Photonic Crystal Slow Light Research Articles

Deep Learning-Based Modelling of Complex Photonic Crystal Slow Light Waveguides

Deep Learning-Based Modelling of Complex Photonic Crystal Slow Light Waveguides

Slow-light effect in symmetry-reduced non-defect photonic crystals

In this study, a two-dimensional low-symmetric photonic crystal (PC) configuration with elliptical geometry is presented and its slow-light (SL) effect is investigated. Reducing the symmetry in the PC unit cell provides slow modes at the higher transverse electric bands. The calculated group index and the corresponding normalized bandwidth equal to {ng, BW}={63.56, 0.0065}. That corresponds to a value of figure of merit (FOM)=0.4344 defined by the product of the average group index and the normalized bandwidth, FOM=〈ng〉BW, which is comparable to the values available in literature. Tracing the whole edges of the Brillouin zone, strongly excited SL modes are observed only along Γ-X but not along Γ-X’ or Γ-M. That condition allows for the design of low-symmetric PC waveguides with finite thicknesses at the expense of lowering group index value. The SL effect is still obtained for the proposed low-symmetric PCs having finite thicknesses, which is numerically proved via finite-difference time-domain methods. It is important to note that non-dispersive SL Bloch modes exist through the non-zero k-vector components of Brillouin zone. Hence, such a defect-free (without either point- or line- defect) SL PC design may have a great potential for the use of compact photonic devices such as in optical switching and biochemical sensing applications.

Low-power all-optical switch based on slow light photonic crystal

Low-power all-optical switch based on slow light photonic crystal

64 Gbps Si photonic crystal slow light modulator by electro-optic phase matching.

We demonstrate 64 Gbps operation in a compact Si photonic crystal optical modulator that employs meander line electrodes and compensate for the phase mismatch between slow light and RF signals. Although low dispersion slow light increases the modulation efficiency, maintaining a sufficiently wide working spectrum, the phase mismatch becomes a limiting factor on the operation speed even when the phase shifter length is as short as 200 μm. Meander line electrodes broke this limit and enhanced the cutoff frequency by up to 31 and 38 GHz using 50 Ω and 20 Ω termination resistors, respectively. This allowed to use a group index of slow light higher than 20, and greatly improved the quality of the modulation characteristics at 25 and 32 Gbps. Clear open eye was observed even at 40-64 Gbps.

Dispersion engineering and thermo-optic tuning in mid-infrared photonic crystal slow light waveguides on silicon-on-insulator.

In this Letter, the design, fabrication, and characterization of slow light devices using photonic crystal waveguides (PhCWs) in the mid-infrared wavelength range of 3.9-3.98μm are demonstrated. The PhCWs are built on the silicon-on-insulator platform without undercut to leverage its well-developed fabrication process and strong mechanical robustness. Lattice shifting and thermo-optic tuning methods are utilized to manipulate the slow light region for potential spectroscopy sensing applications. Up to 20nm wavelength shift of the slow light band edge is demonstrated. Normalized delay-bandwidth products of 0.084-0.112 are obtained as a result of dispersion engineering. From the thermo-optic characterization results, the slow light enhancement effect of thermo-optic tuning efficiency is verified by the proportional relationship between the phase shift and the group index. This work serves as a proof of concept that the slow light effect can strengthen light-matter interaction and thereby improve device performance in sensing and nonlinearity applications.

Experimental simulation of ranging action using Si photonic crystal modulator and optical antenna.

Time of flight light detection and ranging (LiDAR) has been tested and used as a key device for auto-driving of vehicles. Frequency-modulated continuous-wave (FMCW) LiDAR potentially achieves a high sensitivity. In this study, we fabricated and tested two components of FMCW LiDAR based on Si photonics. The ranging action was also experimentally simulated. A Si photonic crystal slow light Mach-Zehnder modulator was driven by linearly frequency-chirped signals to generate quasi-frequency-modulated signal light. Then, the light was inserted into a fiber delay line of 20-320 m. Its output was irradiated to a photonic crystal slow beam steering device that acted as an optical antenna via a free-space transmission. The detected light was mixed with the reference light branched after the modulation in balanced photodiodes. A sufficiently sharp beat spectrum was observed, whose frequency well agreed with that expected for the delay line. The experimental simulation of the FMCW LiDAR, thus, was achieved.

Thermally controlled Si photonic crystal slow light waveguide beam steering device.

The doubly periodic Si photonic crystal waveguide radiates the guided slow light into free space as an optical beam. The waveguide also functions as a beam steering device, in which the steering angle is changed substantially by a slight variation in the wavelength generated due to the large angular dispersion of the slow light. A similar function is obtained when the wavelength is fixed and the refractive index of the waveguide is changed. In this study, we tested two kinds of integrated heater structures and observed the beam steering using the thermo-optic effect. For a p-i-p doped waveguide, the heating current was made to flow directly across the waveguide and a beam steering range of 21° was obtained with a relatively low heating power and high-speed response of the order of 100 kHz, maintaining a narrow beam divergence of 0.1-0.3° and a 120 resolution points. We also performed a preliminary life test of the device but did not observe any severe degradation in the temperature variation of 80-430 K for the duration up to 20‒40 h. For a TiN heater device, we obtained the comparable beam steering characteristics, but the required heating power increased, and the response speed decreased drastically.

Si Photonic Crystal Slow-Light Modulators with Periodic p–n Junctions

We theoretically optimized and demonstrated the periodic p–n junction in silicon photonic crystal slow-light modulators to balance the efficiency and speed of phase shifters and reduce the power consumption compared with those of previous linear and interleaved p–n junctions. In particular, sawtooth and wavy junctions, whose profiles match with the distribution of the slow-light mode, theoretically prove effective in achieving these objectives. However, the sawtooth junction requires a high-resolution process. Therefore, we finally employed the wavy junction and obtained 25- and 32-Gb/s operations in a 200-μm device with extinction ratios of 4 and 3 dB, respectively, for an excess modulation loss of 1 dB.

Time-reversal constraint limits unidirectional photon emission in slow-light photonic crystals.

Photonic crystal waveguides are known to support C-points-point-like polarization singularities with local chirality. Such points can couple with dipole-like emitters to produce highly directional emission, from which spin-photon entanglers can be built. Much is made of the promise of using slow-light modes to enhance this light-matter coupling. Here we explore the transition from travelling to standing waves for two different photonic crystal waveguide designs. We find that time-reversal symmetry and the reciprocal nature of light places constraints on using C-points in the slow-light regime. We observe two distinctly different mechanisms through which this condition is satisfied in the two waveguides. In the waveguide designs, we consider a modest group velocity of vg≈c/10 is found to be the optimum for slow-light coupling to the C-points.This article is part of the themed issue 'Unifying physics and technology in light of Maxwell's equations'.

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.

Tunable slow light in 1-D photonic crystal

We study the effect of different defect layer refractive indices and thicknesses on group velocity and dispersion properties of slow light in one-dimensional photonic crystal. It is found that the increase of the refractive index, number of the defects and defect layer thickness will induce the decrease of group velocity. We take advantage of these variations to introduce a novel technique to tune and control the slowing light in photonic crystals.

Giant gain enhancement in photonic crystals with a degenerate band edge

We propose a new approach leading to giant gain enhancement. It is based on unconventional slow-wave resonance associated with a degenerate band edge (DBE) in the dispersion diagram for a special class of photonic crystals supporting four modes at each frequency. We show that the gain enhancement in a Fabry-P\'erot cavity (FPC) when operating at the DBE is several orders of magnitude stronger when compared to a cavity of the same length made of a standard photonic crystal with a regular band edge (RBE). The giant gain condition is explained by a significant increase in the photon lifetime and in the local density of states. We have demonstrated the existence of DBE operated special cavities that provide for superior gain conditions for solid-state lasers, quantum cascade lasers, traveling wave tubes, and distributed solid-state amplifiers. We also report the possibility to achieve low-threshold lasing in FPC with DBE compared to RBE-based lasers.

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

A Proposal for Loss Engineering in Slow-Light Photonic Crystal Waveguides

The authors present a loss engineering method for slow-light photonic crystal (PhC) waveguides. Our proposed method for engineering the loss is based on optofluidic techniques to produce low propagation loss in the low dispersions slow-light regime. We numerically demonstrate that this approach allows one to control the propagation loss (from 180 to 120 dB/cm) by selective infiltration of suitable fluids into air holes of a PhC waveguide around the group velocity of c/55 to c/70. It is proposed that fabrication imperfections, i.e., roughness and other deviations from an ideal structure that lead to propagation losses, can be compensated by a selective nanoinfiltration method. A loss of 150 dB/cm for loss-engineered waveguides, compared to 230 dB/cm for conventional waveguides, is numerically demonstrated, while both waveguide structures maintain the same group index-bandwidth product.

Silica-Clad Silicon Photonic Crystal Waveguides for Wideband Dispersion–Free Slow Light

We comprehensively calculated the photonic bands of the waveguide modes in practical lattice-shifted photonic crystal waveguides, which are completely cladded by silica. We assumed various lattice shifts and found that the shift of the second rows and the mixed shift of the first and third rows along the waveguide generate low-dispersion slow light with group indices of 34-36, which is higher than those with a conventional shift of the third rows, maintaining a wide bandwidth over 10 nm at telecom wavelengths. We fabricated the waveguides using a CMOS-compatible process and confirmed correspondence with the calculation results. We also compared 25-Gb/s photonic crystal slow light Mach-Zehnder modulators and confirmed the improvement of the modulation efficiency by second-row shifts.

Theoretical and experimental investigation of low-volgage and low-loss 25-Gbps Si photonic crystal slow light Mach–Zehnder modulators with interleaved p/n junction

In this study, we investigated the performance of Si lattice-shifted photonic crystal waveguide (LSPCW) Mach-Zehnder modulators theoretically and experimentally. The LSPCW increases the phase shift in modulator to 2.3 - 2.5 times higher, which allows for size reduction and high performance. On-chip passive loss was reduced to less than 5 dB by optimizing each component. We obtained 25 Gbps clear open eye and 3 dB extinction ratio at a drive voltage of 1.5 – 1.75 V for 200 μm phase shifter with linear p/n junction when we added a modulation loss of 7 dB. This modulation loss was reduced to 0.8 dB, maintaining other performance, by employing interleaved p/n junction and optimizing doping concentrations.

Slow light in photonic crystals waveguide constructed with symmetrically perturbed structure

Symmetrically perturbed photonic crystal waveguide can be constructed by inserting perturbative dielectric rods into photonic crystal waveguide structure with whose rods’ radius distributed according to a certain proportion. Slow light properties in this new structure are studied by using the plane wave expansion method (PWM). In this paper, schemes of adjusting radius of perturbative dielectric rods and adjusting the dielectric constant of perturbative dielectric rods are proposed to optimize slow light properties. The result shows that the scheme for adjusting radius of perturbative rods can realize larger average slow light bandwidth and efficiently control the NDBP value of the waveguide, but it contributes little to obtain smaller group velocity. The scheme for adjusting dielectric constant of perturbative rods can realize smaller group velocity, but can only obtain smaller slow light bandwidth and cannot efficiently enlarge NDBP value of waveguide. Both optimization schemes proposed in this paper realize group velocity that is two magnitudes smaller than the vacuum speed of light meanwhile maintaining large NDBP and low GVD region. Our results provide important theoretical basis for the potential application offered by symmetrically perturbed photonic crystal in future optical networks.

Improvement of Raman amplifier bandwidth by means of slow light in photonic crystal based waveguide structure

In this paper, we investigate the enhancement of Raman amplification bandwidth with self phase modulation (SPM) effect in silicon based photonic crystal waveguides. The Maxwell equations are solved using finite difference time domain method considering two photon absorption, free carrier absorption, Kerr and SPM effects. We also study the effects of shape, width and average power on the Raman amplification bandwidth. Then by changing the photonic crystal geometrical parameters, pump and signal group velocity are reduced to achieve higher Raman bandwidth.

Strongly coupled slow-light polaritons in one-dimensional disordered localized states

Cavity quantum electrodynamics advances the coherent control of a single quantum emitter with a quantized radiation field mode, typically piecewise engineered for the highest finesse and confinement in the cavity field. This enables the possibility of strong coupling for chip-scale quantum processing, but till now is limited to few research groups that can achieve the precision and deterministic requirements for these polariton states. Here we observe for the first time coherent polariton states of strong coupled single quantum dot excitons in inherently disordered one-dimensional localized modes in slow-light photonic crystals. Large vacuum Rabi splittings up to 311 μeV are observed, one of the largest avoided crossings in the solid-state. Our tight-binding models with quantum impurities detail these strong localized polaritons, spanning different disorder strengths, complementary to model-extracted pure dephasing and incoherent pumping rates. Such disorder-induced slow-light polaritons provide a platform towards coherent control, collective interactions, and quantum information processing.

Slow light in photonic crystal with combinations of gradient dielectric constant

A line defect waveguide formed by a new kind of two-dimensional photonic crystal with combinations of gradient varied dielectric was designed and the properties of slow light will been studied by using the Plane Wave expansion Method (PWM). The result shows that the group velocity is slower when the corresponding combination of gradient dielectric constants are with higher dielectric constant and lower interval. In our new structure of photonic crystal, the corresponding group velocity can reach as low as 0.06c (c: light velocity in vacuum), two orders magnitude lower than light velocity. Meanwhile, the group velocity dispersion effect is relatively small.