Omnidirectional Reflector Research Articles

High-quality-factor Bragg onion resonators with omnidirectional reflector cladding.

We propose to approximate a spherically symmetric Bragg resonator in an onionlike geometry. We develop a transfer-matrix theory for calculation of the quality factors and analyze the effect of the onion stem on cavity Q factors. We find that it is possible to achieve significant inhibition of spontaneous emission in an onion resonator with omnidirectional cladding layers.

Optimizing omnidirectional reflection by multilayer mirrors

Periodic layered media can reflect strongly for all incident angles and polarizations in a given frequency range. Quarter-wave stacks at normal incidence are commonplace in the design of such omnidirectional reflectors. We discuss alternative design criteria to optimize these systems.

Omnidirectional reflective contacts for light-emitting diodes

An electrically conductive omnidirectional reflector (ODR) is demonstrated in an AlGaInP light-emitting diode (LED). The ODR serves as p-type contact and comprises the semiconductor, a metal layer and an intermediate low-refractive index dielectric layer. The dielectric layer is perforated by an array of AuZn microcontacts thus enabling electrical conductivity. It is shown that the ODR significantly increases light extraction from an AlGaInP LED as compared to a reference LED employing a distributed Bragg reflector (DBR). External quantum efficiencies of 18% and 11% are obtained for the ODR- and the DBR-LED, respectively.

Second Order Approximation for Band Gap Characterization of One-Dimensional Dielectric Omnidirectional Reflector

A derivation of approximate analytical expressions for band edges ω± of the first band gap of a multilayer periodic structure is presented for both TE and TM waves at arbitrary angles of incidence. It is found that the approximate expressions give an excellent agreement with the numerical results as verified by the band edge variation with respect to the filling fraction [Formula: see text] and refractive index contrast Δn. The analytical expressions for the band edges are further employed to derive a semi numerical optimization of the relative gap width [Formula: see text] with respect to the filling fraction ν. The result is again shown to be in good agreement with the numerical result.

GaAs-based near-infrared omnidirectional reflector

We introduce a compound-semiconductor-based omnidirectional reflector. A four-layer-pair stack of GaAs/AlAs was grown epitaxially using molecular-beam epitaxy, and was then converted to a GaAs/Al2O3 multilayer stack by selective oxidation of the AlAs layers. The resultant one-dimensional photonic crystal exhibited omnidirectional reflection properties in near-IR wavelength range below 1 μm. Reflectance spectra measured at various incidence angles and polarizations were observed to be in good agreement with theoretically simulated results.

Si-based omnidirectional reflector and transmission filter optimized at a wavelength of 1.55 μm

Omnidirectional (omni-) reflector and transmission filter optimized at a wavelength of 1.55 μm have been realized using Si/SiO2 one-dimensional photonic crystals (1D PCs). Photonic band structures in the PCs with and without a defect layer have been also analyzed. The 1D PCs fabricated have very large omnidirectional photonic band gaps (omni-PBGs), and their measured reflectance spectra are in very good agreement with the calculated results. The omni-PBG in a six-pair Si/SiO2 1D PC (no defect layer, a filling factor η=0.406) exists in a normalized frequency range of 0.223–0.297. Introducing a defect layer generates a defect state in the PBG, and it behaves like electronic impurity levels in the gap with a change in thickness of the defect layer (dD). A six-pair Si/SiO2 1D PC with the structure parameters of η=4.060, dH (Si thickness)=95.3 nm, dL (SiO2 thickness)=234.9 nm, and dD=2.336⋅dH could act as a 1.55 μm-transmission filter for normal incidence light.

A Dielectric Omnidirectional Near-infrared Reflector

We have studied both theoretically and experimentally an omnidirectional reflector operating at the wavelength region of 1.3 <TEX>$\mu$</TEX>m. The omnnidirectional reflector, a special case of one-dimensional photonic crystals, was prepared by alternately sputtering two dielectric materials, amorphous silicon and silicon dioxide. Measured reflectance spectra, very consistent with simulated results at all angles and polarizations, clearly showed the existence of an omnidirectional photonic bandgap.

Wide bandwidth, large, and tunable polarization mode dispersions in multilayered omnidirectional reflectors

We numerically demonstrate the generation of large, broadband, and tunable polarization mode dispersion (PMD) with omnidirectionally reflecting dielectric stacks with embedded cavities. Both first-order and second-order polarization mode dispersion can be generated by similar configurations. Our examples show that the first-order PMD can be tuned from 0 to 30 ps over an 80 GHz bandwidth and second-order PMD from 0 to 50 ps/nm can be generated over a 100 GHz bandwidth. We also show that in our configuration there is no significant modal distortion of the light beams when the beam diameters exceed 3.5 mm.

Design of omnidirectional high reflectors with quarter-wave dielectric stacks for optical telecommunication bands.

A design for omnidirectional high reflectors with quarter-wave dielectric stacks in the optical telecommunication band that uses conventional optical thin-film design theory is described. The omnidirectional bandwidth is derived in units of wavelength and investigated as a function of its high- and low-refractive-index values in the near infrared. The results show that the high refractive index should be larger than 2.26 for an omnidirectional high reflector and that the low refractive index for maximum onmidirectional bandwidth should be approximately 1.5. It is shown that one can design broad-bandwidth omnidirectional high reflectors for S, C, and L bands for optical telecommunication simply by connecting the band edges of omnidirectional high reflectors.

Silicon-based photonic crystal heterostructure

A silicon-based one-dimensional photonic crystal heterostructure is proposed, and its applications to an omnidirectional reflector and a transverse electric/transverse magnetic (TE/TM) splitter are illustrated. The photonic crystal heterostructure consists of two sets of multilayers, each has six periods of bilayers of poly-silicon and silicon dioxide. When the incident light comes from a medium with refractive index lower than 1.16, there is a frequency range over which of both TE and TM polarizations will be totally reflected at any incident angle. In such a case, the photonic crystal heterostructure can be used as a low loss omnidirectional dielectric reflector. However, when the refractive index of the medium is higher than 1.16, the heterostructure will serve as a good TE/TM splitter of very low crosstalk within certain ranges of light frequency and incident angle.

Making an omnidirectional reflector

The effect of having a finite number of layers on the design of omnidirectional reflectors was investigated. It was shown that the structure should be finished with a low-index layer having a thickness larger than a quarter-wave to increase reflectivity, whereas layers below may remain of quarter-wave optical thickness at normal incidence angle. This general trend has been used for designing and realizing two a-Si-SiO(2) (amorphous silicon and silicon dioxide) omnidirectional reflectors in the near-infrared range on a silicon and a silica substrate, respectively. Owing to the decrease of absorption of recrystallized silicon as compared with a-Si in the visible range, the transmissivity of the structure realized on silica substrate was dramatically increased in the visible range upon annealing, whereas the high reflectivity and the omnidirectional effect were maintained in the near-infrared range.

Low-loss infrared dielectric material system for broadband dual-range omnidirectional reflectivity

A material system for broadband thermal IR applications based on branched polyethylene and tellurium is introduced. This system exhibits low absorption losses from 3.5 to 35 microm , has a large index contrast, and is readily deposited as a thin film. These unique features were used to investigate the formation of an omnidirectional reflector that exhibits two distinct, broadband omnidirectional ranges at thermal wavelengths. Reflectivity measurements are presented that confirm the existence of two omnidirectional ranges in the solar atmospheric windows extending from 8 to 12 microm and from 4.5 to 5.5 microm . The measurements are in good agreement with simulations.

Expanding high reflection range in a dielectric multilayer reflectorby disorder and inhomogeneity

A systematical study is presented on the impact of disorder andinhomogeneity on the high reflection range (HRR) in a one-dimensionalbinary dielectric multilayer reflector for various incident angles. It isfound that the introduction of disorder can expand the HRR for bothtransverse electric (TE) and transverse magnetic (TM) waves, irrespectiveof the incident angle. The HRR is found to become wider as the degree ofdisorder increases, until the effect becomes saturated. On the other hand,for the TE wave only, the introduction of inhomogeneity may broaden thewidth of the HRR regardless of the incident angle. While for the TM wave,the effect of inhomogeneity depends on the angle of incidence, and it canexpand the HRR only for the case of a small incident angle. The phenomenonis expected to find significant applications in the optimal design ofbroadband binary dielectric omnidirectional reflectors.

Ellipsometric investigation of the Si/SiO 2 interface formation for application to highly reflective dielectric mirrors

We present a study of the Si/SiO 2 interface formation during the growth of Si on SiO 2 using in situ ellipsometry measurements performed at one wavelength (λ=0.4 μm). Variable angle X-ray photoelectron spectroscopy (XPS) is also used to confirm the ellipsometric measurements. In situ ellipsometry and XPS provide evidence for the formation of a SiO x (0< x<2) interlayer between the SiO 2 and Si layers. The variation of x with depth is analysed from (cos Δ, tan ψ) trajectories and the results obtained allow reproduction of the spectroscopic ellipsometry spectra too. The effects of the substrate temperature on the growth are investigated and show that the higher the substrate temperature the larger the interface. The utilisation of a SiO 2/Si multilayer stack for elaborating an omnidirectional reflector, angle-insensitive in the near infrared range, is also presented.

An omnidirectional dual-shaped reflector antenna

A dual-shaped reflector antenna with an omnidirectional pattern and vertical polarization is presented. A radial-mode conical feed illuminates a rotationally symmetric dual-shaped reflector system able to drive the main beam in a nearly horizontal direction and able to control both the main lobe direction and its pattern. © 2000 John Wiley & Sons, Inc. Microwave Opt Technol Lett 27: 371–374, 2000.

SiO 2 / TiO 2 omnidirectional reflector and microcavity resonator via the sol-gel method

Thin films of SiO2 and TiO2 were used to fabricate one-dimensional photonic crystal devices using the sol-gel method: an omnidirectional reflector and microcavity resonator. The reflector consisted of six SiO2/TiO2 bilayers, designed with a stopband in the near infrared. Reflectivity over an incident angle range of 0°–80° showed an omnidirectional band of 70 nm, which agrees with theoretical predictions for this materials system. The microcavity resonator consisted of a TiO2 Fabry–Perot cavity sandwiched between two SiO2/TiO2 mirrors of three bilayers each. We have fabricated a microcavity with resonance at λ=1500 nm and achieved a quality factor of Q=35. We measured a resonance frequency modulation with a change in incident angle of light and defect layer thickness.

Perfect Mirrors, Past and Present

In their report “A dielectric omnidirectional reflector” (27 Nov. p. 1679), Yoel Fink et al. describe a model for an omnidirectional reflector in which it is possible to design and build mirrors that have a reflectance that is very high, regardless of the angle at which light is incident—thus the name omnidirectional mirror. These “perfect mirrors” are composed of multiple, dielectric (transparent, nonmetallic) layers that will not incur the light energy losses found in metallic mirrors and will perhaps make possible new applications. Fink et al. state that they are unaware of any previous knowledge of this phenomenon.

An Omnidirectional Reflector and Microcavity Resonatorvia the Sol-Gel Method

AbstractThin films of sol-gel SiO2 and TiO2 were used to fabricate two types of onedimensional photonic crystals: an omnidirectional reflector and microcavity resonator. The reflector consisted of six SiO2/TiO2 bilayers, designed with a stopband in the near infrared. Reflectance measurements over an incident angle range of 0–80° showed an omnidirectional band of 70 nm, which agrees with theoretical predictions for this materials system. The microcavity resonator consisted of a TiO2 Fabry-Perot cavity sandwiched between two SiO2/TiO2 mirrors of three bilayers each. We have fabricated a microcavity with resonance at λcavity = 1500nm and achieved a quality factor of Q=35. We measured a modulation in the cavity resonance frequency with a change of defect layer thickness and incident angle of light. This work demonstrates the feasibility of fabricating photonic crystals via the sol-gel method.

A dielectric omnidirectional reflector

A design criterion that permits truly omnidirectional reflectivity for all polarizations of incident light over a wide selectable range of frequencies was used in fabricating an all-dielectric omnidirectional reflector consisting of multilayer films. The reflector was simply constructed as a stack of nine alternating micrometer-thick layers of polystyrene and tellurium and demonstrates omnidirectional reflection over the wavelength range from 10 to 15 micrometers. Because the omnidirectionality criterion is general, it can be used to design omnidirectional reflectors in many frequency ranges of interest. Potential uses depend on the geometry of the system. For example, coating of an enclosure will result in an optical cavity. A hollow tube will produce a low-loss, broadband waveguide, whereas a planar film could be used as an efficient radiative heat barrier or collector in thermoelectric devices.