Intra-chip Optical Interconnects Research Articles

Integrated optical beam steering device using switchable nanoantennas and a reflective metalens

In this paper, an integrated optical device is proposed in which a reflective meta-lens and five switchable nano-antennas are combined to provide optical beam steering at the standard telecommunication wavelength of 1550 nm. For this purpose, a graphene-based switchable power divider is designed and integrated with nano-antennas to control the flow of the light entering the device. To achieve a higher angular accuracy in the radiated beams, a new algorithm is proposed and utilized to optimize the location of feeding nano-antennas in accordance with the reflective meta-lens. In order to achieve a minimum fluctuation in the light intensity when the beams are rotated in the space, an algorithm is developed to select optimum unit cells for the engineered meta-lens. The whole device is numerically analyzed using Electromagnetic full-wave simulations illustrating an optical beam steering with high accuracy (better than 1 degree) in the beam direction, and a low variation (less than 1 dB) in the radiated light intensity. The proposed integrated device can be used for many applications such as inter- and intra-chip optical interconnects, optical wireless communication systems, and advanced integrated LIDARs.

Effect of Sputtering Parameters on the Optical Properties of Luminescent Eu-Doped SiOx Thin Films

Luminescent silicon (Si) based structures that are compatible with the existing complementary metal oxide semiconductor industry has drawn significant interest with the vision of developing an integrated optoelectronic platform on monolithic Si [1]. Such luminescent structures find several applications in inter- and intra-chip optical interconnects, microdisplay, biological detections, and solid-state lighting [2, 3]. The outstanding optical properties and chemical stability of rare earth materials make doping Si with rare earths a promising approach to obtain bright emission from Si-based structures [4].In this work, we have investigated the influence of sputtering power and argon (Ar) partial pressure on the optical properties and the composition of europium (Eu) doped silicon oxide (SiOx) thin films. For this purpose, the thin films were fabricated by electron cyclotron resonance plasma enhanced chemical vapor deposition (ECR-PECVD) with in-situ magnetron sputtering on a p-type 3" Si (100) substrate. Silane (SiH4) and oxygen (O2) were used as precursor gases, and a solid 99.9% pure Eu as the sputtering target. Ar was used as the sputtering gas and brought into the chamber through a gas injection ring. A set of samples was prepared by changing the Ar flow rate ranging from 10 to 40 cubic centimeters per minute at standard temperature and pressure (sccm) in steps of 10 sccm. Another set of samples was fabricated by varying the sputtering power from 20 to 30W in steps of 5W. Post-deposition annealing was performed at a higher temperature from 900°C to 1200°C in ambient nitrogen and a mixture of nitrogen (95%) and hydrogen (5%) atmosphere. The effect of hydrogen passivation has also been studied in this work. The compositional analysis was performed by Rutherford backscattering spectrometry (RBS) and elastic recoil detection analysis (ERDA). We found that the doping concentration was strongly influenced by the sputtering power, and it is slightly affected by the Ar partial pressure. The thickness, refractive index, and absorption coefficient of the thin films were obtained from variable angle spectroscopic ellipsometry (VASE) analysis. The dependency of the thickness and the optical parameters on sputtering power and Ar partial pressure was also analyzed. In addition, the luminescence properties of Eu doped SiOx films have been investigated. A bright white emission was observed due to the transition between 4f65d and 4f7energy levels of Eu2+ ion from all the films, which can be seen by the naked eye under daylight conditions. The relation between intensity and doping concentration was also discussed.[1] Lin, R. Huang, H. Wang, Y. Wang, Y. Zhang, Y. Guo, ... and H. Li, “Dense nanosized europium silicate clusters induced light emission enhancement in Eu-doped silicon oxycarbide films,” J. Alloys Compd., vol. 694, pp. 946–951, 2017, doi: 10.1016/j.jallcom.2016.10.132.[2] D. Thomson, A. Zilkie, J. E. Bowers, T. Komljenovic, G. T. Reed, L. Vivien, ... and J. M. Hartmann, , “Roadmap on silicon photonics,” J. Opt. (United Kingdom), vol. 18, no. 7, pp. 1–20, 2016, doi: 10.1088/2040-8978/18/7/073003.[3] H. Jeong, S. Lee, G. Y. Sung, and J. H. Shin, “Design and fabrication of Tb3+-doped silicon oxy-nitride microdisk for biosensor applications,” IEEE Photonics Technol. Lett., vol. 23, no. 2, pp. 88–90, 2011, doi: 10.1109/LPT.2010.2091268.[4] L. Li, J. Zheng, Y. Zuo, B. Cheng, and Q. Wang, “Strong Eu2+ light emission in Eu silicate through Eu3+ reduction in Eu2O3/Si multilayer deposited on Si substrates,” Nanoscale Res. Lett., vol. 8, no. 1, p. 194, 2013, doi: 10.1186/1556-276x-8-194.

Selective dry etching of GaAs/AlOx for realizing electrical isolation on monolithic integrated photonic crystal laser array

Extremely high etch selectivity of GaAs over AlOx is mandatory to fabricate a current blocking trench (CBT) between adjacent lasers in a photonic crystal (PhC) laser array for intra-chip optical interconnections. In this paper, selective etching of GaAs on AlOx with PhC structure is reported for the first time by using inductively coupled plasma etching with Cl2/BCl3-containing gas mixture. The etched profile of the CBT on GaAs/AlOx heterostructure with PhC structure was significantly different from that without PhC structure. By optimizing the etching conditions, the etching of the CBT was stopped at AlOx, and extremely high etch selectivity of GaAs over AlOx was achieved. Thus, we successfully fabricated the CBT in the PhC laser array. The horizontal electrical resistance between adjacent laser cavities with a 0.4 μm or wider CBT reached the instrumentation-limit of 100 MΩ, confirming the electrical isolation effect of the CBT.

Energy-Efficiency Switchable Grating Coupler for Intra-Chip Wireless Optical Interconnection

A switchable compact plasmonic grating coupler is proposed and demonstrated theoretically. It is excited by TM standing waves and can be used to modulate the intensity of radiated light by moving the position of standing waves. The coupler used for connection is composed of five gold strips and a reflector, and it’s footprint is only $2.19\,\,\mu \text{m}\,\,\times 3.19\,\,\mu \text{m}$ . The calculation results show that the bandwidth of coupler is 110 nm from 1.5 to $1.61~\mu \text{m}$ , and the extinction ratio can reach 20 dB at $\lambda = 1.55\,\,\mu \text{m}$ with the ON-radiation efficiency bigger than 82%. Meanwhile, the thermal loss rate is no more than 8%. The switchable coupler, with features of compact size, high energy efficiency, fast response, and ability of vertical light emission, can be used for dense optical interconnection systems. It provides a new approach for intra-chip wireless optical interconnection.

Photonic Crystal Circular Defect (CirD) Laser

We describe the design of photonic crystal circular defect (CirD) lasers to construct a compact optical module with a wavelength division multiplexing function for the application of inter-chip or intra-chip optical interconnects. Subsequently, we investigated the characteristics of CirD lasers including the quality factor of the cavity, the lasing threshold, and the modulation speed with a three-dimensional finite-difference time-domain method and two-dimensional rate equations. Finally, we demonstrated the single mode lasing and wavelength tuning behaviors of the CirD lasers using optical pumping technology under room-temperature continuous-wave conditions.

Intra-chip optical interconnection based on polarization division multiplexing photonic integrated circuit

We proposed a polarization division multiplexed (PDM) silicon photonic integrated circuit (PIC) suitable for intra-chip optical interconnection. The PDM PIC is composed of two micro-ring modulators, two gain peaked germanium photodetectors and the multiplexer as well as demultiplexer based on polarization rotator splitter. 20 Gb/s on-chip PDM system is successfully demonstrated with clear eye diagrams. Under 10−9 bit error ratio level, the power penalty is about 1 dB between the single and dual polarization tributes, which validates the superior link performance of the proposed PIC.

Enabling inter- and intra-chip optical wireless interconnect by the aid of hybrid plasmonic leaky-wave optical antennas

In this paper, we propose a new method to provide optical link in Photonic Integrated Circuits (PICs). The proposed method uses two hybrid plasmonic leaky-wave optical antennas, operating at the standard optical telecommunication wavelength of 1.55μm, to provide inter-chip interconnect between two layers in a photonic chip and also intra-chip interconnect between two different photonic ICs. Linearly tapered couplers are designed to couple the optical signal from the silicon waveguide to the hybrid plasmonic antennas. The performance of the proposed optical link is verified using numerical full wave simulation. The proposed structure is planar, and can be fabricated using standard CMOS technology which makes it the superior candidate for realization of future multi-layered Photonic Integrated Circuits.

Integrated Optical-Phased Array Nanoantenna System Using a Plasmonic Rotman Lens

An integrated linear optical-phased array with five switchable beams at a spacing of 15 $^\circ$ operating at the standard telecommunication wavelength of $1.55 \mu \text{m}$ is proposed, and numerically studied. To realize the desired phase shift between adjacent antenna elements, a plasmonic Rotman lens is designed. In order to conduct power into a selected input port, a graphene-based power splitter with switchable output is proposed. The antenna array is composed of six plasmonic nanoantennas designed in compatibility with the feeding plasmonic waveguides. The proposed integrated system can be used for applications such as optical wireless communication, inter- and intrachip optical interconnects, and low-cost integrated LIDAR systems.

Observation of the Uller-Zenneck wave.

The Uller-Zenneck wave has been theoretically predicted to exist at the planar interface of two homogeneous dielectric materials of which only one must be dissipative. Experimental confirmation of this century-old prediction was obtained experimentally by exciting the Uller-Zenneck wave as a Floquet harmonic of nonzero order at the periodically corrugated interface of air and crystalline silicon in the 400-to-900-nm spectral regime. Application for intrachip optical interconnects at 850 nm appears promising.

Transparent Ga2O3 Photodetectors for Harsh Optoelectronics

We fabricate fully transparent solar-blind DUV PD devices employing β-Ga2O3 as an active layer and indium zinc oxide (IZO) as the transparent electrodes, exhibiting the average transmittance up to 80% from visible to near IR wavelength region without image blurring. The β-Ga2O3 MSM PDs for the use in harsh environments show a very low dark current (~1 nA), no sign of breakdown even at a bias higher than 200 V, excellent thermal stability, and intrinsic solar blindness. The dark current of β-Ga2O3 MSM PDs under significantly different oxygen concentration in the ambiences is similar, indicating that the high inertness to surface effect due to superior crystallinity nature of β-Ga2O3. The working temperature is up to 700 K and the responsivity is 0.32 mA/W at 10 V bias under 185-nm illumination. β-Ga2O3 PDs can be fully recovered after 700-K operation, showing excellent thermal reliability and robustness. The intriguing optoelectronic and electrical properties of β-Ga2O3 promise a new generation of stable, solar-blind DUV PDs for the extremely harsh electronic applications, such as sensing, imaging, and intrachip optical interconnects in high-temperature environments.

Focus on silicon photonics

Si photonics has attracted much interest over the last decade as it allows integration of photonic and electronic devices on a small Si chip, providing high-capacity, energy-efficient, and cost-effective optical interconnects. Active optical cables based on Si photonics are now commercially available for rack-to-rack interconnects in supercomputers and data centers. Shorter reach optical interconnects, i.e., inter- and intra-chip optical interconnects, have been studied as the next challenge for higher speed computing. Science and technology of advanced materials is essential for rapid development in this field, and therefore it is very appropriate to publish this focus issue in STAM.This focus issue introduces state-of-the-art material and device technologies in Si photonics. The topics cover not only technologies for the optical interconnects but also optical sensing as a novel application.We are grateful to the authors who contributed to this focus issue, and hope that it will help researchers understand the frontiers of Si photonics, stimulating novel concepts in material and device technologies, as well as novel developments in and applications of Si photonics.

Energy‐efficient and temperature‐stable oxide‐confined 980 nm VCSELs operating error‐free at 38 Gbit/s at 85°C

980 nm vertical-cavity surface-emitting lasers (VCSELs) operating error-free at 38 Gbit/s at 85°C with record 177 fJ of dissipated heat per bit are presented. The VCSELs are particularly well suited for short, very-short and ultra-short-reach interboard, chip-to-chip and intrachip optical interconnects, where temperature stability, bandwidth and VCSEL density and the minimisation of waste heat are critical. Error-free operation at 40 Gbit/s at 75°C and at 42 Gbit/s at 25°C is achieved. To date, 980 nm VCSELs are the most energy-efficient of all VCSELs at 85°C at any wavelength.

Optical interconnect transmitter based on guided-wave silicon optical bench

An optical interconnect transmitter based on guided-wave silicon optical bench is demonstrated. The guided-wave silicon optical bench (GW-SiOB) is developed on a silicon-on-insulator (SOI) substrate. The three-dimensional guided-wave optical paths on the silicon optical bench are realized using trapezoidal waveguides monolithically integrated with 45° micro-reflectors. Such three-dimensional guided-w ave optical paths of SiOB would simplify and shrink the intra-chip optical interconnects located on a SOI substrate. The clearly open eye patterns operated at a data rate of 5 Gbps verifies the proposed GW-SiOB is suitable for intra-chip optical interconnects.

Enhanced UV emission of Y-doped ZnO nanoparticles

Y-doped ZnO (Zn1−xYxO) nanoparticles with different Y concentrations were synthesized by sol–gel method. The effects of Y concentration on the optical properties of the as-synthesized Zn1−xYxO nanoparticles were investigated. Photoluminescence (PL) measurements revealed that doping of Y in ZnO induced an evidence increase of UV emission. At Y concentration x=0.07, the UV emission intensity reached maximum, which is about 9 times higher than that of the compared pure ZnO nanoparticles. Exceeding this concentration, the formation of Y2O3 causes a decrease in the UV emission intensity. This enhanced UV emission enables Zn1−xYxO system potential applications in supersensitive UV detector sinimaging, photosensing, and intrachip optical interconnects.

Monolithic integration of III-V nanowire with photonic crystal microcavity for vertical light emission

A novel photonic structure formed by the monolithic integration of a vertical III-V nanowire on top of a L3 two-dimensional photonic crystal microcavity is proposed to enhance light emission from the nanowire. The impact on the nanowire spontaneous emission rate is evaluated by calculating the spontaneous emission factor β, and the material gain at threshold is used as a figure of merit of this vertical emitting nanolaser. An optimal design is identified for a GaAs nanowire geometry with r = 155 nm and L~1.1 μm, where minimum gain at threshold (gth~13×10³ cm⁻¹) and large spontaneous emission factor (β~0.3) are simultaneously achieved. Modification of the directivity of the L3 photonic crystal cavity via the band-folding principle is employed to further optimize the far-field radiation pattern and to increase the directivity of the device. These results lay the foundation for a new approach toward large-scale integration of vertical emitting nanolasers and may enable applications such as intra-chip optical interconnects.

An optically pumped nanophotonic InP/InGaAlAs optical amplifier integrated on a SOI waveguide circuit

A small footprint integrated optical amplifier on Silicon-on-insulator is proposed in this article. By choosing to use optical pumping to drive the device, electrical contacting is avoided and the active waveguide can be made as thin as 100 nm, maximizing the optical confinement in the quantum well layers. Furthermore, the optical pumping is done through the silicon waveguide layer to optimally use the pump light. This leads to a compact device with high gain. We show 8 dB gain in pulsed regime in a 100 μm long device using a peak pump power of only 4.5 mW, while a comparable gain using an electrically pumped device would require an order of magnitude higher power consumption. This is an important step towards a CMOS-compatible optical amplifier for intra-chip optical interconnects.

SOI-based trapezoidal waveguide with 45° microreflector for noncoplanar optical interconnect

A silicon on insulator (SOI)-based trapezoidal waveguide with a 45° reflector for noncoplanar optical interconnect is demonstrated. The proposed waveguide is fabricated on an orientation-defined (100) SOI substrate by using a single-step anisotropic wet-etching process. The optical performances of proposed waveguides are numerically and experimentally studied. Transmittance of -4.51 dB, alignment tolerance of ±20 μm, cross talk of -53 dB, and propagation loss of -0.404 dB/cm are achieved The proposed waveguide would be a basic element and suitable for the future intrachip optical interconnects.

Fabrication and Evaluation of Nano-Scale Optical Circuits Using Sol-Gel Materials With Photo-Induced Refractive Index Variation Characteristics

Nano-scale optical circuits with core thickness of ~ 230 nm and core width of ~ 1 mum were fabricated and evaluated, using the photo-induced refractive index variation sol-gel materials, whose refractive index gradually increases by UV light exposure and baking. Propagation loss of linear waveguides was 1.86 dB/cm for TE mode and 1.89 dB/cm for TM mode at 633 nm in wavelength, indicating that there were small polarization dependences of ~ 0.03 dB/cm. Spot sizes of guided beams along core width direction and along core thickness direction were, respectively, 0.6 and 0.3 mu m for both TE mode and TM mode. Bending loss of S-bending waveguides was reduced from 0.44 to 0.24 dB for TE mode with increasing the bending curvature radius from 5 to 60 mu m. Although the bending loss for TM mode was slightly higher than that for TE mode, the difference was less than 10%. Branching loss of Y-branching waveguides was reduced from 1.33 to 0.08 dB for TE mode, and from 1.34 to 0.12 dB for TM mode with decreasing the branching angle from 80deg to 20deg. From these results, it is concluded that the photo-induced refractive index variation sol-gel materials can realize miniaturized optical circuits with sizes of several tens of microns and guided beam confinement within a cross section area less than 1.0 mum2 with small polarization dependences, indicating potential applications to intra-chip optical interconnects.

Enhancing UV photoconductivity of ZnO nanobelt by polyacrylonitrile functionalization

UV photodetector fabricated using a single ZnO nanobelt (NB) has shown a photoresponse enhancement up to 750 times higher than that of a bare ZnO NB after coating with ∼20nm plasma polymerized acrylonitrile (PP-AN) nanoscale film. The mechanism for this colossal photoconductivity is suggested as a consequence of the efficient exciton dissociation under UV illumination due to enhanced electron transfer from valence band of ZnO NB to the PP-AN and then back to the conduction band of ZnO. This process has demonstrated an easy and effective method for improving the performance of the nanowire∕NB-based devices, possibly leading to supersensitive UV detector for applications in imaging, photosensing, and intrachip optical interconnects.

ZnO Nanowire UV Photodetectors with High Internal Gain

ZnO nanowire (NW) visible-blind UV photodetectors with internal photoconductive gain as high as G approximately 108 have been fabricated and characterized. The photoconduction mechanism in these devices has been elucidated by means of time-resolved measurements spanning a wide temporal domain, from 10-9 to 102 s, revealing the coexistence of fast (tau approximately 20 ns) and slow (tau approximately 10 s) components of the carrier relaxation dynamics. The extremely high photoconductive gain is attributed to the presence of oxygen-related hole-trap states at the NW surface, which prevents charge-carrier recombination and prolongs the photocarrier lifetime, as evidenced by the sensitivity of the photocurrrent to ambient conditions. Surprisingly, this mechanism appears to be effective even at the shortest time scale investigated of t < 1 ns. Despite the slow relaxation time, the extremely high internal gain of ZnO NW photodetectors results in gain-bandwidth products (GB) higher than approximately 10 GHz. The high gain and low power consumption of NW photodetectors promise a new generation of phototransistors for applications such as sensing, imaging, and intrachip optical interconnects.