Silicon Nanowaveguides Research Articles

Impact of propagation losses on photon-pair generation in silicon

We present a semi-classical numerical model to estimate the impact of linear scattering loss and nonlinear absorption losses on the biphoton flux and their quantum correlations generated via spontaneous four-wave mixing in silicon nanowaveguides. The counter-intuitive observed enhancement of pair correlations with increasing loss is attributed to the dominant effect of reduced accidental counts from multiphoton generation, with a corresponding trade-off for the source brightness. Silicon nanowaveguides are shown to be capable of generating highly correlated paired photons with coincidental-to-accidental ratio as high as ∼3400 and spectral brightness 2.8×104 pairs s−1 GHz−1 mW−1, even in the presence of linear propagation loss of 1 dB cm−1 and nonlinear losses (two-photon absorption and free-carrier absorption), over a length of 1 cm with an input pump power of 10 mW. Loss and the corresponding Langevin noise are modeled using distributed beam splitters along the waveguide length to encapsulate the phenomenological coupling to the background reservoir (vacuum fluctuations). The proposed numerical model is more general compared to previous analytical models, particularly for including dispersion and wavelength-dependent losses, and more accurate for noise estimation in the high-photon-flux regime such as optical parametric amplifiers and squeezed state generation.

Ultracompact single-layer optical MEMS accelerometer based on evanescent coupling through silicon nanowaveguides

In this paper, a novel optical MEMS accelerometer is proposed based on evanescent coupling between parallel silicon nanowaveguides. The coupling length between nanowaveguides changes due to the input acceleration, leading to a great change of coupling efficiency. As a result, the applied acceleration can be obtained by measuring the transmission of waveguiding light. Simulation results with optical displacement sensing sensitivity of 32.83%/upmu m within measurement range of 1.68 g is obtained. This design shows high compactness with no need of assembly, suggesting great potential in applications such as integrated photonic circuits.

Wideband Rapidly Tunable Delay-Controlled Optoelectronic Oscillator

With the recent advancements in photonic integrated circuits, integrated RF photonics has increasingly been considered as a viable avenue to meet the demands of many emerging applications in telecommunication, sensing and signal processing. In this paper, an integrated, rapidly tunable, wideband optical delay line is used to implement an optoelectronic oscillator that can be continuously tuned from 594 MHz to 634 MHz. The oscillator exhibits a relatively flat modulation response at high frequencies (larger than 400 MHz), accommodating wideband fast frequency chirp generation with applications in radar and imaging. The on-chip variable delay line is characterized by continuously delaying an ultra-wide band 0.5-5 GHz monocycle pulse between 0-100 ps. The photonic integrated chip consists of silicon nanowaveguides, phase modulators, multimode interference coupler, a Mach-Zehnder interferometer, and a photodiode. The chip is fabricated on IME 180 nm Silicon Photonics platform and has an area of 0.55 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

Standing-wave spectrometry in silicon nano-waveguides using reflection-based near-field scanning optical microscopy**Project supported by National Key R&D Program of China (Grant No. 2017YFA0303800), National Natural Science Foundation of China (Grant No. 61575218), and Defense Industrial Technology Development Program, China (Grant No. JCKY201601C006).

Utilizing reflection-based near-field scanning optical microscopy (NSOM) to image and analyze standing-wave patterns, we present a characterization technique potentially suitable for complex photonic integrated circuits. By raster scanning along the axis of a straight nano-waveguide in tapping mode and sweeping wavelength, detailed information of propagating waves in that waveguide has been extracted from analyses in both space and wavelength domains. Our technique needs no special steps for phase stabilization, thus allowing long-duration and environment-insensitive measurements. As a proof-of-concept test, in a silicon single-mode waveguide with a few of etched holes, the locations and reflection strengths of the inner defects have been quantified. The measurement uncertainty of the reflection amplitude is less than 25% at current stage. Our technique paves the way for non-destructively diagnosing photonic circuits on a chip with sub-wavelength spatial resolution and detailed information extraction.

High-Q inverted silica microtoroid resonators monolithically integrated into a silicon photonics platform.

We report on the monolithic integration of a new class of reflown silica microtoroid resonators with silicon nanowaveguides fabricated on top of the silica film. Connectivity with other silicon photonics devices is enabled by inversion of the toroid geometry, defined by etching a circular opening rather than a disk in an undercut silica membrane. Intrinsic quality factors of up to 2 million are achieved and several avenues of process improvement are identified that can help attain the higher quality factors (> 108) that are possible in reflown microtoroids. Moreover, due to the microtoroid being formed by standard microfabrication and post-processing by local laser induced heating, these devices are in principle compatible with monolithic co-fabrication with other electro-optic components.

Imaging of guided waves using an all-fiber reflection-based NSOM with self-compensation of a phase drift.

A phase-resolved reflection-based near-field scanning optical microscopy (NSOM) technique with an original all-fiber configuration is presented. Our system consists of an intrinsically phase-stable common-path interferometer. The reflection from the waveguide input facet or from an integrated fiber Bragg grating is used as the reference beam. This arrangement effectively suppresses the phase drift caused by environmental fluctuations. By raster scanning a silicon atomic force microscope probe, we measure the complex near fields of the propagating and stationary waves in silicon nanowaveguides. Our robust, align-free, cost-effective, and shot-noise-limited near-field imaging technique paves the way for versatile optical characterizations of nanophotonic structures on a chip.

Optical free-carrier generation in silicon nano-waveguides at 1550 nm

We report on time-resolved pump and probe characterization of linear and nonlinear optical generation of free carriers in a silicon strip nano-waveguide at the 1550 nm communication band. Analytical expressions were developed to extract the carrier density averaged along the waveguide length from the measured free-carrier absorption for different input pump power levels. This allows us to discriminate the contributions from two-photon absorption (TPA) and single-photon absorption (SPA), obtaining TPA and SPA coefficients of (1.5 ± 0.1) cm/GW and (1.9 ± 0.1) m−1, respectively. Our results reveal that the effective TPA within the waveguide is higher than the value reported for bulk silicon. In addition, we find that for the waveguide under test, the carrier generation via SPA plays an important role up to ∼300 mW, and therefore, it must be taken into account to correctly assess free-carrier effects in silicon photonic devices.

Nanowaveguide Enhanced Photothermal Interferometry Spectroscopy

We report a new optical nanowaveguide enhanced photothermal (PT) interferometry spectroscopy method for trace molecule detection. Absorption of pump evanescent field of an optical nanowaveguide heats up the trace molecules surrounding the waveguide, causing the temperature of waveguide to rise via thermal conduction and modulating the refractive index and dimension of the nanowaveguide. The phase of a probe beam propagating through the same nanowaveguide is then modulated and can be detected with optic fiber interferometry. Numerical simulation with silica, cyclic transparent optical polymer, and silicon nanowaveguides with proper dimensions can achieve PT index modulation of 10 to over 8000 times that of the commercial HC-1550-02 photonic bandgap fiber. Experiments with 12-mm-long, 800-nm-diamter silica nanofiber demonstrated a lower detection limit of 600 parts per billion (ppb) acetylene at ambient conditions.

Frequency-degenerate parametric generation through IFWM effect in nanowaveguides

We investigate highly efficient frequency-degenerate parametric generation through inverse four-wave-mixing (IFWM) in silicon nanowaveguides, which exhibits distinctly from traditional FWM phenomenon and manifests itself as a unique process producing signal and idler photon pairs with frequencies at the center of two pumps. The influences of dispersion, nonlinear coefficient and frequency detuning on the IFWM process are numerically analyzed in detail. On this basis, the optimal condition for high gain IFWM and the nanowaveguide with high nonlinearity and large normal dispersion are proposed. These results substantiate the feasibility of such frequency-degenerate parametric generation in CMOS-compatible integrated platforms, which could find important potential in signal-processing systems for photonics networks and entangled qubits generation for quantum optics.

Nonlinear carrier dynamics in silicon nano-waveguides

The understanding of free-carrier dynamics in silicon photonic nano-waveguides and micro-cavities is fundamental to several nonlinear optical phenomena. Through time-resolved pump and probe experiments, a complex and nonlinear carrier recombination dynamics is revealed. Our results show that the carrier lifetime varies as the recombination evolves, with faster decay rates at the initial stages (with lifetime of ∼800 ps) and much slower lifetimes at later stages (up to ∼300 ns). The large surface-to-volume ratio in nano-waveguides enables clear observation of the effect of carrier trapping, manifesting as a decay curve that is highly dependent on the initial carrier density. Further, we demonstrate faster recombination rates by operating at high carrier density. Our results, along with a theoretical framework based on trap-assisted recombination statistics applied to nano-waveguides, can impact the dynamics of several nonlinear nanophotonic devices in which free carriers play a critical role, and open further opportunities to enhance the performance of all-optical silicon-based devices.

All-Optical Wavelength Conversion Based on Four-Wave Mixing in Dispersion-Engineered Silicon Nanowaveguides

We demonstrate experimentally all-optical wavelength conversion based on four-wave mixing in dispersion-engineered silicon nanowaveguides with a picosecond pulse pump. We find that the conversion efficiency is significantly limited by nonlinear losses induced by the two-photon absorption and freecarrier absorption. Using a picosecond pulse pump centered at 1,550 nm, we show that the input continuous-wave signals can efficiently be converted into a broadband idler pulse in silicon waveguides with various dimensions. Conversion efficiencies versus signal wavelengths are different for silicon waveguides with different dimensions due to the variation in the phase mismatch; we obtain a conversion efficiency of – 32 dB in silicon nanowaveguides with a length of 5.8 mm. Such on-chip optical wavelength converters can find important potential applications in highly-integrated optical circuits for all-optical ultrafast signal processing.

A Theoretical and Experimental Study of Silicon Y-Branch Micro-Loop Reflectors

Y-branch micro-loop reflectors based on silicon nano-waveguides are designed, fabricated, and tested for maximizing the reflectivity in this letter. Our theoretical analysis suggests that a maximum reflectivity and a suppressed wavelength-sensitivity can be achieved by optimizing the waveguide dimensions of the Y-branch, given the phase distortion of the Y-branch is eliminated. The micro-loop reflector is fabricated on a silicon-on-insulator wafer, and the experimental results agree with the numerical analysis. The highest reflectivity achieved is 99.1% in the design and 89% in the experiment. This optimized Y-branch micro-loop reflector acts as a high-reflectivity reflector for chip-scale laser systems and integrated optical sensors.

Phase Sensitive Parametric Process in Silicon Nano-Waveguides at C-Band Wavelengths

Using numerical and analytical tools, we compare different phase sensitive parametric schemes namely one pump and two pump schemes for silicon nano-waveguides. The conditions required to achieve phase sensitive gain are obtained, and it is shown that a real continuous wave phase sensitive parametric gain could be possible in the presence of two photon absorption.

Fabrication, Characterization and Loss Analysis of Silicon Nanowaveguides

Low loss silicon waveguides are the key to the realization of high performance photonic integrated circuits. In this paper, fabrication, characterization and loss analysis of silicon nanowaveguides are presented. Silicon nanowaveguides are fabricated on silicon-on-insulator (SOI) wafers with 0.13 μm complementary metal-oxide-semiconductor (CMOS) technology. To reduce the propagation loss, both photolithography and etching processes are optimized to make the waveguide sidewalls smooth. Propagation losses of 2.4 ± 0.2 and 0.59 ± 0.32 dB/cm are obtained at 1550 nm wavelength for TE and TM modes, respectively. A theoretical method is used to estimate the propagation losses for TE and TM modes. Scattering losses from both sidewalls and top/bottom surface are considered. The calculated results show that loss comes from sidewall roughness is the main source of propagation loss for TE mode while for TM mode, losses from both sidewall and top/bottom surface contribute comparably to the total propagation loss. The theoretically estimated propagation loss agrees well with the measured results.

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.

Ultrafast nonlinear optical studies of silicon nanowaveguides

Results of a self-consistent ultrafast study of nonlinear optical properties of silicon nanowaveguides using heterodyne pump-probe technique are reported. The two-photon absorption coefficient and free-carrier absorption effective cross-section were determined to be 0.68cm/GW, and 1.9x10(-17) cm2, respectively and the Kerr coefficient and free-carrier-induced refractive index change 0.32x10(-13) cm2/W, and -5.5x10(-21) cm3, respectively. The effects of the proton bombardment on the linear loss and the carrier lifetime of the devices were also studied. Carrier lifetime reduction from 330ps to 33ps with a linear loss of only 14.8dB/cm was achieved using a proton bombardment level of 10(15)/cm2.

Characterization of Nonlinear Optical Crosstalk in Silicon Nanowaveguides

We investigate optical crosstalk on a signal in a silicon nanowaveguide due to the presence of another signal by direct radio frequency crosstalk level measurements in a pump-probe configuration and by bit-error-rate-based characterization. We quantify this degradation as a function of the modulation frequency and power of the auxiliary signal. Our results indicate that two-photon and free-carrier absorption are primary facilitators of crosstalk in silicon nanowaveguides.

First Demonstration of a 10-Gb/s RZ End-to-End Four-Wave-Mixing Based Link at 1884 nm Using Silicon Nanowaveguides

We demonstrate a double-stage four-wave mixing (FWM) scheme in silicon nanowaveguides which allows effective optical time-division-multiplexed data generation and reception in the 2-μm region. The scheme is based on a first mixing stage which unicasts a high-speed return-to-zero stream from the C-band to 1884-nm, followed by a second mixing stage which wavelength converts the data from 1884-nm down to the O-band for detection. The 10-Gb/s data traverses an aggregate record distance of 909 nm in the cascaded wavelength-conversion and unicast stages, with a power penalty of 2.5 dB. This scheme effectively overcomes the lack of commercially-available high-performance sources and receivers at 2 μm by relying on telecommunication band components along with ultrabroad FWM silicon devices.

Continuous-wave mid-infrared frequency conversion in silicon nanowaveguides

We report the first demonstration of cw wavelength conversion from the telecommunications band to the mid-IR (MIR) region via four-wave mixing in silicon nanowaveguides. We measure a parametric bandwidth of 748 nm by converting a 1636 nm signal to produce a 2384 nm idler and show continuously tunable wavelength conversion from 1792 to 2116 nm. This report indicates that the advantages of silicon photonics may be leveraged to create devices for a large range of MIR applications that require cw operation.

Raman amplification of optical pulses in silicon nanowaveguides: Impact of spectral broadening of pump pulses

We consider the Raman amplification problem for silicon waveguides in the regime in which both the pump and signal pulses are relatively short but wide enough that their duration exceeds the phonon lifetime (about 3 ps in silicon). We use the coupled pump-signal equations for numerical simulations that include all competing nonlinear effects such as self- and cross-phase modulations, two-photon and free-carrier absorptions, and changes in the refractive index induced by the free carriers. However, numerical simulations do not provide much physical insight. For this reason, we also develop an approximate analytic approach for solving the Raman amplification problem. We introduce the concept of an effective Raman gain and show analytically how it depends on the pump bandwidth. As the pump spectrum broadens inside the silicon waveguide, the effective Raman gain is reduced considerably. We obtain an analytical form of the nonlinear phase accumulated during propagation inside a silicon waveguide and use it to calculate the total spectral broadening experienced by a pump pulse. Using this result, we can predict changes in the effective Raman gain as a function of pump pulse energy. A comparison of our predictions with the recent experimental data shows that our model is reasonable and captures the essential physics.