (>1 MHz) fibre coupled electro‐optic switches. Hybrid interfacing different platforms is a promising route to exploiting the advantages of existing technology and has permitted the demonstration of the multiplexing of four identical sources of single photons to one output. Since this is an integrated technology, it provides scalability and can immediately leverage any improvements in transmission, detection and photon production efficiencies.     

硅基光导波结构的特性研究

研究了在硅基底上采用等离子增强化学汽相淀积（ＰＥＣＶＤ）氮氧化硅（ＳｉＯＮ）所形成的包括通道波导、条汉志、倒脊型波导等多种结构的特性，通过对测试结果进一步分析，总结出有关结论并提出一些改进方法。