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We numerically investigate two Fano resonances with high Q-factors based on a permittivity-asymmetric metastructure composed of two pea-shaped cylinders. By employing different materials to break the permittivity-asymmetry, the quasi-bound state of the continuum spectrum (BIC) resonance at 982.87 nm is excited, showing the Q-factor as high as 8183.7. The electromagnetic fields and vectors are analyzed by using the finite-difference time-domain (FDTD) method, and the resonance modes are identified as magnetic dipole (MD) responses and MDs by multipole decomposition in Cartesian coordinates, displaying that the light is confined within a pea-shaped cylinder to achieve localized field enhancement. In addition, the sensing performances of the metastructure are evaluated, and an optical refractive index sensor can be obtained with the sensitivity of 152 nm/RIU and maximum figure of merit (FOM) of 832.6. This proposed structure offers a new, to the best of our knowledge, way to achieve Fano resonant excitation on all-dielectric metastructures and can be used in nonlinear optics, biosensing, optical switches, and lasers.

We have designed three mode order converters using the sub-wavelength grating on the silicon-on-insulator substrate. The proposed mode order converters can separately realize the mode order conversion from the fundamental transverse electric mode to the first-order transverse electric mode (TE 0 -to-TE 1 ), the second-order transverse electric mode (TE 0 -to-TE 2 ) and the third-order transverse electric mode (TE 0 -to-TE 3 ) with compact device sizes and good performances. The simulation results show that the mode order conversion efficiencies of TE 0 -to-TE 1 , TE 0 -to-TE 2 and TE 0 -to-TE 3 are larger than 94.4%, 95.7% and 83.7% in the wavelength ranging from 1.5 µm to 1.6 µm, the corresponding device length are 8.72 µm, 4.98 µm and 14.54 µm. In addition, the mode order converter can be fabricated with only once etching which will be advantage to the device fabrication.

We present a two-element back-illuminated symmetric-connected uni-traveling-carrier photodiode array (SC-PDA) integrated with sub-wavelength-gratings based beam-splitter (SWGs-BS) for high power optoelectronic applications. The SC-PDA with SWGs-BS, a top-illuminated SC-PDA and a single PD were fabricated and tested. The proposed SC-PDA with SWGs-BS demonstrates a 3dB bandwidth of 23.8GHz@60mA, a saturation current of 87.9mA@12GHz and a maximum output RF power of ~16dBm@12GHz. Compared to the top-illuminated SC-PDA and the single PD, the proposed SC-PDA with SWGs-BS achieved high-power handling capability at low coupling complexity and requires no phase-matching techniques in the system.

We propose a planar dielectric reflector with focusing ability using concentric circular subwavelength gratings (CC-SWGs). The two-dimensional focusing ability of CC-SWGs is investigated by the rigorous coupled-wave analysis (RCWA) and finite element method (FEM). By designing the concentric circular pattern of the grating surface, a focusing reflector with high numerical aperture (NA) and high reflectivity is constructed. A CC-SWG reflector with a diameter of 32.6μm and a focal length of 6μm is investigated, which exhibits high focusing ability at normal incidence with a radially polarized plane wave. At the reflection focal plane, the full-width-half-maximum (FWHM) of the electric field intensity is 0.89μm. Numerical aperture value as high as 0.93 is achieved for the reflector with very high reflectivity of 92%.

We present a concise approach to design focusing reflectors using concentric circular subwavelength high-contrast gratings (CCSW-HCGs). With this method, we design a high-numerical-aperture, high-reflectivity CCSW-HCG focusing reflector. By designing the CCSW-HCG structure, the grating can give the target phase front of reflected light while maintaining a high reflectivity. The properties of focusing and reflection are numerically studied with the finite element method. Simulation results show that, when the incident wavelength is 1550 nm, the reflectivity is 0.8538, and the numerical aperture is up to 0.9213. At the reflection focal plane, the full width at half-maximum of the field distribution is 0.9845 μm. Furthermore, the CCSW-HCG can simultaneously focus the reflected and transmitted light and has a strong robustness.

A reflection-enhanced dual-absorption InP-PIN/GaAs-DBR photodetector was fabricated and characterized. The photodetector is monolithically integrated using a heteroepitaxy growth of an InGaAs/InP dual-absorption "PINIP" structure on the GaAs/AlGaAs Bragg reflectors. These features lead to an increase in quantum efficiency over a wide wavelength range while maintaining a high speed. The measured quantum efficiency was increased by 48.8% in comparison with that without reflectors. A quantum efficiency of 64% at a wavelength of 1522 nm and a 3 dB bandwidth of 26 GHz at a reverse bias of 3 V were simultaneously obtained in the device.

A subwavelength plasmonic waveguide composed of a pair of comb-shape nanorod chains is proposed. The electromagnetic energy can be transported in the waveguide via the interaction strength of magnetoinductive coupling as well as conduction current exchange. Finite Element Method simulation results reveal that for such a waveguide composed of 50 pairs of 400 nm-long-nanorods, a passband ranging from zero to cutoff frequency 156.2 THz, and an effective propagation length of 20.87 μm can be achieved simultaneously. The proposed mechanism of energy transport in the nanoscale has potential applications in subwavelength transmission lines for a wide range of integrated optical devices.

Hybrid integrated photodetectors with flat-top steep-edge spectral responses that consist of an Si-based multicavity Fabry-Perot (F-P) filter and an InP-based p-i-n absorption structure (with a 0.2 μm In(0.53)Ga(0.47)As absorption layer), have been designed and fabricated. The performance of the hybrid integrated photodetectors is theoretically investigated by including key factors such as the thickness of each cavity, the pairs of each reflecting mirror, and the thickness of the benzocyclobutene bonding layer. The device is fabricated by bonding an Si-based multicavity F-P filter with an InP-based p-i-n absorption structure. A hybrid integrated photodetector with a peak quantum efficiency of 55% around 1549.2 nm, the -0.5 dB band of 0.43 nm, the 25 dB band of 1.06 nm, and 3 dB bandwidth more than 16 GHz, is simultaneously obtained. Based on multicavity F-P structure, this device has good flat-top steep-edge spectral response.

Based on the generalized stochastic nonlinear Schrödinger equation, the effect of intrapulse Raman scattering (IRS) on broadband amplitude noise of supercontinuum (SC) generated in the normal dispersion regime is investigated numerically. The results show that, in the normal dispersion regime, where the IRS contributes less to the bandwidth of the SC spectrum, the broadband amplitude noise of SC is amplified significantly in the process of SC generation because of the existence of IRS effect. Using fiber with an optimal negative dispersion slope, the IRS effect can be suppressed, and thus the SC amplitude noise is reduced without spectral bandwidth loss.

This paper presents the design and analysis of a Si-based tunable flattop photodetector realized by the introduction of a stepped Fabry-Perot cavity, which can be thermally tuned via applying tuning power on its tuning electrode. By using a transfer matrix method, the spectral response of the photodetector is simulated in detail, indicating a flattop line shape can be achieved with an optimum step height. A trade-off residing in this device between the free spectrum range and the ease of fabrication of step height is also revealed and analyzed. In the final design of the photodetector, 1 dB linewidth of 0.5 nm, 3 dB linewidth of 0.8 nm, 6 dB linewidth of 1.2 nm, peak quantum efficiency of 40%, tuning efficiency of 91 mW/nm are theoretically obtained. We discuss the epitaxial growth and fabrication of the photodetector in the end, exhibiting the mature technique available for this device.

An integrated reconfigurable four-channel wavelength-division-multiplexed drop module for use in the long-wavelength was demonstrated using a tunable wavelength-selective photodetector array. The array consists of an InP-based p-i-n absorption structure and a GaAs-based multistep Fabry-Pérot filtering cavity. The high quality GaAs/InP heteroepitaxy was realized by employing a thin low temperature buffer layer. The GaAs-based multistep cavity was fabricated by wet etching and regrowth. The dropped central wavelengths were 1538, 1550, 1559, and 1570nm. The tunable range reached 10nm with a tuning power efficiency of 14.2nm/W. A spectral linewidth less than 0.5nm (FWHM), a 3dB bandwidth of 9.2GHz, and the peak quantum efficiencies from 13% to 20% were simultaneously obtained, in agreement with the theoretical simulation.

We demonstrate a wavelength-selective photodetector that combines a Fabry-Perot filtering cavity (FPC) with a taper absorption cavity (TAC). The taper cavity shows a nonresonant effect but exhibits an absorption enhancement effect, so that high speed, high quantum efficiency, wide tuning range, and an ultranarrow spectral linewidth can be achieved simultaneously. Device performance was theoretically investigated by including key factors such as taper angle, finite-size diffracting-beam input, and lateral walk-off in the taper cavity. The device was fabricated by bonding a GaAs-based FPC, which can be tuned via thermal-optic effect, with an InP-based TAC. An integrated device with a spectral linewidth of 0.6 nm (FWHM), a wavelength tuning range of 10.2 nm(1518.0-1528.2 nm), a 3 dB bandwidth of 12 GHz, and a quantum efficiency of approximately 70% was demonstrated, and the absorption layer thickness is only 0.3 microm.

A new, to our knowledge, scheme of all-optical switching is put forward by use of an all-fiber Mach-Zehnder interferometer incorporating ytterbium-doped fiber Bragg gratings. The device utilizes the characteristics of the sharp change of group velocity in transmission with the detuning parameter, delta = 2pi n(1/lambda - 1/lambdaB). The switching is achieved by changing the Bragg wavelength of the ytterbium-doped arm of the interferometer. A very small shift of the Bragg wavelength can lead to a pi phase shift between the two arms, so the power needed to realize complete switching is much lower than that of other schemes. In addition, the device can compensate the dispersion of an optical pulse through the positive group velocity dispersion in transmission provided by the gratings.