Anjin Liu

Reduced divergence angle of photonic crystal vertical-cavity surface-emitting laser

The reduced divergence angle of the photonic crystal vertical-cavity surface-emitting laser (PC-VCSEL) was investigated in both theory and experiment. The photonic crystal waveguide possessed the weakly guiding waveguide characteristic, which accounted for the reduction of the divergence angle. The three-dimensional finite-difference time-domain method was used to simulate the designed PC-VCSEL, and a calculated divergence angle of 5.2° was obtained. The measured divergence angles of our fabricated PC-VCSEL were between 5.1° and 5.5° over the entire drive current range, consistent with the numerical results. This is the lowest divergence angle of the fabricated PC-VCSEL ever reported.

Polymer embedded silicon nitride thermally tunable Bragg grating filters

Optical waveguides have been fabricated and investigated that employ silicon nitride as core and polymer as surrounding cladding material. Buried single-mode waveguides showed a propagation loss of ∼0.7 dB/cm at 1550 nm. Depending on the thickness of the silicon nitride core, the optical mode can be designed to expand largely into the polymer cladding. Using such a low-confinement structure and a metal strip heater embedded in the polymer cladding material beneath the core region, efficient thermally tunable waveguide Bragg grating filters were demonstrated. Wavelength tuning >57 nm was achieved at ∼220 mW of electrical heating power.

Polarization-insensitive subwavelength grating reflector based on a semiconductor-insulatormetal structure

We present a polarization-insensitive subwavelength grating reflector based on a semiconductor-insulator-metal structure. The polarization-insensitive characteristic originates from the combined effect of the TM-polarized high-reflectivity high-index-contrast subwavelength grating and the TE-polarized metallic (Au) subwavelength grating with the addition of the insulator layer. The overlapped high reflectivity (>99.5%) bandwidth between the transverse electric polarization and the transverse magnetic polarization is 89 nm. This polarization-insensitive subwavelength grating reflector can be used in the applications without a preferred polarization.

Optical properties of the crescent and coherent applications

By out-of-particle surface plasmon (SP) excitation in the near infrared range, the influences of key parameters on the basic optical properties of the Au crescent are qualitatively studied from the mode dispersion. Based on the coherent control of SP wave, a crescent pair sensor with the intensified extracted signal and the controllability of sensing is proposed. In addition, the crescent half replaced by Ag functioning as a position detector is also proposed. The particular phase of the detecting light as a detection parameter is used to improve the accuracy of the position detection.

Phase-locked ring-defect photonic crystal vertical-cavity surface-emitting laser

Phase-locked oxide-confined ring-defect photonic crystal vertical-cavity surface-emitting laser is presented. The coupled-mode theory is employed to illustrate the two supermodes of the device, in-phase and out-of-phase supermode. Experimental results verify the two supermodes by the characteristics of the spectra and the far field patterns. At the lower current, only the out-of-phase supermode is excited, whereas under the higher current, the in-phase supermode also appears at the shorter wavelength range. In addition, the measured spectral separation between the two supermodes agrees well with the theoretical result.

Lateral cavity photonic crystal surface-emitting laser with ultralow threshold

A lateral cavity photonic crystal (PhC) surface-emitting laser is realized on a commercial epitaxial waveguide wafer without a distributed Bragg reflector first. Energy is coupled from the lateral resonance to surface-emitting light through the Γ band edge of the PhC with a square lattice. Electrically driven 1553.8 nm surface-emitting lasing action is observed at room temperature. The threshold current density of 667 A/cm2 is ultralow for the surface-emitting laser.

High efficiency operation of butt joint line-defect-waveguide microlaser in two-dimensional photonic crystal slab

Butt joint line-defect-waveguide microlasers are demonstrated on photonic crystal slabs with airholes in a triangular lattice. Such microlaser is designed to increase the output power from the waveguide edge directly. The output power is remarkably enhanced to 214 times higher by introducing chirped structure in the output waveguide. The lasing mode operates in the linear dispersion region of the output waveguide so that the absorption loss due to the band-edge effect is reduced. The laser resonance is illustrated theoretically using the finite difference time domain method. A practical high power efficiency of 20% is obtained in this microlaser.

The impact of imperfect symmetry on band edge modes of a two-dimensional photonic crystal with square lattice

A theoretical analysis has been performed by means of the plane-wave expansion method to examine the dispersion properties of photons at high symmetry points of an InP based two-dimensional photonic crystal with square lattice. The Q factors are compared qualitatively. The mechanism of surface-emitting is due to the photon manipulation by periodic dielectric materials in terms of Bragg diffraction. A surface-emitting photonic crystal resonator is designed based on the phenomenon of slow light. Photonic crystal slabs with different unit cells are utilized in the simulation. The results indicate that the change of the air holes can affect the polarization property of the modes. So we can find a way to improve the polarization by reducing the symmetry of the structure.

Fabrication and Characterization of Integrable GaAs-Based High-Contrast Grating Reflector and Fabry–Pérot Filter Array With GaInP Sacrificial Layer

Integrable GaAs-based high-contrast gratings (HCGs) are fabricated and characterized, targeting applications in high-speed vertical-cavity surface-emitting lasers (VCSELs). A Ga0.51In0.49P sacrificial layer beneath the GaAs layer is employed to create a low index surrounding HCG strips by selective etching. Experimental results show that the finite-size HCG has a reflectivity of 93% from 1270 to 1330 nm for the transverse magnetic polarization, which is consistent with the calculated results. An HCG-based Fabry-Perot filter array formed by the different HCGs, air gap, and GaAs substrate is demonstrated. The measured resonance wavelengths of the filter arrays are consistent with the theoretical results, which implies that the resonance wavelength of such filters can be tuned by parameters of the HCG itself.

Thermo-optic simulations of silicon nitride / polymer hybrid waveguides

We perform the thermal and optical simulations of silicon nitride / polymer hybrid waveguides with different heating schemes by finite element method. Both the top and buried microheaters are adopted to realize tuning function by the thermo-optic effect. We find the buried microheater is more energy-efficient than the top microheater in creating a uniformed temperature environment in the waveguide region. On the other hand, the top electrode tends to create a strong temperature gradient through the waveguide, which in turn distorts the optical mode. This distortion, however, is different for TE and TM modes. This thermally induced birefringence effect is thoroughly investigated in this paper.