Siqi Yan

Slow-light-enhanced energy efficiency for graphene microheaters on silicon photonic crystal waveguides

Slow light has been widely utilized to obtain enhanced nonlinearities, enhanced spontaneous emissions and increased phase shifts owing to its ability to promote light–matter interactions. By incorporating a graphene on a slow-light silicon photonic crystal waveguide, here we experimentally demonstrate an energy-efficient graphene microheater with a tuning efficiency of 1.07 nmmW−1 and power consumption per free spectral range of 3.99 mW. The rise and decay times (10–90%) are only 750 and 525 ns, which, to the best of our knowledge, are the fastest reported response times for microheaters in silicon photonics. The corresponding figure of merit of the device is 2.543 nW s, one order of magnitude better than results reported in previous studies. The influence of the length and shape of the graphene heater to the tuning efficiency is further investigated, providing valuable guidelines for enhancing the tuning efficiency of the graphene microheater.

Generation of Terahertz Vortices Using Metasurface With Circular Slits

We propose a metal device containing circular slits to generate a terahertz (THz) orbital angular momentum beam with numerical simulations. The estimation of the polarization extinction ratio is above 20 dB over the bandwidth ranging from 0.3 to 3 THz, and a mode purity of TC = 1 or -1 is close to 1 over a wide bandwidth range, except for the area near the deteriorated valley. We analyze the OAM spectrum and find that the main noise comes from an OAM mode of TC = -3 or 3. When multiple concentric circular slits are employed, a larger transmittance is obtained without the sacrifice of mode purity. The design of such a device is simple with a size of micrometer order, revealing an option to generate a broadband THz vortex beam.

Double metal subwavelength slit arrays interference to measure the orbital angular momentum and the polarization of light

We put forward a double-slit interference device based on two metal subwavelength slit arrays to measure the orbital angular momentum (OAM) and the polarization of beams simultaneously. The subwavelength slit serves as a localized spatial polarizer, and each slit array can be regarded as a wide diffraction-slit. When an OAM beam is normally incident upon the two slit arrays, the interference fringes twist, and the displacement depends on the topological charge of OAM beams. We present a detailed theoretical analysis of this measurement model. This model does not need additional reference light and is a linear model.

Chip-integrated optical power limiter based on an all-passive micro-ring resonator

Recent progress in silicon nanophotonics has dramatically advanced the possible realization of large-scale on-chip optical interconnects integration. Adopting photons as information carriers can break the performance bottleneck of electronic integrated circuit such as serious thermal losses and poor process rates. However, in integrated photonics circuits, few reported work can impose an upper limit of optical power therefore prevent the optical device from harm caused by high power. In this study, we experimentally demonstrate a feasible integrated scheme based on a single all-passive micro-ring resonator to realize the optical power limitation which has a similar function of current limiting circuit in electronics. Besides, we analyze the performance of optical power limiter at various signal bit rates. The results show that the proposed device can limit the signal power effectively at a bit rate up to 20 Gbit/s without deteriorating the signal. Meanwhile, this ultra-compact silicon device can be completely compatible with the electronic technology (typically complementary metal-oxide semiconductor technology), which may pave the way of very large scale integrated photonic circuits for all-optical information processors and artificial intelligence systems.

Operation bandwidth optimization of photonic differentiators

We theoretically investigate the operation bandwidth limitation of the photonic differentiator including the upper limitation, which is restrained by the device operation bandwidth and the lower limitation, which is restrained by the energy efficiency (EE) and detecting noise level. Taking the silicon photonic crystal L3 nano-cavity (PCN) as an example, for the first time, we experimentally demonstrate that the lower limitation of the operation bandwidth does exist and differentiators with different bandwidths have significantly different acceptable pulse width range of input signals, which are consistent to the theoretical prediction. Furthermore, we put forward a novel photonic differentiator scheme employing cascaded PCNs with different Q factors, which is likely to expand the operation bandwidth range of photonic differentiator dramatically.

Manipulation of orbital angular momentum beams based on space diffraction compensation

We put forward a technique to manipulate the size of orbital angular momentum (OAM) beams based on space diffraction compensation. Paraxial Fresnel diffraction which carries a negative spatial quadratic phase distribution can be regarded as a negative diffractive effect. To compensate the negative diffraction, we employ a 4f Fourier lens system containing a phase mask to generate an inverse quadratic phase. The size of OAM beams can be easily controlled by designing the phase mask profile without changing the OAM. The applications of space diffraction compensation in OAM demultiplexing, ring fiber coupling for OAM beams and optical manipulation of micro particles are also discussed.

Bandwidth-adaptable silicon photonic differentiator employing a slow light effect

A photonic differentiator (DIFF) plays a crucial role in photonic circuits. Despite the fact that a DIFF having a terahertz bandwidth has been reported, the practical bandwidth is limited to being a bandpass response. In this Letter, we propose the concept of a bandwidth-adaptable DIFF, which exploits the slow light effect in a photonic crystal waveguide (PhCW) to overcome the inherent bandwidth limitation of current photonic DIFFs. We fabricated a PhCW Mach-Zehnder interferometer (PhCW-MZI) on the silicon-on-isolator material platform to validate our concept. Input Gaussian pulses with full width to half-maximums (FWHMs) ranging from 2.7 to 81.4 ps are accurately differentiated using our PhCW-MZI. Our all-passive scheme circumvents the bandwidth bottlenecks of previously reported photonic DIFFs and can greatly broaden the application area of photonic DIFFs.

Photonic arbitrary waveform generator based on Taylor synthesis method

Arbitrary waveform generation has been widely used in optical communication, radar system and many other applications. We propose and experimentally demonstrate a silicon-on-insulator (SOI) on chip optical arbitrary waveform generator, which is based on Taylor synthesis method. In our scheme, a Gaussian pulse is launched to some cascaded microrings to obtain first-, second- and third-order differentiations. By controlling amplitude and phase of the initial pulse and successive differentiations, we can realize an arbitrary waveform generator according to Taylor expansion. We obtain several typical waveforms such as square waveform, triangular waveform, flat-top waveform, sawtooth waveform, Gaussian waveform and so on. Unlike other schemes based on Fourier synthesis or frequency-to-time mapping, our scheme is based on Taylor synthesis method. Our scheme does not require any spectral disperser or large dispersion, which are difficult to fabricate on chip. Our scheme is compact and capable for integration with electronics.

Temporal imaging using a time pinhole

We experimentally demonstrate a temporal imaging system based on a time pinhole. In accordance with the spatial pinhole-imaging counterpart, it consists of two sections of dispersion fibers connected by a temporal shutter, which is experimentally realized by a logic AND-gate with a short pulse. Both theoretical analysis and experimental results show that the output waveform is the scaled and broadened profile of the input waveform. Specifically, the output waveform is reversed if the signs of the dispersion on both sides of the time-gate are identical, otherwise it is non-reversed if the signs of the dispersion are opposite. Furthermore, we adjust the duration of the temporal shutter by changing the spectral width of the pulse, and investigate the effect of the shutter time on the performance of the output waveform.

Advances on silicon-based integrated microwave photonics

Microwave photonic systems have huge potential for both existing and future applications, including radar, radiofrequency sensing and modern wireless communications due to their distinct advantages in terms of ultra-wide bandwidth, flexible tunability, and immunity to electromagnetic interference. There is a strong research trend in microwave photonic systems towards integration and miniaturization, resulting in multiple radio frequency functions on a single chip which is both compact and light weight. Thus integrated microwave photonics has attracted a lot of attentions and achieves significant improvements in last ten years. In this paper, we will review some research progresses on silicon-based integrated microwave photonics in our group, including highly efficient micro heater on silicon photonic chip, chip-scale microwave waveform generation, on-chip true time delay, and microwave photonic processing and measurement. Our schemes are all fabricated on silicon-on-insulator chips and have advantages of compactness and capability to integrate with electronic units. These chips may motivate the great application potentials in silicon-based integrated microwave photonics.