Introduction to Optical Coatings

Almost every lens receives an antireflective coating to maximize transmission or image brightness, and to minimize ghost images. In fact, complex lens designs involving six or more elements could not realize their maximum potential if it were not for antireflective coatings.

For mirrors, coatings have replaced solid castings of polished metal in all but a few specialized applications. Mirror coatings perform with better reflectivity than solid metal mirrors, are lighter in weight and cost less to produce.

In the last 15 years, uses for optical coatings have expanded beyond their original applications as antireflectors for lenses and reflectors for mirrors. For example, some coatings are used as transmissive electrodes to activate electro-optic materials and highly durable coatings can improve the resistance of sensitive optical components to harsh environments as well.

Mirror coatings reflect light, and antireflective coatings transmit light by reducing reflection. It is easy to forget that optical coatings are components because they always work with lenses, prisms, windows or solid mirror substrates, whose imaging properties occupy most of the systems-design effort.

From the perspective of development and manufacturing, coatings can be classified as either metallic, dielectric or hybrid and as single-layer or multi-layer. Metallic coatings are usually deposited by evaporating a metal, such as aluminum or gold, in a chamber so that the vapor condenses upon the substrate. Other methods include ion-beam-assisted deposition, sputtering and electrolytic deposition.

Dielectric coatings are made of dielectric materials (electrically non-conductive) such as magnesium fluoride (MgF2). Hybrid coatings consist of dielectric layers deposited upon a metallic base layer. Purely dielectric coatings may be single layer, or they may be stacked to form multi-layer coatings with improved characteristics. Hybrid coatings are, of course, always multi-layered.

Applications engineers usually classify coatings as reflective or antireflective and broadband or narrowband. Broadband coatings handle many colors, i.e., a broad range of wavelengths, whereas narrowband coatings are designed for one color, i.e., a narrow range of wavelengths.

A high-performance coating is not just one coating but several thin films deposited on top of each other. Any one of the layers might exhibit modest performance. Working together, however, a reflective stack of layers can achieve very high reflectivity (99.9%) and an antireflective stack can achieve very low reflectivity (0.1%).

Each layer is very thin, typically one-quarter to one-half the wavelength of light, or about 10 to 20 millionths of an inch. Design and manufacture of these multi-layer coatings is complex and difficult, but computers and vacuum deposition techniques make them cost effective.

Constraints upon Performance

Coatings are designed to work under specific conditions of illumination, tilt and environment because their performance changes with wavelength of light, polarization of light, intensity of light, angle of incidence, humidity and temperature.

The simpler, single-layer coatings, like MgF2 or aluminum, exhibit more modest performance and more latitude in application when compared to the high-efficiency, multi-layer coatings.

Choosing a Coating

Choice of a coating is most influenced by the reflectivity or transmission required at certain wavelengths, but altogether there are seven issues involved in the design and manufacture of a high-quality coating. These issues are:
1. Wavelength
2. Reflectivity or transmission
3. Polarization of light
4. Angle of incidence
5. Substrate
6. Intensity or power of light
7. Environmental conditions.

Each of these design issues is discussed below.

1. Wavelength

 All coatings exhibit different reflectivity or transmission at different wavelengths. They may be classified as either broadband or narrowband.

 Broadband coatings handle large regions of the spectrum. For example, a broadband antireflective coating for visible light will reduce the reflective loss at the surface of a glass element for wavelengths between 400nm (violet light) and 700nm (red light). A broadband mirror coating, such as aluminum, can effectively reflect light as short as 350nm in the ultraviolet and as long as 10,000nm in the infrared.

 Narrowband coatings are designed to work in just one narrow region of the spectrum. V-coats are narrowband antireflective coatings that reduce reflections at a glass surface over a small range of wavelengths. For example, a high-efficiency V-coat designed for helium-neon laser light of 632.8nm might reduce reflective loss to just 0.1%. Its reflectivity might rise to more than that of uncoated glass at 500nm, just 133nm toward the blue. A narrowband mirror for helium-neon lasers might reflect 99.5% of red 632.8nm radiation but only 80% of blue-green light at 500nm.

 By classifying all coatings, reflective (mirrors) or antireflective (transmitters), as either narrowband or broadband, the wide variety of coatings can be organized in a simple, logical format:
A. Broadband
   1. Metallic
   2. Dielectric
      a. Single layer
      b. Multi-layer
   3. Hybrid (metal and dielectric)
B. Narrowband (multi-layer, dielectric)
　

The broadband category includes every kind of coating structure: metallic, dielectric and hybrid. Narrowband coatings are limited to multi-layer dielectric structures because they always achieve their performance with complex optical interference effects between the layers.

2. Reflectivity or Transmission

The reflectivity required of a coating completes its fundamental specification. Reflectivity usually defines the behavior of both reflective and antireflective coatings. For reflective mirror coatings, high reflectivity is desired. The opposite is true for antireflective coatings; low reflectivity characterizes "high-efficiency" performance.

The performance of single-layer coatings is less efficient than that of multi-layer coatings, but single layers are the most forgiving and the least expensive because of their simplicity.

Broadband multi-layer coatings reflect or transmit over a broad range of wavelengths with performance exceeding that of single-layer coatings. When compared at specific wavelengths, a broadband multi-layer coating can outperform a broadband single-layer coating by a factor of ten. For example, a magnesium fluoride (MgF2) single-layer antireflective coating might exhibit reflectivity of about 2% at 550nm, whereas a multi-layer coating designed for the same central wavelength of 550nm might exhibit 0.2% reflectivity.

Narrowband multi-layer coatings can be designed to outperform broadband coatings at specific wavelengths. Narrowband antireflective coatings are often called V-coats. The terminology originates in the appearance of graphs that plot reflectivity against wavelength. A V-coat can perform with 0.1% reflectivity at a specific wavelength, but its reflectivity rises quickly for shorter and longer wavelengths. The graph of its performance looks like the letter "V."

3. Polarization of Light

We all observe the effects of polarization every day. Most materials around us–asphalt roads, window glass, water and vegetation–reflect one kind of polarization better than the other; we say that they cause glare. Glare often results from the efficient reflection of horizontal, s-polarized light. Polaroid® sunglasses absorb the horizontally polarized light and thereby reduce glare from horizontal surfaces.

Light is composed of many colors or wavelengths, and each ray of specific wavelength can be analytically decomposed into one or two linear polarizations. Polarization, which refers to the direction of the electric field vector in a ray of light, is a concept that grows out of the electromagnetic wave theory of light.

A "polarized" ray of light is one that maintains a constant state of polarization over time; "unpolarized" light is polarized at any given instant in time but is always changing.

Engineers loosely refer to polarization as either "vertical" or "horizontal." The terms are used with reference to the plane of a surface rather than to our usual sense of up and down or sideways. In other words, when intersecting a vertical surface, a horizontal polarization can be straight up and down if the ray of light approaches from one side.

The precise terms for polarization are s-polarization and p-polarization. The first, s-polarization, is derived from the German word senkrecht, meaning perpendicular, since the electric field vector of s-polarized light is perpendicular to its plane of incidence. The second term, p-polarization, is taken from the German word parallel. This polarization is parallel to, or within, the plane of incidence (Figure G-1).

Figure G-1: The s-polarization is the component of polarization that is perpendicular and p-polarization is the component that is parallel to the plane of incidence which is defined by the incident and reflective ray.

A rule of thumb states that s-polarizations are reflected more efficiently than p-polarizations. This rule leads to unofficial terms for the two polarizations: skipping and plunging. Skipping-, or s-polarization, "skips" off a surface with more intensity than plunging-, or p-polarization, which "plunges" through the surface. Coatings can be designed in which this rule does not hold, but they are special cases.

4. Angle of Incidence

Angle of incidence is defined as the angle of light impinging upon a surface as measured from the normal to that surface. Light with an angle of incidence of 0° comes straight down onto the surface; when light approaches at large angles of incidence, it skims over the surface.

Optical coatings are designed for peak performance at a specified angle of incidence. If the angle of incidence is changed during operation, for example by tilting the optical component, then performance will usually degrade. Narrowband coatings are most sensitive to this specification; a 15° tilt can alter reflectivity at its nominal wavelength by a factor of five and shift the wavelength of peak performance by about 10nm. Broadband coatings exhibit slightly more tolerance to tilt, but a 30° change in angle of incidence will dramatically alter their performance.

A coating’s sensitivity to angle of incidence presents a challenge to the design of fast systems (small f-numbers, substantial light-gathering power) and wide-angle systems. In a fast optical system, the converging or diverging cones of light intersect surfaces at many different angles. The rays at the center of a cone may approach a surface at the angle of incidence to which the coating was designed, but the outer rays may intersect at considerably larger or smaller angles. Likewise, a wide-angle system will contain rays whose angles of incidence cover a broad range.

Designers restrict the most sensitive coatings to planar surfaces in collimated beams. By definition, the rays in a collimated beam travel parallel to each other. Therefore, they all intercept a planar surface at the same angle of incidence.

5. Substrate

The exact same coating will perform differently when deposited upon different substrate materials. This means that the exact formula for the structure and material of a coating, especially a multi-layer coating, will be tailored to the substrate. The origin of this variability lies in the fact that different substrates have different optical characteristics. When tailored properly, nearly identical performance can be measured for the same class of coatings applied to different substrates.

6. Intensity or Power of Light

Some coatings are "soft" while others are "hard." For imaging applications where the intensity of light is rather low, soft coatings withstand the radiant flux; however, for high-power laser applications, such as welding or surgery, soft coatings would be destroyed by the radiant flux. Hard coatings have been designed for these high-power applications.

Basic thin-film design philosophy is the same for soft and hard coatings; both are designed as stacks of thin layers. Their differences lie in the details of their prescriptions, such as material composition, and techniques of application.

Specifications that define the "softness" or "hardness" of a coating are written in terms of the threshold intensity that will damage the coating. For example, a typical hard infrared coating is rated for pulsed mode at 1 gigawatt/cm2. A 20-nanosecond, Q-switched laser pulse with a peak power density of less than one gigawatt/cm2 should not damage the coating.

Coatings are rated for their damage threshold under pulsed and continuous irradiation. Thresholds for damage are higher for pulsed modes of operation.

7. Environmental Conditions

Optical coatings must be handled with care. The harder coatings, which are resistant to laser damage, tend to resist scratching and abrasion, but even they are softer than many glasses. The softer coatings will be marred by careless or vigorous rubbing.

Today’s coatings are much more durable than those used before 1940. Early coatings would stain easily because their porous microscopic structure trapped finger oils. They could not be cleaned once they had been touched.

Varying humidity or temperature can alter the performance of a coating. Those containing water-absorbing layers exhibit sensitivity to changes in relative humidity because absorbed water changes the layer’s refractive index. Temperature also affects refractive index and even thickness.

In the vast majority of cases, a coating’s sensitivity to the environment is small enough to be ignored. In critical or unusual applications, more sensitive coatings are placed on components that can be protected from the environment. For example, a multi-element lens system might feature hard, durable coatings on its outer elements but softer, more sensitive coatings on its internal elements.