High Reflectivity Band Research Articles

Design and experimental approach of optical reflection filters with graded refractive index profiles

A novel optical multilayer filter with graded refractive index profiles using a quarterwave stack was designed. This filter exhibits a high reflectance band, a sharp cutoff and a wide transmittance region as the result of sidelobe suppression. A 31 layer TiO2/SiO2 high reflectance filter with a graded refractive index profile was fabricated by helicon plasma sputtering on BK7 and Si (100) substrates. The deposition system was operated under ambient gas pressure of 1.8×10−1 Pa at room temperature. The measured transmittance of the spectrum was in good accord with calculated results. Transmission electron microscopic observations confirmed the expected microstructure of the filter.

Multiple solutions provided by analytical synthesis in admittance matching; application to dichroic mirrors

Abstract The paper studies the application of the complete admittancematching in the design of dichroic mirrors. The matching stacks were analytically synthesized and all solutions with one, two and three periods were investigated in order to obtain a large-transmission (LT) band and to preserve the high-reflectance (HR) band. The method is applied in the design of two dichroic mirrors, a one-side matched mirror with HR at 1·44 μm and L T at 1·064 μm and a two-sided matched mirror with HR at 1·064 μm and L T at 0·809 μm.

Multiple solutions provided by analytical synthesis in admittance matching; application to dichroic mirrors

The paper studies the application of the complete admittancematching in the design of dichroic mirrors. The matching stacks were analytically synthesized and all solutions with one, two and three periods were investigated in order to obtain a large-transmission (LT) band and to preserve the high-reflectance (HR) band. The method is applied in the design of two dichroic mirrors, a one-side matched mirror with HR at 1·44 μm and L T at 1·064 μm and a two-sided matched mirror with HR at 1·064 μm and L T at 0·809 μm.

Spectral characteristics of an InP/InGaAs distributed absorbing Bragg reflector

An InP/In0.53Ga0.47As distributed absorbing Bragg reflector is investigated by reflectance and group-delay-time spectroscopy. The measured reflectance is suppressed and shows a minimum at the low-wavelength side of the high-reflectance band. This suppression of reflectance is due to an enhancement of optical absorption. The enhancement of optical absorption originates from an increase in the overlap of the optical field with the absorbing InGaAs layers since an intense optical field is confined near the surface in the spectral range of the enhancement.

Deep crustal structure and rheology of the Gascoyne volcanic margin, western Australia

Interpretation of deep seismic reflection data across the Gascoyne Margin reveals six distinct seismic facies units related to the tectono-magmatic breakup history. On the outer Exmouth Plateau four large scale units are identified: (1) an extensively block-faulted upper crust; (2) a middle-crustal unit of discontinuous, undulatory reflectors; (3) a reflection-free deep crustal unit; and (4) a lower-crustal band of low-frequency, high-amplitude reflectors. Two additional units are found near the continent-ocean boundary (COB); (5) seaward-dipping reflectors (SDR); and (6) landward-dipping reflectors in the lower crust below the SDR. The lower-crustal high-reflectivity band, located near the top of a high-velocity unit (Vp > 7 kms−1), is interpreted as magmatic underplating. There is a spatial correlation between the underplated area and the presence of extensive upper-crustal block-faulting and intrusive rocks in the shallow crust. The undulatory middle-crustal reflector unit is also only identified in the outer plateau area, and is interpreted as a zone in which the upper-crustal faults terminate. The inner parts of the margin consist of a deep basin showing little upper-crustal faulting and no evidence of middle crustal deformation or underplating. Theoretical modeling of the effect of rifting and magmatic underplating on crustal strength profiles suggests that the brittle-ductile transition may migrate at least 5 km upwards during several million years after the underplating event. Based on the seismic interpretation and crustal strength modeling we propose that the seismic structure of the outer Exmouth Plateau is severely modified by a transient change in the crustal rheological structure associated with magmatic underplating.

Wide bandwidth AlAs/AlGaAs tandem Bragg reflectors grown by organometallic vapor phase epitaxy

Wide bandwidth AlAs/Al0.6Ga0.4As tandem Bragg reflectors were grown by organometallic vapor phase epitaxy. Quarter-wave reflector stacks designed for different wavelengths were placed in cascade in epitaxially grown structures to expand the high reflectance bands. Intermediate low-index layers were put in between every two stacks to suppress the transmission peaks in the centers of the combined high reflectance bands. While a single-stack structure showed a full width half-maximum bandwidth of 500 Å, the two-stack and the three-stack structures effectively doubled and tripled this bandwidth to approximately 1000 and 1500 Å, respectively.

Recent developments of laser optical coatings in Hungary

Optical coating suppliers specialized to meet special needs of laser developers often have to face new challenges to help them in building high-performance, high-efficiency lasers. Some aspects of laser optical coating design and manufacturing are discussed. First an unconventional coating deposition technology, termed the reactive electron beam evaporation under reduced oxygen pressure, is reviewed. The technology is applied for the TiO 2 /SiO 2 and the Ta 2 O 5 /SiO 2 material pairs producing optical coatings with low absorption and scattering losses. Then some concrete application problems and their solutions are presented using our technology: high reflectance UV mirrors containing UV-grade HfO 2 layers; high-damage-threshold, visible/near-infrared power optics; mirrors with extended high reflectance bands (all-dielectric broadband reflectors); and elements of a specially designed mirror set for femtosecond pulse Ti:sapphire lasers. The mirror set for the Ti:sapphire laser has been optimized for both phase and amplitude characteristics.

The Anomalous High-reflection Band for the Herpin-equivalent Gradient Index Film

Abstract A high-reflection band in the short-wavelength region for Herpin-equivalent gradient index layer system was found and analysed numerically. The band shape and edge position are sensitive to the number of steps for which the gradient index layer is divided and the Herpin transfer wavelength. The existence of the band is independent of the index profile. The existence of the band was also experimentally verified.

On the high reflectance bands of two- and three-layer dielectric systems

Abstract The relative width of the high reflectance bands of two- and three-layer dielectric systems is investigated. It is shown explicitly that the maximum value of the width in question is obtained for systems whose total optical thickness of the outer layers is equal to that of the inner layer, and the systems have to be made of those two of the three given materials whose ratio of refractive indices is the highest.

Two- and three-layer dielectric systems with the same high reflectance bands

In this paper we show that there exists an unlimited number of dielectric three-layer systems with the same high reflectance bands as those of a given two- or three-layer system made of two materials.

High reflectance bands of some simple two-material dielectric multilayers

The high reflectance bands and other optical properties of the two structures [(HL) m H] n and [(HL) p (LH) p ] l are discussed and compared with those of the structures (HL) k and [(H/2)L(H/2) k .

Modulation of light by an electrically tunable multilayer interference filter

We describe the structure and performance of a new, nonabsorbing, perpendicular geometry light modulator based on an electrically tunable, multilayer interference filter. The device consists of a GaAs/AlAs quarter-wave stack with aluminum contacts, grown by molecular beam epitaxy. A strong, applied electric field changes the refractive indices through the Franz–Keldysh effect, shifting the transmission spectrum and modulating the intensity of light tuned to the edges of the high reflectance band. Modulation ratios up to 2.5:1 have been obtained for photon energies 0.05–0.2 eV below the GaAs band gap. The device has applications in arrays and for modulation of high power light.

Multilayered coatings with suppression of two neighboring high-reflection bands

Multilayered coatings with suppression of two neighboring high-reflection bands

Utilization of Kard’s equations to suppress the high frequency reflectance bands of periodic multilayers

Kard’s equations for the R/T polynomial of a nonabsorbing multilayer are used to design reflectors with suppressed high frequency reflection bands.

Origin of the high-reflectivity band at ? = 2 for a multilayer film structure with layers of unequal thickness

Origin of the high-reflectivity band at ? = 2 for a multilayer film structure with layers of unequal thickness

Simplified description of multilayer dielectric reflectors

The spectral-absorptance curves of quarter-wave reflectors are quite asymmetric in general as a result of absorption in the layers. The maximum electric field E in an infinite stack is in the center of the high-index (low-index) layer at the low-frequency (high-frequency) end of the high-reflection band. Thus, the absorptance minimum of a stack having much greater absorption in the high-index material occurs at ω > ωc (band center). Operation at ω > ωc could increase the damage resistance if the high-index material is easily damaged. At ωc, E decays rapidly. As ω departs from ωc in an infinite reflector, E penetrates deeper into the coating, with no decay at the band edges. The peak-to-peak distance in an infinite dielectric stack is constant for all frequencies in the band. By using the continuity of E and dE/dz, E can be obtained from E = 0 at the metallic substrate or, in an infinite stack, from E = 0 at the reflector surface for ω = ωc or E = 0 in the center of the high- or low-index layer at the band edges. Simple closed-form approximations for the spectral absorptance and the phase of E are accurate and sufficient for present applications.

Reststrahlen crystals as wavelength-selective laser reflectors

In this Note, we point out that the high-reflectance reststrahlen bands of many crystals may be used as wavelength-selective mirrors for far-infrared lasers (8–150 μ). Wavelength coincidences between selected laser transitions and the peaks of some crystal reststrahlen bands are given. Using the reststrahlen band at 9 μ in fused quartz, we have produced oscillation on the 9.0065 μ transition in XeI, with the exclusion of all other transitions.

Suppression of Reflection Peaks in Passbands of Multilayer Dielectric Filters

In multilayer dielectric mirrors with one or more high reflectance bands in the near-ir and visible regions, large anomalous reflection peaks usually appear in the passbands. Their origin is traced to layer thickness inequality caused by unequal dispersions of the high and low index materials, and the common practice of monitoring layer thicknesses at stop bands. A procedure is described which takes account of the variations in materials dispersion in different coating laboratories, suppresses the anomalous reflection peaks, and locates the high reflection bands at their required wavelengths, with a relatively simple monitoring procedure.

Multilayer Mirrors with High Reflectance Over an Extended Spectral Region

The design of mirrors composed of multilayer stacks of dielectric films is considered. High reflectance over an extended spectral range is attained by positioning the stacks so that on a wavelength scale their high-reflectance bands are either contiguous or overlapping. Certain precautions must be taken in the choice of stacks to avoid deep-reflectance minima from developing within the extended high-reflectance region. Some of these are discussed and illustrated with both computional and experimental curves. The techniques of extending the high-reflectance regions are applicable not only to mirrors, but also to low- and high-pass cutoff filters and to multilayer polarizers.

Improvements in Heat-Reflecting Filters

A nonabsorbing periodically stratified medium (abbreviated PSM) acts as a band-reflecting filter. Within a high-reflectance band, the reflectance tends to 100 percent in the limit as the number of periods increases indefinitely. A high-reflectance band is usually approximately centered on any wavelength such that the combined optical thickness of the layers constituting one period equals an integral number of half-waves. The integer in question is called the order of the high-reflectance band. Certain orders may be suppressed.A heat-reflecting filter can be made up of PSM’s having some unsuppressed high-reflectance bands in the infrared and only suppressed bands in the visible. The longer the wavelengths reflected by the first order, the greater the number of orders in the visible which must be suppressed. Present heat-reflecting PSM’s have only a single order suppressed. A method is described for designing a PSM which has two successive orders suppressed.