Fast Lights Research Articles

Enhancement of the group delay in quadratic coupling optomechanical systems subjected to an external force

We theoretically study the effect of the quadratic coupling strength on optomechanical systems subjected to a continuous external force. Quadratic coupling strength originates from strong coupling between the optical and the mechanical degrees of freedom. We show that the quadratic coupling strength reduces the amplitude of the dispersion spectra at the resonance in both blue- and red-sideband regimes. However, it increases (decreases) the amplitude of the absorption spectrum in the blue- (red-)sideband regime. Furthermore, in both sideband regimes, the effective detuning between the pump and the cavity deviates with the quadratic coupling strength. Thereby, appropriate selection of the quadratic coupling strength results in an important magnification (in absolute value) of the group delay for both slow and fast light exiting from the optomechanical cavity.

Exceptional-point-engineered cavity magnomechanics

We theoretically study the role of the exceptional point (EP) in a non-Hermitian cavity magnomechanical (CMM) system, with a tunable dissipative magnon-photon coupling. We find that the EP emerging in such a system can radically change the properties of photons and phonons. As a result, flexible on-off optical transmission, coherent switching of slow and fast lights, and enhanced mechanical cooling deep into the ground state can be achievable by approaching the EP. Detailed comparisons between this system and its Hermitian counterpart are given. Our results show that EP-assisted CMM devices can serve as tools for applications in optical communications and electromechanical switching or sensing.

Slow and Fast Lights in Metal/Dielectric Composite of Cylindrical Nanoinclusions in Passive and Active Linear Dielectric Host Matrices

This paper presents theoretical discussions and computational numerical results obtained from the study of extreme values of the speed of light in metal/dielectric composite, where the cylindrical nanoinclusions are uniformly distributed in the linear dielectric host matrix. The results testify that, within our approach, at the region of anomalous dispersion, light can travel with a group velocity greater than the speed of light in vacuum. In a composite with passive host matrix, the light pulse is absorbed within a very small distance. The problem of absorption can be reduced considerably by using an active host matrix.

Nonreciprocal transmission and asymmetric fast–slow light effect in an optomechanical system with two -symmetric mechanical resonators

We explore nonreciprocal transmission behaviors in an optomechanical system, in which two dissipative cavity modes are coupled with each other and also with two parity-time-symmetric mechanical modes. Two cavities, one of which is probed by a weak field, are driven by two strong control fields, respectively. With the active-passive mechanical-resonator scheme, such a closed-loop four-mode system can show an amplification behavior of the probe field with three transmission windows based on optomechanically induced transparency (OMIT). Due to the breaking of the time-reversal symmetry corresponding to the relative phase between two control fields, the amplified nonreciprocal transmission can be realized in the middle OMIT window and its direction can be controlled via the phase modulation. In addition, the system can also show asymmetric group velocities of light propagation, i.e. the tunable asymmetric fast–slow light effects, for example, slow lights along a direction and fast lights along the other one. It is of interest that the dynamic asymmetric fast-to-slow light conversion can be realized periodically by phase modulation. Such a system of three OMIT windows, acting as the schemes of the directional amplifier and all-optical switch of the direction and velocity of light, may provide underlying applications in the photonic network and information communicating process involving multi-signal transmission.

Dual-gate transistor amplifier in a multimode optomechanical system.

We present a dual-gate optical transistor based on a multimode optomechanical system, composed of three indirectly coupled cavities and an intermediate mechanical resonator pumped by a frequency-matched field. In this system, two cavities driven on the red mechanical sidebands are regarded as input/ouput gates/poles and the third one on the blue sideband as a basic/control gate/pole, while the resonator as the other basic/control gate/pole. As a nonreciprocal scheme, the significant unidirectional amplification can be resulted by controlling the two control gates/poles. In particular, the nonreciprocal direction of the optical amplification/rectification can be controlled by adjusting the phase differences between two red-sideband driving fields (the pumping and probe fields). Meanwhile, the narrow window that can be analyzed by the effective mechanical damping rate, arises from the extra blue-sideband cavity. Moreover, the tunable slow/fast light effect can be observed, i.e, the group velocity of the unidirectional transmission can be controlled, and thus the switching scheme of slow/fast light effect can also utilized to realize both slow and fast lights through opposite propagation directions, respectively. Such an amplification transistor scheme of controllable amplitude, direction and velocity may imply exciting opportunities for potential applications in photon networks and quantum information processing.

All-optically controlled slow and fast lights in graphene-coated tilted fiber Bragg grating

We demonstrate all-optically controlled slow and fast lights in tilted fiber Bragg grating (TFBG) with a graphene film. By employing graphene’s efficient photothermal effect, the global resonances in TFBG are shifted with slopes of 1.1 and 1.01 pm mW−1 for cladding and core modes, respectively. This enables changes of group delay and thus supports tunable fast and slow lights from −156 to 20 ps in cladding modes and up to 400 ps in core modes. Our demonstration with an all-in-fiber scheme possesses advantages, such as low-cost manufacture, simple and compact configuration, and multi-wavelength operation.

All-optical photon switching, router and amplifier using a passive-active optomechanical system

We propose a feasible and flexible scheme to realize all-optical photon switching, router and amplifier by using a passive-active optomechanical system consisting of a passive cavity, an active cavity and a common movable membrane oscillator. Due to the cooperation between the gain effect of the active cavity and the interference effect between the probe fields, some novel phenomena arise, such as the frequency-independent perfect reflection first proposed in this paper, controllable photon routing, tunable fast and slow lights and so on. Based on the results, a photon switching of absorption-amplification and fast-slow-light transitions, and a photon router with controllable direction and intensity can be realized by adjusting relevant parameters such as the strengths of control fields and the relative phase between probe fields. Such a scheme may provide a promising platform for controllable photon transport and photonic device fabrication.

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.

Nested Configuration of Silicon Microring Resonator With Multiple Coupling Regimes

We propose and demonstrate a nested configuration of a silicon microring resonator possessing multiple coupling regimes in one passive device. For different resonance notches of the transmission intensity spectrum, diverse notch depths and bandwidths owing to multiple coupling regimes are designed and experimentally achieved. The phase response of the proposed configuration is also investigated. Experimental observation of fast and slow lights at different resonance wavelengths further verifies the realization of multiple coupling regimes.

Few-cycle self-induced-transparency solitons

We reveal the existence of an optical self-induced-transparency soliton family in a two-level absorbing system down to the few-cycle limit. Based on the few-cycle envelope approximation, we introduce a systematical method of reducing the Maxwell-Bloch equations beyond the slowly varying envelope approximation and characterize the parameter space to achieve slow and fast lights. Verified by direct numerical simulations with full vectorial Maxwell-Bloch equations, we also give the relationships for a number of optical cycles on area theory and pulse group velocity, which demonstrate possible coherent photon-matter interactions.

Strong tunable slow and fast lights using a gain-clamped semiconductor optical amplifier

Previously demonstrated slow light is still far from applications, particularly due to the limited bandwidth and control speed. Although semiconductor-based slow light has the high bandwidth and sub-nanosecond control speed, slow light was observed only in the absorption regime with attenuation, while fast light observed in the gain regime with amplification. The large power difference in two regimes makes the use of the optical delay impractical. We report novel slow light in the gain regime, with a high power comparable to that of fast light, utilizing the anomalous gain characteristic in a gain-clamped semiconductor optical amplifier. The slow light is tunable to fast light with the current as the only variable. Additional high speed operation, fast delay control, and wide range of operation wavelength make the present approach practical.

Active chromatic control on the group velocity of light at arbitrary wavelength in benzocyclobutene polymer

A novel method is proposed to generate slow and fast lights at arbitrary signal wavelength in benzocyclobutene (BCB) polymer, which eliminates the requirement on the optical nonlinearity or the resonant effect at the signal wavelength with the help of the thermo-optic nonlinear effect induced by a control beam at a different but fixed wavelength. The signal group velocity can be precisely tuned simply by scanning the position of the BCB sample along the light propagation direction. Another advantage is the ability to control chromatically the group velocity of the signal beam by adjusting the control beam in the BCB sample, which, in essence, is to control the refractive index change experienced by the signal beam. This method provides an active chromatic control on the group velocity of light at arbitrary signal wavelength and therefore may have important potential applications in optical communication network and optical information processing.

Generalized fast and slow lights using multi-state microring resonators for optical wireless links

We propose an interesting result on fast and slow light generation using nonlinear microring resonators. When a soliton pulse is generated within the first ring resonator, the specific signals can be generated using the chaotic filter characteristics of light pulses travelling within the microring devices. We have demonstrated that two different frequency bands can be generated and they are suitable for use in optical wireless links. The other application of fast and slow lights is also presented, whereas signal security can be formed using the add/drop device to retrieve the original signal from the chaotic noisy signals. The key advantage is that the original signal can be kept in secret without any tapping due to the problem of fast light pulses and noisy signals, which are difficult to intercept.

SLOW AND FAST LIGHTS WITH MOVING AND STATIONARY REFRACTIVE INDEX GRATINGS IN SOLIDS AT ROOM TEMPERATURE

We reviewed the recent progress on slow and fast lights in solids at room temperature based on moving and stationary refractive index gratings. A dispersive photorefractive phase coupling associated with moving gratings results in slow and fast lights. In principle, such phase-coupling-induced slow and fast lights can be observed in any nonlinear wave mixing process with a dispersive phase coupling effect. The slow and fast lights in the stationary gratings are also discussed. One advantage of the stationary gratings is the possibility to engineer the dispersion slope of the grating through designing the grating structure and parameters. As an example, we show that the dispersion slope of the gratings is enhanced significantly by stratifying a series of identical volume index gratings with homogeneous optical buffer layers sandwiched between every two neighboring grating layers. The slow and fast lights, therefore, can be controlled more effectively in such specifically designed grating structures than in the homogeneous gratings. Another advantage is the high transparency of the slow and fast lights with appropriate grating structure and parameters. Issues such as the pulse broadening effect and the pulse distortion are addressed. The slow and fast light techniques have many important potential applications such as optical delay lines and optical buffers.

Electrically tunable slow and fast lights in a quantum-dot semiconductor optical amplifier near 155 μm

We have demonstrated both slow light in the absorption regime and fast light in the gain regime of a 1.55 microm quantum-dot semiconductor optical amplifier at room temperature. The theory with coherent population oscillations and four-wave mixing effects agrees well with the experimental results. We have observed a larger phase delay at the excited state than that at the ground state transition, likely due to the higher gain and smaller saturation power of the excited state.

The curing light dilemma

At this time, all the light-curing technologies on the market work adequately when used as directed by the manufacturers. The trend in the profession is to accept faster curing of resin-based composite. I agree completely. I have been using fast curing lights for many years, and I do not see any scientific or clinical reasons to change.

CURING RESTORATIVE RESIN: A SIGNIFICANT CONTROVERSY

Over the last several years, resin-curing lights have been introduced that cure resin in significantly less time than that in the past. Times of three seconds or less have been suggested by companies and are used by dentists. Dentists are eager to accept these fast lights, because of the significant reduction in practice time. However, there has been controversy on the subject. At almost the same time that the fast lights were introduced, several companies also introduced lights that cure at a slower rate, which is supposed to cause less resin shrinkage and stress in the tooth.