Xi Chen

Engineering of fast mode conversion in multimode waveguides

We propose fast and robust mode conversion in multimode waveguides based on Lewis-Riesenfeld invariant theory. The design of mode converters using the multimode driving for dynamical invariant is discussed. Computer-generated planar holograms are used to mimic the shaped pulses driving the states in three-level quantum systems. We show that the invariant-based inverse engineering scheme reduces mode converter length as compared to the common adiabatic scheme.

Giant bistable lateral shift owing to surface-plasmon excitation in Kretschmann configuration with a Kerr nonlinear dielectric

We investigate the reflection of a TM-polarized light beam from a Kretschmann configuration with a Kerr nonlinear dielectric. It is found that there exists a hysteretic response between the lateral shift of the reflected beam and the intensity of the incident beam. In contrast to the lower switch threshold, the higher switch threshold of optical bistability is more sensitive to the variations of the angle of incidence and the thickness of metal film. It is also found that the peak value of the lateral shift is strongly dependent on the thickness of metal film.

Controllable Goos-Hanchen shifts and spin beam splitter for ballistic electrons in a parabolic quantum well under a uniform magnetic field

The quantum Goos-Hanchen shift for ballistic electrons is investigated in a parabolic potential well under a uniform vertical magnetic field. It is found that the Goos-Hanchen shift can be negative as well as positive, and becomes zero at transmission resonances. The beam shift depends not only on the incident energy and incidence angle, but also on the magnetic field and Landau quantum number. Based on these phenomena, we propose an alternative way to realize the spin beam splitter in the proposed spintronic device, which can completely separate spin-up and spin-down electron beams by negative and positive Goos-Hanchen shifts.

Short and robust directional couplers designed by shortcuts to adiabaticity

We propose short and robust directional couplers designed by shortcuts to adiabaticity, based on Lewis-Riesenfeld invariant theory. The design of directional couplers is discussed by combining invariant-based inverse engineering and perturbation theory. The error sensitivity of the coupler is minimized by optimizing the evolution of dynamical invariant with respect to coupling coefficient/input wavelength variations. The proposed robust coupler devices are verified with beam propagation simulations.

Double-periodic quasi-periodic graphene superlattice: non-Bragg band gap and electronic transport

Electronic band gap and transport in quasi-periodic graphene superlattice (GSL) of double-periodic sequence are investigated. It is found that such quasi-periodic structure can possess a zero-averaged wave-number (zero- ) gap which is associated with an unusual Dirac point. Different from Bragg gap, the zero- gap is less sensitive to the incidence angle, and robust against the lattice constants. The locations of Dirac point and multi-Dirac-points in the GSLs of various sequences are also compared. The control of electron transport over the zero- band gap in GSLs may facilitate the development of many graphene-based electronics.

Vibrational mode multiplexing of ultracold atoms

Sending multiple messages on qubits encoded in different vibrational modes of cold atoms or ions along a transmission waveguide requires us to merge first and then separate the modes at input and output ends. Similarly, different qubits can be stored in the modes of a trap and be separated later. We design the fast splitting of a harmonic trap into an asymmetric double well so that the initial ground vibrational state becomes the ground state of one of two final wells, and the initial first excited state becomes the ground state of the other well. This might be done adiabatically by slowly deforming the trap. We speed up the process by inverse engineering a double-function trap using dynamical invariants. The separation (demultiplexing) followed by an inversion of the asymmetric bias and then by the reverse process (multiplexing) provides a population inversion protocol based solely on trap reshaping.

Giant negative and positive lateral shifts in graphene superlattices

Electronic analogue of generalized Goos-Hänchen shifts is investigated in the monolayer graphene superlattice with one-dimensional periodic potentials of square barriers. It is found that the lateral shifts for the electron beam transmitted through the monolayer graphene superlattice can be negative as well as positive near the band edges of zero-k̄ gap, which are different from those near the band edges of Bragg gap. These negative and positive beam shifts have close relation to the Dirac point. When the condition qAdA = −qBdB = mπ (m = 1, 2, 3...) is satisfied, the beam shifts can be controlled from negative to positive when the incident energy is above the Dirac point, and vice versa. In addition, the beam shifts can be greatly enhanced by the defect mode inside the zero-k̄ gap. These intriguing phenomena can be verified in a relatively simple optical setup, and have potential applications in the graphene-based electron wave devices.

Short-length and robust polarization rotators in periodically poled lithium niobate via shortcuts to adiabaticity.

Conventional narrowband spectrum polarization devices are short but not robust, based on quasi-phase matching (QPM) technique, in periodically poled lithium niobate (PPLN) crystal. In this paper, we propose short-length and robust polarization rotators by using shortcuts to adiabaticity. Beyond the QPM condition, the electric field and period of PPLN crystal are designed in terms of invariant dynamics, and further optimized with respect to input wavelength/refractive index variations. In addition, the stability of conversion efficiency on the electric field and period of PPLN crystal is also discussed. As a consequence, the optimal shortcuts are fast as well as robust, which provide broadband spectrum polarization devices with short length.

Energy flux and Goos–Hänchen shift in frustrated total internal reflection

Using Yasumoto and Õishis energy flux method, a generalized analytical formulation for analyzing the Goos-Hänchen (GH) shift in frustrated total internal reflection is provided, from which the GH shift given by Artmans stationary phase method is shown to equal the GH calculated by Renards conventional energy flux method plus a self-interference shift. The self-interference shift, originating from the interference between the incident and reflected beams, sheds light on the asymptotic behavior of the GH shift in such optical tunneling process in term of energy flux.

Negative and positive lateral shifts: a result of beam reshaping caused by interference

The origin of negative and positive lateral shifts of the light beams in a dielectric slab configuration is investigated from the viewpoint of the interference between multiple light beams, due to the multiple reflections and transmissions inside the slab. It is shown that the whole reflected and transmitted beams with negative shifts can be obtained from the successively reflected and transmitted constituents, respectively, with positive displacements predicted from geometrical optics. When the slabs thickness is much smaller in comparison with the width of the beam waist, the reshaping reflected and transmitted beams retain the profile of the incident beam. The physical mechanism presented here could also be applicable in investigating other counterintuitive phenomena, such as the superluminal traversal time in quantum tunneling and dispersive media.