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HIGH REFLECTORS
High Reflector Coatings (HR) - is a film or system of films deposited on a substrate to increase the overall reflectivity of the substrate surface.
HR's primary function is to increase the reflectivity of a substrate surface, and in most cases, move or steer the light to another location in a optical system. HR's are divided up into two basic areas: Metals and Dielectric Multilayers. Metal coatings are widely used to steer light in an optical system and, in most everyday uses, are thr more economical reflector alterative. Metal coatings can be extremely broadband, but have a tendency to be fragile and hard to clean. Dielectric films are used for more narrowband applications where high reflectivity, low absorptivity, and robust coatings are necessary. This section of Optical coating design will be divided up into 2 areas:
Metal Coatings
- Bare Metal
- Protected Metal
- Enhanced Metal
Dielectric Multilayers
- The Basic HR
- Ratio Stacks
Metal Coatings
Bare Metal
Metal mirrors have been made for centuries and are fairly common in many optical systems. The mirror's main purpose is to collect light and steer it to another location in the optical system. Metal mirrors offer high efficiencies and are the coating of choice for very large optics, such as telescope mirrors. However, their drawback is that they are extremely fragile and difficult to maintain over long periods of time. The most common metals used for highly reflective surfaces are Aluminum (Al), Gold (Au), and Silver (Ag). Their reflective performances are shown in the figure below:
Here are just a few notes on each of these three popular metals:
Metals Average % ReflectivityThere are also other metals that are used frequently as mirrors. The figure below outlines the refelctance performances of Copper (Cu), Nickel (Ni), Platinum (Pt), and Rhodium (Rh).
THE INDEX DATA FOR ALL OF THE ABOVE METALS CAN BE FOUND ON THE MATERIALS PAGE.
Protected Metal
Most metal thin films are suscepible to damage if cleaned or wiped. Therefore, in most cases, it is necessary to protect the metal film with some type of transparent dielectric layer to increase the mirrors robustness. The figure below shows the optimum thicknesses for the dielectric film layer that would produce maximum and minimum reflectivity when the design wavelength was 550nm.
Protective layers not only need to have their thickness optimized, but their index needs to be as low as possible for broadband higher reflectivity. The figure below outlines the difference between a film with an index of 1.45 and one with and index of 2.0. The minimums on the curve for the film with the 2.0 index are lower than that of the 1.45 film. Therefore, if the only protective film alternative is one with a high index, care needs to be taken to be sure that the optical thickness is at the reflectivity maximum. If the optical thickness is missed, reflectivity will be sacrificed.
Another important point to remember when designing a protected metal mirror is that with the addition of a dielectric layer, within a region where the metal has absorption, the metal absorption increases at non-normal incidence. The figure below shows the effect of a protective layer of n=1.45 on aluminum where the incident angle is 45 degrees:
Enhanced Metal
Enhancing the reflectivity of a metal layer will not only boost the reflectivity around the design wavelength, it will also narrow the high reflectivity region. When starting the design for the enhancement layers it is important to have the low index layer closest to the metal and the high index layer closest to the medium, as shown:
Medium / (H L)m Metal /
Substrate
where m is the number of repeating periods of HL
The figure below shows an enhanced aluminum with a high index of 2.40 and a low index of 1.46. As one can see, by increasing the number of periods, the reflectivity increases, but the high reflectivity region narrows.
Dielectric Multilayers
The Basic HR
The high reflector design is based on alternating high and low refractive index layers, nH and nL such that a "stopband" (or area of high
reflectivity) is created that is centered around the design wavelength, l0. The most basic HR design has each layer arranged such that the optical thickness of
each of the indiviual layers equals a quarter of the design wavelength, or a QWOT.
The design in its basic form resembles:
Medium / (HL)m H / Substrate
where m is the number of periods of the multilayer stack.
The following figure below outlines all of the characteristics of a basic (HL)mH high reflector design
From the figure above one can see that the high reflector has a 1st order harmonic stopband and successive harmonic ordered stopbands at lower wavelengths. In between the harmonic stopbands is an area know as the "passband" where light is transmitted and not reflected. One will also notice that for this basic design there are no 2nd or 4th order stopbands. Later in this tutorial, determining your harmonic stopbands will be addressed.
If you have designed your HR to be of the period m and desire to know what the reflectivity of the system will ultimately be the following formula to calculate the reflectance, R, is the following:
Now, if you have a specific reflectivity in mind, and want to know how many periods to start with in your design, you can use the formula below solved for the period m:
So, for example, if we are building a HR on glass (ns=1.52)with nL=1.46 (SiO2) and nH=2.40 (TiO2) at l0=550nm, and we wanted to have a reflectivity of 99.9%. If we use equation-2h above, m=7 periods. The example is shown in the figure below:
Air / (HL)7 H / Glass
There will also be some variables used to describe the stopband of the high reflector. The first is g which is the ratio of the design wavelength to the performance wavelegth, where,
And, the half-width of the stopband Dg , where,
The stopband can be described Dg when the spectral performance is viewed in terms of g:
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Ratio Stacks
The most interesting phenomenon associated with high reflector designs is the reflectivity profile at other wavelengths other than the design wavelength. The peak that is created from the QWOT layers at the design wavelength is referred to as the first order harmonic. We have seen in a figure above that for the basic high reflector stack their are also higher order harmonics that have high reflectivity as well. This section of HR design is going to cover deisgn modifications to the basic HR to manipulate which higher order has high reflectivity or is highly attenuated. The technique used is referred to as a ratio stack.
A traditional HR design has both of the high and low index layers at perfect QWOT at the design wavelength. Therefore, the low to high index ratio is 1:1. If we alter the ratios of the high index layer to the low index layer we can alter which higher orders will be present and which ones will be suppressed. The figure below shows the performances of a 1:1, 2:1 and 3:1 high reflector stacks when plotted vs. g:
The figure below was taken from Philip Baumeister's lecture notes and illustrates when stopbands and passbands occur for a 1:1, 2:1, and 3:1 ratio stacks.
The above figure illustrates that in the different ratio designs, if the edge of the shaded area falls in the center of a wave, the order will have maximum reflectivity. If the edge of the shaded region falls at a nodal point, there will be no reflection band at that order. If the edge falls within the wave but not at the center, the reflection band will be present, but not at maximum reflectivity.
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Metal-coated mirrors are mirrors (optical reflectors) based on a thin metal coating, produced e.g. with an vacuum evaporation or sputtering technique. The metallic coating is placed on a substrate, which is often consisting of a glass (e.g. fused silica), and sometimes a metal such as copper.
Common metal mirror coatings consist of thin films of aluminum, silver or gold; less common are beryllium, copper, chrome and various nickel/chrome alloys. The metallic coating is often protected (enhanced) with an additional dielectric layer.
The central features of metallic mirrors are very broadband reflections, although with a limited reflectivity, and low chromatic dispersion. Further, the angular dependence of the reflectivity is relatively weak.
Metal mirrors play a special role in infrared optics, i.e., in spectral regions where dielectric mirrors are hard to realize.
For a first surface mirror, the reflective coating is on the side of the incident light. The light only slightly penetrates the coating, but does not reach the substrate.
Figure 1:
Comparison of second surface and first surface mirrors. In the former case, the incident light travels through the transparent mirror substrate, and a secondary reflection from the front surface can occur.Second surface mirrors have the reflecting coating on the other side of the substrate, so that the coating can be better protected. The light propagates through the substrate before and after the reflection. This type of mirror is also common for household applications. In technical applications, problems can arise from the Fresnel reflection at the first surface (which can lead to ghost images, for example, and to some power losses), and in some applications from the chromatic dispersion of the glass.
The article on first surface mirrors contains more details.
Often, the metal layer of a first surface mirror is covered with a thin layer or multiple layers of a dielectric material such as amorphous SiO2 (silica) or Si3N4 (silicon nitride), which protects the coating against oxidation (tarnish) and scratches. Such enhanced or protected mirror coatings are definitely more abrasion-resistant than uncoated ones, but still they tend to be more sensitive than dielectric mirrors. This implies that greater care is generally required for the cleaning of metal-coated optics; they may even have to be replaced when e.g. fingerprints have been made. Also, metallic mirrors are more or less sensitive to humidity and corrosive gases.
Multilayer protection coatings can also be used to enhance the reflectivity (see below); this leads to enhanced metal coatings e.g. for enhanced silver mirrors, also called protected silver mirrors. The resulting metal / dielectric coating effectively combines the large bandwidth of a metallic mirror with the higher reflectivity and damage threshold of a dielectric mirror. Note, however, that the chromatic dispersion can be substantially modified by such a multilayer coating.
Early mirrors where made from massive pieces of speculum metal, an alloy of two thirds copper and one third tin, which can be polished to obtain a smooth surface with high reflectivity. Such metal mirrors were used in telescopes, for example, They suffered from relatively rapid tarnishing, which made frequent re-polishing necessary. In the 19th century, such mirrors were largely replaced with silvered glass mirrors. These do not only exhibit less tarnishing, but also the glass provides are more rigid substrate, which is nevertheless less heavy. Since then, it became common to use only thin metallic coatings on substrates.
A big advantage of metal-coated mirrors over dielectric mirrors is that the reflectivity is quite uniform over a wide spectral range and also quite insensitive to the angle of incidence and polarization. Also, metal-coated mirrors can be fabricated easily and are thus relatively cheap. Therefore, they are often used as general-purpose mirrors. Also, they are sometimes required for ultrashort pulses with ultrabroad bandwidth, where it is difficult to obtain sufficient reflection bandwidth from dielectric mirrors (although chirped dielectric mirrors nowadays also offer very large bandwidths). In that context, it also relevant that metal-coated mirrors exhibit very weak chromatic dispersion; the reflection phase-shift exhibits a very small wavelength dependence. This is useful, for example, for use as reference mirrors in white-light interferometers.
Metal-coated mirrors can also work for extremely long infrared wavelengths, e.g. up to 20 μm (&#; infrared optics). In that region, it is difficult to work with dielectric mirrors, as dielectrics then exhibit strong absorption.
A disadvantage of metallic mirrors, when compared with dielectric mirrors, are the significant reflection losses. This limitation is of fundamental nature, as metals inevitably absorb some of the incident light (even if they are very pure). As a consequence, the reflectivity is limited, e.g. to roughly 98% for protected silver mirrors. Another consequence is the relatively low optical damage threshold: the absorbed light leads to heating, and as the heat is deposited in a quite thin layer, this can easily lead to damage. Both in terms of average power and peak power, the damage threshold is low. For high average powers, substantial thermal lensing and thermal beam distortions can arise from heating effects even well below the damage threshold.
The reflection losses may be reduced with dielectric multilayer coatings. Such enhanced coatings also lead to a correspondingly higher optical damage threshold. For example, the damage threshold of an enhanced coating silver mirror for nanosecond pulses from a -nm YAG laser can be several J/cm2, whereas it may be only 0.5 J/cm2 for a simple protected silver mirror (and even much less for aluminum). For comparison, dielectric mirrors can stand several tens of J/cm2.
It is possible to obtain partial transparency of a metal coating by making it very thin. Such mirrors can also be used as beam splitters. However, the power losses are substantial, so that the sum of transmittance and reflectance is well below 1. It is more common to use dielectric mirrors for such applications, but partially transmissive metal-coated mirrors are used if a very high operation bandwidth is required and high power losses can be tolerated.
Protected aluminum mirrors, e.g. with a SiO2 coating, are used for many broadband applications in the visible and ultraviolet spectral region, reaching reflectivities well above 90% in the visible but often below 90% in the UV (similar to bare aluminum). Compared with silver mirrors, such enhanced aluminum mirrors have a lower tendency for tarnishing when exposed to humidity.
Protected silver coatings (again usually with a SiO2 coating) are similarly suitable for wavelengths from about 500 nm to 20 μm. Due to their lower reflection losses, they also exhibit substantially higher damage thresholds than aluminum mirrors. Therefore, they are often preferred for applications involving lasers.
Gold mirrors are similar, but can be used only for about 600 nm and longer wavelengths. Here, reflectivities well over 95% (sometimes around 99%) are achieved. It is possible, for example, to obtain an average reflectivity of 97% between 700 and nm with a protected gold mirror. Sometimes, an unprotected gold mirror is preferred in order to avoid any dispersion from a protective coating; this is possible as gold does not tend to get oxidized.
Gold-coated copper mirrors (often fabricated with electrochemical techniques) can be used with high-power infrared lasers, such as CO2 lasers. Here, a thin reflecting gold coating is placed on a massive copper substrate. Substantial amounts of heat can be tolerated due to the high thermal conductivities of gold and copper.
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