Slow-light Waveguides Research Articles

Designing a low-threshold quantum-dot laser based on a slow-light photonic crystal waveguide.

We numerically investigate and design a compact electrically pumped edge-emitting photonic crystal waveguide (PCW) quantum dot (QD) laser operating at room temperature. Use of a narrowband folded directional coupler as the output mirror has made the proposed structure an edge-emitting single-mode laser. Moreover, we propose a set of rate equations to model the performance of the PCW-QD laser. In the proposed model, we take the effects of the homogeneous and inhomogeneous broadenings and the slow-light effects on the modal gain and loss coefficient into account. Simulations show that threshold current as low as ∼26 μA can be achieved for the PCW-QD laser with a 50-μm-long cavity and output power in the range of micro-watts. The proposed low-threshold edge-emitting PCW-QD laser is a promising light source for the off-chip and on-chip photonic network applications.

Ultra-wide band dispersionless slow light waveguides

In conventional slow light waveguide, the delay time of signal in a waveguide is inversely proportional to its bandwidth. As a measure of storage density of optical pulse, the delay-bandwidth product is limited to a small constant depending on waveguiding approach. Breaking the limit is fundamental problem in optical signal and quantum information processing fields. The solution to this problem is becoming increasingly important, especially with high-speed signal processing system. So far, there have been neither practical nor realizable techniques that can solve this problem. In this paper, we propose a mechanism to explore the possibility for breaking the delay-bandwidth limit by using ultra-flat photonic band cluster. We show that based on multiple micro-cavities having corresponding multiple ultra-flat photonic bands, a slow light with arbitrary bandwidth in frequency can be achieved with ultraslow group velocity and ultralow group velocity dispersion, leading to a broken delay-bandwidth limit. This work demonstrates that arbitrary bandwidth dispersionless slow light can be realized with a two-dimensional multiple-microcavity photonic-crystal line-defect waveguide.

Fan-beam steering device using a photonic crystal slow-light waveguide with surface diffraction grating.

Compact non-mechanical beam steering devices are desired not only for current common applications, but also for advanced applications such as light detection and ranging. We use a Si photonic crystal slow-light waveguide with a diffraction grating, which radiates the guided mode to free space and steers a fan beam by sweeping the wavelength. Due to its large angular dispersion, slow light enhances the steering range without degrading the beam quality, resulting in more resolution points. We fabricated 600μm devices and observed a 23° steering range and a beam divergence of 0.23°, which resulted in 100 resolution points.

Temporal imaging based on four-wave mixing in slow-light photonic crystal waveguide

We have proposed a temporal imaging system based on four-wave mixing (FWM) in the dispersion engineered slow-light photonic crystal waveguide (PCW). Dispersion relations of the modified PCW are calculated through the 3D plane wave expansion method. Time lens is demonstrated by solving the couple-mode equations describing the FWM process inside the PCW directly. Intensity and phase evolutions of the signal, pump and idler waves during the FWM process are calculated to investigate temporal imaging. Meanwhile, temporal magnifications with different magnification factors are realized by tuning the total dispersion of the input signals. Furthermore, influences of dispersion and free-carrier effects inside PCW on the temporal imaging performance are analyzed. The simulation results show the capability to realize temporal imaging system based on the FWM process in slow-light engineered PCW.

Free Carrier Front Induced Indirect Photonic Transitions: A New Paradigm for Frequency Manipulation on Chip

Nonlinear degenerate four wave mixing and cross phase modulation are established approaches for all optical frequency manipulation in a silicon chip. These approaches require exact group velocity and/or phase velocity matching of pump, signal, and idler. On the other hand, several experimental demonstrations were presented recently, where frequency of light was changed by a free carrier front propagating in a silicon waveguide. This Doppler-like effect is less known, but has important advantages for frequency manipulation on chip. It requires no phase velocity matching and is not dependent on the shape and duration of the pump pulse. It also allows packet switching and can operate in a pump power independent regime. Here, we shortly review the work on front induced indirect transitions in silicon slow light waveguides. We consider three possible interaction regimes: transmission through the front, reflection from the front, and moving with the front called surfing. We derive analytical equations for the f...

Dispersionless and slow unidirectional air waveguide

Unidirectional waveguides and slow light waveguides both play the key roles in the modern communication system. The unidirectional waveguides do not support slow light. The traditional slow light waveguides are always limited to the group velocity dispersion. To combine both the functionalities of unidirectional transmission and slow light into one waveguide, we add a surface defect into unidirectional waveguide to modulate the odd mode dispersion. Through a structure optimizing, a dispersionless and slow unidirectional waveguide is obtained. The waveguide with compound functionalities is confirmed through the frequency-domain and time-domain simulations.

Design of Silicon Phoxonic Crystal Waveguides for Slow Light Enhanced Forward Stimulated Brillouin Scattering

We propose and design the silicon phoxonic crystal waveguides to achieve forward stimulated Brillouin scattering (FSBS). The waveguides with honeycomb-lattice structure are able to support the photonic and phononic band gaps simultaneously. By the optimized design, the corresponding optical and acoustic defect modes are tightly confined in the line defect to enhance acousto-optic coupling, resulting in the intramode and intermode FSBS. The three-dimensional (3D) full-vectorial theoretical formulation is applied to analyze the influences of structural parameters on FSBS gain. By the coupled-mode equations, we explore the FSBS process taking into account the slow light enhanced factor, two-photon and free-carrier absorptions in the slow-light phoxonic crystal waveguide. The results indicate that 18 dB Stokes amplification is obtained with 100 mW pump power in a short waveguide length of 200 μm. Such approach enables the realization of on-chip FSBS with CMOS technologies.

Optimal interfacing with coupled-cavities slow-light waveguides: mimicking periodic structures with a compact device.

We present a design for optimal interfacing (I/O coupling) with slow-light structures consisting of coupled cavities. The I/O couplers are based on adding a small set of cavities with varying coupling coefficients at edges of the coupled cavities waveguide in order to match its impedance with that of the I/O waveguides. I/O efficiencies exceeding 99.9% are shown to be possible over a bandwidth which is larger than 50% of that of the coupled cavities waveguide. Consequently, the reflections at the edges of the slow-light structure are practically eliminated. We discuss the properties of the perfectly impedance matched slow-light structure as an effective (super-structure) cavity and study the impact of the number of cavities comprising the I/O coupler. We also consider in details the impact of errors and disorder in the I/O coupling sections.

Efficient coupling into slow-light one-dimensional fishbone waveguide by mode converter method

The high-order mode in a fishbone waveguide accommodates the easy manipulation of group index dispersion. To minimize Fresnel reflection, group velocity mismatch can be compensated for by a casual step taper, but with low coupling efficiency. Here, we propose a mode converter to boost the coupling efficiency to in and out of the slow-light fishbone waveguide. Split, phase, and side slots in a mode converter are employed to compensate for the mode mismatch in terms of the width of the lobes, the phase offset, and the power ratio between neighboring lobes. A high coupling efficiency of ∼70% leads to the efficient compact true-time delay required for phased array antennas.

Quantum Nonlinear Optics with a Germanium-Vacancy Color Center in a Nanoscale Diamond Waveguide.

We demonstrate a quantum nanophotonics platform based on germanium-vacancy (GeV) color centers in fiber-coupled diamond nanophotonic waveguides. We show that GeV optical transitions have a high quantum efficiency and are nearly lifetime broadened in such nanophotonic structures. These properties yield an efficient interface between waveguide photons and a single GeV center without the use of a cavity or slow-light waveguide. As a result, a single GeV center reduces waveguide transmission by 18±1% on resonance in a single pass. We use a nanophotonic interferometer to perform homodyne detection of GeV resonance fluorescence. By probing the photon statistics of the output field, we demonstrate that the GeV-waveguide system is nonlinear at the single-photon level.

Optimised photonic crystal waveguide for chiral light–matter interactions

We present slow-light photonic crystal waveguide designs that provide a ×8.6 improvement of the local density of optical states at a fully chiral point over previous designs.

Tuning out disorder-induced localization in nanophotonic cavity arrays.

Weakly coupled high-Q nanophotonic cavities are building blocks of slow-light waveguides and other nanophotonic devices. Their functionality critically depends on tuning as resonance frequencies should stay within the bandwidth of the device. Unavoidable disorder leads to random frequency shifts which cause localization of the light in single cavities. We present a new method to finely tune individual resonances of light in a system of coupled nanocavities. We use holographic laser-induced heating and address thermal crosstalk between nanocavities using a response matrix approach. As a main result we observe a simultaneous anticrossing of 3 nanophotonic resonances, which were initially split by disorder.

Slow-light-enhanced energy efficiency for graphene microheaters on silicon photonic crystal waveguides

Slow light has been widely utilized to obtain enhanced nonlinearities, enhanced spontaneous emissions and increased phase shifts owing to its ability to promote light–matter interactions. By incorporating a graphene on a slow-light silicon photonic crystal waveguide, here we experimentally demonstrate an energy-efficient graphene microheater with a tuning efficiency of 1.07 nmmW−1 and power consumption per free spectral range of 3.99 mW. The rise and decay times (10–90%) are only 750 and 525 ns, which, to the best of our knowledge, are the fastest reported response times for microheaters in silicon photonics. The corresponding figure of merit of the device is 2.543 nW s, one order of magnitude better than results reported in previous studies. The influence of the length and shape of the graphene heater to the tuning efficiency is further investigated, providing valuable guidelines for enhancing the tuning efficiency of the graphene microheater.

Wideband and low-dispersion engineered slow light using liquid infiltration of a modified photonic crystal waveguide.

We present a wideband and low-dispersion slow-light photonic crystal waveguide with a large normalized delay-bandwidth product that can be exploited in many ultra-compact all-optical devices, such as modulators and switches. The proposed new approach is based on infiltrating optical fluid into the first and second rows of the shifted air holes adjacent to the line-defect waveguide in a hexagonal lattice of photonic crystal. The simulation results show that the normalized delay-bandwidth product can be enhanced to a large value of 0.469 with a wide bandwidth operation of 36.8nm in the C-band frequency optical communication window. Furthermore, by means of two-dimensional finite-difference time-domain calculations, the low-dispersion slow-light propagation is demonstrated by simulating the temporal Gaussian pulse width broadening.

Wideband slow-light propagation with no distortion in a nanofiber-plane-grating composite waveguide

A nanofiber-plane-grating composite slow-light waveguide to achieve wideband slowlight propagation with no distortion is proposed. The waveguide is formed by embedding a tapered nanofiber into a V-groove on a plane-grating surface. By optimizing the waveguide structural parameters, a slow-light effect with bandwidth of about 1453 GHz is obtained. Based on finite-difference time-domain (FDTD) method, we analyze the waveguide’s optical properties and slow-light characteristics. Simulation results show that a picosecond optical pulse propagating in the slow-light waveguide can be delayed for about 980 fs and without distortion. The group velocity of the optical pulse can be reduced to about 0.3c (c is the speed of light in vacuum). This study will provide important theoretical basis and innovative ideas for the development of new-type slow-light elements.

Erratum: Atom-field dressed states in slow-light waveguide QED [Phys. Rev. A93, 033833 (2016)

Erratum: Atom-field dressed states in slow-light waveguide QED [Phys. Rev. A<b>93</b>, 033833 (2016)

Resonance fluorescence spectra from coherently driven quantum dots coupled to slow-light photonic crystal waveguides

Using a polaron master equation approach we investigate the resonance fluorescence spectra from coherently driven quantum dots (QDs) coupled to an acoustic phonon bath and a photonic crystal waveguide with a rich local density of photon states (LDOS). Resonance fluorescence spectra from QDs in semiconductor crystals are known to show strong signatures of electron-phonon interactions, but when coupled to a structured photonic reservoir, the QD emission properties are also determined by the frequency dependence of the LDOS of the photon reservoir. Here, we investigate the simultaneous role of coupled photon and phonon baths on the characteristic Mollow triplet spectra from a strongly driven QD. As an example structured photonic reservoir, we first study a photonic crystal coupled cavity waveguide, and find that photons and phonons have counter-interacting effects near the upper mode-edge of the coupled-cavity waveguide, thus establishing the importance of their separate roles in determining the emission spectra. The general theory is developed for arbitrary photonic reservoirs and is further applied to determine resonance the fluorescence spectra from a realistic, disordered W1 photonic crystal waveguide showing important photon-phonon interaction effects that are directly relevant to emerging experiments and theoretical proposals.

Novel optoelectronic characteristics from manipulating general energy-bands by nanostructures

This paper summarizes our research work on optoelectronic devices with nanostructures. It was indicated that by manipulating so called “general energy-bands” of fundamental particles or quasi-particles, such as photon, phonon, and surface plasmon polariton (SPP), novel optoelectronic characteristics can be obtained, which results in a series of new functional devices. A silicon based optical switch with an extremely broadband of 24 nm and an ultra-compact (8 mm × 17.6 mm) footprint was demonstrated with a photonic crystal slow light waveguides. By proposing a nanobeam based hetero optomechanical crystal, a high phonon frequency of 5.66 GHz was realized experimentally. Also, we observed and verified a novel effect of two-surface-plasmon-absorption (TSPA), and realized diffraction-limit-overcoming photolithography with resolution of ~1/11 of the exposure wavelength.

Atom-field dressed states in slow-light waveguide QED

We discuss the properties of atom-photon bound states in waveguide QED systems consisting of single or multiple atoms coupled strongly to a finite-bandwidth photonic channel. Such bound states are formed by an atom and a localized photonic excitation and represent the continuum analog of the familiar dressed states in single-mode cavity QED. Here we present a detailed analysis of the linear and nonlinear spectral features associated with single- and multi-photon dressed states and show how the formation of bound states affects the waveguide-mediated dipole-dipole interactions between separated atoms. Our results provide a both qualitative and quantitative description of the essential strong-coupling processes in waveguide QED systems, which are currently being developed in the optical and the microwave regime.

Enhanced photoresponsivity in graphene-silicon slow-light photonic crystal waveguides

We demonstrate the enhanced fast photoresponsivity in graphene hybrid structures by combining the ultrafast dynamics of graphene with improved light-matter interactions in slow-light photonic crystal waveguides. With a 200 μm interaction length, a 0.8 mA/W photoresponsivity is achieved in a graphene-silicon Schottky-like photodetector, with an operating bandwidth in excess of 5 GHz and wavelength range at least from 1480 nm to 1580 nm. Fourfold enhancement of the photocurrent is observed in the slow light region, compared to the wavelength far from the photonic crystal bandedge, for a chip-scale broadband fast photodetector.