Slow-light Waveguides Research Articles

Slow-light effect in a silicon photonic crystal waveguide as a sub-bandgap photodiode.

We demonstrate a Si sub-bandgap photodiode in a photonic crystal slow-light waveguide that operates at telecom wavelengths and can be fabricated using a Ge-free, standard Si-photonics CMOS process. In photodiodes based on absorption via mid-bandgap states, the slow-light enhancement enables performance that is well balanced among high responsivity, low dark current, high speed, wide working spectrum, and CMOS-process compatibility, all of which are otherwise difficult to achieve simultaneously. Owing to the slow-light effect and supplemental gain at a high reverse bias, the photodiode shows a responsivity of 0.15 A/W with a low dark current of 40 nA, which is attributed to no particular processes such as ion implantation and excess exposure of the Si surface. The maximum responsivity was 0.36 A/W. The modest gain allows for sufficient frequency bandwidth to observe an eye opening at up to 30 Gb/s.

应用于相控阵雷达的光子晶体慢光波导光实时延迟线

应用于相控阵雷达的光子晶体慢光波导光实时延迟线

Phase-matched second-harmonic generation in slow-light photonic crystal waveguides

We present an analytical description of phase-matched second-harmonic generation in photonic crystal waveguides in the presence of loss. In particular, we investigate the case where the second-harmonic modes suffer from radiative losses. We use the adjoint field formalism to develop a coupled-mode theory that uses the quasinormal Bloch modes as the basis for modal expansion. To test our analytical description numerically, we propose a design in a lithium niobate photonic crystal slab waveguide, where a slow-light mode at the fundamental harmonic frequency is phase matched to a leaky mode at the second-harmonic frequency. The results of the numerical experiment agree with our analytical predictions.

Quantum theory of the emission spectrum from quantum dots coupled to structured photonic reservoirs and acoustic phonons

Electron-phonon coupling in semiconductor quantum dots plays a significant role in determining the optical properties of excited excitons, especially the spectral nature of emitted photons. This paper presents a comprehensive theory and analysis of emission spectra from artificial atoms or quantum dots coupled to structured photon reservoirs and acoustic phonons, when excited with incoherent pump fields. As specific examples of structured reservoirs, we chose a Lorentzian cavity and a coupled cavity waveguide, which are of current experimental interest. For the case of optical cavities, we directly compare and contrast the spectra from three distinct theoretical approaches to treat electron-phonon coupling, including a Markovian polaron master equation, a non-Markovian phonon correlation expansion technique and a semiclassical linear susceptibility approach, and we point out the limitations of these models. For the cavity-QED polaron master equation, we give closed form analytical solutions to the phonon-assisted scattering rates in the weak excitation approximation, fully accounting for temperature, cavity-exciton detuning and cavity dot coupling. We show explicitly why the semiclassical linear susceptibility approach fails to correctly account for phonon-mediated cavity feeding. For weakly coupled cavities, we calculate the optical spectra using a more general reservoir approach and explain its differences from the above approaches in the low Q limit of a Lorentzian cavity. We subsequently use this general reservoir to calculate the emission spectra from quantum dots coupled to slow-light photonic crystal waveguides, which demonstrate a number of striking photon-phonon coupling effects. Our quantum theory can be applied to a wide range of photonic structures including photonic molecules and coupled-cavity waveguide systems.

Theory and experiments of disorder-induced resonance shifts and mode-edge broadening in deliberately disordered photonic crystal waveguides

We study both theoretically and experimentally the effects of introducing deliberate disorder in a slow-light photonic crystal waveguide on the photon density of states. We first introduce a theoretical model that includes both deliberate disorder through statistically moving the hole centres in the photonic crystal lattice and intrinsic disorder caused by manufacturing imperfections. We demonstrate a disorder-induced mean blueshift and an overall broadening of the photonic density of states for various amounts of deliberate disorder. By comparing with measurements from a GaAs photonic crystal waveguide, we find good qualitative agreement between theory and experiment which highlights the importance of carefully including local field effects for modelling high-index contrast perturbations. Our work also demonstrates the importance of using asymmetric dielectric polarizabilities for modelling positive and negative dielectric perturbations when modelling a perturbed dielectric interface in photonic crystal platforms.

One-chip integration of optical correlator based on slow-light devices

We propose and demonstrate an on-chip optical correlator, in which two types of photonic crystal slow-light waveguides are integrated and operated as an optical delay scanner and a two-photon-absorption photodetector. The footprint of the device, which was fabricated using a CMOS-compatible process, was 1.0 × 0.3 mm(2), which is substantially smaller than that of conventional optical correlators with free-space optics. We observed optical pulses using this device and confirmed the correspondence of pulse waveforms with those observed using a commercial correlator when the pulse width was 5-7 ps. This device will achieve one-chipping of an optical correlator and related measurement instruments.

Investigation of light trapping effect in hyperbolic metamaterial slow-light waveguides

A hyperbolic metamaterial waveguide can reduce the group velocity of light down to zero by tuning a waveguide structure. The light trapping capability of a tapered hyperbolic metamaterial waveguide is comprehensively studied in this work. Although a pulse of light cannot be trapped forever in such a waveguide, the duration that light remains trapped can be flexibly prolonged by adjusting the volume filling ratio of the metamaterial or the material dispersion, in order to achieve a sufficiently long trapping time for practical applications.

Role of Bloch mode reshaping and disorder correlation length on scattering losses in slow-light photonic crystal waveguides

Intrinsic disorder in photonic crystal waveguides occurs via rapid fluctuations of the air-dielectric interface and is typically characterized by a quadratic mean surface roughness and a surface correlation length. We theoretically study the impact of correlation length on extrinsic scattering losses and discuss the numerical implementation for several different waveguide designs. The role of correlation length is found to be strongly influenced by the underlying Bloch modes which are dependent on waveguide design and frequency, and can thus be partly controlled via spatial-dispersion engineering. For most frequencies and waveguide designs, we find an asymptotical increase in losses as the correlation length increases; however, we show that for some frequencies and designs, a maximum scattering loss is achieved for a finite correlation length. Our results also demonstrate the importance of choosing an appropriate correlation function for modeling quickly varying disorder.

Slow-light enhanced subwavelength plasmonic waveguide refractive index sensors.

We introduce slow-light enhanced subwavelength scale refractive index sensors which consist of a plasmonic metal-dielectric-metal (MDM) waveguide based slow-light system sandwiched between two conventional MDM waveguides. We first consider a MDM waveguide with small width structrue for comparison, and then consider two MDM waveguide based slow light systems: a MDM waveguide side-coupled to arrays of stub resonators system and a MDM waveguide side-coupled to arrays of double-stub resonators system. We find that, as the group velocity decreases, the sensitivity of the effective index of the waveguide mode to variations of the refractive index of the fluid filling the sensors as well as the sensitivities of the reflection and transmission coefficients of the waveguide mode increase. The sensing characteristics of the slow-light waveguide based sensor structures are systematically analyzed. We show that the slow-light enhanced sensors lead to not only 3.9 and 3.5 times enhancements in the refractive index sensitivity, and therefore in the minimum detectable refractive index change, but also to 2 and 3 times reductions in the required sensing length, respectively, compared to a sensor using a MDM waveguide with small width structure.

Germanium nanopyramid arrays showing near-100% absorption in the visible regime

Solar energy is regarded as one of the most plentiful sources of renewable energy. An extraordinary light-harvesting property of a germanium periodic nanopyramid array is reported in this Letter. Both our theoretical and experimental results demonstrate that the nanopyramid array can achieve perfect broadband absorption from 500- to 800-nm wavelength. Especially in the visible regime, the experimentally measured absorption can even reach 100%. Further analyses reveal that the intrinsic antireflection effect and slow-light waveguide mode play an important role in the ultra-high absorption, which is helpful for the research and development of photovoltaic devices.

Post-process wavelength tuning of silicon photonic crystal slow-light waveguides.

Silicon photonic crystal waveguides have enabled a range of technologies, yet their fabrication continues to present challenges. Here, we report on a post-processing method that allows us to tune the operational wavelength of slow-light photonic crystal waveguides in concert with optical characterization, offsetting the effects of hole-radii and slab thickness variations. Our method consist of wet chemical surface oxidation, followed by oxide stripping. Theoretical modelling shows that the changes in optical behavior were predictable, and hence controlled tuning can be achieved by changing the number of processing cycles, where each cycle removes approximately 0.25 nm from all exposed surfaces, producing a blueshift of 1.6±0.1 nm in operating wavelength.

Unambiguous demonstration of soliton evolution in slow-light silicon photonic crystal waveguides with SFG-XFROG.

We demonstrate the temporal and spectral evolution of picosecond soliton in the slow light silicon photonic crystal waveguides (PhCWs) by sum frequency generation cross-correlation frequency resolved optical grating (SFG-XFROG) and nonlinear Schrödinger equation (NLSE) modeling. The reference pulses for the SFG-XFROG measurements are unambiguously pre-characterized by the second harmonic generation frequency resolved optical gating (SHG-FROG) assisted with the combination of NLSE simulations and optical spectrum analyzer (OSA) measurements. Regardless of the inevitable nonlinear two photon absorption, high order soliton compressions have been observed remarkably owing to the slow light enhanced nonlinear effects in the silicon PhCWs. Both the measurements and the further numerical analyses of the pulse dynamics indicate that, the free carrier dispersion (FCD) enhanced by the slow light effects is mainly responsible for the compression, the acceleration, and the spectral blue shift of the soliton.

Flat band slow light in silicon photonic crystal waveguide with large delay bandwidth product and low group velocity dispersion

Flat band slow light is achieved in a silicon photonic crystal waveguide with large normalised delay bandwidth product (NDBP) while maintaining smaller values of group velocity dispersion. By altering the waveguide width through shifting the position of two rows of air holes adjacent to the waveguide, it is possible to achieve a high NDBP value. The NDBP values of 0.40, 0.41 and 0.43 over bandwidths of 11, 14 and 18 nm are, respectively, reported. Towards the applicability of the proposed slow light waveguide for buffering applications on silicon, buffering capacity as high as 140 bits can be achieved with the proposed design approach.z

Design of a low-power all-optical NOR gate using photonic crystal quantum-dot semiconductor optical amplifiers

Design and simulation of a novel all-optical logic gate (AOLG) based on photonic crystal quantum-dot semiconductor optical amplifiers (PC-QDSOAs) is reported for the first time. For this purpose, a flat-band slow-light QD-based active photonic crystal waveguide (APCW) is designed and employed for implementation of an all-optical NOR gate in a symmetric Mach-Zehnder interferometer configuration. Then, the performance of the NOR gate is investigated using a comprehensive nonlinear state space model (NSSM). Simulation results demonstrate that PC-QDSOAs present an interesting platform for realizing ultracompact AOLGs with low power consumption.

20 µm long slow-light Bragg reflector waveguide modulator with over 20 GHz modulation bandwidth

A slow-light Bragg reflector waveguide optical modulator is designed and optimized for operations with a large modulation bandwidth and ultra-low power consumption. Devices with various modulator lengths are fabricated and characterized. Extinction ratios over 15 and 30 dB are obtained with reverse bias voltages below 1 and 1.5 V, respectively. In a 20 µm long modulator, a 3 dB modulation bandwidth exceeds 20 GHz with a bias voltage of only −600 mV. The total energy consumption of the modulator is estimated to be lower than 100 fJ/bit, including both the load and dynamic power dissipations.

Enhanced four-wave mixing in graphene-silicon slow-light photonic crystal waveguides

We demonstrate the enhanced four-wave mixing of monolayer graphene on slow-light silicon photonic crystal waveguides. 200-μm interaction length, a four-wave mixing conversion efficiency of −23 dB is achieved in the graphene-silicon slow-light hybrid, with an enhanced 3-dB conversion bandwidth of about 17 nm. Our measurements match well with nonlinear coupled-mode theory simulations based on the measured waveguide dispersion, and provide an effective way for all-optical signal processing in chip-scale integrated optics.

Control of delay-bandwidth product in slow-light photonic crystal waveguides with asymmetric microfluidic infiltration

In this work, we report on the optofluidic tuning of the delay bandwidth product of a slow-light photonic crystal waveguide. The proposed tuning method is obtained by infiltrating the central two rows of the W09 waveguide. These infiltrated rows consist of holes that are infiltrated asymmetrically by different liquids with different refractive indices. The simulation results show that by changing the refractive index of the infiltrated liquids, we can achieve a very flat band corresponding with low group velocity and dispersion. It is found that the values of the bandwidth and group index—the bandwidth product of the proposed waveguide—can be improved by changing the refractive index of the infiltrated liquid. This approach allows us to control the group velocity, dispersion and delay bandwidth product of the slow-light photonic crystal waveguide by choosing a suitable refractive index of the two infiltrated liquids.

Optical Gain, Phase, and Refractive Index Dynamics in Photonic Crystal Quantum-Dot Semiconductor Optical Amplifiers

In this paper, the gain, phase, and refractive index dynamics of photonic crystal quantum-dot semiconductor optical amplifiers (PC-QDSOAs) are investigated for the first time. For this purpose, we establish a comprehensive nonlinear state space model with 1088 state variables to simulate the carrier dynamics of the inhomogeneously broadened InAs/GaAs QD active region embedded in a flat-band slow-light photonic crystal waveguide. The gain and phase recovery responses are monitored during the amplification of an ultrashort pump pulse. Simulation results show that changes in refractive index in the PC-QDSOA active region are much stronger than the index changes in the conventional QDSOAs. In addition, the gain and phase recovery time in PC-QDSOAs are similar to those in the conventional QDSOAs based on ridge waveguide structures. Also, we found that the injected current and the consumed power in PC-QDSOAs can be two orders of magnitude less than those values in the conventional QDSOAs.

On the limitations of the first-order nonlinear Schrödinger equation in slow-light photonic crystal structures

We investigate the limitations of the first-order nonlinear Schrodinger equation for describing slow-light enhanced optical nonlinearities in photonic crystal structures. We do this by introducing the concept of generalized Bloch modes to describe light propagation in nonlinear photonic crystal structures. By considering how the nonlinear effects perturb the eigenproblem that describes such generalized Bloch modes, we derive a formalism that allows us to determine the impact of nonlinearities up to an arbitrary order. Based on our formalism, we derive expressions for the first- and second-order nonlinear terms experienced by a single generalized Bloch mode. We then theoretically demonstrate that the second-order terms can in silicon-based slow-light photonic crystal waveguides already become significant at optical powers as small as 100 mW. Additionally, we show that the significance of the second-order term is closely related with the structural group index dispersion or second-order dispersion parameter of the structures considered.

Wide-bandwidth zero-dispersion slow light in MKRs with a two-ring parallel connection structure based on an analogue of electromagnetically induced transparency

Based on an analog of electromagnetically induced transparency (EIT) in microfiber knot resonators with a two-ring parallel connection structure, a wide bandwidth and zero-dispersion slow-light waveguide is proposed. The EIT-like transmission spectrum with the variation of wavelengths for different coupling coefficients is numerical simulated. By appropriately adjusting the coupling coefficients of the two knot rings, a group time delay of about 72.4 ps with a flat wavelength bandwidth of about 82.7 pm and a full width at half-maximum of about 228 pm at one of the induced transparency windows is achieved theoretically. And the group time delay dispersion at the same induced transparency window nearly reaches to zero with a wavelength bandwidth of about 82.7 pm.