Ye-Long Xu

Experimental demonstration of a unidirectional reflectionless parity-time metamaterial at optical frequencies

Invisibility by metamaterials is of great interest, where optical properties are manipulated in the real permittivity-permeability plane. However, the most effective approach to achieving invisibility in various military applications is to absorb the electromagnetic waves emitted from radar to minimize the corresponding reflection and scattering, such that no signal gets bounced back. Here, we show the experimental realization of chip-scale unidirectional reflectionless optical metamaterials near the spontaneous parity-time symmetry phase transition point where reflection from one side is significantly suppressed. This is enabled by engineering the corresponding optical properties of the designed parity-time metamaterial in the complex dielectric permittivity plane. Numerical simulations and experimental verification consistently exhibit asymmetric reflection with high contrast ratios around a wavelength of of 1,550 nm. The demonstrated unidirectional phenomenon at the corresponding parity-time exceptional point on-a-chip confirms the feasibility of creating complicated on-chip parity-time metamaterials and optical devices based on their properties.

Nonreciprocal light propagation in a silicon photonic circuit.

An engineered metallic-silicon waveguide allows for direction-dependent light propagation. Optical communications and computing require on-chip nonreciprocal light propagation to isolate and stabilize different chip-scale optical components. We have designed and fabricated a metallic-silicon waveguide system in which the optical potential is modulated along the length of the waveguide such that nonreciprocal light propagation is obtained on a silicon photonic chip. Nonreciprocal light transport and one-way photonic mode conversion are demonstrated at the wavelength of 1.55 micrometers in both simulations and experiments. Our system is compatible with conventional complementary metal-oxide-semiconductor processing, providing a way to chip-scale optical isolators for optical communications and computing.

Acoustic asymmetric transmission based on time-dependent dynamical scattering.

An acoustic asymmetric transmission device exhibiting unidirectional transmission property for acoustic waves is extremely desirable in many practical scenarios. Such a unique property may be realized in various configurations utilizing acoustic Zeeman effects in moving media as well as frequency-conversion in passive nonlinear acoustic systems and in active acoustic systems. Here we demonstrate a new acoustic frequency conversion process in a time-varying system, consisting of a rotating blade and the surrounding air. The scattered acoustic waves from this time-varying system experience frequency shifts, which are linearly dependent on the blade’s rotating frequency. Such scattering mechanism can be well described theoretically by an acoustic linear time-varying perturbation theory. Combining such time-varying scattering effects with highly efficient acoustic filtering, we successfully develop a tunable acoustic unidirectional device with 20 dB power transmission contrast ratio between two counter propagation directions at audible frequencies.

Experimental realization of Bloch oscillations in a parity-time synthetic silicon photonic lattice

As an important electron transportation phenomenon, Bloch oscillations have been extensively studied in condensed matter. Due to the similarity in wave properties between electrons and other quantum particles, Bloch oscillations have been observed in atom lattices, photonic lattices, and so on. One of the many distinct advantages for choosing these systems over the regular electronic systems is the versatility in engineering artificial potentials. Here by utilizing dissipative elements in a CMOS-compatible photonic platform to create a periodic complex potential and by exploiting the emerging concept of parity-time synthetic photonics, we experimentally realize spatial Bloch oscillations in a non-Hermitian photonic system on a chip level. Our demonstration may have significant impact in the field of quantum simulation by following the recent trend of moving complicated table-top quantum optics experiments onto the fully integrated CMOS-compatible silicon platform.

Acoustic rainbow trapping by coiling up space

We numerically realize the acoustic rainbow trapping effect by tapping an air waveguide with space-coiling metamaterials. Due to the high refractive-index of the space-coiling metamaterials, our device is more compact compared to the reported trapped-rainbow devices. A numerical model utilizing effective parameters is also calculated, whose results are consistent well with the direct numerical simulation of space-coiling structure. Moreover, such device with the capability of dropping different frequency components of a broadband incident temporal acoustic signal into different channels can function as an acoustic wavelength division de-multiplexer. These results may have potential applications in acoustic device design such as an acoustic filter and an artificial cochlea.

Highly efficient and perfectly vertical chip-to-fiber dual-layer grating coupler

A novel high-efficiency silicon-chip-to-fiber grating coupler is investigated here. By introducing a dual layer grating structure with an inter-layer lateral shift to mimic 45° tilted mirror behavior, perfectly vertical coupling is successfully demonstrated. Our numerical results show that a peak silicon-chip-to-fiber coupling efficiency about 70% is possible near 1550 nm. Meanwhile, for the entire telecom C-band, i.e. wavelengths from 1530 nm to 1565 nm, the coupling efficiency is > 50% and the back reflection is less than < 1%. Our proposed high-performance silicon perfectly vertical coupling structure is suitable for interfacing with multi-core fiber platform, which may play an important role in the future CMOS photonic integration technology.

Asymmetric diffraction based on a passive parity-time grating

Optical structures with balanced loss and gain provide an efficient platform to study the features of light propagation under non-Hermitian parity-time symmetry. Here, we report a feasible design of one-dimensional parity-time symmetric diffraction grating, where the real and imaginary parts of refractive index are separately modulated. Due to the spontaneous breaking of parity-time symmetry at the exceptional point, asymmetric diffractions are observed between a pair of oblique incident light. This asymmetric phenomenon, determined by the modulation direction of the introduced parity-time symmetry, is also polarization-dependent. The coupled-mode theory is implemented to theoretically analyze the polarization dependent asymmetric diffraction, showing consistence with numerical simulations. Our findings may provide a feasible way for manipulating light and instructively inspire the development of diffraction optics.

Unidirectional Transmission Based on a Passive PT Symmetric Grating With a Nonlinear Silicon Distributed Bragg Reflector Cavity

A silicon waveguide consisting of passive parity-time (PT) symmetry optical potentials connected with a distributed Bragg reflector (DBR)-based resonator is proposed to achieve nonreciprocal waveguide transmission. The unidirectional reflectionless waveguide implementing PT symmetry is discussed by consistent coupling mode theory and finite-difference time-domain (FDTD) simulation results. Due to the high field confinement in the DBR cavity, transmission of a light pulse through the device is analyzed in the nonlinear optical regime using FDTD simulations. The combination of the nonlinear DBR cavity and the passive PT symmetric grating results in unidirectional transmission in the silicon waveguide, while still keeping the reflection minimum in the desired direction. The numerical simulation shows an extinction ratio of about 20 dB at the telecom wavelength of around 1550 nm.

Optical Isolation by Time-Dependent Sinusoidal-Shaped Structures

Design of nonmagnetic optical isolators integrated with silicon waveguides is important for rapidly growing silicon photonics in optical communications. We introduce a silicon waveguide consisting of an array of complex sinusoidal-shaped structures that create the designed time-dependent modulation of refractive indices. These time-dependent sinusoidal-shaped structures are engineered to have in-phase mode conversion only in one direction, thus leading to asymmetric optical mode conversion in the silicon waveguide and one-way light transmission with high contrast ratios, confirmed by numerically simulated field maps and calculated transmission at the wavelength of 1550 nm. Our design may offer another practical design to the compact chip-scale optical isolators in silicon waveguides.

Nonreciprocal light propagation on an integrated silicon photonic chip

We have designed, fabricated, and tested a CMOS compatible silicon waveguide system that demonstrates nonreciprocal light propagation on-chip, by mimicking microscopic non-Hermitian optical potentials for guided light and thus spontaneously breaking parity-time symmetry. Experiments were performed at 1.55 µm wavelength for potential telecommunication applications.