High efficiency phase holograms are designed and implemented using aperiodic two-dimensional (2D) high-contrast gratings (HCGs). With our design algorithm and an in-house developed rigorous coupled-wave analysis (RCWA) package for periodic 2D HCGs, the structural parameters are obtained to achieve a full 360-degree phase-tuning range of the reflected or transmitted wave, while maintaining the power efficiency above 90%. For given far-field patterns or 3D objects to reconstruct, we can generate the near-field phase distribution through an iterative process. The aperiodic HCG phase plates we design for holograms are pixelated, and the local geometric parameters for each pixel to achieve desired phase alternation are extracted from our periodic HCG designs. Our aperiodic HCG holograms are simulated using the 3D finite-difference time-domain method. The simulation results confirm that the desired far-field patterns are successfully produced under illumination at the designed wavelength. The HCG holograms are implemented on the quartz wafers, using amorphous silicon as the high-index material. We propose HCG designs at both visible and infrared wavelengths, and our simulation confirms the reconstruction of 3D objects. The high-contrast gratings allow us to realize low-cost, compact, flat, and integrable holograms with sub-micrometer thicknesses.
In recent years, subwavelength dielectric gratings have been engineered for use as planar focusing elements at optical communication frequencies. Pioneering designs were based on aperiodic one-dimensional gratings, which were polarization-sensitive and designed bar by bar. In this paper, we present our recent designs which eliminated the polarization dependence by using a novel two-dimensional hexagonal lattice and algorithm to build the lens. In this way, lens can be designed algorithmically, with the inherent geometry requiring the use of only one period for the hexagonal lattice. We propose a unique geometry for designing two-dimensional grating lenses: dielectric posts arrayed in concentric circles. Because it is straightforward to space concentric rings apart at varying distances, we no longer need to restrict the design to a uniform grating period. By choosing two periodicities to work with, we managed to algorithmically design a two-dimensional lens, but with the advantage that our smallest feature sizes are up to twice as large as those of lenses designed with only one period. This increases the ease of fabrication for lenses working at current wavelengths and opens up the possibility for working with shorter wavelengths. Furthermore, this concentrically arrayed grating lens can be designed using phase information calculated for a periodic hexagonal lattice, even though the two designs show very little geometric resemblance. Also, we found that the grating lens is suitable not only for focusing plane waves, but also for imaging point sources. Finally, we show that bifocal lenses can be crated from diffraction gratings using our algorithm as well.
We present a unique heterogeneous integration approach for VCSELs on silicon using eutectic bonding. An electrically pumped III-V – silicon heterogeneous VCSEL is demonstrated using a high-contrast grating (HCG) reflector on silicon. CW output power >1.5 mW, thermal resistance of 1.46 K/mW, and 5 Gb/s direct modulation is demonstrated. We also explore the possibility of an all-HCG VCSEL structure that would benefit from stronger thermal performance, larger tuning efficiency, and higher direct modulation speeds.
Metal-cavity submonolayer (SML) quantum-dot (QD) microlasers are demonstrated at room
temperature under continuous-wave electrical injection for 2-μm-radius devices and pulsed
operation for 0.5-μm-radius devices. Compared to our previous quantum well devices, the superior
optical properties of SML QDs provide the possibility for further size reduction. Size-dependent
lasing characteristics are extracted from measurements to investigate the device physics for future
size reduction. An optical cavity model using the transfer matrix and the effective index method
including metal dispersion is developed and used for both the design and the experimental results
analysis. The laser uses an active region consisting of three groups of SML QDs, and each group
consists of 10 stacks of 0.5-monolayer InAs QD layers. The cylindrical microcavity is formed by
hybrid metal-distributed Bragg reflectors (DBRs) mirrors with an optimized SiNx passivation layer
on the sidewall to reduce the metal loss and to avoid the leakage current. The transverse optical
modes are solved using the Maxwell equations, and the resonance condition is determined by roundtrip
phase matching. Vertically-correlated QDs are modeled as quantum disks, and the wave
functions and eigenenergies in both conduction and valance bands are solved from Schrodinger
equation. Carrier-dependent material gain is calculated using Fermi’s golden rule and included in the
model. The lasing wavelengths, quality factors, and confinement factors for cavity modes are the
inputs for the rate-equation model, which predicts the light output power vs. current behavior and
has shown excellent agreement with experiments. Size-dependent physical quantities such as
leakage current and spontaneous emission coupling factor are extracted and investigated. Further
size reduction using only four pairs of DBRs is proposed.