Science letter
www.beneq.com
Anti-reflective coatings for laser wavelengths
Kalle Niiranen1
December 29, 2016
Optical applications by Atomic Layer Deposition (ALD) are demonstrated by anti-reflective coatings at
laser wavelengths. Three wavelengths spanning from deep ultraviolet (266 nm) to visible (633 nm) and
infra-red (1064 nm) regions are presented.
Introduction
Anti-reflective coatings are optical thin films structures
with the aim of reducing reflection from a surface by
destructive interference. In many cases the decrease in
reflectivity can be directly translated to the increase of
transmission through a surface, such as a lens, because
only extremely low levels of absorption are acceptable.
Particularly for optical laser applications where optical
losses and undesired reflections are critical to minimize,
anti-reflective coatings are essential to incorporate.
In the case of lasers the target wavelengths are precise
and discrete, and the coatings require accurate deposition with low losses (ranging typically from < 100
ppm to < 1 ppm) in the resulting filter. High uniformity in deposition enables the coating of multiple
substrates in a single process run, allowing batch production. Atomic Layer Deposition is a particularly suitable method to achieve these targets either on planar
or 3D-substrates.
2
Design
Coatings designed for laser-applications require very
low levels of absorption due to the possible high power
output of the laser. To this end, care must be employed in the design phase of the coating in terms of
materials and their deposition chemistries. In particular, interface properties are among the most important
identifiable issues a designer must take into account.
Much of the knowledge in terms of material selection is
derived from experience. However, as the optical target
is typically a single wavelength, simple designs for the
film stack are possible.
Selected anti-reflective coatings are the neodymiumdoped yttrium aluminium garnet laser wavelengths of
266 nm (4th harmonic) and 1046 nm, representing the
ultraviolet and infra-red -wavelengths, respectively. A
much used visible wavelength red Helium-Neon laser at
633 nm is also presented.
1 Beneq
Oy, Espoo. Contact: [email protected]
www.beneq.com
Visiting address: Olarinluoma 9, Espoo
FI-02201 Espoo, Finland
[email protected]
3
Deposition
The deposition of films for filters is usually carried out
by determining the growth rate and optical constants
of single material films and then proceeding to construct the film stack. The development and up-scaling
to batch production phase are possible to be performed
in a single Beneq P400 reactor, so it was employed
for all depositions at temperatures 200 °C < T < 300
°C. The optical measurements were conducted using a
PerkinElmer Lambda 900 spectrophotometer. As the
wavelength range of interest in this application is the
neighbourhood of a single wavelength, simple Cauchy
models of the optical parameters of the films were employed.
3.1
266 nm AR
An anti-reflective coating for the Nd:YAG wavelength
of 266 nm was deposited by using thermal silicon dioxide, SiO2 and aluminum oxide, Al2 O3 on a fused silica
substrate. The resulting design yields a 4-layer solution
with total thickness of about 150 nm. The measured
reflectance against air is presented in Figure 1.
Reflectance, S-Pol, 8° (%)
1
14
Measurement
Design
12
10
8
266 nm
6
4
2
0
220
240
260
280
300
320
Wavelength (nm)
Figure 1: Reflectance of the 266nm anti-reflective coating on a
fused silica substrate measured against air at 8° incidence.
1–2
Because of the conformity to design of the reflectance
spectra of Figure 1 and of transmission (not pictured),
the removal of the fused silica – air interface, representing reflectance of about 4 %, results in reflection less
than 0.25 % into fused silica.
3.2
633 nm AR
The red He-Ne laser wavelength of 633 nm allows for
more material choices for design, as the absorption coefficients of multiple materials are typically much lower
at visible wavelength photon energies. Here, a 3-layer
solution was provided. As the absorption levels need to
be low, measurement of transmission is a good indicator
of filter performance. Figure 2 presents the transmission results of the coating on a fused silica substrate.
www.beneq.com
Transmission, S-Pol, 0° (%)
Science letter
100
95
1064 nm
90
Measurement
Design
85
80
900
950
1000
1064 1100 1150 1200
Wavelength (nm)
Figure 3: Transmission of a 1064 nm double-sided coated antireflective filter on a D263T glass substrate.
1064 nm. The result deviates at wavelengths other than
the target as a result of our method of obtaining the
optical constants of the optical films most precisely just
at the neighbourhood of the target wavelength for these
single wavelength applications.
4
Summary
Three different wavelengths were considered as a target for an anti-reflective coating with minimized optical
losses. Details of the deposited films are summarized
in Table 1.
Figure 2: Transmission of the 633nm anti-reflective coating on a
fused silica substrate measured against air at normal incidence.
From Figure 2, results indicate good congruity with the
design. The predicted transmission T into fused silica
can be evaluated by removing the substrate-air interface reflection (R ≈ 3.5 %) and is therefore projected
at T > 99.9 %.
Table 1: Summary of the deposited films
Wavelength (nm)
Layers
Thickness (nm)
266
633
1064
4
3
2
150
275
280
Laser wavelength anti-reflection coatings by Atomic
Layer Deposition have been demonstrated for
ultraviolet- visible- and infrared wavelength reA very important laser for medical and industrial apgions. Because of the nature of ALD, the substrate
plications (such as etching, marking and cutting) is the
shape typically does not have a major effect on the
Nd:YAG at 1064 nm. An anti-reflective coating at this
deposition accuracy. Therefore results derived from
near-infrared wavelength was constructed on a Schott
planar substrates, such as glass or lenses as presented
D263T glass substrate using a two-layer solution comhere can be almost directly applied to curved lenses or
prised of tantalum pentoxide, Ta2 O5 and silicon dioxmore complex 3D-shapes.
ide, SiO2 . The two-sided transmission for the performance of the filter is depicted in Figure 3.
The conformity to design is well demonstrated in Figure
3 at the vicinity of the target wavelength of
3.3
1064 nm AR
www.beneq.com
Visiting address: Olarinluoma 9, Espoo
FI-02201 Espoo, Finland
[email protected]
2–2