Photonic Crystal Holes Research Articles

Study on influence of oxide aperture shape on modal characteristics of VCSELs

The effects of oxide aperture shape whose size is small, on the mode characteristics of external cavity oxide-confined photonic crystal vertical cavity surface emitting laser (VCSEL) is studied extensively based on the finite-difference time-domain algorithm. A new three-dimensional model of optical field in photonic crystal VCSEL is built and the influence of the variation in aperture shape on the characteristics of far-field and spectrum is pointed out. It is found that the modal characteristics of PhC-VCSEL are influenced by the shape of oxide aperture, the influence on spectrum feature is more apparent. It can be explained from the distribution of modal field by the fact that the symmetrical characteristic of prismatic oxide aperture is not identical to that of high-order mode. But the influence is less apparent while the depth of photonic crystal hole is deeper. The reason for this matter is illustrated. The results can be used to guide the fabrication of external cavity oxide-confined photonic crystal VCSELs.

Effect of rapid thermal annealing on the luminescence of self-assembled InAs quantum dots embedded in GaAs-based photonic crystal nanocavities

Postgrowth rapid thermal annealing (RTA) was used to modify the optical properties of self-assembled InAs quantum dots (QDs) embedded in a photonic crystal nanocavity. Compared with the reference QDs in the unpatterned regions, it was found that the effect of RTA on the luminescence blueshift of QDs embedded in the nanocavities is similar to that from the unpatterned regions. However, InAs QDs in nanocavities exhibit an anomalous variation of luminescent linewidth during the course of RTA. This could be attributed to the deterioration of InAs QDs at the edge of photonic crystal air holes at high annealing temperatures. The stable position of cavity modes during the RTA temperatures shows the RTA technique could be applied to tune the spectral coupling of QDs/cavities when the QD spectra are on the red side of a cavity mode.

Decimated Photonic Crystal Defect Cavity Lasers

We present the design, fabrication, and characterization of decimated lattice photonic crystal membrane lasers. Based on our simulated designs, decimated photonic crystal lattices have reduced number of holes, which enhance the current and heat conductive paths from the optical cavity with a minimal impact on the cavity quality factor and resonance. Our calculations indicate lower operating temperature using the decimation lattice designs. Two decimated cavity designs are demonstrated and compared to conventional photonic crystal defect cavity lasers in InP-based membranes. The decimated designs show improved laser performance with a small reduction of measured cavity quality factor. Reduction of the photonic crystal holes around the defect cavity to as few as two periods still enables photopumped operation of photonic crystal defect lasers. The improvement in the thermal properties is characterized by varying the photopumping duty cycle.

Nanofabrication of photonic crystal-based devices using electron beam spot lithography

This paper reports on photonic crystal-based devices fabricated via the electron beam spot lithography technique. This technique produces uniform, submicron, and nanometer scale periodic holes over large areas in the millimeter range and the write time is a factor of over 120 times faster than the conventional direct-write technique. Generic design guidelines of this technique are established and the patterning of photonic crystal holes’ arrays in the square and hexagonal lattice configurations are investigated. The single spot lithography technique could potentially be useful as a mass fabrication approach, particularly for very large areas of 2D photonic crystal-based devices and possibly for other device fabrication, for a wide range of scientific and technological applications.

Self-aligned mask renewal for anisotropically etched circular micro- and nanostructures

The top–down fabrication of high aspect ratio circular micro- and nanostructures in silicon nitride is presented. A new method is introduced to increase the aspect ratio of anisotropically etched holes by a factor of more than two with respect to the results obtained from an established dry-etching process. The method is based on the renewal of an etching mask after a first etching step has been completed. Mask renewal is done by line-of-sight deposition of a masking layer on the surface of the sample, which is mounted at an angle with respect to the deposition direction. No additional alignment step is required. The proof of principle is performed for silicon nitride etching through a mask of titanium, but the method has great potential to be applicable to a wide variety of substrate–mask combinations and to find entrance into various engineering fields. Two specific applications are highlighted. Firstly, a thick silicon nitride hardmask is used for the fabrication of deeply etched photonic crystal holes in indium phosphide (InP). For holes of 280 nm diameter, a record aspect ratio of 20 and an overall selectivity of 28.5 between a positive-tone resist layer and InP are reported. Secondly, the use of perforated silicon nitride membranes for droplet formation for applications in food engineering or pharmaceutics is addressed. Preliminary results show a potential for the self-aligned mask renewal method to exceed state-of-the-art membrane quality in terms of pore size, aspect ratio and membrane stability.

Monolithically integrated cantilevers with self-aligned tips for wavelength tuning in a photonic crystal cavity-based channel-drop filter

A technology to monolithically integrate micro-bimorph cantilevers equipped with tips that are self-aligned with respect to the holes of a 2D photonic crystal cavity-based channel-drop filter is presented. On electrostatic actuation, the tips move into the holes and provide electromechano-optical modulation of light. The technology allows the fabrication of tips on specific photonic crystal holes by controlling the hole diameter and the sacrificial layer thickness. The integrated device is both mechanically and optically characterized. A 180 pm wavelength shift at the first band edge of the photonic crystal cavity-based channel-drop filter is measured on the application of a 2 V dc voltage to the cantilever. This CMOS-compatible device is designed to operate in the C-band of the telecommunication wavelengths and constitutes a promising candidate for future integrated all-optical devices.

Novel postetch process to realize high quality photonic crystals in InP

Thermally driven reflow of material during annealing was positively used to obtain near-vertical sidewall profiles for high-aspect-ratio nanostructures in InP fabricated by dry etching. This is very promising for achieving high optical quality in photonic crystal (PhC) components. Nearly cylindrical profiles were obtained for high-aspect-ratio PhC holes with diameters as small as 200–350 nm. Mini stop bands (MSBs) in line-defect PhC waveguides were experimentally investigated for both as-etched and reshaped hole geometries, and their spectral characteristics were used to assess the quality of PhC fabrication. The spectral characteristics of the MSB in PhC waveguides with reshaped holes showed significant improvement in performance with a transmission dip as deep as 35 dB with sharp edges dropping in intensity more than 30 dB for ∼4 nm of wavelength change. These results show potential for using high extinction drop-filters in InP-based monolithic photonic integrated circuit applications. Finally, it is proposed that other nanostructure geometries may also benefit from this reshaping process.

Design and focused ion beam fabrication of single crystal diamond nanobeam cavities

We present the design and fabrication of nanobeam photonic crystal cavities in single crystal diamond for applications in cavity quantum electrodynamics. First, we describe three-dimensional finite-difference time-domain simulations of a high quality factor (Q∼106) and small mode volume [V∼0.5(λ/n)3] cavity whose resonance corresponds to the zero-phonon transition (637 nm) of the nitrogen-vacancy color center in diamond. This high Q/V structure, which would allow for strong light-matter interaction, is achieved by gradually tapering the size of the photonic crystal holes between the defect center and the mirror regions of the nanobeam. Next, we demonstrate two different focused ion beam (FIB) fabrication strategies to generate thin diamond membranes and nanobeam photonic crystal resonators from a bulk crystal. These approaches include a diamond crystal “side-milling” procedure as well as an application of the “lift-out” technique used in transmission electron microscopy sample preparation. Finally, we discuss certain aspects of the FIB fabrication routine that are a challenge to the realization of the high Q/V designs.

Lattice constant effects of photonic crystals on the extraction of guided mode of GaN based light emitting diodes

We have fabricated a series of square-lattice hole photonic crystal (2PhC) arrays simultaneously at the un-current injection region on a special sample of GaN based light emitting diode (LED) by using focus ion beam milling (FIBM). The lattice constants of the 2PhC arrays vary from 230 to 1500 nm, while the 2PhC arrays have a constant area of about 9 μm × 18 μm and a fixed depth of 150±10 nm which approaches but does not penetrate the active layer. Microscopic electroluminescence images and spectral measurements consistently confirm that the top emitting intensities from different 2PhCs are all enhanced compared with the unpatterned region. It is demonstrated that the output coupling of propagating guided modes is realized by the diffracted transmission of the 2PhCs. The enhancement factors of the guided modes compared with the unpatterned region are plotted as function of the lattice constant. It is found that the highest enhancements for the extraction of guided modes were obtained for the lattice constant of 230 and 460 nm of 2PhCs. The results are discussed by the two-dimensional rigorous coupled wave analysis (RCWA).

Study of two-dimensional photonic crystal microcavities filled with polymer

We present numerical study of microcavity biosensor in photonic crystal (PC) with triangular lattice of air holes patterned perpendicularly to an InP-based confining heterostructure. The microcavity is formed by varying the radius of one air hole. The 2D finite difference time domain (FDTD) method algorithm (fullwave simulator) is used to compute the light transmission efficiency and the quality factor (Q) when the refractive index (RI) filled in the air holes of water and polymer. The detected spectrum has a Lorentzian line shape, and the peak occurs when the PC cavity is at resonance. The resonance wavelength of this cavity will shift accordingly due to the variation of RI. The polymer filling of photonic crystal holes can be used to measure gas, fluids, biolayers, or bound chemical.

Structural Effects on Highly Directional Far-Field Emission Patterns of GaN-Based Micro-Cavity Light-Emitting Diodes With Photonic Crystals

This study theoretically and experimentally investigates the highly directional far-field emission patterns of GaN photonic crystal (PhC) micro-cavity light-emitting diodes (MCLEDs) depending on varying structural parameters. Angular-spectra-resolved electroluminescence measurements reveals the behavior of guided-mode extraction which is significantly affected by the structural parameters of GaN PhC MCLEDs, where the GaN cavity thickness decides the extracted guided mode numbers, PhC lattice constant influences the distribution of far-field emission, and PhC hole depth affects the interaction with guided modes. The proposed GaN ultrathin MCLED (uMCLED) with PhC lattice constant of 420 nm and deep hole depth of 250 nm exhibited a maximum output light extraction efficiency of 248% under one-watt input power compared to GaN non-PhC uMCLED and produced a directional far-field emission pattern at half intensity near 17^ . The present results indicate that highly directional light extraction enhancement could contribute to developments of many applications, especially for etendue-limited applications such as pico- projectors.

Displacement Sensing With a Mechanically Tunable Photonic Crystal

We demonstrate a mechanically tunable photonic crystal (PC) designed for highly position-sensitive reflection. Our device consists of a multilayer PC membrane attached to a silicon microelectromechanical system structure for lateral actuation. Inside each of the PC holes there is a pillar, which is attached to the substrate. By moving the PC membrane with respect to the pillars we change the shape of the PC holes and thereby modulate the PC reflectivity. Our measurements show a reflectivity change of more than 80% for a 115-nm lateral displacement.

Highly confined and continuous single-mode operation of self-align photonic crystal oxide VCSEL

In this letter, we report on single-mode operation of originally multi-mode oxide VCSEL by using etched photonic crystal air holes and unique trench structure. The device fabrication utilized conventional photolithography; with simplified lithography step of self-aligning the photonic crystal and trench structures to the laser aperture for efficient and vigorous device processing. The fabricated photonic crystal VCSEL with trench device exhibits a single-mode output power of 0.7 mW, threshold current of 3.5 mA, slope efficiency of 0.10 W/A, and continuous single-mode output spectra at wide operating current range. The results are compared with conventional multi-mode oxide VCSEL of similar device geometry. In addition, theoretical analysis is presented for developing further understanding of the photonic crystal VCSEL.

Divergent Far-Field III–Nitride Ultrathin Film-Transferred Photonic Crystal Light-Emitting Diodes

The divergent far-field distribution of III–nitride ultrathin film-transferred light-emitting diodes (FTLEDs) with ellipse holes of triangular photonic crystal (PhC) lattice have been experimental and theoretical studies. Angular-and-spectral-resolved measurement revealed guided modes extraction behaviors which obtain good agreement with theoretical model according to two-dimensional free photon band structure. The azimuthal evolution of the guided modes' diffraction behavior according to Bragg's diffraction has also been discussed by angular-resolved monochromatical mapping. Finally, the PhC FTLEDs show light enhancement with the divergence far-field pattern that exhibited 130% at a driving current of 200 mA as compared with non-PhC (without PhC) FTLEDs. This could lead to promising LEDs with unusual divergent far-field properties for specific applications.

Sensitivities of InGaAsP photonic crystal membrane nanocavities to hole refractive index

The sensitivities of resonant wavelengths of photonic crystal (PhC) membrane nanocavities with embedded InAs quantum dots to the ambient refractive index are reported for use in (bio) chemical sensing. The resonances for the different modes of several point-defect type cavities are obtained by photoluminescence measurements. Systematic trends of the variation of sensitivity with increase of the overlap of the modes with the PhC holes are observed for varying cavity type as well as for a given mode within a cavity type. A maximum sensitivity of approximately 300 nm/RIU (refractive index unit) is observed, corresponding to approximately 25% mode overlap with the holes and complete infiltration with the aqueous solution.

High-efficiency, large-bandwidth silicon-on-insulator grating coupler based on a fully-etched photonic crystal structure

A grating coupler for interfacing between single-mode fibers and photonic circuits on silicon-on-insulator is demonstrated. It consists of columns of fully etched photonic crystal holes, which are made in the same lithography and etching processes used for making the silicon-on-insulator wire waveguide. The holes have a diameter of around 143 nm, and are defined with electron-beam lithography. A peak coupling efficiency of 42% at 1550 nm and 1 dB bandwidth of 37 nm, as well as a low back reflection, are achieved. The performance of the proposed fully etched grating coupler is comparable to that based on the conventional shallowly etched grating, which needs additional fabrication steps.

Study on current spreading of photonic crystal vertical cavity surface emitting lasers

The distribution of injected current in the active region of external cavity oxide-confined photonic crystal vertical cavity surface emitting lasers is studied extensively. An advanced three-dimensional model of current distribution is used to analyse the effects of photonic crystal structures on current density distribution and series resistance of the device. It is found that the deeper the photonic crystal holes are, the worse the circular symmetry of the current density distribution is and the higher the series resistance is,especially when the ethching depths of holes are larger than 2 μm. Different patterns of photonic crystal structures have a great imfluence on current density distribution and circular symmetry. The results are beneficial to the research and the design of external cavity oxide-confined photonic crystal vertical cavity surface emitting lasers.

Enhanced Absorption in Amorphous Silicon Solar Cells Using Plasmonic and Photonic Crystals – Measurement and Simulation

AbstractWe develop experimentally and theoretically plasmonic and photonic crystals for enhancing thin film silicon solar cells. Thin film amorphous silicon (a-Si:H) solar cells suffer from decreased absorption of red and near-infrared photons, where the photon absorption length is large. Simulations predict maximal light absorption for a pitch of 700-800 nm for photonic crystal hole arrays in silver or ZnO/Ag back reflectors, with absorption increases of ~12%. The photonic crystal improves over the ideal randomly roughened back reflector (or the ‘4n2limit’) at wavelengths near the band edge. We fabricated metallic photonic crystal back-reflectors using photolithography and reactive-ion etching. We conformally deposited a-Si:H solar cells on triangular lattice hole arrays of pitch 760 nm on silver back-reflectors. Electron microscopy demonstrates excellent long range periodicity and conformal a-Si:H growth. The measured quantum efficiency increases by 7-8 %, relative to a flat reflector reference device, with enhancement factors exceeding 6 at near-infrared wavelengths. The photonic crystal back reflector strongly diffracts light and increases optical path lengths of solar photons.

Experimental demonstration of an ultracompact polarization beamsplitter based on a multimode interference coupler with internal photonic crystals

The fabrication and characterization of a compact InP-based polarization beamsplitter (PBS) is presented. A multimode interference (MMI) coupler with an internal air hole photonic crystal (PhC) section is utilized to separate the two polarizations. The PhC structure in the middle of the MMI is polarization dependent, so that one polarization is reflected and the other one is transmitted; both are collected by the respective output ports of the MMI coupler. The obtained experimental results show that the PBS as short as ~400 µm has an extinction ratio as large as 15 dB.

Theoretical demonstration of DBR-assisted super-periodic photonic-crystal light-emitting diodes with narrow divergence angles

Highly-directive light-emitting diodes showing high extraction efficiency are designed and demonstrated based on a two-dimensional super-periodic photonic-crystal structure. With this specific design, a super-periodic photonic-crystal structure can generate a highly directional beam by satisfying the multiple-resonance conditions of distributed Bragg reflectors and the topmost slab. Dependence of the power efficiency on the thickness of the topmost slab and the etching depth of the photonic-crystal holes is then analyzed by the three-dimensional finite-difference time-domain method. From subsequent analyses, it has been determined that >80% of the extracted power could be funneled into a small divergence angle of 30°.