Xiaofeng Duan

Enhanced absorption of graphene strips with a multilayer subwavelength grating structure

The optical absorption of graphene strips covered on a multilayer subwavelength grating (MSG) surface is theoretically investigated. The absorption of graphene strips with MSG is enhanced in the wavelength range of 1500 nm to 1600 nm by critical coupling, which is associated with the combined effects of a guided resonance of MSG and its photonic band gap effect. The critical coupling of the graphene strips can be controlled by adjusting the incident angle without changing the structural parameters of MSG. The absorption of graphene strips can also be tuned by varying key parameters, such as grating period, strip width, and incident angle.

Theoretical analysis and design guideline for focusing subwavelength gratings

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%.

Reconfigurable multi-channel WDM drop module using a tunable wavelength-selective photodetector array

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.

Add-drop filters based on asymmetric high-order microring resonators

Add/drop filters are key components of Wavelength Division Multiplexing (WDM) communication systems. Free spectral range(FSR) is a key parameter for Add/drop filters, the FSR should operate within the entire C-band (1530-1562nm).And flat-top drop-port response with a sharp rolloff is also import, Flatness of the passband, sharp roll-off from passband to stop band are necessary to minimize the pulse broadening and the packing efficiency of wavelength channels. In this paper, we proposed an asymmetric approach to design high-order microring filters, The aim is to achieve large extension ratios and adequate suppression of the spurious interstitial mode, meanwhile, flat-top and steep-side response in filter could be obtained by this approach. Our simulation results showed an extended FSR of 40nm, reducing the interstitial peak suppression from 5dB to 35dB and a boxlike filter response with sharpe factor(SF) of 0.68. And a quality-factor of 2961 and a 3-dB bandwidth of 0.52nm is achieved.

Extremely Low-Threshold Current Density InGaAs/AlGaAs Quantum-Well Lasers on Silicon

An InGaAs/AlGaAs quantum-well laser structure was grown on a silicon substrate by adopting a three-step grown thin (1.8 μm) and simple buffer layer. The whole structure was grown by metalorganic chemical vapor deposition. The material quality was characterized by transmission electron microscopy, photoluminescence spectra, and electrochemical capacitance-voltage profiler. It shows that the threading dislocation density and interface roughness are effectively reduced, and the active regions optical properties grown on a silicon substrate are comparable to that grown on a GaAs substrate. Broad stripe lasers have also been fabricated. The cavity length and stripe width are 1 mm and 15 μm, respectively. An extremely low-threshold current density of 313 A/cm2 has been achieved under pulsed condition at room temperature. Meanwhile, the laser can operate under continuous wave condition at the temperature of 240 K. The above results make us more confident for realizing better performance of Si-based III-V semiconductor lasers by further improvement of the material growth method.

Mushroom-Mesa Photodetectors Using Subwavelength Gratings as Focusing Reflectors

A novel grating-integrated mushroom-mesa photodetector is demonstrated, which uses an undercut mushroom-mesa structure and a grating focusing reflector to increase bandwidth and efficiency simultaneously. The mushroom-mesa p-i-n structure is heterogeneously integrated on a non-periodic concentric circular subwavelength grating by a wafer bonding process. High speed can be obtained by using a mushroom-mesa structure for reducing its RC time constant. Meanwhile, high quantum efficiency can be obtained by using a grating reflector for its focusing ability. The external quantum efficiency measured on a device of 30 μm in diameter is 45.28% at a wavelength of 1.55 μm, which is enhanced by 21.4% in contrast to the device without reflector. And a 3-dB bandwidth of 30 GHz at a reverse bias of 3 V is simultaneously measured for our photodetector.

High-reflectivity high-contrast grating focusing reflector on silicon-on-insulator wafer

A high-contrast grating (HCG) focusing reflector providing phase front control of reflected light and high reflectivity is proposed and fabricated. Basic design rules to engineer this category of structures are given in detail. A 1550 nm TM polarized incident light of 11.86 mm in focal length and 0.8320 in reflectivity is obtained in experiment. The wavelength dependence of the fabricated HCGs from 1530 nm to 1580 nm is also tested. The test results show that the focal length is in the range of 11.81–12 mm, which is close to the designed focal length of 15 mm. The reflectivity is almost above 0.56 within a bandwidth of 50 nm. At a distance of 11.86 mm, the light is focused to a round spot with the highest concentration, which is much smaller than the size of the incident beam. The FWHM of the reflected light beam decreases to 120 nm, and the intensity increases to 1.18.

Influence of GaAs substrate on the transmission performance of epitaxially grown Fabry-Peerot f ilter

The influence of GaAs substrate on the transmission performance of a multi-film Fabry-Peerot filter (FPF), fabricated by metalorganic chemical vapor deposition epitaxial growth on GaAs substrate, is investigated using the transfer matrix method. On the basis of the theoretical simulation, we determine that the quality of the resonant transmission peak of this epitaxially grown FPF (EG-FPF) deteriorates through splitting when the substrate is taken into account. Rapid periodic oscillation of peak-transmittivity along with the alteration of substrate thickness is also observed in the simulation results. Finally, a remarkably improved transmission performance of the EG-FPF is obtained by thinning the substrate down to a suitable thickness range through well-controlled grinding and polishing.

Transient simulation of UTC-PD using drift-diffusion model

In this paper, the reliability of transient simulation results of uni-traveling-carrier photodiodes (UTC-PDs) is demonstrated by compared with experimental and reported simulation results, when the parameters of drift-diffusion model are set correctly.

High-speed uni-traveling-carrier photodetector with the new design of absorber and collector

High-speed uni-traveling-carrier photodetector (UTC-PD) and three modified uni-traveling-carrier photo-detectors (MUTC-PDs) incorporating different doping profile are designed and simulated for a wavelength of 1550 nm in this letter. According to comparison of simulated results, UTC-PD-1, MUTC-PD-2 and MUTC-PD-3 with constant doping profile, step-constant doping profile and Gaussian doping profile in InGaAs absorber are respectively achieved the 3-dB bandwidth of 45.8 GHz, 48.2 GHz and 178.9 GHz, while the MUTC-PD-4 incorporating Gaussian doping profile in InGaAs absorber and InP collector is obtained the 3-dB bandwidth of up to 182.9 GHz under the same reverse bias voltage of 1.5 V. Afterwards, the high-speed response characteristics of different area are simulated and achieved the 3-dB bandwidth of 26 GHz, 55 GHz, 118 GHz and 182.9 GHz at the area of 100 μm2, 40μm2, 20 μm2 and 14 μm2.