Group Velocity Control Research Articles

Topological states switching and group velocity control in two-dimensional non-reciprocal Hermitian photonic lattice

We proposed a model with non reciprocal coupling coefficients, in which the imaginary parts γ indicate the phase delay or exceed term. The distributions of band structure and the group velocity are both characterized as a function of the coupling. we studied the system’s topological states and group velocity control. The results show that the movement and breaking of Dirac points exist in the energy band of the system. By changing the coupling coefficients, the conversion between any topological states corresponds to different Chern number. Topological edge states exist in topological non-trivial systems that correspond to the two different Chern numbers. Besides, it is also found that both the coupling coefficient and the wave vector can cause the oscillation of the pulse group velocity. At the same time, the topological state can suppress the amplitude of the group velocity profiles. Our findings enrich the theory of light wave manipulation in high-dimensional photonic lattices and provide a novel view for realizing linear localization and group velocity regulation of light waves, which has potential application in high-speed optical communication and quantum information fields.

All-optical group velocity manipulation in Mach-Zehnder fiber interferometers incorporating with Brillouin random lasing resonance

We proposed and experimentally demonstrated a novel scheme of light group velocity control via a Mach-Zehnder fiber interferometer incorporated with Brillouin random lasing resonance in a half-open linear cavity. A theoretical model of the proposed Brillouin random lasing resonance-based Mach-Zehnder interferometer (BRLR-MZI) for the light group velocity manipulation was derived, predicting optical power-dependent transmittance spectrum shift as well as the enhancement of the temporal advancement. Thanks to Brillouin random lasing-induced fast light effect, the proposed scheme not only achieves a flexible all-optical manipulation of its transmittance spectrum but also overcomes the saturation effect for the light speed control based on a traditional MZI. In experimental validation, sinusoidally modulated signals with the modulation frequency of 160 kHz ultimately experienced a maximum advancement of 4031 ns (corresponding to the group index of 0.875), which exhibits an advancement improvement of 1364 ns compared with the traditional MZI-based scheme. It suggests a promising platform for exploring applications in fields of all-optical signal processing, microwave photonic, and ultra-sensitive fiber sensing.

Optical image addition and subtraction based on electromagnetically induced transparency effect

The electromagnetically induced transparency (EIT) effect is a quantum coherence effect between different atomic transition channels. The absorption of the atomic ensemble is significantly reduced or even close to zero, and at the same time, this effect is accompanied by strong spectral dispersion near the resonant frequency of atoms, which is widely used in group velocity control, light pulse storage, and coherent manipulation on the light field. In the light pulse storage based on the EIT effect, the retrieval field is determined by the signal, coupling, and readout fields, enabling the retrieval field to be dynamically controlled by manipulating the spatial frequency components of the interacting light fields. In this paper, according to the EIT effect, we achieve experimentally the optical image addition and subtraction in the imaging plane of a 4<inline-formula><tex-math id="M2">\begin{document}$f$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20221560_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20221560_M2.png"/></alternatives></inline-formula> system through the coherent manipulation of the retrieval field via the interacting signal, coupling, and readout fields. Specifically, we first store the spatial frequency spectrum of a double-slit mask in the praseodymium-doped yttrium silicate crystal located in the confocal plane of the 4<inline-formula><tex-math id="M3">\begin{document}$f$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20221560_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20221560_M3.png"/></alternatives></inline-formula> system based on the EIT effect. Then, we utilize a specially designed mask containing the target objects and perform spatial filtering by using the spatial frequency spectrum of the double-slit mask through the retrieval of stored light pulse. By moving the double-slit mask, the addition and subtraction between the images of target objects can be achieved in the imaging plane of the 4<inline-formula><tex-math id="M4">\begin{document}$f$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20221560_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20221560_M4.png"/></alternatives></inline-formula> system. We present a theoretical model to describe the addition and subtraction between the target images through the EIT-based light pulse storage technique. The experimental results accord well with the theoretical prediction. Compared with the traditional scheme with a cosine grating filtering the spatial frequency spectrum, our method does not require the preparation of a spatial frequency filter, and only produces the first-order positive and negative diffraction images without the influence of the zeroth-order diffraction image. The optical image addition and subtraction based on the EIT effect provide a new approach to optical image processing. This approach is not limited to image addition or subtraction, and it can be extended to achieve more complex operations such as differentiation, enhancement, encryption, and decryption with rational design. Therefore, it can be widely used in areas such as coherent manipulation of light fields and dynamic optical image processing.

Imaginary coupling induced Dirac points and group velocity control in the non-reciprocal Hermitian lattice.

We propose a mechanism to achieve the group velocity control of bifurcation light via an imaginary coupling effect in the non-reciprocal lattice. The physical model is composed of two-layer photonic lattices with non-reciprocal coupling in each unit cell, which can support a real energy spectrum with a pair of Dirac points due to the hermicity. Furthermore, we show that the systems experience topological phase transition at the Dirac points, allowing the existence of topological edge states on the left or right boundaries of respective lattice layers. By adjusting the imaginary coupling and the wave number, the group velocity of the light wave can be manipulated, and bifurcation light transmission can be achieved both at the Dirac points and the condition without the group velocity dispersion. Our work might guide the design of photonic directional couplers with group velocity control functions.

A hybrid kinetic WGVC–WENO scheme for compressible flows

In this paper, we propose a hybrid kinetic weighted group velocity control–weighted essentially non-oscillatory (WGVC–WENO) scheme which combines the WGVC–WENO reconstruction technique He et al. (2014) with the hybrid kinetic method proposed in Liu et al. (2015). The new scheme is based on the idea of hybridization. The hybrid kinetic method is adopted for flux calculations, and the WGVC–WENO technique is adopted for physical reconstructions. In smooth flow regions, the collision-related flux method and WGVC reconstruction technique play a leading role. The kinetic flux vector splitting (KFVS) method and WENO reconstruction technique are dominant in discontinuous regions. The third-order TVD Runge–Kutta method is used for temporal discretization. Many one-dimensional and two-dimensional numerical examples show that the new scheme has the properties of high accuracy, low dissipation, and good shock-capturing ability.

Numerical study of sum frequency ultrashort pulse compression in borate crystals

Second harmonic generation (SHG) of picosecond pulses in type II phase-matched nonlinear optical crystals is not widely employed because of the lower efficiencies compared to the type I phase-matching scheme. The limited efficiencies come from the difference between group velocities (GV difference) of the ordinary and extraordinary polarized input pulses. However, if the input pulses are delayed before the nonlinear crystal, a short second harmonic pulse can be generated with a slowly widening temporal overlap. Furthermore, this GV difference can be controlled by the tilting of the pulse fronts, and optimal GV difference can be obtained to achieve powerful output pulses with durations an order of magnitude lower than those of the input pulses. In this work, we present numerical results of SHG pulse compression in a beta barium borate (BBO) nonlinear crystal, which is ideal for SHG of high-power 1030 nm thin-disk lasers. The pulse compression is controlled by predelay and tilting of the pulse fronts. We find optimal parameters to achieve five-fold increase in output power and 20-fold pulse compression of 1.7 ps input pulses. Finally, we consider the experimental aspects of the group velocity control.

Group Velocity Control of Gaussian Pulse Signal With Pump Phase Modulation

Pump phase modulation is an important method for signal group velocity control. By controlling the phase difference between pump and signal, the fast and slow light can be changed easily, and the degree of signal delay or advancement can be greatly improved. In this letter, the fast and slow light of a Gaussian signal with pump phase modulation is studied for the first time. The Gaussian pulse signal is similar to the actual network transmission signal, so it is significant to investigate it. Unlike a sinusoidal signal, the duty ratio of a Gaussian signal can be adjusted, leading to the difference in pump phase modulation of the Gaussian signal. The experimental and simulation results show that the smaller the duty ratio of the Gaussian signal is, the smaller the impact of the pump phase adjustment on the fractional delay and advancement will be. Moreover, it is also found that at a certain duty ratio, two signal peaks appear within a certain adjustment range of phase difference.

Photonic emulation of two-dimensional materials with antiferromagnetic order

We introduce an electromagnetic metamaterial - a spin-valley photonic topological insulator (SV-PTI) - that emulates a wide class of theoretically predicted gapped two-dimensional materials with antiferromagnetic order coupled to the valley degree of freedom. First-principles electromagnetic simulations and an analytic model reveal that a single parameter controls the propagation properties of the chiral states at the domain wall between two SV-PTIs: their group velocity, polarization, and the existence/absence of topological protection. The latter is determined by the geometry of the line of the least frequency separation between the bandgap-forming propagation bands. Topologically-protected compression of electromagnetic energy via group velocity control is enabled by SV-PTIs.

Group velocity control by gain-assisted fibre Mach-Zehnder interferometers

We report the group velocity control of light in optical fiber by gain-imbedded unbalanced fiber Mach–Zehnder interferometer. Slow and fast light effects were observed at the frequencies near the minimum of the transmission spectrum. Controllable group velocity between 0.719 c to 0.620 c (c is the light speed in a vacuum) was obtained via the cross-intensity modulation effect, by changing the gain index in the long arm. With the power compensation provided by the introduced Erbium doped fiber, this proposal offers a simple and operable solution to realize lossless slow and fast light propagation in optical fiber.

Comparative Study of Light Manipulation in Three-Level Systems Via Spontaneously Generated Coherence and Relative Phase of Laser Fields**Supported by Vietnamese Ministry of Science and Technology under Grant No. DTDLCN.17/17

We present a comparative study of the control of group velocity in three-level atomic systems including Λ-, Ladder-, and V-types via spontaneously generated coherence (SGC) and relative phase of laser fields. In the steady regime, expressions of group index are derived as an analytical function of controllable parameters related to SGC and relative phase. It is shown that the group velocity can be further slowed down or speeded up under SGC in which modifications of group velocity due to the SGC in Λ- and V-systems are stronger than that in Ladder-system. In particular, when interference strength of SGC p increases from 0.7 to 1.0, the light propagation is switched from superluminal to subluminal modes (for Λ- and Ladder-systems) or from subluminal to superluminal modes (for V-system). In the presence of relative phase, the group velocity changes between subluminal and superluminal modes with a period of 2π. In resonant region, the largest positive value of group index reaches at the relative phase φ = π while the largest negative value of group index reaches at φ = 0 and 2π for the Λ- and Ladder-systems, otherwise, for the V-system the largest positive group index occurs at φ = 0 and 2π, and the largest negative group index occurs at φ = π.

A physical model for solving the dredging thermal protection system of hypersonic vehicle leading edge

The dredging thermal protection system is considered as a thermal protection method to prevent hypersonic vehicles whose leading edges work in the serious aerodynamic heating that seriously affects the performance and safety of the aircraft. This paper uses a special matrix splitting scheme and a high order accurate upwind compact difference scheme with group velocity control to solve the three-dimensional unsteady governing equations of the leading edge of the hypersonic vehicle. Fractal theory is introduced into the heat pipe wick model. The governing equation of the porosity of liquid components in the wick is derived. On the gas-liquid interface in the heat pipe, the temperature and pressure of the interface are determined by the Clapeyron equation that characterizing the relationship between saturation temperature and saturation pressure, assuming the temperature is the local saturation temperature. The distributions of temperature and density under the condition of different wedge half angles and heat pipes are studied by the coupled numerical simulation of the outer flow region of the aircraft and the internal region of heat pipes. The numerical results show that the high order accurate coupled computational method is reasonable and feasible.

微环谐振器中各参数对光速控制输出脉冲畸变的影响仿真分析

微环谐振器中各参数对光速控制输出脉冲畸变的影响仿真分析

Stark shift in Y2SiO5:Er3+ by the photon echo beating method

The coefficient of a linear Stark shift of the 4F9/2–4I15/2 transition of Er3+ ion in Y2SiO5 is measured (14.3 kHz V−1 cm) by splitting the upper and lower levels of the transition by a pulsed electric field and measuring the splitting frequency from the period of beatings of the temporal form of a photon echo signal (stark photon echo beating method). The linear Stark coefficients of Er3+ transitions in Y2SiO5 have not been measured before, and this is a valuable result for a number of applications such as gradient echo memory, group velocity control via spectral hole burning, etc.

Tunable slow- and fast-light devices based on molecular-aggregate nanofilms

We study the tunability of nanofilms composed of linear molecular aggregates for slow- and fast-light performance. In order to describe a wide range of intensity field regimes, we consider a two-exciton model for the molecular aggregate where exciton creation or annihilation may occur. Our simulations show fractional delays and advancements of the order of those found in longer propagating media. This nanometric device presents two functionalities: (i) it allows the existence of slow or fast light in a different bandwidth of pulsed signals and (ii) such dual performance can be tuned by a small perturbation of the input signal amplitude for a fixed pulse temporal width. Both effects are tested under the usual presence of disorder in these molecular systems. This study concludes that a molecular-aggregate nanofilm presents a wide control of group velocity in the GHz and THz bandwidths.

Ultrafast group-velocity control via cascaded quadratic nonlinearities in optical parametric amplification.

Slow and fast light are ubiquitous in optical amplifiers. In this Letter, we show for the first time, to the best of our knowledge, that optical parametric amplification (OPA) in chirped quasi-phase-matching structures can act as a platform for group-velocity control in the femtosecond regime. The resonant cascaded nonlinear phase underlies the group-velocity control, which manifests an unusual effect that both slow and fast light can be achieved under the normal condition of signal amplification. As numerically demonstrated in the OPA based on the lithium niobate crystal, the signal and idler pulse can be significantly delayed in time comparable to the signal duration and can also keep high fidelity for durations down to 100fs until the crystal dispersion becomes effective. The broad bandwidth, large group delay, and direct compatibility with integrated optics will make the proposed platform attractive to both fundamental research and applied science.

Absorption-free superluminal light propagation in a Landau-quantized graphene

In recent years, control of group velocity of light has attracted enormous interest. One of the main challenges is to realize an absorption-free fast or slow light propagation. Here, we study dispersion and absorption properties of a weak probe field in a Landau-quantized graphene and report a gain-assisted superluminal light propagation. Moreover, an attempt is made to develop an analytical expression and necessary parameters for switching the group velocity of the probe field from subluminal to superluminal. It’s worth mentioning that large dephasing rate in graphene offers feasibility of superluminal propagation of ultrashort light pulses. Additionally, dynamical behavior of dispersion and absorption of a weak probe field in a closed-type graphene system is investigated, and it is found that the absorption and dispersion can be dramatically affected by both the relative phase of applied fields and the Rabi frequencies in such a way that a large transient gain can be achieved and a transient absorption can be completely eliminated.

Force-induced transparency and conversion between slow and fast light in optomechanics

The optomechanics can generate fantastic effects of optics due to appropriate mechanical control. Here we theoretically study effects of slow and fast lights in a single-sided optomechanical cavity with an external force. The force-induced transparency of slow/fast light and the force-dependent conversion between the slow and fast lights are resulted from effects of the rotating-wave approximation (RWA) and the anti-RWA, which can be controlled by properly modifying the effective cavity frequency due to the external force. These force-induced phenomena can be applied to control of the light group velocity and detection of the force variation, which are feasible using current laboratory techniques.

Waveguide Bandgap in Crystalline Bandgap Slows Down Surface Plasmon Polariton

Next generation on-chip optical devices require light manipulation in time and space, that is, control of group velocity of light in subwavelength dimensions. A waveguide in plasmonic crystal fulfills such requirements offering nanoscale light confinement in the dispersion-tunable plasmonic crystal matrix. However, there has been no direct access to the local dispersion of the waveguide mode itself, and the group velocity of light could not be evaluated. Herein, for the first time, we experimentally clarify the dispersion of the waveguide modes by use of angle-resolved cathodoluminescence scanning transmission electron microscopy. Their group velocity can be extremely slowed down by the existence of a bandgap formed in the waveguide in the energy range of the plasmonic crystal bandgap.

Using electric fields for pulse compression and group-velocity control

In this article, we experimentally demonstrate a way of controlling the group velocity of an optical pulse by using a combination of spectral hole burning, the slow-light effect, and the linear Stark effect in a rare-earth-ion–doped crystal. The group velocity can be changed continuously by a factor of 20 without significant pulse distortion or absorption of the pulse energy. With a similar technique, an optical pulse can also be compressed in time. Theoretical simulations were developed to simulate the group-velocity control and the pulse compression processes. The group velocity as well as the pulse reshaping are solely controlled by external voltages which makes it promising in quantum information and quantum communication processes. It is also proposed that the group velocity can be changed even more in an Er-doped crystal while at the same time having a transmission band matching the telecommunication wavelength.Received 21 October 2016DOI:https://doi.org/10.1103/PhysRevA.95.032104©2017 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasLight propagation, transmission & absorptionQuantum state engineeringStark effectPhysical SystemsRare-earth doped crystalsTechniquesCoherent controlOptical pumpingRabi modelAtomic, Molecular & OpticalGeneral Physics

Stopping Narrow-Band X-Ray Pulses in Nuclear Media.

A control mechanism for stopping x-ray pulses in resonant nuclear media is investigated theoretically. We show that narrow-band x-ray pulses can be mapped and stored as nuclear coherence in a thin-film planar x-ray cavity with an embedded ^{57}Fe nuclear layer. The pulse is nearly resonant to the 14.4keV Mössbauer transition in the ^{57}Fe nuclei. The role of the control field is played here by a hyperfine magnetic field which induces interference effects reminiscent of electromagnetically induced transparency. We show that, by switching off the control magnetic field, a narrow-band x-ray pulse can be completely stored in the cavity for approximately 100ns. Additional manipulation of the external magnetic field can lead to both group velocity and phase control of the pulse in the x-ray cavity sample.