Waveguide-integrated optical modulators with two-dimensional materials

Waveguide-integrated optical modulators are indispensable for on-chip optical interconnects and optical computing.To cope with the ever-increasing amount of data being generated and consumed,ultrafast waveguide-integrated optical modulators with low energy consumption are highly demanded.In recent years,two-dimensional(2D)materials have attracted a lot of attention and have provided tremendous opportunities for the development of high-performance waveguide-integrated optical modulators because of their extraordinary optoelectronic properties and versatile compatibility.This paper reviews the state-of-the-art waveguide-integrated optical modulators with 2D materials,providing researchers with the developing trends in the field and allowing them to identify existing challenges and promising potential solutions.First,the concept and fundamental mechanisms of optical modulation with 2D materials are summarized.Second,a review of waveguide-integrated optical modulators employing electro-optic,all-optic,and thermo-optic effects is provided.Finally,the challenges and perspectives of waveguide-integrated modulators with 2D materials are discussed.

Fundamentals and applications of photonic waveguides with bound states in the continuum

Photonic waveguides are the most fundamental element for photonic integrated circuits(PICs).Waveguide properties,such as propagation loss,modal areas,nonlinear coefficients,etc.,directly determine the functionalities and performance of PICs.Recently,the emerging waveguides with bound states in the continuum(BICs)have opened new opportunities for PICs because of their special properties in resonance and radiation.Here,we review the recent progress of PICs composed of waveguides with BICs.First,fundamentals including background physics and design rules of a BIC-based waveguide will be introduced.Next,two types of BIC-based waveguide structures,including shallowly etched dielectric and hybrid waveguides,will be presented.Lastly,the challenges and opportunities of PICs with BICs will be discussed.

On-chip digitally tunable positive/negative dispersion controller using bidirectional chirped multimode waveguide gratings

A silicon-based digitally tunable positive/negative dispersion controller(DC)is proposed and realized for the first time using the cascaded bidirectional chirped multimode waveguide gratings(CMWGs),achieving positive and negative dispersion by switching the light propagation direction.A 1×2 Mach-Zehnder switch(MZS)and a 2×1 MZS are placed before and after to route the light path for realizing positive/negative switching.The device has Q stages of identical bidirectional CMWGs with a binary sequence.Thus the digital tuning is convenient and scalable,and the total dispersion accumulated by all the stages can be tuned digitally from−(2^(Q)−1)D0 to(2^(Q)−1)D_(0) with a step of D_(0) by controlling the switching states of all 2×2 MZSs,where D_(0) is the dispersion provided by a single bidirectional CMWG unit.Finally,a digitally tunable positive/negative DC with Q=4 is designed and fabricated.These CMWGs are designed with a 4-mm-long grating section,enabling the dispersion D_(0) of about 4.16 ps∕nm in a 20-nm-wide bandwidth.The dispersion is tuned from−61.53 to 63.77 ps∕nm by switching all MZSs appropriately,and the corresponding group delay is varied from−1021 to 1037 ps.

Acousto-optic modulation of photonic bound state in the continuum

Photonic bound states in the continuum(BICs)have recently been studied in various systems and have found wide applications in sensors,lasers,and filters.Applying BICs in photonic integrated circuits enables low-loss light guidance and routing in low-refractive-index waveguides on high-refractive-index substrates,which opens a new avenue for integrated photonics with functional single-crystal materials.Here,we demonstrate high-quality integrated lithium niobate microcavities inside which the photonic BIC modes circulate and further modulate these BIC modes acoustooptically by using piezoelectrically actuated surface acoustic waves at microwave frequencies.With a high acoustooptic modulation frequency,the acousto-optic coupling is well situated in the resolved-sideband regime.This leads to coherent coupling between microwave and optical photons,which is exhibited by the observed electro-acoustooptically induced transparency and absorption.Therefore,our devices serve as a paradigm for manipulating and controlling photonic BICs on a chip,which will enable many other applications of photonic BICs in the areas of microwave photonics and quantum information processing.

Genetically optimized on-chip wideband ultracompact reflectors and Fabry–Perot cavities

Reflectors are an essential component for on-chip integrated photonics. Here, we propose a new method for designing reflectors on the prevalent thin-film-on-insulator platform by using genetic-algorithm optimization.In simulation, the designed reflector with a footprint of only 2.16 μm× 2.16 μm can achieve ~97% reflectivity and 1 dB bandwidth as wide as 220 nm. The structure is composed of randomly distributed pixels and is highly robust against the inevitable corner rounding effect in device fabrication. In experiment, we fabricated on-chip Fabry–Perot(FP) cavities constructed from optimized reflectors. Those FP cavities have intrinsic quality factors of>2000 with the highest value beyond 4000 in a spectral width of 200 nm. The reflectivity fitted from the FP cavity resonances is >85% in the entire wavelength range of 1440–1640 nm and is beyond 95% at some wavelengths.The fabrication processes are CMOS compatible and require only one step of lithography and etch. The devices can be used as a standard module in integrated photonic circuitry for wide applications in on-chip semiconductorlaser structures and optical signal processing.

Toward calibration-free Mach-Zehnder switches for next-generation silicon photonics

Silicon photonic Mach–Zehnder switches(MZSs)have been extensively investigated as a promising candidate for optical systems.However,conventional 2×2 MZSs are usually prone to the size variations of the arm waveguides due to imperfect fabrication,resulting in considerable random phase imbalance between the two arms,thereby imposing significant challenges for further developing next-generation N×N MZSs.Here we propose a novel design toward calibration-free 2×2 and N×N MZSs,employing optimally widened arm waveguides,enabled by novel compact tapered Euler S-bends with incorporated mode filters.With standard 180 nm CMOS foundry processes,more than thirty 2×2 MZSs and one 4×4 Benes MZS with the new design are fabricated and characterized.Compared with their conventional counterparts with 0.45-μm-wide arm waveguides,the present 2×2 MZSs exhibit significant reduction in the random phase imbalance.The measured extinction ratios of the present 2×2 and 4×4 MZSs operating in the all-cross state are 27-49 dB and∼20dB across the wavelength range of∼60nm,respectively,even without any calibrations.This work paves the way toward calibration-free large-scale N×N MZSs for next-generation silicon photonics.

Compact electro-optic modulator on lithium niobate

Fast electro-optic modulators with an ultracompact footprint and low power consumption are always highly desired for optical interconnects.Here we propose and demonstrate a high-performance lithium niobate electro-optic modulator based on a new 2×2 Fabry–Perot cavity.In this structure,the input and reflected beams are separated by introducing asymmetric multimode-waveguide gratings,enabling TE_(0)−TE_(1)mode conversion.The measured results indicate that the fabricated modulator features a low excess loss of∼0.9dB,a high extinction ratio of∼21dB,a compact footprint of∼2120μm^(2),and high modulation speeds of 40 Gbps OOK and 80 Gbps PAM4 signals.The demonstrated modulator is promising for high-speed data transmission and signal processing.

Broadband meta-converters for multiple Laguerre-Gaussian modes

Metasurface provides miniaturized devices for integrated optics.Here,we design and realize a meta-converter to transform a plane-wave beam into multiple Laguerre-Gaussian(LG)modes of different orders at various diffraction angles.The metasurface is fabricated with Au nano-antennas,which vary in length and orientation angle for modulation of both the phase and the amplitude of a scattered wave,on a silica substrate.Our error analysis suggests that the metasurface design is robust over a 400 nm wavelength range.This work presents the manipulation of LG beams through controlling both radial and azimuthal orders,which paves the way in expanding the communication channels by one more dimension(i.e.,radial order)and demultiplexing different modes.

Circular Bragg lasers with radial PT symmetry:Design and analysis with a coupled-mode approach

Parity–time(PT) symmetry has been demonstrated in the frame of classic optics. Its applications in laser science have resulted in unconventional control and manipulation of resonant modes. PT-symmetric periodic circular Bragg lasers were previously proposed. Analyses with a transfer-matrix method have shown their superior properties of reduced threshold and enhanced modal discrimination between the radial modes. However, the properties of the azimuthal modes were not analyzed, which restricts further development of circular Bragg lasers. Here, we adopt the coupled-mode theory to design and analyze chirped circular Bragg lasers with radial PT symmetry. The new structures possess more versatile modal control with further enhanced modal discrimination between the azimuthal modes. We also analyze azimuthally modulated circular Bragg lasers with radial PT symmetry, which are shown to achieve even higher modal discrimination.