Unpredicted Internal Geometric Reconfiguration of an Enclosed Space Formed by Heteroepitaxy.

Epitaxial lateral overgrowth (ELO) over a free-standing dielectric mask is an unexplored territory in selective epitaxy growth (SEG) of semiconductors. By shrinking the dielectric mask dimension to the micron scale, the growth fronts from ELO are able to converge and coalesce, thus providing the freedom to engineer the interfacial structure between the epi-layer and dielectric mask. We demonstrate, herein, anomalous adatom diffusion and migration at the Ge/SiO2 interface upon SEG on a Si (100) wafer. We find, depending on the oxide strip length, a polyhedral cavity or tunnel can form on the oxide layer. More importantly, we observe a thermally induced substantial internal surface reconfiguration process of Ge atoms that connects two tunnels and one cavity in order to form a single tunnel. Defect-free Ge above the oxide strips is obtained after coalescence. Our findings yield new insight into adatom migration in an enclosed space, and the cavity and tunnel show the first known three-dimensional geometric configuration in selective heteroepitaxial structures.

Compact spectrum splitter for laterally arrayed multi-junction concentrator photovoltaic modules.

A spectrum splitter based on micro-prism arrays is demonstrated for laterally arrayed multi-junction concentrator photovoltaic modules. The conjugate micro-optics design delivers high-transmission, efficient spectrum splitting with minimum aberration, a low profile, and low-cost fabrication, thus allowing large-scale production of micro-concentrator photovoltaic modules. A dispersive optic prototype based on a four-prism design is fabricated and characterized through outdoor measurements showing excellent agreement with our design model.

Loss reduction of silicon-on-insulator waveguides for deep mid-infrared applications.

We report that propagation loss of optical waveguides based on a silicon-on-insulator (SOI) material platform can be greatly reduced. Our simulations show that the loss, including SiO2 absorption and substrate leakage, but no scattering loss, is 0.024 and 0.53 dB/cm in the deep mid-infrared at 4.8 and 7.1 µm wavelengths, where the material absorption in SiO2 is 100 and 1000 dB/cm, respectively. The loss becomes negligible, compared to scattering loss in Si waveguides. This is enabled by using the TE10 mode in a pedestal waveguide. We also show that the TE10 mode can be excited in the proposed waveguide by the fundamental mode with a coupling efficiency of >94%. Low propagation loss, high coupling efficiency, and fabrication-friendly design would make it promising for practical use of SOI devices in the deep mid-infrared.

Mid-IR supercontinuum generated in low-dispersion Ge-on-Si waveguides pumped by sub-ps pulses.

Ge-on-Si is an attractive material platform for mid-IR broadband sources on a chip because of its wide transparency window, high Kerr nonlinearity and CMOS compatibility. We present a low-loss Ge-on-Si waveguide with flat and low dispersion from 3 to 11 µm, which enables a coherent supercontinuum from 2 to 12 µm, generated using a sub-ps pulsed pump. We show that 700-fs pump pulses with a low peak power of 400 W are needed to generate such a wide supercontinuum, and the waveguide length is around 5.35 mm.

Linear-regression-based approach for loss extraction from ring resonators.

This Letter proposes and demonstrates a linear-regression-based loss-extraction model that is applicable for all-pass rings and symmetrically and asymmetrically coupled add-drop rings. The model is derived by transforming the transmission spectra of a ring into linear relationships with no approximation. An all-pass ring resonator is fabricated to verify the model, and the experimental results indicate that the proposed model is more reliable than previously reported models [Opt. Express14, 3872 (2006)OPEXFF1094-408710.1364/OE.14.003872; Opt. Express15, 10553 (2007)OPEXFF1094-408710.1364/OE.15.010553; and J. Lightwave Technol.16, 1308 (1998)JLTEDG0733-872410.1109/50.701410].

Epitaxy and characterization of GaInP/AlInP light-emitting diodes on As-doped Ge/Si substrates.

We investigate the impact of threading dislocation density (TDD) and thermal conductivity of substrates on the performance of GaInP/AlInP light-emitting diodes (LEDs) for the integration of III-V optoelectronics on Si. We utilized an arsenic (As) doped Ge/Si substrate that showed a reduced TDD compared to undoped Ge/Si. Compared to LEDs on undoped Ge/Si, the leakage current density for LEDs on As-doped Ge/Si substrate is reduced by four orders of magnitude and the light output is increased six-fold. An increased junction temperature causes light output saturation for LEDs on bulk Ge at high injection current densities. The light output of LEDs on As-doped Ge/Si shows good linearity with injection current density and its junction temperature is ~25 ± 5 °C lower than that of LEDs on bulk Ge at high injection current densities due to better thermal conductivity of the Ge/Si substrate.

Bilayer dispersion-flattened waveguides with four zero-dispersion wavelengths.

We propose a new type of bilayer dispersion-flattened waveguides that have four zero-dispersion wavelengths. Low and flat dispersion can be achieved by using two different material combinations, with a much smaller index contrast as compared to the previously proposed slot-assisted dispersion-flattened waveguides. Without using a nano-slot, dispersion becomes less sensitive to waveguide dimensions, which is highly desirable for high-yield device fabrication. Ultra-low dispersion, high nonlinearity, and fabrication-friendly design would make it promising for practical implementation of nonlinear photonic functions. The proposed waveguide configuration deepens our understanding of the dispersion flattening principle.

Robust polarization-insensitive strip-slot waveguide mode converter based on symmetric multimode interference.

Strip-slot waveguide mode converters for TE0 have been widely investigated. Here we demonstrate a polarization-insensitive converter numerically and experimentally. The polarization-insensitive performance is achieved by matching the optical field distribution of the 2-fold image of the Multimode Interference (MMI) and the TE0 (TM0) mode of a slot waveguide. The working principle for this MMI-based mode converter is thoroughly analyzed with the quantitatively evaluated optical field overlap ratio that is theoretically derived from the orthonormal relation of eigenmodes. Based on the analysis, the MMI-based polarization-insensitive converters are then simulated and fabricated. The simulation and measurement results indicate that the proposed scheme is a robust design since it is not only polarization-insensitive but also wavelength-insensitive and fabrication-tolerant. Moreover, the mode converter is as small as 1.22 µm × 4 µm while the measured conversion efficiencies are 95.9% for TE0 and 96.6% for TM0. All these excellent properties make the proposed mode converter an ideal solution for coupling light between strip and slot waveguides when both TE and TM polarizations are considered.

Effect of a breather soliton in Kerr frequency combs on optical communication systems.

In this study, we numerically investigate the effect of Kerr-comb-generated breather soliton pulses on optical communication systems. The breather soliton pulse amplitude and spectrum envelope oscillate periodically in time. Simulations show that the spectrum of each comb line in the breather soliton state has multiple sub-teeth due to the periodic oscillation of the comb spectrum. In the simulation, the comb output is modulated with different formats. We find that the sub-teeth distort quadrature phase-shift-keyed signals but have less of an effect on on-off-keyed signals.

Athermal and flat-topped silicon Mach-Zehnder filters.

Athermal and flat-topped transmissions are the two main requirements for a silicon WDM filter. A Mach-Zehnder (MZ) filter which simultaneously satisfies these two requirements has been experimentally demonstrated in this paper. A combination of strip waveguide and hybrid strip-slot waveguide is introduced for athermalization, and two-stage interference is utilized for flat-topped transmission. The temperature dependent wavelength shift is measured to be ~-5 pm/K while the best 1 dB bandwidth is 5.5 nm with 14.7 nm free spectral range (FSR). The measured minimum insertion loss is only 0.4 dB with a device dimension of 170 µm × 580 µm. Moreover, Such a MZ filter is compatible with the state-of-art CMOS-fabrication process and its minimum feature size is as large as 200 nm.

Corrigendum: On-chip light sources for silicon photonics.

[This corrects the article DOI: 10.1038/lsa.2015.131.].

Soliton breathing induced by stimulated Raman scattering and self-steepening in octave-spanning Kerr frequency comb generation.

We investigate the impact of stimulated Raman scattering (SRS) and self-steepening (SS) on breather soliton dynamics in octave-spanning Kerr frequency comb generation. SRS and SS can transform chaotic fluctuations in cavity solitons into periodic breathing. Furthermore, with SRS and SS considered, bandwidth of the soliton breathes more than two times stronger. The simultaneous presence of SRS and SS also make the soliton breathe slower and degrades the coherence of the soliton.

Nonlinear conversion efficiency in Kerr frequency comb generation.

We analytically and numerically investigate the nonlinear conversion efficiency in ring microresonator-based mode-locked frequency combs under different dispersion conditions. Efficiency is defined as the ratio of the average round trip energy values for the generated pulse(s) to the input pump light. We find that the efficiency degrades with growth of the comb spectral width and is inversely proportional to the number of comb lines. It depends on the cold-cavity properties of a microresonator only and can be improved by increasing the coupling coefficient. Also, it can be increased in the multi-soliton state.

Post-fabrication trimming of athermal silicon waveguides.

We experimentally demonstrate the post-fabrication trimming of polymer-coated athermal silicon waveguides by exploiting the photosensitivity of As(2)S(3) chalcogenide glass to near-bandgap visible light. Our technique enables compensation of fabrication tolerances and modification of specific circuit functionalities after fabrication. Moreover, our athermal and trimmable waveguide technology is highly resilient to high optical power, and thus extremely appealing for nonlinear applications. Finally, it enables to fix the absolute wavelength and spectral response of silicon devices with extremely low dependence from temperature and power.

Generation of two-cycle pulses and octave-spanning frequency combs in a dispersion-flattened micro-resonator.

We show that octave-spanning Kerr frequency combs with improved spectral flatness of comb lines can be generated in dispersion-flattened microring resonators. The resonator is formed by a strip/slot hybrid waveguide, exhibiting a flat and low anomalous dispersion between two zero-dispersion wavelengths that are separated by one octave from near-infrared to mid-infrared. Such flattened dispersion profiles allow for the generation of mode-locked frequency combs, using relatively low pump power to obtain two-cycle cavity solitons on a chip, associated with the octave-spanning comb bandwidth. The wavelength dependence of the optical loss and of the coupling coefficient and thus wavelength dependent Q-factor are also considered.

Direct-write thermocapillary dewetting of polymer thin films by a laser-induced thermal gradient.

A positive-tone 2D direct-write technique that can achieve sub-wavelength patterning by non-linear overlap effects in a conventional polymer system is described. The technique involves relatively inexpensive free-space optics, skips the usual development step, and promises the possibility of a lithographic method that is solvent-free.

Infrared absorption of n-type tensile-strained Ge-on-Si.

We analyze the IR absorption of tensile-strained, n-type Ge for Si-compatible laser applications. A strong intervalley scattering from the indirect L valleys to the direct Γ valley in n+ Ge-on-Si is reported for the first time to our knowledge. The intervalley absorption edge is in good agreement with the theoretical value. On the other hand, we found that the classical λ2-dependent Drude model of intravalley free-carrier absorption (FCA) breaks down at λ<15 µm. A first-principle model has to be employed to reach a good agreement with the experimental data. The intravalley FCA loss is determined to be <20 cm(-1) for n=4×10(19) cm(-3) at λ=1.5-1.7 µm, an order lower than the results from Drude model. The strong Lâ†’Γ intervalley scattering favors electronic occupation of the direct Γ valley, thereby enhancing optical gain from the direct gap transition of Ge, while the low intravalley free-electron absorption at lasing wavelengths leads to low optical losses. These two factors explain why the first electrically pumped Ge-on-Si laser achieved a higher net gain than the theoretical prediction using λ2-dependent free-carrier losses of bulk Ge and indicate the great potential for further improvement of Ge-on-Si lasers.

Co-polymer clad design for high performance athermal photonic circuits.

Ubiquitous, low power consumption and high bandwidth density communication will require passive athermal optical filters for WDM transceivers in Si-CMOS architecture. Two silicon-polymer composite structures, deposited using initiated chemical vapor deposition (iCVD), poly(perfluorodecyl acrylate) (pPFDA) and poly(perfluorodecyl acrylate-co-divinyl benzene) p(PFDA-co-DVB), are analyzed as candidates for thermal compensation. The addition of DVB to a fluorinated acrylate backbone reduces the C-F bond density, increases the density in the copolymer and thereby increases refractive index. The addition of DVB also increases the volume expansion coefficient of the copolymer, resulting in an increased thermo-optic (TO) coefficient for the copolymer system. The increased index and TO coefficient of the co-polymer gives improved bend loss, footprint and FSR performance for athermal silicon photonic circuits.

Light trapping limits in plasmonic solar cells: an analytical investigation.

We analytically investigate the light trapping performance in plasmonic solar cells with Si/metallic structures. We consider absorption enhancements for surface plasmon polaritons (SPPs) at planar Si/metal interfaces and localized surface plasmon resonances (LSPRs) for metallic spheres in a Si matrix. We discover that the enhancement factors at Si/metal interfaces are not bound to the conventional Lambertian limit, and strong absorption can be achieved around plasmonic resonant frequencies. In addition, those enhancements are greatly reduced as the fields decay away from the Si/metal interfaces. Therefore, localized plasmonic resonances can be used as efficient light trapping schemes for ultrathin Si solar cells (< 50 nm), while photonic guided mode enhancement is more appropriate for thicker films.