High-performance Photonic Devices Research Articles

Post-Growth Laser Annealing for High-Performance Ge Photodiodes on Si

Near-infrared (NIR) laser light is used to irradiate Ge epitaxial layers on Si as a post-growth annealing process with a low thermal budget for high-performance photonic device applications. In contrast to ordinary furnace/lamp annealing, where both of Ge and Si are heated, NIR laser irradiation enables a preferential annealing of Ge layers due to the optical absorption selectively in Ge. Ge pin photodiodes on Si, fabricated applying a preferential annealing of Ge with an NIR laser (1.07 µm in wavelength), show a significant reduction of dark leakage current with keeping high photodetection efficiencies, resulting from the reduction of threading dislocation density in Ge. The results indicate that the NIR laser annealing is promising to incorporate the fabrication of Ge devices at the back end of line in the Si process.

Silicon-Germanium-Silica Monolithic Photonic Integration Platform for High-Performance Optical Data Communication Systems

Recently, data communication systems are now facing an explosive traffic increase. In order to cope with such an explosive traffic increase, densely-integrated photonic circuits, which would provide compact, high-functionality, energy-efficient and cost-effective network modules, are required. The silicon (Si) photonic platform, which enables monolithic integration of compact photonic devices made of Si and germanium (Ge) on Si wafer, is one of the breakthrough technologies that satisfy these requirements. Waveguide cores on a scale of 100 nm and micrometer-order bending of silicon wire waveguides provide us ultra-compact integrated photonic devices with fast operation speed.However, the hurdles to practical applications of the Si photonic platform are still very high. In middle and long distance applications, for example, photonic devices with a large dynamic range, low optical losses, and low polarization dependence are required. The present Si photonic platform, however, can not provide such high-performance photonic devices. This is because the device performances are extremely sensitive to geometric errors. For example, the geometric tolerance required for passive photonic devices, such as wavelength filters for wavelength division multiplexing (WDM) systems, is far below that of the errors in the present fabrication technology.To overcome these obstacles, we have developed a photonic platform on which Si-, Ge- and silica-based photonic devices are monolithically integrated[1]. Figure 1 shows the conceptual structure of the Si-Ge-silica monolithic photonic platform. On this platform, Si wire waveguides and a Ge layer grown on them are mainly used for dynamic and active devices, which require compactness and high operation speed, while silica waveguides are used for high-performance passive devices. Thanks to the large fabrication tolerance of silica-based waveguide devices, we can build passive decvices with low loss, low crosstalk and low polarization dependece. The silica waveguide can also serve as an interface to external optical fiber. In the Si-Ge-silica monolithic integration, it is important to deposit index-controllable silica at a low temperature so as not to damage active/dynamic devices based on Si and Ge.On this platform, we have developed various photonic devices, such as silica-based arrayed-waveguide-grating wavelength filters (AWG), Si-based electrically-driven modulation devices, and Ge-based photodetectors. The silica-based AWG exhibits high performance in optical loss, crosstalk and polarization dependence. Moreover, the performance of active/dynamic devices has also been improved in order to meet practical applications. For example, the responsivity, frequency bandwidth and polarization-dependent loss of fabricated Ge photodetectors (PDs) are almost comparable to those of indium-phosphide-based high-performance devices. These photonic devices can be monolithically integrated on a Si chip. As an example, Fig. 2 shows an integrated AWG-PD for a 16-channel WDM receiver with a 1.6-nm channel spacing. Inter-channel crosstalk of the device is -22 dB, which is significantly lower than those based on silicon wire waveguides. Each channel works at a data rate of 25 Gbps, and total receiver capacity reaches 400 Gbps.Features of the photonic circuits based on the Si-Ge-silica photonic integration platform are compactness, CMOS compatible fabrication and robustness. The size of the integrated device is typically less than 1-cm square, which can be installed in a small package. The CMOS compatible fabrication process simplifies the assembly processes and reduces fabrication cost. The platform is also endurable against the flip-chip bonding of electronic circuits. For example, we have already mounted 4-channel high-speed electronic amplifiers for PDs on a photonic chip consisting of 4-channel wavelength filters and PDs.Thus, using the Si-Ge-silica monolithic photonic platform, we can construct compact, highly-functional high-density photonic-electronic integration devices for sustainable growth of data communications industries.[1] T. Hiraki et al., IEEE Photonics Journal, vol. 5, 4500407 (2013).

Radial Epitaxy of Silicon for Optoelectronic Applications

At the heart of the dramatic transition in semiconductor device technology from planar to three-dimensional (3D) architectures is the ability to shrink devices to nanoscale dimensions with near atomic level control of materials’ structure and composition. The 3D structures serve as building blocks for high-performance electronic and photonic devices such as photovoltaic cells, light-emitting diodes, multi-gate field effect transistors and require unprecedented control in dimensions and materials properties. Epitaxial crystal growth, along with lithographic and etching technologies, provides one of the enabling approaches to create such structures. The standard for epitaxy in electronics technology is chemical vapor deposition (CVD) growth, which enables preparation of 3D semiconducting nanomaterials with modulated composition and electrical dopant profiles, with silicon (Si) epitaxy being arguably the world’s best studied crystal growth system. It is thus important to understand Si epitaxy on nanoscale structures, since any change in crystal growth kinetics at small sizes would have a large effect on dimensional control from a technological perspective and would also be of great interest from a fundamental perspective for understanding atomic scale processes and fabricating novel structures. We show that Si epitaxial growth at the nanoscale becomes size dependent at dimensions significantly larger than the onset of thermodynamic limits, and that this previously unknown behavior within the mesoscopic size regime for one of the best known crystal growth systems is a general phenomenon. A monotonic reduction in homoepitaxial growth rate with size is observed for facet widths below certain scale and the results modeled by an area-dependent precursor incorporation rate. The presence of n and p type dopants results in even greater reductions in low temperature radial growth rates, and a critical thicknesses for Si single crystal radial epitaxy is found for phosphorus. The results provide new insights on the nature of CVD crystal growth at small dimensions and have significant implications for 3D nanoscale fabrication of technologically important Si electronic device architectures. Si radial p-i-n junction nanowire structure is a suitable platform to integrate fundamental understanding and applications since the structure provides an approach for realizing concurrent maximization of light absorption and photogenerated carrier separation in photovoltaic (PV) devices. Additionally, NW arrays embedding radial p-(i)-n junctions enhance light absorption significantly in wide range of wavelengths. Si radial p-i-n junctions consisted of core Si NWs and Si shells. Core Si NWs were prepared by top-down consisting of lithographic technique and deep reactive ion etching. After formation of Si NWs, the color of Si substrate in NW regions turned to be black from shiny metallic surface. The change of color indicates enhanced light absorption in visible wavelengths. Electrically doped Si radial shell was prepared by epitaxial growth on the surfaces of Si NWs via low-pressure chemical vapor deposition. PV characteristics of crystalline Si radial p-i-n junction arrays prepared by top-down approach and epitaxial shell growth were investigated by current–voltage measurements under dark and Air Mass 1.5G conditions. Though the unoptimal dimensions of NWs and top electrode configuration, the open circuit voltage, short circuit current density, and photoconversion efficiency of the device were 0.46 V, 40 mA/cm2, and 10%, respectively. The high short circuit current density comparable to that of commercialized Si PV cell indicates that efficient photogenerated generation/collection is occurred in Si radial p-i-n junction.

Structural and optical characteristics of Ge_1−xSnx/Ge superlattices grown on Ge-buffered Si(001) wafers

We report an investigation on low dimensional Ge1−xSnx/Ge heterostructures. A series of strained-layer Ge1−xSnx/Ge superlattices with various Sn contents up to a threshold value that affords a direct bandgap is achieved by the technique of low temperature growth using molecular beam epitaxy. The Sn composition, strain status, and crystallographic are systematically characterized by cross-sectional transmission electron microscope and x-ray diffraction. Optical absorption measurements were carried out at room temperature to determine the bandgap energies of the Ge1−xSnx/Ge superlattices. Analyzing the direct transition energies reveals the room-temperature quantum confinement in the Ge1−xSnx/Ge superlattices. Present investigation demonstrates the growth and the quantum confinement of Ge1−xSnx/Ge superlattices, moving an important step forward toward the development of high-performance photonic devices based on Sn-containing group-IV low-dimensional structures.

Dual-Wavelength Harmonically Mode-Locked Fiber Laser With Topological Insulator Saturable Absorber

Dual-wavelength passively harmonic mode locking (HML) operation is demonstrated in an erbium-doped fiber laser with a microfiber-based topological insulator (TI) saturable absorber. It was found that the dual-wavelength pulse-trains possess different HML orders due to the different cavity nonlinear effects experienced by the two wavelengths. By properly adjusting the cavity parameters, dual-wavelength HML pulses with repetition rates of 388 and 239 MHz were achieved. Moreover, wavelength switchable operation of dual-wavelength HML pulses was also obtained. The experimental results reveal that the microfiber-based TI could indeed be employed as a high-performance dual-function photonic device with the saturable absorption and high nonlinear effects for the applications in fields of ultrafast and nonlinear photonics.

Photonic crystal slab fabricated on the platform of lithium niobate-on-insulator

We report on a photonic crystal slab patterned on a 690 nm thick LiNbO3 thin film bonded to SiO2 on lithium niobate substrate. The transmission spectrum is measured and a broad and clear photonic bandgap ranging from 1335 to 1535 nm with a maximum extinction ratio of more than 20 dB is observed. The bandgap is simulated by plane wave expansion and 3D finite-difference time-domain methods. Such a deep and broad bandgap structure can be used to form high-performance photonic devices and circuits on the platform of lithium niobate-on-insulator.

Effects of SiO2 hard masks on si nanophotonic waveguide loss for photonic device integration

As the basic building block for photonic device integration, silicon nanophotonic waveguide requires low-loss propagation for high-performance ultra-compact photonic device. We experimentally study silicon dioxide hard masks grown by two different methods, i.e., thermal oxidation and plasma-enhanced chemical vapor deposition (PECVD) for silicon nano-waveguides fabrication and their effects on the propagation loss. It is found that the denser and smoother quality of thermally grown silicon dioxide increases the etch selectivity against silicon and reduces the line edge roughness transferred to the silicon nano-waveguide sidewalls, hence resulting in a lower loss as compared with the PECVD silicon dioxide hard mask. With thermally grown silicon dioxide as a hard mask, the silicon nano-waveguides loss can be halved for a 650-nm-wide nano-waveguide, and the loss is comparable with a waveguide fabricated with a resist etch mask.

Magneto-Optical Thin Films for On-Chip Monolithic Integration of Non-Reciprocal Photonic Devices.

Achieving monolithic integration of nonreciprocal photonic devices on semiconductor substrates has been long sought by the photonics research society. One way to achieve this goal is to deposit high quality magneto-optical oxide thin films on a semiconductor substrate. In this paper, we review our recent research activity on magneto-optical oxide thin films toward the goal of monolithic integration of nonreciprocal photonic devices on silicon. We demonstrate high Faraday rotation at telecommunication wavelengths in several novel magnetooptical oxide thin films including Co substituted CeO2−δ, Co- or Fe-substituted SrTiO3−δ, as well as polycrystalline garnets on silicon. Figures of merit of 3~4 deg/dB and 21 deg/dB are achieved in epitaxial Sr(Ti0.2Ga0.4Fe0.4)O3−δ and polycrystalline (CeY2)Fe5O12 films, respectively. We also demonstrate an optical isolator on silicon, based on a racetrack resonator using polycrystalline (CeY2)Fe5O12/silicon strip-loaded waveguides. Our work demonstrates that physical vapor deposited magneto-optical oxide thin films on silicon can achieve high Faraday rotation, low optical loss and high magneto-optical figure of merit, therefore enabling novel high-performance non-reciprocal photonic devices monolithically integrated on semiconductor substrates.

Complexity in electro-optic delay dynamics: modelling, design and applications

Nonlinear delay dynamics have found during the last 30 years a particularly prolific exploration area in the field of photonic systems. Besides the popular external cavity laser diode set-ups, we focus in this article on another experimental realization involving electro-optic (EO) feedback loops, with delay. This approach has strongly evolved with the important technological progress made on broadband photonic and optoelectronic devices dedicated to high-speed optical telecommunications. The complex dynamical systems performed by nonlinear delayed EO feedback loop architectures were designed and explored within a huge range of operating parameters. Thanks to the availability of high-performance photonic devices, these EO delay dynamics led also to many successful, efficient and diverse applications, beyond the many fundamental questions raised from the observation of experimental behaviours. Their chaotic motion allowed for a physical layer encryption method to secure optical data, with a demonstrated capability to operate at the typical speed of modern optical telecommunications. Microwave limit cycles generated in similar EO delay oscillators showed significantly improved spectral purity thanks to the use of a very long fibre delay line. Last but not least, a novel brain inspired computational principle has been recently implemented physically in photonics for the first time, again on the basis of an EO delay dynamical system. In this latter emerging application, the computed result is obtained by a proper 'read-out' of the complex nonlinear transients emerging from a fixed point, the transient being issued by the injection of the information signal to be processed.

Towards low energy consumption integrated photonic circuits based on Ge/SiGe quantum wells

AbstractDespite being an indirect bandgap material, germanium (Ge) recently appeared as a material of choice for low power consumption optical link on silicon. Thanks to a low energy difference between direct and indirect energy bandgap, optical transitions around the direct gap can be used to achieve strong electroabsorption or photodetection in a material already used in microelectronics circuits. However, many challenges have to be addressed such as the growth of germanium-rich structures on silicon or the modeling of these structures around both direct and indirect bandgaps. This paper will explore recent achievements in Ge/SiGe quantum wells structures. Quantum confined Stark effect has been studied for different quantum well designs and light polarization. Both absorption and phase variations have been characterized and will be reported. Carrier recombination processes is also an intense research topic, in order to evaluate the competition between direct and indirect band gap emission as a function of temperature. Main results and conclusion will be introduced. Finally, high performance photonic devices (modulator and photodetector) that have already been demonstrated will be presented. At the end the challenges faced by Ge/SiGe QW as a new photonic platform will be presented.

High Performance Photonic Devices for Multiplexing/Demultiplexing Applications in Multi Band Operating Regions

High Performance Photonic Devices for Multiplexing/Demultiplexing Applications in Multi Band Operating Regions

Low-voltage high-performance silicon photonic devices and photonic integrated circuits operating up to 30 Gb/s

We present high performance silicon photonic circuits (PICs) defined for off-chip or on-chip photonic interconnects, where PN depletion Mach-Zehnder modulators and evanescent-coupled waveguide Ge-on-Si photodetectors were monolithically integrated on an SOI wafer with CMOS-compatible process. The fabricated silicon PIC(off-chip) for off-chip optical interconnects showed operation up to 30 Gb/s. Under differential drive of low-voltage 1.2 V(pp), the integrated 1 mm-phase-shifter modulator in the PIC(off-chip) demonstrated an extinction ratio (ER) of 10.5dB for 12.5 Gb/s, an ER of 9.1dB for 20 Gb/s, and an ER of 7.2 dB for 30 Gb/s operation, without adoption of travelling-wave electrodes. The device showed the modulation efficiency of V(π)L(π) ~1.59 Vcm, and the phase-shifter loss of 3.2 dB/mm for maximum optical transmission. The Ge photodetector, which allows simpler integration process based on reduced pressure chemical vapor deposition exhibited operation over 30 Gb/s with a low dark current of 700 nA at -1V. The fabricated silicon PIC(intra-chip) for on-chip (intra-chip) photonic interconnects, where the monolithically integrated modulator and Ge photodetector were connected by a silicon waveguide on the same chip, showed on-chip data transmissions up to 20 Gb/s, indicating potential application in future silicon on-chip optical network. We also report the performance of the hybrid silicon electronic-photonic IC (EPIC), where a PIC(intra-chip) chip and 0.13μm CMOS interface IC chips were hybrid-integrated.

Condensation‐ and Crystallinity‐Controlled Synthesis of Titanium Oxide Films with Assessed Mesopores

Materials with controlled microstructures based on sol– gel science have been widely applied to intelligent adsorbents, catalyst supports, photocatalysts, and ceramics in highperformance electronic, photonic, and magnetic devices. Sol–gel reactions of metal alkoxides, such as hydrolysis and condensation, in general lead to the formation of inorganic oxide networks. Compositional design in the frameworks is possible in mixed alkoxide systems. Non-hydrolytic routes have often facilitated the fabrication of mixed metal oxides [1] and inorganic–organic hybrid materials. [2] Likewise, self-adjusted reactions between appropriate inorganic acidity and alkalinity pairs are a powerful method of constructing not only single and/or mixed metal oxides but also metal phosphates. [3] Although these studies have only focused on the initial formation of inorganic networks, subsequent condensation has not been discussed as adequately. Condensation reactions of transition-metal oxides are too fast and it is still difficult to control microstructures atomically in the final metal oxides. Control of the sol–gel reactions has been investigated further by using chemical modification of metal alkoxides with organic chelating agents; the hydrolysis reaction rates of the hydrophobic metal species are lower than those of the original precursors. [4] However, once the designed alkoxides have been hydrolyzed by the presence of water, it becomes difficult to manage subsequent condensation reactions between the metal species, which is a limitation of using sol–gel chemistry with highly reactive transition-metal chlorides and alkoxides. Herein we demonstrate an effective way for preventing

Ordered Arrays of Dual-Diameter Nanopillars for Maximized Optical Absorption

Optical properties of highly ordered Ge nanopillar arrays are tuned through shape and geometry control to achieve the optimal absorption efficiency. Increasing the Ge materials filling ratio is shown to increase the reflectance while simultaneously decreasing the transmittance, with the absorbance showing a strong diameter dependency. To enhance the broad band optical absorption efficiency, a novel dual-diameter nanopillar structure is presented, with a small diameter tip for minimal reflectance and a large diameter base for maximal effective absorption coefficient. The enabled single-crystalline absorber material with a thickness of only 2 μm exhibits an impressive absorbance of ∼99% over wavelengths, λ = 300-900 nm. These results enable a viable and convenient route toward shape-controlled nanopillar-based high-performance photonic devices.

A high performance photonic pulse processing device

This paper presents an all optical fiber based implementation of a hybrid analog-digital computational primitive that provides a basis for complex processing on high bandwidth signals. A natural implementation of a hybrid analog/digital photonic processing primitive is achieved through the integration of new nonlinear fiber, and exploitation of the physics of semiconductor device to process signals in unique ways. Specifically, we describe the use of a semiconductor optical amplifier to implement leaky temporal integration of a signal and a highly Ge-doped nonlinear fiber for thresholding. A straightforward correspondence between our computational primitive and leaky-integrate-and-fire neurons permits leveraging of a large body of research characterizing the computational capabilities of these devices and the emerging pulse processing computational paradigm as a means to implement practical signal processing algorithms in hybrid computing platforms. An experimental demonstration of the behavior of the pulse processing primitive is presented.

Field enhancement and power distribution characteristics of subwavelength-diameter terahertz hollow optical fiber

Fields in subwavelength-diameter terahertz hollow optical fiber (STHOF) can be intensified by large discontinuity of the electric field at high index contrast interfaces. The influences of fiber geometry and refractive index of the dielectric region on the fiber characteristics, such as power distribution, enhancement factor, have been discussed in detail. By appropriate design, the intensity in the central region of STHOF may be enhanced by a factor of greater than 1.5 compared with subwavelength-diameter terahertz fiber without the central hole and the loss can be reduced. For its compact structure and simple fabrication process, the fiber may be very useful in many miniaturized high performance and novel terahertz photonic devices.

Field and dispersion properties of subwavelength-diameter hollow optical fiber

We have investigated the basic properties of subwavelength- diameter hollow optical fiber with exact solutions of Maxwell's equations. The characteristics of modal field and waveguide dispersion have been studied. It shows that the subwavelength-diameter hollow optical fibers have interesting properties, such as enhanced evanescent field, local enhanced intensity in the hollow core and large waveguide dispersion that are very promising for many miniaturized high performance and novel photonic devices.

Property-tailorable PFCB-containing polymers for wavelength division devices

This paper demonstrates the application of property-tailorable perfluorocyclobutyl (PFCB) polymers to the fabrication of arrayed-waveguide grating (AWG) devices for use in wavelength division multiplexing (WDM) optical networks. This novel series of PFCB polymers exhibits low optical attenuation, high thermal stability, and tailorable refractive index, facilitating the design of high-performance photonic devices. The design, fabrication, and characterization of AWGs are reported in this paper. The fabricated devices show high thermal stability and low on-chip losses. Optimization of devices through the tailoring of material properties and fabrication process parameters is discussed. The successful fabrication of AWG highlights the potential of this material for other devices in photonic applications

Enhanced absorption and electro-optic Pockels effect of electrostatically self-assembled CdSe quantum dots

The spectrum and electro-optic properties of CdSe quantum dots are studied. Spectrum wavelength shifts that are due to the quantum size effect and to the electro-optic Stark effect are investigated. It is found that CdSe quantum dot-polymer composites formed by an electrostatic self-assembly (ESA) technique exhibit high internal electric fields. Using the second-order perturbation theory of the 1s-1s energy shift (Stark effect), we estimate the internal field of the ESA film to be as high as 2.6 x 10(8) V/m. This value results in a much higher absorption coefficient and electro-optic coefficients for ESA films than for their bulk crystal counterparts or for spin-coated film samples. The relationships among unusual spectra, film structure, and high electro-optic response are analyzed. These results are useful both for understanding the physical mechanisms of semiconductor quantum dots and for developing high-performance photonic devices.

Growth of high optical quality InAs quantum dots in InAlGaAs∕InP double heterostructures

Two methods were studied to improve the optical quality of InAs quantum dot nanostructures grown in lattice-matched InAlGaAs∕InP double heterostructures of a center wavelength around 1.55μm. By either inserting InAlAs∕InGaAs superlattices between the InAlGaAs waveguide and the upper InAlAs cladding layer, or depositing a tensile-strained InGaAs strain-balance layer after the quantum dot formation, the optical quality of the quantum dot samples is greatly improved and a strong room-temperature photoluminescence is observed. The InGaAs strain-balance layer can be used to reduce the overall strain of the heterostructure, which makes it possible to stack a large number of quantum dot layers to improve the uniformity of the dot size distribution and increase the optical gain volume for high performance photonic device applications.