Slow Light Effect Research Articles

Nanosensing and slow light application based on Fano resonance in waveguide coupled equilateral triangle resonator system

Plasmonic Fano resonance effect is investigated in a novel metal-insulator-metal waveguide structure, which consists of a stub MIM waveguide and a triangle shaped resonator. The transmission properties of the proposed plasmonic waveguide structure are numerically investigated by the two dimensional finite-difference-time -domain (FDTD). The coupling between the stub resonator and the triangle resonator results in a Fano resonance. The transmission characteristics and the slow light effect, sensing performance of the proposed plasmonic structure are studies in detail. By proper design, the sensitivity and the optical delay time of the proposed structure can be up to 800 nm/RIU and 0.053 ps. The above characteristics indicate the proposed plasmonic structure may have application in slow light device, plasmonic filter, all optical switch and nanosensing.

Synchronous Slow and Fast Light Based on Plasmon-Induced Transparency and Absorption in Dual Hexagonal Ring Resonators

Slow and fast light effects have many potential applications. In this paper, slow and fast light were obtained synchronously based on plasmon-induced transparency (PIT) and plasmon-induced absorption (PIA). The proposed structure consisted of dual hexagonal ring resonators coupled with two parallel metal-insulator-metal waveguides. The theoretical model established using temporal coupled-mode theory showed that the core–core separation and resonance wavelength detuning between two hexagonal rings were the key factors to PIT and PIA. The finite-difference time-domain simulation results revealed that the structural parameters have an important influence on the transmission characteristics and group indices. Moreover, there was a feasible way for adjusting the slow and fast light effects without changing the geometric dimensions. This proposed structure can provide a way toward controlling the velocity of light in highly integrated devices.

Deep tissue imaging with acousto-optical tomography and spectral hole burning with slow light effect: a theoretical study.

Biological tissue is a highly scattering medium that prevents deep imaging of light. For medical applications, optical imaging offers a molecular sensitivity that would be beneficial for diagnosing and monitoring of diseases. Acousto-optical tomography has the molecular sensitivity of optical imaging with the resolution of ultrasound and has the potential for deep tissue imaging. Here, we present a theoretical study of a system that combines acousto-optical tomography and slow light spectral filters created using spectral hole burning methods. Using Monte Carlo simulations, a model to obtain the contrast-to-noise ratio (CNR) deep in biological tissue was developed. The simulations show a CNR > 1 for imaging depths of ∼5 cm in a reflection mode setup, as well as, imaging through ∼12 cm in transmission mode setups. These results are promising and form the basis for future experimental studies.

Analysis of Kerr nonlinearity enhancement in silicon nitride waveguides using one-dimensional slow light structure

A corrugated based slow-light silicon nitride waveguide is studied numerically to enhance Kerr nonlinearity by increasing the group index of the waveguide at the band edge. The structure is optimized taking into account the limitations existing in CMOS-compatible platforms. Bandwidth limitations of the structure are investigated at different wavelengths for various slow-down factors. Finite size realizations of the structure are studied using transfer matrix method to obtain the desired transmission and group velocity at the band edge. Optical loss corresponding to slow-light effect is modeled, and the limitations imposed by the slow-light loss on the nonlinear phase shift saturation are considered. It is shown that by employing slow-down factor of 3, the nonlinear phase shift is improved by a factor of 5. Also, in comparison with a straight waveguide, shorter corrugated waveguides (up to nine times) can provide the same accumulated nonlinear phase shift.

Dynamic electromagnetically induced transparency based on a metal-graphene hybrid metamaterial

A novel design of a dynamic electromagnetically induced transparency (EIT) metamaterial is proposed. The metamaterial consists of two kinds of cut-wire metal resonators with a graphene strip placed between them. The destructive interference between the two resonators gives rise to a transparency window. By varying the Fermi energy of the graphene through external gating, the EIT effect can be manipulated dynamically and the maximum modulation depth can reach up to 81%. The efficient modulation is controlled only in the EIT window with slight changes in transmission dips, which may avert the additional noises at adjacent frequencies in the modulation process. Moreover, the actively controlled slow-light effect and sensing performances can also be realized as the corresponding EIT window is modulated. This work provides a strategy to achieve a tunable EIT effect in a metal-graphene hybrid structure and exhibits potential applications in designing terahertz modulators, environmental sensors and slow-light devices.

Novel tunable terahertz graphene metamaterial with an ultrahigh group index over a broad bandwidth

We propose a grating-coupled graphene metamaterial structure at a terahertz band. An obvious tunable plasmon-induced transparency can be achieved by changing the graphene Fermi level, and there is a very high absorption peak in the spectrum. Thus, this structure can be used to realize a terahertz modulator or absorption device with two outstanding functions. Using phase changes, we have also studied the slow light effect, and we find that there is a broad ultrahigh-group-index band, and the maximum group index can reach 216. This research may open up new avenues for slow light devices.

Analysis of the slow-light effect in silicon wire waveguides with metamaterials

Slow-light devices have been attracting a great amount of attention owing to their potential applications in future optical networks and information-processing systems. In this paper we propose two types of silicon wire waveguides with metamaterials, which are potentially applicable to the deceleration of light speed, and we analyze the slow-light effect by using the homogenization-based mode analysis method. The derived dispersion relation indicates that a group index larger than 70 can be obtained with a propagation loss of ∼0.7 dB/μm near the resonant frequency of the metamaterial.

Large group delay in a microwave metamaterial analog of electromagnetically induced reflectance.

Recently reported metamaterial (MM) analogs of electromagnetically induced reflectance (EIR) enable a unique route to endow classical optical structures with aspects of quantum optical systems. This method opens up many fascinating prospects on novel optical components, such as slow light units, highly sensitive sensors, and nonlinear devices. Here we designed and simulated a microwave MM made from aluminum thin film to mimic the EIR system. High reflectance of about 99 percent and also a large group index at the reflectance window of about 243 are demonstrated, which mainly arise from the enhanced coupling between radiative and nonradiative elements. The interaction between the elements of the unit cell, induced directly or indirectly by the incident electromagnetic wave, leads to a reflectance window, resembling the classical analog of EIR. This reflectance window, caused by the coupling of radiative-nonradiative modes, can be continuously tuned in a broad frequency regime. The strong normal phase dispersion in the vicinity of this reflectance window results in the slow light effect. This scheme provides an alternative way to achieve tunable slow light in a broad frequency band and can find important applications in active and reversibly tunable slow light devices.

Design of an ultra-compact graphene-based integrated microphotonic tunable delay line.

The design of a continuously tunable optical delay line based on a compact graphene-based silicon Bragg grating is reported. High performance, in terms of electro-optical switching time (tswitch < 8 ns), delay range (Δτ = 200 ps), and figure of merit FOM = Δτ/A = 1.54x105 ps/mm2, has been achieved with an ultra-compact device footprint (A ~1.3 x 10-3 mm2), so improving the state-of-the-art of integrated optical delay lines. A continuous and complete tunability of the delay time can be achieved with a very low delay loss ( = 0.03 dB/ps) and a weak power consumption ( = 0.05 mW/ps). A flat bandwidth B = 1.19 GHz has been calculated by exploiting the slow-light effect in the device. This performance makes the proposed optical delay line suitable for several applications in Microwave Photonics (MWP), such as beamsteering/beamforming, for which large delay range, flat and wide bandwidth and small volume are required.

Slow-Light Effect and Mode Selection of Double Fiber Ring With a Fiber Bragg Grating

A novel slow-light reflector composed of a double-ring resonator with a uniform fiber Bragg grating (FBG) incorporated is proposed in this paper, which can be used as a mirror of narrow-linewidth fiber laser, based on its enlarged group delay, and as a high-sensitivity sensor. Its characteristics are investigated theoretically and experimentally in this paper. Compared with the single fiber ring the amplitude of resonance is modulated, the effective mode spacing is expanded greatly, due to the Vernier effect, giving much better mode selectivity. Mode hopping is believed to be suppressed effectively in applications of narrow-linewidth lasers, and resolution will be enhanced in sensor applications.

High-gain, low-threshold and small-footprint optical parametric amplifier for photonic integrated circuits

A new model of an optical parametric amplifier is proposed based on a silicon-slab slotted photonic crystal waveguide (SPCW). The slot is considered to be filled with silicon nanocrystal material (SiNC/SiO2) having a high Kerr nonlinearity. The extreme optical confinement (spatial and temporal) of the SPCW has enhanced the optical nonlinearity, and thus, a high parametric gain is attained in a small length of the waveguide. Further, for analyses of pulse propagation, the coupled nonlinear Schrodinger’s equations have been modified to incorporate the enhancements in the linear and the nonlinear coefficients due to the slow-light effect. Simulations have been performed on both the high group index region and the low-dispersion regions of the guided band. The simulations on the high group index region, centered at ≈1584 nm, depict a 22.6 dB parametric gain and a 20.9 dB conversion efficiency at a waveguide length of 152 μm, with an effective pump power and peak input signal power of 700 mW and 0.25 Mw, respectively. The pulsating signal, with a pulse width of 5 ps, also experienced a negligible deterioration in its pulse shapes in this length of the SPCW. On the other hand, the simulations on the region of negligible dispersion, centered at 1530 nm, have produced an over 10 dB parametric gain and conversion efficiency through a long range of wavelengths, i.e., 1490–1568 nm, which covers half of the S-band and the complete C-band of the optical communication windows. The significant gain at a micrometer scale length of the waveguide is expected to advance the possibility of on-chip integration of a high-speed all-optical amplifier in photonic integrated circuits.

Optical-response properties in hybrid optomechanical systems with quadratic coupling

We theoretically investigate the optical-response properties of the four-mode quadratically coupled optomechanical system (OMS), in which two standard OMSs with quadratic coupling are coupled to each other via a common waveguide. In the presence of a strong control field applied to one cavity and a weak probe field applied to the other, we show that by suitably tuning the system parameters, there appears the normal mode splitting, optomechanically induced absorption, and double or triple electromagnetically induced transparency phenomena in the probe absorption spectrum. In particular, the explicit physical explanations for those fantastic phenomena are detailed discussed. Moreover, we also show that our proposal can be exploited to implement the optical switch as well as the slow and fast light effects.

Slowing down light using terahertz semiconductor metamaterial for dual-band thermally tunable modulator applications.

Compared to the neighboring infrared and microwave regions, the terahertz regime is still in need of fundamental technological advances. We have designed a terahertz (THz) semiconductor metamaterial (MM) waveguide system, which exhibits a significant slow-light effect, based on a classical electromagnetically induced transparency phenomenon. The potential of MMs for THz radiation originates from a resonant electromagnetic response that can be tailored for specific applications. By appropriately adjusting the distance between the two radiative and nonradiative modes, a flat band corresponding to a nearly constant group index (of the order of 4924) in the THz regime can be achieved. Finite-difference time-domain simulations show that the incident pulse can be slowed down. The proposed device from a paucity of naturally occurring materials has useful applications in electronic or photonic properties at terahertz frequencies. This proposed compact configuration may find potential applications in plasmonic slow-light systems, optical buffers, and thermal and electromagnetic modulating applications and temperature sensors.

Fundamental Limitations to the Ultimate Kerr Nonlinear Performance of Plasmonic Waveguides

Plasmonic waveguides can greatly enhance nonlinear light–matter interactions through strong field confinement. However, achieving high performance nonlinear plasmonic devices remains challenging because of optical losses and material damage. Here we investigate the ultimate Kerr nonlinear performance of plasmonic waveguides. We account for optical damage by requiring that the local electric field intensity does not exceed the damage threshold of the nonlinear material. This allows us to factorize the fundamental limitations into those stemming from the constituent materials’ linear and nonlinear properties, and from the mode characteristics. We define quality coefficients for the metal and for the nonlinear dielectric so that these materials can be selected appropriately, and illustrate their utility by application to surface plasmon polaritons (SPPs). We further propose the concept of nonlinear effectiveness in order to quantify a mode’s ability to exploit the material’s nonlinearity. We find that the full exploitation of the material’s maximum nonlinearity requires a uniform field in addition to slow light effects. This is exemplified by the discovery that the maximum nonlinearity of Metal–Dielectric–Metal structures can be stronger than that of the bulk material. These counterintuitive insights provide deep understanding into the ultimate performance of nonlinear waveguides, and point to novel approaches to achieve practical, high performance nonlinear plasmonic devices.

Slow light enhanced gas sensing in photonic crystals

Infrared spectroscopy allows for highly selective and highly sensitive detection of gas species and concentrations. Conventional gas spectrometers are generally large and unsuitable for on-chip applications. Long absorption path lengths are usually required and impose a challenge for miniaturization. In this work, a gas spectrometer is developed consisting of a microtube photonic crystal structure. This structure of millimetric form factors minimizes the required absorption path length due to slow light effects. The microtube photonic crystal allows for strong transmission in the mid-infrared and, due to its large void space fraction, a strong interaction between light and gas molecules. As a result, enhanced absorption of light increases the gas sensitivity of the device. Slow light enhanced gas absorption by a factor of 5.8 in is experimentally demonstrated at 5400 nm. We anticipate small form factor gas sensors on silicon to be a starting point for on-chip gas sensing architectures.

密集波分复用器慢光效应仿真研究

密集波分复用器慢光效应仿真研究

Mode selection and dispersion engineering in Bragg-like slot photonic crystal waveguides for hybrid light–matter interactions

We introduce a family of slot photonic crystal waveguides (SPhCWs) for the hybrid integration of low-index active materials in silicon photonics with energy-confinement factors of ∼30% in low-index regions. The proposed approach, which is based on a periodic indentation of the etched slot in the middle of the SPhCW, makes it possible to reconcile a simultaneously narrow and wide slot for exploiting the two modes of even symmetry of a SPhCW. The resulting mode-selection mechanism allows a flexible choice of the modes to be used. Furthermore, the proposed structure offers tremendous flexibility for adjusting the dispersive properties of the slot-confined modes, in particular of their slow-light effects. Flat band slow light in a bandwidth of about 60 nm with a group velocity dispersion factor |β2| below 1 ps2/mm is numerically demonstrated by this approach, corresponding to a normalized delay bandwidth product of around 0.4. These results, obtained from hollow-core periodic waveguides that are directly designed in view of hybrid integration of active materials in mechanically robust structures (not based on free-standing membranes) could pave the way for the realization of on-chip slow-light bio-sensing, active hybrid-silicon optoelectronic devices, or all-optical hybrid-silicon nonlinear functionalities.

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.

Full C-band Si photonic crystal waveguide modulator.

The slow-light effect in silicon lattice-shifted photonic crystal waveguide (LSPCW) Mach-Zehnder modulators allows compact phase shifters, while limiting the working spectrum Δλ. We optimized the structural parameters of the LSPCW and extended Δλ to cover the full C-band in exchange for moderately decreasing the group index ng. We obtained Δλ=42 nm with ng=8-9 in a fabricated device and observed 25Gbps eye opening in the 200μm modulator in the full C-band.

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 nbgd to ∼ 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.