Slow and fast light: basic concepts and recent advancements based on nonlinear wave‐mixing processes

After a review of the basic concepts of slow and fast light, recent advancements based on nonlinear wave‐mixing processes are described. As a nonlinear medium, the authors focus on a liquid crystal light valve showing that it allows obtaining a large control of the group delay, with a maximum fractional delay of 1, and a deceleration of light pulses down to group velocities as small as 0.2 mm/s. A theoretical model accompanies the observations and accounts for them in the general framework of two‐wave mixing in the light valve. At the end, a high‐sensitivity interferometer is presented as an example of slow light applications.     

Slow and fast light in liquid crystal light valves

We show that fast and slow light results from multiple scatterings in a liquid crystal light valve, where nondegenerate two-wave mixing occurs in the Raman-Nath regime of optical diffraction. The large nonlinear response and dispersive characteristics of the liquid crystals allow us to obtain group velocities as slow as less than 0.2 mm/s, which is attractive for the realization of ultrahigh precision interferometers and metrology measurements.     

Liquid refractive index sensor based on slow light in slotted photonic crystal waveguide

A high sensitive and compact refractive index sensor based on slotted photonic crystal waveguide (S-PhCW) is demonstrated. This design is worked on a Mach–Zehnder interferometer (MZI) configuration with S-PhCW as the measuring arm, which can be used to detect any changes in refractive index that correspond to different concentration of the measuring liquid. Combining the slow light enhancement in photonic crystal waveguide (PhCW) with the advantage of excellent optical confinement in slot waveguide, the sensitivity of this simple scheme can reach to 2.3 × 10⁹ nm/RIU with the active region of only 1 mm long.     

高灵敏度色散型慢光干涉仪的仿真研究

设计了一种基于慢光介质色散特性的新型干涉装置,研究其物理过程,证明利用慢光介质可以有效提高干涉仪的光谱灵敏度,并且光谱灵敏度与慢光介质的群折射率成正比。同时,进一步具体分析慢光介质GaAs的色散特性,给出研究色散型慢光干涉仪的一般性方法,并证明使用慢光介质GaAs在近红外区域可以提高光谱灵敏度2倍左右。     

Observation of ultraslow light propagation in a ruby crystal at room temperature

We have observed slow light propagation with a group velocity as low as 57.5+/-0.5 m/s at room temperature in a ruby crystal. A quantum coherence effect, coherent population oscillations, produces a very narrow spectral "hole" in the homogeneously broadened absorption profile of ruby. The resulting rapid spectral variation of the refractive index leads to a large value of the group index. We observe slow light propagation both for Gaussian-shaped light pulses and for amplitude modulated optical beams in a system that is much simpler than those previously used for generating slow light.     

基于光子晶体马赫-曾德尔干涉仪的太赫兹开关及强度调制器

通过对二维正方晶格光子晶体线缺陷模色散曲线慢光特性的研究, 利用外电场对液晶分子取向的调控作用, 在填充液晶的正方晶格波导的马赫-曾德尔干涉仪结构中实现了开关和强度调制等功能. 利用平面波展开法计算了光子晶体波导的线缺陷模, 分析了液晶折射率的变化对缺陷模的影响. 计算表明, 液晶折射率仅改变了0.1, 线缺陷模有效折射率改变达0.168, 该特性可以更为有效地实现对相位的控制, 进而实现高消光比开关和强度调制功能, 这种高效的相位调节器件在集成光系统中将有很好的应用前景.     

Enhanced localization of light in slow wave slot photonic crystal waveguides

A flexible design of slot photonic crystal waveguide with a wide comb is investigated. Introduction of a carefully designed comb within the photonic crystal waveguide allows an accurate dispersion engineering in order to achieve slow light and increase the optical confinement within the comb. The strong light confinement results in an extremely small nonlinear effective area around 0.015 μm². We report experimental realization of a comb photonic crystal waveguide with measured group indices higher than 100 in a Mach-Zehnder interferometer configuration and extract losses limited to 3.7?dB for a 100?μm device at n_g=37.     

液晶空间光调制器在光信息综合实验中的应用

提出了一种把液晶空间光调制器应用于光电信息综合实验的光学实验系统。系统中的空间光调制器采用光寻址液晶光阀和电寻址液晶光阀。给出了利用这一系统开设的四个光学信息处理实验。实践表明,该系统具有较好的教学效果。     

Pattern formation and direct measurement of the spatial resolution in a photorefractive liquid crystal light valve

In a photorefractive liquid crystal light valve, acting as a Kerr-like nonlinear optical medium, we show the appearance of optical patterns induced by a single mirror feedback. The spatial wavelength of the patterns scales with the distance between the mirror and the valve and the contrast of the patterns decreases for decreasing this distance. We use these properties to setup a new optical scheme for the measurement of the spatial resolution of the nonlinear device.     

Coupling loss minimization of slow light slotted photonic crystal waveguides using mode matching with continuous group index perturbation

We experimentally demonstrate highly efficient coupling into a slow light slotted photonic crystal waveguide. With optical mode converters and group index tapers that provide good optical mode matching and impedance matching, a nearly flat transmission over the entire guided mode spectrum of 68.8 nm range with 2.4 dB minimum insertion loss is demonstrated. Measurements also show up to 20 dB baseline enhancement and 30 dB enhancement in the slow light region, indicating that it is possible to design highly efficient and compact devices that benefit from the slow light enhancement without increasing the coupling loss.     

Slow light engineering in polyatomic photonic crystal waveguides based on square lattice

In this paper, the slow light properties of the polyatomic Photonic Crystal (PhC) which has multiple different air holes in each primitive cell are investigated. A slow light waveguide with “U-type” group index-frequency curve, which results in nearly constant group index over large bandwidth, is achieved using this new photonic crystal geometry based on the square lattice. Also, the radius and position of the innermost rows of small air holes have been modified to investigate the feasibility of controlling the dispersion relation by subtle structural modification. Numerical results demonstrate that decreasing the group velocity effectively and meanwhile maintaining a large Normalized Delay-Bandwidth Product (NDBP) can be achieved by only modifying the radius of the innermost rows of small air holes. Shifting the innermost rows of small air holes toward the waveguide core is highly beneficial to enlarge the slow light bandwidth, but it contributes nothing to the promotion of NDBP. Our results provide important theoretical basis for the potential application offered by the polyatomic photonic crystal in future optical networks.     

Slow light performance enhancement of graphene-based photonic crystal waveguide

We propose a graphene-based photonic crystal (PC) slow light waveguide, which is realized by creating periodical air holes in a silicon layer to achieve spatially varying chemical potentials of graphene. The structure is optimized around 30 THz, and a large group index of 166.6 is obtained, with a very low propagation loss of ?2.1 dB/um. The corresponding normalized delay-bandwidth product reaches as high as 4.00. Furthermore, the slow light performance can be dynamically tuned by changing a bias voltage. The center frequency of the slow light waveguide can be tuned between 19.1 THz and 27.4 THz. Our results suggest that graphene-based PC structures are very promising for slow light devices.     

Slow light Mach-Zehnder fiber interferometer

A slow light structure Mach-Zehnder fiber interferometer is theoretically demonstrated. The sensitivity of the interferometer is significantly enhanced by the dispersion of the slow light structure. The numerical results show that the sensitivity enhancement factor varies with the coupling coefficient and reaches its maximum under critical coupling conditions.     

Broadrange tunable slow and fast light in quantum dot photonic crystal structure

Slow and fast light processes, based on both structural and material dispersions, are realized in a wide tuning range in this article. Coherent population oscillations(CPO) in electrically tunable quantum dot semiconductor optical amplifiers lead to a variable group index ranging from the background index(nbgd) to ～ 30. A photonic crystal waveguide is then dispersion engineered and a group index of 260 with the normalized delay-bandwidth product(NDBP) of 0.65 is achieved in the proposed waveguide. Using comprehensive numerical simulations, we show that a considerable enhancement of slow light effect can be achieved by combining both the material and the structural dispersions in the proposed active QDPCW structure. We compare our developed FDTD results with analytical results and show that there is good agreement between the results, which demonstrates that the proposed electrically-tunable slow light idea is obtainable in the QDPCW structure.We achieve a total group index in a wide tuning range from nbgdto ～ 1500 at the operation bandwidth, which shows a significant enhancement compared with the schemes based only on material or structural dispersions. The tuning range and also NDBP of the slow light scheme are much larger than those of the electrically tunable CPO process.     

Control of slow light in a ring-out-ring structure

This paper proposes a ring-out-ring structure of coupled optical resonators to yield coupled-resonator-induced transparency (CRIT). Considering the insertion loss of the coupler,it theoretically deduces the transmission and the effective phase shift. The influences of the insertion loss of the coupler on the transmittance,the effective phase shift,the group index and the CRIT linewidth are fully studied. We find that the increase in multiple m can effectively enhance the normal dispersion and the group index of the proposed structure. Moreover,the specific expression of the group index at resonance is theoretically deduced and discussed for the proposed structure with two rings. The result shows that the multiple m between the lengths of ring 1 and ring 2 can enhance the group index to m times that of the structure with two equal-sized rings at resonance. The control of slow light in the proposed structure is important for applications of highly sensitivity gyroscopes,optical delay lines and optical buffers,etc.     

Enhancing the sensitivity of fiber Mach-Zehnder interferometers using slow and fast light

We demonstrate theoretically and experimentally that the sensitivity of a fiber Mach-Zehnder (M-Z) interferometer can be enhanced by coupling fiber ring resonators with it. The experimental results agree well with theoretical predications that combining slow light with fast light will further increase the sensitivity of a fiber M-Z interferometer.     

Nonlinear performance in silicon nitride slow light photonic crystal waveguides with elliptical holes

We propose a slow-light photonic crystal waveguide, which uses a combination of circular and elliptical air holes arranged in a hexagonal lattice with the background material of silicon nitride (refractive index n = 2.06). Large value of normalized delay bandwidth product (NDBP = 0.3708) is obtained. We have analyzed nonlinear performance of the structure. With our proposed geometry strong SPM is observed at moderate peak power levels. Proposed photonic crystal waveguide has slow light applications such as reduction in length and power consumption of all-optical and electro-optic switches at optical frequency.     

基于Ⅲ-Ⅴ与Ⅱ-Ⅵ族半导体材料色散特性的高灵敏度慢光干涉仪

分析了Ⅲ-Ⅴ与Ⅱ-Ⅵ族半导体材料的光学特性,证明半导体慢光介质不但可以提高干涉仪的光谱灵敏度,而且可以获得远大于气体慢光介质的工作光谱范围.实验证明,基于慢光介质GaAs的干涉仪光谱灵敏度相对于传统的干涉仪提高约3.2倍. 关键词： 干涉仪 非线性光学 Ⅲ-Ⅴ与Ⅱ-Ⅵ族半导体材料     

Effect of disorder on slow light velocity in optical slow-wave structures

Slow-wave optical structures such as coupled photonic crystal cavities, coupled microresonators, and similar coupled-resonator optical waveguides are being proposed for slowing light because of the nature of their dispersion relationship. Since the group velocity becomes small, slow light and enhanced light-matter interaction may be observed at the edges of the waveguiding band. We derive a model of the effects of disorder on slow light in such structures, obtaining a relationship between the root-mean-square variation in the coupling coefficients and how slow the light is at the band edge.     

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.