arXiv subjects

Zhicheng Xiao

Publications and source records attributed to Zhicheng Xiao.

7 recordsLinked to original sources

arXiv2019

Controlling photonic spin Hall effect via exceptional points

The photonic spin Hall effect (SHE), featured by a spin-dependent transverse shift of an impinging optical beam driven by its polarization handedness, has many applications including precise metrology and spin-based nanophotonic devices. It is highly desirable to control and enhance the photonic SHE. However, such a goal remains elusive, due to the weak spin-orbit interaction of light, especially for systems with optical loss. Here we reveal a flexible way to modulate the photonic SHE via exceptional points, by exploiting the transverse shift in a parity-time (PT) symmetric system with balanced gain and loss. The underlying physics is associated with the near-zero value and abrupt phase jump of the reflection coefficients at exceptional points. We find that the transverse shift is zero at exceptional points, but it is largely enhanced in their vicinity. In addition, the transverse shift switches its sign across the exceptional point, resulting from spontaneous PT-symmetry breaking. Due to the sensitivity of transverse shift at exceptional points, our work also indicates that the photonic SHE can enable a precise way to probe the location of exceptional point in photonic systems.

physics.optics↗

arXiv2019

Highly-Linear Magnet-Free Microelectromechanical Circulators

This paper reports the first demonstration of a magnet-free, high performance microelectromechanical system (MEMS) based circulator. An innovative circuit based on the commutation of MEMS resonators with high quality (Q) factor using RF switches is designed and implemented. Thanks to the high Q factor, a much smaller modulation frequency can be achieved compared to the previous demonstrations, reducing the power consumption and enabling the use of high power-handling switches. Furthermore, the MEMS resonators greatly reduce the required inductance value, guaranteeing much smaller form factor compared to the previous LC demonstrations. The demonstrated circulator shows broad BW (15 dB-IX BW=34.7 MHz for an operational frequency around 2.5 GHz), low IL (4 dB), high IX (30 dB), high linearity (P1dB=28 dBm; IIP3=40 dBm) and at the same time low power consumption, addressing several of the current limitations hindering the full development of magnet-free circulators.

physics.app-ph↗

arXiv2019

Radio Frequency Magnet-free Circulators Based on Spatiotemporal Modulation of Surface Acoustic Wave Filters

In this paper, a new generation of magnet-free circulators with high performance is proposed. Circulators are crucial devices in modern communication systems due to their ability to enable full-duplexing and double the spectral efficiency directly in the physical layer of the radio-frequency (RF) front-end. Traditionally, Lorentz reciprocity is broken by applying magnetic bias to ferrite materials, therefore conventional circulators are bulky and expensive. In this paper, this problem is addressed by replacing the magnetic bias with periodic spatiotemporal modulation. Compared to previous works, the proposed circulator is constructed using surface acoustic wave (SAW) filters instead of transmission lines (TL), which reduces the modulation frequency by at least a factor of 20 and ensures ultra-low power consumption and high linearity. The miniaturized high quality (Q) factor SAW filters also lead to a low-loss non-reciprocal band with strong isolation (IX) and broad bandwidth (BW) on a chip scale, therefore addressing such limitations in previous magnet-free demonstrations. Furthermore, compared to the conventional differential circuit configuration, a novel quad configuration is developed, which doubles the intermodulation-free bandwidth.

eess.SP↗

arXiv2018

Broadband Cyclic-Symmetric Magnet-less Circulators and Theoretical Bounds on their Bandwidth

In this paper, we explore theoretically and experimentally broadband spatiotemporally modulated (STM) magnet-less circulators realized by combining three-port non-reciprocal junctions with three identical bandpass filters. We develop a rigorous theory for the proposed circuit, which allows to optimize their design and to derive a global bound on the maximum possible bandwidth (BW) for cyclic-symmetric magnetless circulators. We verify our theory with simulations and measurements of a printed circuit board (PCB) prototype based on a differential wye junction and second-order Chebyshev bandpass filters, resulting in a measured fractional BW of 13.9% at a center frequency of 1 GHz.

eess.SP↗

arXiv2018

Non-reciprocal Components Based on Switched Transmission Lines

Non-reciprocal components, such as isolators and circulators, are critical to wireless communication and radar applications. Traditionally, non-reciprocal components have been implemented using ferrite materials, which exhibit non-reciprocity under the influence of an external magnetic field. However, ferrite materials cannot be integrated into IC fabrication processes, and consequently are bulky and expensive. In the recent past, there has been strong interest in achieving non-reciprocity in a non-magnetic IC-compatible fashion using spatio-temporal modulation. In this paper, we present a general approach to non-reciprocity based on switched transmission lines. Switched transmission lines enable broadband, lossless and compact non-reciprocity, and a wide range of non-reciprocal functionalities, including non-reciprocal phase shifters, ultra-broadband gyrators and isolators, frequency-conversion isolators, and high-linearity/high-frequency/ultra-broadband circulators. We present a detailed theoretical analysis of the various non-idealities that impact insertion loss and provide design guidelines. The theory is validated by experimental results from discrete-component-based gyrators and isolators, and a 25GHz circulator fabricated in 45nm SOI CMOS technology.

eess.SP↗

arXiv2012

Goos-Hänchen and Imbert-Fedorov Shifts of Vortex Beams at Air-Left-Handed Material Interfaces

In this paper, we present a systematic study of beam shifts and angular momenta of paraxial vortex beams at air-left-handed material (LHM) interfaces. It is shown that, compared to their counterparts at air-right-handed material (RHM) interfaces, the spatial Goos-Hänchen (GH) and Imbert-Fedorov (IF) shifts remains the same, while the angular GH and IF shifts are reversed, at air-LHM interfaces. The spatial and angular shifts of paraxial vortex beams have their respective origins in transverse angular momenta and transverse linear momenta. The spatial GH and IF shifts remain unreversed as a result of the both reversions of transverse angular momenta and z-component linear momentum, while the angular GH and IF shifts are reversed due to that the z-component linear momentum is reversed and the transverse linear momenta are unreversed, at air-LHM interfaces. In addition, we perform a quantitative analysis on spin-orbit angular momentum conversion and orbit-orbit angular momentum conversion, which further helps us understand the essence of vortex beam shifts at air-LHM interfaces and their fundamental distinctions with those at air-RHM interfaces.

physics.optics↗

arXiv2012

Experimental observation of the spin Hall effect of light on a nano-metal film via weak measurements

We theorize the spin Hall effect of light (SHEL) on a nano-metal film and demonstrate it experimentally via weak measurements. A general propagation model to describe the relationship between the spin-orbit coupling and the thickness of the metal film is established. It is revealed that the spin-orbit coupling in the SHEL can be effectively modulated by adjusting the thickness of the metal film, and the transverse displacement is sensitive to the thickness of metal film in certain range for horizontal polarization light. Importantly, a large negative transverse shift can be observed as a consequence of the combined contribution of the ratio and the phase difference of Fresnel coefficients.

physics.optics↗