Microstructure Intro Apr08

April 2008

Micro-Textured Surface Treatments For
Enhanced Performance Optics and Windows
Technology Overview
Many unique and useful optical functions can be realized by the fabrication of
microstructures in the surface of a window or optic. The field of optical microstructures
can be divided into two main areas based on the relative size of the microstructures,
namely diffractive and sub-wavelength structures. Diffractive microstructures such as
gratings that effect wavelength separation contain microstructures with dimensions
that are comparable to or larger than the wavelength of light on which they operate.
Sub-wavelength micro-structures are sized to be much smaller than the wavelengths
of light that will pass through the optic or window. With dimensions that range from
tens of nanometers to just a few micrometers, sub-wavelength microstructures have
been demonstrated operating on light with wavelengths ranging from the ultraviolet
through the very long wave infrared.
Sub-wavelength microstructures that provide an anti-reflection (AR) function
are currently under development, along with microstructures that effect wavelength or polarization filtering.
In general, many critical optical systems that are limited by conventional thin-film coating technology can
realize a performance, lifetime, durability, or power handling capability increase by integrating a sub-
wavelength microstructure. For many applications these benefits are obtained with comparable or
reduced costs.

Technology Drivers: Anti-Reflection
The suppression of reflected light from the many optical surfaces found in a
typical military system, is an essential function that is currently addressed by thin-
film AR coating technology. AR microstructures have been shown to outperform
AR coatings in the following areas;
• Performance: Reflected light levels as low as 0.01% (-40 dB) have been
recorded with typical values in the 0.5% range.
• Bandwidth: Reflected light levels below 0.5% have been demonstrated over a spectral range
spanning five octaves in the visible through near infrared (400 to 2000nm). Good AR performance
has been shown for many types of infrared transmitting materials over the important wavelength
regions from 1 to 5 micron, 3 to 12 micron, and 7 to 16 micron.
• Viewing Angle: Because of the graded index nature of AR microstructures, reflected light levels
remain low over a wide incident angle range. Some measurements have shown good performance
over a ±45 degree range without the polarization effects found with thin-film AR coatings. Theoretical
models indicate that AR microstructures can remain effective at incident angles beyond 60 degrees.
• Radiation Hardness: AR microstructures retain the radiation hardness of the window or optic material.
Initial proton exposure trials have shown no performance degradation for exposure levels up to 1000
KRad(Si).
• Mechanical Durability: AR microstructures retain the mechanical durability of the window or optic
material. Initial sand and rain erosion testing indicates that the lifetime of AR microstructures will be
many times longer than any type of thin-film AR coating.
• Temperature Stability: AR microstructures retain the thermal properties of the window or optic
material. Temperature cycling tests over the range of 77K to 350K have shown no damage to the
microstructures or degradation in AR performance.
• Laser Damage Threshold: At multiple laser wavelengths ranging from the deep ultraviolet into the
infrared region, AR microstructures have been shown to exhibit high pulsed laser damage thresholds.
For example at the important 1064nm wavelength of the Nd:YAG laser, AR microstructures in float
glass and fused silica windows have been measured with damage thresholds in the 40 to 60 J/cm2

1

range, a value at least three times larger than the highest reported damage threshold of a single-layer
thin-film AR treatment. Similar results have been demonstrated at 351nm, 532nm, and 1538nm. An
initial surface absorption test indicated that the lifetime of AR microstructured windows passing
continuous wave laser energy, may also be significantly longer than thin-film AR coating treatments.
• Cost: In applications where conventional molding techniques can be employed, AR microstructures
can be replicated in high volumes for low costs. In other cases where batch processing is required,
AR microstructures should have costs that are similar to thin-film AR coating costs.

Technology Drivers: Bandpass, Rejection, and Polarization Filtering
Band-pass and rejection filters are needed for a wide variety of military and
commercial devices that utilize broad-band visible or infrared light. For example, an
imaging system gains increased functionality and new information when discrete
portions of the observed light spectrum are analyzed separately using optical filters.
In addition, the performance of laser-based communications, targeting, and weapons
systems depends on both wavelength and polarization discriminating filters. Flat
panel and projection display systems incorporate a number of polarizing and color
filtering devices. Microstructure-based optical filters can provide advantages over
conventional absorptive and multi-layer thin-film interference filters in the following areas;
• Unique Functionality: A single microstructure resonant filter can be designed to pass or reject
multiple wavelength bands that are not simple harmonics. This allows the fabrication of laser
hardening filters that protect against multiple laser sources, or the creation of a filter function that
matches the signature of a specific target, or the emission of a specific chemical. Microstructure
resonant filters can be fabricated as extremely sensitive detectors of biological or chemical agents
borne in air or water. Such sensors, combined with retro-reflector technology, provide a stand-off, or
remote sensing capability. In a similar manner, microstructure filters can act as robust temperature
sensors in extreme environments, allowing temperature measurements over a 2000° range. C
• Unique Polarization Effects: Asymmetric microstructures allow for the simultaneous filtering of both
the polarization and wavelength of a light source. Information can be encoded at multiple
wavelengths and on discrete polarization states enabling sophisticated laser communications
systems and security devices such as hidden friend-or-foe tags and identification marks.
• Simple Fabrication: For bandpass filters used in telecommunications or in astronomical imaging
systems, a conventional thin-film interference filter requires many layers of materials to affect the
required performance. In contrast, performance equivalent to a 100 layer stack of thin films can be
obtained with a single microstructured layer to produce a near infrared telecommunications filter. For
long wave infrared filters, the thickness of the stack of thin-film layers in an interference filter becomes
extreme, typically as much as 10 to 20 microns. The film stress in such a stack is a major concern. A
long wave infrared filter based on microstructures just 2 microns high, has been shown to introduce
little or no stress to the window or optic.
• Mechanical Durability: It is expected that a microstructure-based filter will retain the durability of the
window material in which it is fabricated. In addition the incorporation of hardened materials in the
microstructures may result in increased resistance to rain and sand erosion.
• Temperature Stability: Without the film adhesion and stress issues typical of thin-film interference
filters, microstructure filters can operate for longer periods within environments with extreme
temperature variations.
• Laser Damage Threshold: As described below, AR microstructures have been shown to exhibit laser
damage thresholds 2 to 3 times greater than single-layer thin-film AR coatings. For multiple layer thin
film AR coatings, the damage threshold is greatly reduced to a level at least five times lower than AR
microstructures. It is expected that microstructure filters will exhibit a similarly high power handling
capability relative to multiple-layer thin film interference filters.
• Low Cost: For many display and imaging applications, microstructure filters can be fabricated from a
single master tool using standard high volume replication methods.

2 April 2008

TECHNOLOGY DETAIL
Microstructures For Anti-Reflection (AR)
AR microstructures have been shown to be an effective alternative to thin-film AR
[1-12]
coatings in many infrared and visible-band applications where durability,
radiation resistance, wide viewing angle, or broad-band performance are critical.
These microstructures are fabricated directly into the surface of the window or optic
material, imparting an optical function that minimizes reflections while maintaining
the characteristics of the bulk material with respect to durability, thermal issues, and
radiation resistance.

Three distinct types of surface relief microstructures, commonly known as Motheye,
SWS, and Random textures are under development. The term Motheye is literally
derived from the eye of nocturnal moths that have evolved an AR microstructure as
shown in the SEM images on the right above to avoid detection by their main
predator, the owl. Each type of structure has unique characteristics and optical
properties that can be tailored for specific materials and applications. A brief outline
of the three structure types is given here;

1) MOTHEYE AR Structures: The Motheye structure is an array of surface depressions or pyramidal
protrusions. The tapered surface structure provides a gradual change of the refractive index for light
propagating from air into the bulk optic material. Reflection losses are reduced to a minimum for broad-
band light incident over a wide angular range. A typical Motheye texture profile is depicted on the left
below where the height h and the spacing are indicated. A scanning electron microscope (SEM) image
of Motheye structures fabricated in the surface of a CZT window, are shown on the right below.

n0 Λ
h

n1 MOTHEYE
CdZnTe 70¡

2) SWS Structures: A Sub-Wavelength Structure, or “SWS” effective index texture is depicted in the
profile diagram and SEM image below. An array of holes or posts provides an AR function that is
equivalent to a single layer thin-film coating. The effective index is engineered to be the ideal index for
the particular optic or window material, which is the square root of the material index of refraction. This is
accomplished by tailoring the texture fill factor - the proportions of solid and open areas in the surface.
Structure height h is then set to one quarter-wave optical thickness at the effective index. At one
wavelength reflections are completely eliminated, and over a narrow wavelength band reflections are
suppressed to very low levels. Dual and triple-band AR performance can be obtained with deeper
structures set at multiples of the quarter-wave optical thickness.

n0
Λ
h
n1 SWS CdZnTe 70¡

3 April 2008

3) Random Texture AR Microstructures: A simple fabrication process has been developed for AR
textures that have a random distribution of sub-wavelength sized surface features. The very small and
dense features, as shown in the figures below, provide AR properties that are both extreme and
broadband. Advantages of the Random AR texture include the cost driven benefit of eliminating the
lithography step necessary for fabricating Motheye or SWS structures, and the ability to apply the
structure conformal to challenging topologies, such as micro-lenses and MEMS. Random AR structures
built in glass, plastic, and silicon windows exhibit extreme AR performance for visible and NIR light.

n0

h

n1 RANDOM
Glass 70¡

To achieve high performance AR with surface relief microstructures, optical phenomena such as
diffraction and scattering must be avoided. This requires that the surface structures be fabricated with
feature spacing (Λ in the figures above) smaller than the shortest wavelength of operation within the
material for a given application. In addition, for Motheye and Random AR structures, the height and cross
sectional profile of the surface features must be sufficient to ensure a slowly varying density change. In
general, AR microstructures will exhibit similar characteristics as the bulk material with respect to
mechanical durability, thermal issues, laser power handling capacity, and radiation resistance. The
problems with thin-film AR coating adhesion, stress, off-axis performance degradation, durability and
lifetime, are eliminated.

AR Microstructure References
[1] Bernhard, C. G., "Structural and functional adaptation in a visual system", Endeavour, 26, pgs. 79-84, 1967
[2] Clapham, P.B. and Hutley, M.C., "Reduction of lens reflexion by the 'Moth Eye' principle", Nature, 244, Aug 1973
[3] Thornton, B.S., "Limit of moth's eye principle and other impedance-matching corrugations for solar-absorber
design." JOSA, Vol. 65, No. 3, pgs 267-270, March 1975
[4] Wilson, S.J. & Hutley, M.C., "The optical properties of 'moth eye' antireflection surfaces", Optica Acta, Vol. 29, No.
7, pgs 993-1009, 1982
[5] Southwell, W. H., "Pyramid-array surface-relief structures producing antireflection index matching on optical
surfaces", JOSA A, Vol. 8, No. 3, pgs 549-553, March 1991
[6] Raguin, D.H. & Morris, G.M., "Antireflection structured surfaces for the infrared spectral region", Applied Optics,
Vol. 32, No.7, pg 1154, March 1993
[7] DeNatale, J. F., et. al., "Fabrication and characterization of diamond moth eye antireflective surfaces on Ge", J.
Appl. Phys.,71, (3), pg1388,Feb.1992
[8] Harker, A.B. and DeNatale, J.F., "Diamond gradient index 'moth-eye' antireflection surfaces for LWIR windows.",
SPIE Vol. 1760, Window and Dome Technologies and Materials III, pgs. 261-267, July 1992
[9] MacLeod, B.D., and Hobbs, D.S., “Low-Cost Anti-Reflection Technology For Automobile Displays”, Journal of
the Society for Information Display, Automotive Display Conference, November 2004.
[10] Hobbs, D.S., and MacLeod, B.D., “Design, Fabrication and Measured Performance of Anti-Reflecting Surface
Textures in Infrared Transmitting Materials”, Proc. SPIE Vol. 5786, May 2005.
[11] Hobbs, D.S., and MacLeod, B.D., “Update on the Development of High Performance Anti-Reflecting Surface
Relief Micro-Structures”, Proc. SPIE Vol. 6545, April 2007.
[12] Hobbs, D.S., and MacLeod, B.D., “High Laser Damage Threshold Surface Relief Micro-Structures for Anti-
Reflection Applications”, Proc. SPIE Vol. 6720, Sept. 2007, (Boulder Damage: Received Best Paper Award).
[13] MacLeod, B.D., and Hobbs, D.S., “Long Life, High Performance Anti-Reflection Treatment for HgCdTe Infrared
Focal Plane Arrays”, Proc. SPIE Vol. 6940, March 2008.

4 April 2008

TECHNOLOGY DETAIL

Microstructure-Based Optical Filters
Nature provides a design for one type of surface structure optical filter that is
composed of a single material. As shown in the photograph on the right, a
brilliant metallic blue or violet color can be seen reflected from the wings of
South American butterflies known by their species name, Morpho. The color
is the result of multiple reflections from microstructures projecting from the
wing. Tiny ribs in the structure are spaced about half a wavelength apart for
blue light so that partial reflections from each rib will add constructively. All
other wavelengths pass without reflection and are absorbed by the underlying
wing structure. There are as many as ten ribs on each side of tree-like structures arrayed on the wing
surface creating a large number of reflecting surfaces that account for the observed reflection efficiency –
estimated at over 70%. With this type of structure, the filter bandwidth is determined by the number of
ribs and is equal to approximately the reflected wavelength divided by the number of ribs on each side of
a tree – or about 45 to 50nm in the case of the Morpho Rhetenor butterfly
shown. A thorough analysis of the butterfly wing structure, and methods Λ
for fabricating similar structural filters, was developed by Dr. James J.
Cowan. Dr. Cowan named his artificial butterfly structure “Aztec” after the
microstructure’s resemblance to stepped pyramids built by the Aztec
[1-3]
Indians . Aztec filter structures can be constructed with symmetric or
asymmetric cross sections as shown in the diagrams on the right. In the
case of asymmetric structures, the filtered light is separated from the
residual specular reflection, a useful feature for display and security
applications.

A second type of microstructure filter is based on surface structure wave- n2
[4-20]
guide resonant effects , sometimes referred to as zero-order
diffraction. The resonators create photonic band gaps for free space h
propagating optical beams. Surface relief microstructures etched directly Λ
n1
into an optic or a thin material layer deposited onto an optic, are
composed of microscopic features with dimensions much smaller than
the wavelengths at which the filter is designed to operate. Features such as holes, cones, mesas, lines,
and pillars arranged in a periodic array, function as individual resonators for a narrow range of light
wavelengths. This light is trapped within the wave-guide where it interacts
with the array of resonators and is reflected back with great efficiency. The
size, shape, and composition of the resonators in the array determine the
filter bandwidth, filter pass band profile, and center wavelength. A diagram
showing a typical microstructure resonator design is shown on the right
above. A scanning electron microscope (SEM) image of a typical filter
microstructure, is shown on the left. Here an array of 250nm diameter holes
in a 180nm thick layer of tantalum pentoxide (Ta2O5) deposited on a fused
silica substrate, serve to reflect 780nm wavelength light.

Wave-guide resonant microstructures readily produce narrow-band reflection. To achieve narrow-band
transmission, wave-guide resonant structures are placed between highly reflecting broad-band mirror
structures in a classic Fabry-Perot resonant cavity configuration. (This is directly analogous to placing a
solid etalon within a laser cavity to produce “single-frequency” operation). Thin-film narrow-band
[21]
transmission filters create Fabry-Perot cavities using stacks of non-absorbing dielectric materials . A
cavity resonance is obtained for light propagating in the longitudinal direction. In contrast, structural
wave-guide resonant filters are configured to create a resonance in both the longitudinal and transverse

5 April 2008

directions, effectively reducing the number of layers required to achieve narrow-band transmission. A
microstructure-based narrow-band transmission filter designed for the MWIR, employs just three material
layers with a total thickness of 1400nm. To attain equal performance in a thin-film interference filter
design requires about 25 layers with a total thickness of over 8000nm.

More recently, highly efficient polarizing microstructures have been
[22]
demonstrated . Here the waveguide resonant microstructure design is
implemented with a high degree of asymmetry. Light polarized along the axis of
maximum asymmetry does not encounter discontinuities in the waveguide layer
and therefore reflects with the high efficiency of a dielectric mirror. Light
polarized in the orthogonal direction does encounter the resonators in the
waveguide layer and a resonant reflection will be generated at a wavelength that
is determined by the resonator configuration. For all other light wavelengths with
this polarization state, the microstructure will be anti-reflective producing a high
transmission. Careful design and fabrication of the microstructure aligns the maximum transmission band
with the maximum reflection band for orthogonal polarization states. Polarization extinction ratios better
than 1000:1 have been recorded. The images on the right above show the reflection of polarized visible
light from polarizing filters designed to reflect red, green, or blue light. The top and bottom row of images
were taken through a sheet polarizer with its extinction axis oriented in orthogonal directions. An
alternative design can yield multiple resonant bands with variable efficiency yielding a polarization and
wavelength filter that can be matched to a specific target signature.
Microstructure Filter References
[1] Cowan, J.J., “Aztec surface relief volume diffractive structure”, JOSA A, 7, No. 8, August 1990, pgs 1529-1544
[2] Cowan, J. J., “Electro-Optic Device Allowing Wavelength Tuning”, U.S. Patent 6,707,518, Mar. 16, 2004.
[3] Cowan, J.J., “Advances in Holographic Replication with the Aztec Structure”, Proc. Int. Conf. Holography, 2006
[4] Hobbs, D.S., “Laser Line Rejection or Transmission Filters Based On Surface Relief Microstructures Built On
Infrared Transmitting Materials”, Proc. SPIE 5786-38, March 2005
[5] Raguin, D.H. and Morris, G.M., “Structured surfaces mimic coating performance”, Laser Focus World, April 1997
[6] Mashev, L., Popov, E., “Zero Order Anomaly of Dielectric Coated Gratings”, Optics Communications, 55, 6, 1985
[7] Mazuruk, K., “Bragg diffraction in the vicinity of Woods anomalies”, JOSA A, Vol. 7, No. 8, August 1990, pg.1464
[8] Wang, S.S., Magnusson, R., Bagby, J.S., and Moharam, M.G., “Guided-mode resonances in planar dielectric-
layer diffraction gratings”, JOSA A, Vol. 7, No. 8, August 1990, pg.1470
[9] Magnusson, R. and Wang, S.S., “New principle for optical filters”, Appl. Phys. Lett., 61(9), 31August1992, 1022
[10] Wang, S.S. and Magnusson, R. “Theory and applications of guided-mode resonance filters”, Applied Optics, Vol.
32, No. 14, 10 May 1993, pg 2606
[11] Wang, S.S. and Magnusson, R. “Design of waveguide-grating filters with symmetrical line shapes and low
sidebands”, Optics Letters, Vol. 19, No. 12, June 15, 1994, pg 919
[12] Peng, S. and Morris, M., “Experimental demonstration of resonant anomalies in diffraction from two-dimensional
gratings”, Optics Letters, Vol. 21, No. 8, 15 April 1996, pg 549
[13] Peng, S. and Morris, M., “Resonant Scattering from two-dimensional gratings”, JOSA A, Vol.13, No.5, May 1996
[14] Sharon, A., et. al. “Resonant grating-waveguide structures for visible and near infrared radiation”, JOSA A, Vol.
14, No. 11, Nov. 1997, pg. 2985
[15] Brundrett, D., et. al., “Normal-incidence guided-mode resonant grating filters: design and experimental
demonstration”, Optics Letters, Vol. 23, No. 9, May 1, 1998, pg.700
[16] Tibuleac, S., and Magnusson, R., “Reflection and Transmission Guided-mode Resonance Filters”, JOSA A, Vol.
14, No. 7, July 1997, pg 1617
[17] Priambodo, P.S., Maldonado, T.A., and Magnusson, R., “Fabrication and Characterization of High-Quality
Waveguide-mode Resonant Optical Filters”, Applied Physics Letters, Vol.83(16) October 20,2003, pp 3248-3250.
[18] Magnusson, R., and Wang, S.S., “Transmission band-pass guided-mode resonance filters”, Applied Optics, Vol.
34, 1995, pg 8106
[19] Magnusson, R., et. al., “Efficient Band-pass Reflection and Transmission Filters With Low Sidebands Based on
Guided-Mode Resonance Effects”, U.S. Patent 5,598,300 issued January 28, 1997.
[20] Hobbs, D.S. and Cowan, J. J., “Optical Device For Filtering and Sensing”, U.S. Patent 6,791,757, Sept. 14, 2004.
[21] MacLeod, H.A., Thin Film Optical Filters, Third Edition, Institute of Physics Publishing, Bristol, Philadelphia, 2001
[22] Hobbs, D.S., “Microstructured Optical Device For Wavelength and Polarization Filtering”, US Patent Application
Publication Number 20060262250, November 23, 2006

6 April 2008

Microstructure Intro Apr08

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