Ray-Kuang Lee

Coherent control of optical bistability in an open Λ-type three-level atomic system

We analyze the hybrid absorptive-dispersive optical bistability (OB) behavior in an open Λ-type three-level atomic system by using a microwave field to drive the hyperfine transition between two lower states, along with the consideration of incoherent pumping and spontaneously generated coherence. Different from the closed system, we show that the bistable threshold intensity and related hysteresis loop can be controlled by adjusting the ratio between atomic injection and exit rates. More interestingly, the appearance and disappearance of OB can be transformed mutually by varying the relative phase of three coherent fields under the condition of a strong spontaneously generated coherence. The manipulation of OB behavior through the intensity of the microwave field and the atomic cooperation parameter is also analyzed.

Design of multichannel DWDM fiber Bragg grating filters by Lagrange multiplier constrained optimization.

We present the synthesis of multi-channel fiber Bragg grating (MCFBG) filters for dense wavelength-division-multiplexing (DWDM) application by using a simple optimization approach based on a Lagrange multiplier optimization (LMO) method. We demonstrate for the first time that the LMO method can be used to constrain various parameters of the designed MCFBG filters for practical application demands and fabrication requirements. The designed filters have a number of merits, i.e., flat-top and low dispersion spectral response as well as single stage. Above all, the maximum amplitude of the index modulation profiles of the designed MCFBGs can be substantially reduced under the applied constrained condition. The simulation results demonstrate that the LMO algorithm can provide a potential alternative for complex fiber grating filter design problems.

Local PT Symmetry Violates the No-Signaling Principle

Bender et al. [Phys. Rev. Lett. 80, 5243 (1998)] have developed PT-symmetric quantum theory as an extension of quantum theory to non-Hermitian Hamiltonians. We show that when this model has a local PT symmetry acting on composite systems, it violates the nonsignaling principle of relativity. Since the case of global PT symmetry is known to reduce to standard quantum mechanics A. Mostafazadeh [J. Math. Phys. 43, 205 (2001)], this shows that the PT-symmetric theory is either a trivial extension or likely false as a fundamental theory.

Giant Kerr nonlinearities and slow optical solitons in coupled double quantum-well nanostructure

We investigate the enhancement of Kerr nonlinearities in an asymmetric coupled double quantum-well (CQW) by analyzing the nonlinear optical response. With the consideration of real parameters in AlGaAs-based CQWs, we indicate the possibility to obtain the giant Kerr nonlinearities with the cancellation of linear absorption simultaneously. We also reveal both analytically and numerically that under appropriate conditions a probe field with very low intensity can evolve into a stable shape-preserving waveform that propagates with a slow group velocity. The stability of such slow optical solitons is also demonstrated numerically.

Slow optical solitons via intersubband transitions in a semiconductor quantum well

We show the formation of bright and dark slow optical solitons based on intersubband transitions in a semiconductor quantum well (SQW). Using the coupled Schrodinger-Maxwell approach, we provide both analytical and numerical results. Such a nonlinear optical process may be used for the control technology of optical delay lines and optical buffers in the SQW solid-state system. With appropriate parameters, we also show the generation of a large cross-phase modulation (XPM). Since the intersubband energy level can be easily tuned by an external bias voltage, the present investigation may open possibilities for electrically controlled phase modulators in solid systems.

Dark-bright soliton pairs in nonlocal nonlinear media

We study the formation of dark-bright vector soliton pairs in nonlocal Kerr-type nonlinear medium. We show, by analytical analysis and direct numerical calculation, that in addition to stabilize of vector soliton pairs nonlocal nonlinearity also helps to reduce the threshold power for forming a guided bright soliton. With help of the nonlocality, it is expected that the observation of dark-bright vector soliton pairs in experiments becomes more workable.

Nonlocal dark solitons under competing cubic–quintic nonlinearities

We investigate properties of dark solitons under competing nonlocal cubic-local quintic nonlinearities. Analytical results, based on a variational approach and confirmed by direct numerical simulations, reveal the existence of a unique dark soliton solutions with their width being independent of the degree of nonlocality, due to the competing cubic-quintic nonlinearities.

Optical solitons and wave-particle duality.

We investigate the propagation of optical solitons interacting with linear defects in the medium. We show that solitons exhibit a wave-particle dualism versus power, i.e., depending on the relative size of soliton and defect, responsible for a soliton trajectory dependent on its waist.

Controllable entanglement and polarization phase gate in coupled double quantum-well structures.

By analyzing the nonlinear optical response in an asymmetric coupled double quantum well structure based on the intersubband transitions, we show that a giant Kerr nonlinearity with a relatively large cross-phase modulation coefficient can be used to produce efficient photonphoton entanglement and implement an all-optical two-qubit quantum polarization phase gate. We also demonstrate that such photon-photon entanglement is practically controllable and may facilitate more practical applications in all-optical quantum information and computation.

Ultrafast single-electron transfer in coupled quantum dots driven by a few-cycle chirped pulse

We theoretically study the ultrafast transfer of a single electron between the ground states of a coupled double quantum dot (QD) structure driven by a nonlinear chirped few-cycle laser pulse. A time-dependent Schrodinger equation without the rotating wave approximation is solved numerically. We demonstrate numerically the possibility to have a complete transfer of a single electron by choosing appropriate values of chirped rate parameters and the intensity of the pulse. Even in the presence of the spontaneous emission and dephasing processes of the QD system, high-efficiency coherent transfer of a single electron can be obtained in a wide range of the pulse parameters. Our results illustrate the potential to utilize few-cycle pulses for the excitation in coupled quantum dot systems through the nonlinear chirp parameter control, as well as a guidance in the design of experimental implementation.