OPTICAL COATINGS FOR ELI EXPERIMENTS

PREPARED BY LASER ABLATION

A. BERCEA1, 2, M. FILIPESCU1*, A. MOLDOVAN1, S. BRAJNICOV1, D. COLCEAG1,

V. ION1, L. C. NISTOR3, A. ZORILA2 and M. DINESCU1

1National Institute for Lasers, Plasma and Radiation Physics, 409 Atomistilor, RO-077125,

Magurele, Romania 2Université de Limoges, CNRS, UMR 7315, SPCTS, ENSIL-ENSCI, 87000 Limoges, France

3National Institute of Materials Physics, 077125 Magurele, Ilfov, Romania

Corresponding author: mihaela.filipescu@gmail.com

Received September 25, 2017

Abstract. Dielectric materials (HfO2, Al2O3, SiO2 and Ta2O5) processed as thin

films have been studied extensively for applications in microelectronics, biology,

medicine, etc. In the last decade, antireflection coatings based on these types of

materials have received considerable attention for applications in the field of high

power laser optics. In this paper we report on multi-layers from materials with low

and high refractive indices deposited on quartz substrates by laser ablation. Damage

tests performed with a Ti-Sapphire laser (775 nm wavelength, 220 fs pulse duration)

evidenced higher threshold values for hafnia based heterostructures than for the others

heterostructures.

Key words: ELI, Antireflective Coatings, Heterostructures, Laser Ablation, Laser

Induced Damage Threshold.

1. INTRODUCTION

Lasers have come a long way since the days when they were only capable of

emitting just a series of irregular spikes within each pump pulse [1]. Nowadays,

with the implementation of different amplification techniques, such as the chirped

pulse amplification (CPA) developed in the mid-80s, lasers have been pushed up to

the petawatt power level [2]. Starting with April 2006, a new type of large scale

laser infrastructure, called Extreme Light Infrastructure (ELI), was commissioned

by the European Community to produce the highest peak power achievable at this

time. ELI aims to be the first project dedicated to the fundamental study of laser-

matter interaction in the ultra-relativistic regime (I > 1024

W/cm2) by using exawatt

class lasers [3]. The ELI project consists of several laser facilities. Built in

Romania, one of the pillars of the ELI project, the ELI Nuclear Physics (ELI-NP)

facility will host two 10 PW lasers [4, 5], coherently added to deliver intensities of

the order of 1023

–1024

W/cm2.

Romanian Journal of Physics 63, 606 (2018)

Article no. 606 A. Bercea et al. 2

The basic solution for the ELI-NP facility relies on a laser system consisting

of a Ti:sapphire oscillator, OPCPA Front End, and Ti:sapphire high-energy

amplification operating at ∼0.8 μm wavelength and having a pulse duration below

20 fs [6]. However, the use of such high energies creates a major problem: the

damage of the mirrors coatings facing extreme laser fluences.

One way of improving the performance of the optical components is using

antireflective (AR) coatings [4, 7]. The use of AR coatings is meant to reduce

unwanted reflections from surfaces, keeping only the desired component, for which

the reflectivity can reach 99.99%. The design of an antireflective coating having

high laser damage threshold can be done in two ways. The first method is to use a

single layer coating that has a refractive index equal to the square root of the

refractive index of the substrate [8]. In principle, this is really easy to implement,

but in reality, due to the limited number of available transparent pure substrates

that fulfil this requirement, this approach is not suited for systems such as ELI-NP.

The second method is to use more complicated designs, such as multi-layer

(heterostructures) having different refractive indices. In this case, the unwanted

reflection of light is suppressed as a result of destructive interference. The

materials used for this purpose are dielectric oxides, such as HfO2, Ta2O5, SiO2,

and Al2O3, combined to achieve antireflection coatings having high laser induced

damage thresholds (LIDT) [9, 10].

Hafnium dioxide (hafnia) is a high-index, low-absorption material used for

coatings from the near-UV (below 300 nm) to IR (~10 µm) regions. Due to the

refractive index around 2, hafnia can be combined in multi-layers with silicon

dioxide to form high index / low index contrast multi-layer structures having high

LIDT for UV laser applications [11].

Tantalum oxide (Ta2O5) is an inert material, and a very important one in

optoelectronics. Its high refractive index of 2.27 and low absorption (from VIS to

IR), make it suitable for antireflective coatings and waveguide materials [12, 13].

Silica (silicon dioxide) has a wide area of applications in electronics, optics,

etc. Its refractive index is around 1.46 and it is the main component in optical

fibres. Silica has considerably low thermal expansion, high melting point and is a

good refractory material.

Aluminium oxide (alumina) is a hard dielectric material, having a refractive

index of 1.77 [14] and high corrosion resistance. Aluminium oxide is an important

material in integrated optics [15] than can be easily processed as thin film, and

therefore a good candidate for high LIDT coatings. Al2O3 thin films are widely

employed in various optical components, such as filters and antireflection coatings

[9]. The functionality of these devices relies on interference effects, and their

spectral properties are tailored by constructing film stacks of two or more different

materials.

These materials can be processed/deposited as thin films or multi-layer

structures using a variety of techniques in order to obtain antireflective coatings

3 Optical coatings for ELI experiments prepared by laser ablation Article no. 606

having high LIDT. Glancing angle deposition (GLAD) [8], sol–gel process [16],

ion beam sputtering [17], plasma enhanced chemical vapour deposition [18],

chemical vapour deposition [19], atomic layer deposition [20], radio frequency

sputtering [21], etc., are some of the techniques used to deposit such structures.

The major disadvantages of these techniques are related to the high processes

temperatures, the use of expensive and highly corrosive precursors or subsequent

thermal treatments for crystallization.

Pulsed laser deposition (PLD) is a reliable and relatively inexpensive method

for the obtaining of thin films of simple or complex compounds. PLD is a method

that involves the interaction of a laser beam with a target material (solid or liquid).

The produced plasma expands rapidly and is collected on a substrate, as thin film

[22]. For a long time, the major problem of this method was the relatively small

area of the samples that could be obtained. Today, this problem was solved [23],

whereas the procedure retained its advantages. Thus, fabrication of test samples by

conventional PLD and the up-scaling to samples that could meet the requirements

of the ELI-NP project is a feasible objective. Complex materials, such as multi-

layers, can be obtained through the sequential ablation of two or more targets. The

deposition can be carried out in an inert atmosphere or in the presence of a

reactive gas.

The main purpose of this work is to prove that laser ablation is a method

suitable for obtaining dielectric layers (as heterostructures) to be used as

antireflective coatings for high power ultra-short laser pulses optical set-ups. These

coatings should have high LIDT values (without sacrificing spectral or phase

performance), and low manufacturing costs.

The dielectric layers were investigated from morphological, structural,

compositional and optical point of view.

2. EXPERIMENTAL SECTION

High-purity metal oxide targets (i.e. HfO2, Ta2O5, SiO2, and Al2O3) were

ablated using a Nd:YAG laser system (wavelength: 266 nm, pulse duration: 5–7 ns,

and repetition rate: 10 Hz). Multi-layer structures with alternating high and low

refractive indices have been obtained by laser ablation of the respective targets.

All depositions were carried out in oxygen atmosphere (0.01–0.1 mbar). The

target-substrate distance was set at 4 cm; the laser fluence was optimized for each

type of target. The quartz substrates used to collect the dielectric materials were

heated during the deposition to 600 C.

A parametric study was carried out to identify the optimal deposition

parameters for each layer (wavelength, laser fluence, oxygen pressure), after which

heterostructures consisting in two layers (Al2O3/SiO2, HfO2/SiO2 and Ta2O5/SiO2)

were obtained by sequential laser ablation. The choice of depositing silicon dioxide

Article no. 606 A. Bercea et al. 4

on quartz (SiO2) substrate as bottom layer was taken in order to have a low

refractive index layer with controlled thickness and morphology that can induce a

controlled growth of a top layer having high refractive index. The suitable laser

fluence was found to be 1.5 J/cm2

for both targets. The deposition parameters that

were varied are shown in Table 1.

Table 1

The experimental conditions to obtain heterostructures by PLD

Sample Target Substrate type Poxygen (mbar) NPulses

A1

Al2O3/SiO2/quartz

SiO2 quartz 0.1 36.000

Al2O3 SiO2/ quartz 0.01 36.000

A2

HfO2/SiO2/quartz

SiO2 quartz 0.1 72.000

HfO2 SiO2/ quartz 0.01 72.000

A3

Ta2O5/SiO2/quartz

SiO2 quartz 0.1 72.000

Ta2O5 SiO2/ quartz 0.01 53.000

The heterostructures properties were investigated by secondary ion mass

spectrometry (SIMS), ellipsometry (SE), atomic force microscopy (AFM), and

high-resolution transmission electron microscopy (HR-TEM). Also, tests to

evaluate the laser induced damage threshold (LIDT) of these heterostructures were

performed.

3. RESULTS AND DISCUSSIONS

SIMS investigations were carried out using a Hiden Analytical workstation in

order to investigate the films composition and distribution of elements in the

obtained heterostructures. The main conclusion of the SIMS analysis (sample A2 –

Fig. 1) is that the SiO2 layers exhibit oxygen deficiency, likely due to the fact that a

Si target was used for the growth of SiO2 layers, whereas a hafnia target was used

to obtain HfO2 layers. The oxygen deficiency in the SiO2 layer is suggested by the

different kinetics exhibited by the increase of Si counts at the sample surface and

bottom substrate interface, respectively. The signal of Si ions extracted by the

primary ion beam increases in both cases, due to the fact that in the presence of

oxygen the yield of secondary ions also increases. However, its subsequent

decrease is more dramatic at the surface of the sample than at the interface with the

quartz substrate, thereby suggesting that the SiO2 film has lower oxygen content

with respect to that of the substrate.

Both hafnia and silica layers show good homogeneity. Their thicknesses

(129 nm for hafnia and 133 nm for silica, respectively) were measured using a

profilometer in the crater areas created by SIMS probing in different regions of the

5 Optical coatings for ELI experiments prepared by laser ablation Article no. 606

sample. Scanning the surface of the samples for traces of other elements has

yielded that no impurities are present in the thin layer.

0 50 100 150 200 250 300 350 400 45010

2

103

104

105

106

107

Sig

na

l in

ten

sity (

c/s

)

Depth (nm)

O

Si

Hf

Fig. 1 – SIMS spectra for the A2 heterostructure.

The refractive index, layers thickness and roughness values were inferred by

spectroscopic-ellipsometry. The SE measurements were performed in the

400–1700 nm range, where the materials are considered optically transparent. For

all samples, the incidence angles were set to 60° and 65°. VASE32 software was

used for the fitting of the experimental data.

The initial optical model consists of five layers: the SiO2 substrate, the SiO2

thin layer deposited by PLD, intermix layer, the high refractive index layer (Al2O3,

HfO2 or Ta2O5) and the top rough layer. The refractive indices of the substrate were

taken from literature [24]. The intermix layer is a mixture of SiO2 and,

respectively, Al2O3, HfO2 or Ta2O5. The rough top layer is set to be half air and half

Al2O3, HfO2 or Ta2O5, respectively, in the Bruggeman approximation. As the SiO2

thin layer was deposited on a quartz (SiO2) substrate having identical optical

properties, it is difficult to distinguish between these two layers. In this case, the

optical model for all samples was reduced to four layers: substrate, intermix, thin

films of Al2O3 or HfO2 or Ta2O5 and rough layer. For the calculation of optical

constants, thickness and roughness of sample, a simple Cauchy dispersion formula

was used [25]. The parameters listed in Table 2 were obtained by fitting the

experimental data with a Cauchy dispersion model and assuming that the top layer

consists of 50% air (voids) and 50% Al2O3, HfO2 or Ta2O5, respectively.

Article no. 606 A. Bercea et al. 6

Table 2

The heterostructures characteristics obtained by SE

Sample Material Intermix

(nm)

Thickness

of the top layer

(nm)

Roughness

(nm) MSE

A1 Al2O3/SiO2/quartz 1.205±8.63 68.136±0.602 16.353±0.111 9.886

A2 HfO2/SiO2/quartz 12.630±1.69 112.491±0.343 5.568±0.378 29.3

A3 Ta2O5/SiO2/quartz 20.915±0.666 131.549±0.179 5.738±0.245 24.34

The errors (MSE is the mean square error) presented in Table 2, with three

decimals for thickness, are obtained from software calculation and are not

physically reasonable. An appropriate value should be rounded to a single decimal.

Using the optical model, the values of measured Ψ and Δ are compared with

the theoretical data. The following function is usually employed for the

comparison:

N

i i

ii

i

ii

MNMSE

1

2

exp

,

expmod2

exp

,

expmod

2

1

(1)

where N is the number of (Ψ, Δ) pairs, M is the number of variable parameters in

the model, and σ are the standard deviations on the experimental data points [21].

In this case, for all analyzed samples, N = 650 and M = 2. The unknown

parameters (n – refractive index, d – layer thickness) in the optical model are

varied in order to produce the best fit. Using the described optical model, the

dispersion of refractive index was generated for all samples (Fig. 2).

400 600 800 1000 1200 1400 1600

1.45

1.50

1.55

1.60

1.65

1.70

1.75

1.80

1.85

1.90

Al2O3as deposited

Intermix Al2O3-SiO2

SiO2 ref

Al2O3 ref

n

(nm) a)

Fig. 2

7 Optical coatings for ELI experiments prepared by laser ablation Article no. 606

400 600 800 1000 1200 1400 16001.4

1.5

1.6

1.7

1.8

1.9

2.0

2.1

2.2

HfO2 as deposited

Intermix HfO2-SiO2

SiO2 ref

HfO2 ref

n

(nm)

b)

400 600 800 1000 1200 1400 16001.4

1.5

1.6

1.7

1.8

1.9

2.0

2.1

2.2

2.3

2.4

TaOx as deposited

Intermix TaOx-SiO2

SiO2 ref

Ta2O5 ref

n

(nm)

c)

Fig. 2 (continued) – The refractive index behavior for heterostructure: a) A1, b) A2, and c) A3.

For sample A1, the top layer has a refractive index value much lower than the reference (software database) (n~1.64 at 800 nm), indicating an amorphous layer (as further confirmed by electron diffraction investigations). The thickness of the intermix layer was found to be in the nanometres range, but the error of this thickness was much higher than the value itself. The thickness of the Al2O3 thin layer is 68 nm, with a roughness of 16 nm. This value is in accordance with the thickness value measured by TEM.

Sample A2 has layers with thicknesses of 12 nm for the intermix, 112 nm for the HfO2 and the rough layer is of approximately 5 nm. The values of the refractive index (n~2.09 at 800 nm) are comparable with the literature data, indicating a good quality of the sample obtained by PLD.

In the case of sample A3, the thicknesses of the layers were in the same range as for sample A2, with a roughness of 5 nm, intermix of 20 nm and 131 nm for the Ta2O5 layer. The values of the refractive index for the top layer (2.25 at 800 nm)

Article no. 606 A. Bercea et al. 8

were slightly higher than the values presented in the ellipsometer database, indicating the presence of tantalum sub-oxides in the sample. The values of MSE (Table 2), are higher for samples A2 and A3 due to the presence of defects on the surface (Fig. 3), as can be seen from the AFM images (grains, clusters agglomerations with heights of 20–40 nm). Ellipsometry is a very sensitive technique to the quality of the surface [25], therefore the presence of defects induces errors in the measured data points.

From an optical point of view, the best heterostructures, in terms of high 'n', obtained by pulsed laser deposition on quartz substrates were HfO2/SiO2 and Ta2O5/SiO2.

The surface roughness on large areas (10 mm2) was determined by

ellipsometry; for small areas, an AFM system (XE 100 from Park System) was used. The surface was scanned in non-contact mode. To evidence the fine details

(nanostructures, small pores), areas of 2 2 µm2 were investigated.

All the samples exhibited uniform surfaces, with low roughness (below 3 nm), without micrometric cracks or pores. In the case of heterostructures based on Al2O3 and Ta2O5 (samples A1 and A3, respectively), rare grains, with sizes between 120 and 160 nm, and pores with sizes of 120 to 140 nm were identified.

The HfO2/SiO2/quartz heterostructure (A2) presents a nanostructured surface with very small pores (45–80 nm) and smaller grains (90–110 nm), with respect to the other samples.

a) RMS = 2.2 nm

b) RMS = 1.4 nm

Fig. 3

9 Optical coatings for ELI experiments prepared by laser ablation Article no. 606

c) RMS = 1 nm

Fig. 3 (continued) – 3D AFM images for samples a) A1, b) A2 and c) A3.

The high-resolution electron microscopy (HR-TEM) investigations were

carried out using the JEOL ARM 200F microscope (resolution of 0.19 nm). The

investigations were performed at 200 kV on samples (2 1 mm2) prepared by

mechanical polishing and ionic thinning.

The HRTEM image of sample A1 (Fig. 4a)) shows two amorphous thin

layers corresponding to SiO2 and Al2O3. From the interface between the Al2O3

layer (thickness of 83 nm) and the glue, it can be noticed that the surface layer is

very flat. Due to the fact that the contrast between the SiO2 layer and the quartz

substrate is not evident, the thickness of this layer cannot be determined.

In Fig. 4b) only the crystalline HfO2 layer can be observed. The diffraction

diagram inserted in the figure is indexed with the monoclinic structure: S.G.:P21/c.

The layer is compact (thickness of 110 nm) and shows a columnar growth and low

roughness.

The electron diffraction pattern in Fig. 4c) evidenced only the orthorhombic

phase of Ta2O5 with SG:P21212 spatial group. The layer is very compact and has a

columnar growth, with wide columns. The interface with the glue is very flat,

therefore the roughness of tantalum oxide layer is lower even that of the HfO2 layer

(from A2 heterostructure).

The antireflective coatings used for applications that involve the interaction

with ultra-short high power lasers have to be resistant to the damage induced by

laser; thus, their functional characteristic is the laser induced damage threshold

(LIDT).

Over the years, LIDT was calculated for different wavelengths, with different

pulse durations [26]. In the case of Ti:Sapphire laser, LIDT values for different

coatings were established below 1 J/cm2 [27, 28, 29, 30].

Article no. 606 A. Bercea et al. 10

a) b)

c)

Fig. 4 – HRTEM images on the heterostructure a) A1, b) A2, and c) A3.

In the case of our samples, the LIDT experimental data were obtained using a Ti-Sapphire laser working at a wavelength of 775 nm, with pulse duration of 220 fs. The heterostructures were cleaned in dry air flow and tested in ambient temperature at 24°C±1°C and 35% humidity. A number of 64 testing sites were positioned into the laser beam and irradiated at different energies. The distance between the sites was set at 1 mm. The maximum energy densities per pulse, corresponding to a damage probability of 50% (H50 (N)) and 0% (H0 (N)), respectively, for N pulses per site, were calculated and showed in Table 3. The estimated LIDT [J/cm

2] relative standard uncertainty was ±18%. The LIDTs

for these samples were tested according to the ISO 21254-1, 2, 3, 4:2011 standards recommendations. The damage threshold tests evidenced higher values for hafnia based heterostructures.

11 Optical coatings for ELI experiments prepared by laser ablation Article no. 606

Table 3

Threshold energy densities measured at 0% and 50% damage probability

for all heterostructures

Sample N pulses 10 30 100 300 1000 3000 10000 30000 100000

A1 H0(N)[J/cm2] 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.39

H50(N)[J/cm2] 0.60 0.58 0.53 0.51 0.50 0.50 0.50 0.50 0.47

A2 H0(N) [J/cm2] 0.65 0.65 0.64 0.64 0.64 0.64 0.64 0.64 0.65

H50(N)[J/cm2] 0.83 0.83 0.82 0.82 0.82 0.82 0.82 0.82 0.80

A3 H0(N) [J/cm2] 0.41 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.37

H50(N)[J/cm2] 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.40

a) b)

c)

Fig. 5 – Damage threshold characteristics for sample a) A1, b) A2, and c) A3.

The threshold energy densities versus the number of pulses applied per site,

and the damage probabilities (H50(N) and H0(N)) established by nonlinear fitting

are presented in Figure 5. From the graphical representation of the {H50(N), H0(N)}

Article no. 606 A. Bercea et al. 12

data, the software extrapolates the behaviour of the tested sample to a high number

of pulses per site.

In order to summarize, the 0 % and 50 % energy density values for

N = 105 pulses are presented in Table 4.

Table 4

Summary of LIDTs for the obtained heterostructures

Sample H0(105) (J/cm2) H50(105) (J/cm2)

A1 (Al2O3/SiO2/cuart) 0.39 0.47

A2 (HfO2/SiO2/cuart) 0.65 0.80

A3(Ta2O5/SiO2/cuart) 0.37 0.4

4. CONCLUSIONS

Metal oxide layers were obtained by laser ablation in specific conditions of

temperature and oxygen pressure. The heterostructures contain an amorphous SiO2

bottom layer that cannot be distinguished from the amorphous quartz substrate.

Instead, the top layers of HfO2 and Ta2O5 are crystalline and very compact; the

HfO2 layer exhibits the smallest roughness.

The laser induced damage threshold for all heterostructures was measured

using a Ti:sapphire laser; hafnia based heterostructures were found to have a LIDT

value higher (0.65 J/cm2) than that of others heterostructures (< 0.4 J/cm

2).

The obtained results prove that laser ablation technique allows obtaining

antireflective coatings resistant to high power ultra-short laser pulses, thus

demonstrating their suitability for experimental set-ups containing such laser

systems.

Acknowledgements. Financial support from the Romanian National Authority for Scientific

Research, CNCS – UEFISCDI project number PN–II–PT–PCCA–2013–4–1870 (ARCOLAS) and

from RDI Programme for Space Technology and Avanced Research – STAR, project number

168/2017 is gratefully acknowledged.

REFERENCES

1. Editorial, Extreme light, Nature Materials 15 (2016).

2. M.D. Perry, B.C. Stuart, D. Pennington, G. Tietbohl, J. Britten, C. Brown, S. Herman, J. Miller,

H.T. Powell, B.W. Shore and V. Yanovsky, The Production of Petawatt Laser Pulses,

L. DiMauro, M. Murnane, A. L’Huillier (eds) Applications of High-Field and Short Wavelength

Sources, Plenum Press, New York, 1–9, 1998.

3. ELI White Book, Science and Technology with Ultra-Intense Lasers, G. A. Mourou, G. Korn,

W. Sandner, J. L. Collier (eds), THOSS Media GmbH, Berlin, 2011.

4. D. Ursescu, G. Cheriaux, P. Audebert, M. Kalashnikov, T. Toncian, M. Cerchez, M. Kaluza,

G. Paulus, G. Priebe, R. Dabu, M.O. Cernaianu, M. Dinescu, T. Asavei, I. Dancus, L. Neagu,

13 Optical coatings for ELI experiments prepared by laser ablation Article no. 606

A. Boianu, C. Hooker, C. Barty, C. Haefner, Laser beam delivery at ELI-NP, Romanian Reports

in Physics 68, Supplement, S11–S36 (2016).

5. M.O. Cernaianu, B. De Boisdeffre, D. Ursescu, F. Negoita, C.A. Ur, O. Tesileanu, D. Balabanski,

T. Ivanoaica, M. Ciubancan, M. Toma, I. Dancus, S. Gales, Monitoring and control systems for

experiments at ELI-NP, Romanian Reports in Physics 68, Supplement, S349–S443 (2016).

6. F. Negoita, M. Roth, P.G. Thirolf, S. Tudisco, F. Hannachi, S. Moustaizis, I. Pomerantz,

P. Mckenna, J. Fuchs, K. Sphor, G. Acbas, A. Anzalone, P. Audebert, S. Balascuta,

F. Cappuzzello, M.O. Cernaianu, S. Chen, I. Dancus, R. Freeman, H. Geissel, P. Ghenuche,

L. Gizzi, F. Gobet, G. Gosselin, M. Gugiu, D. Higginson, E. D'humieres, C. Ivan, D. Jaroszynski,

S. Kar, L. Lamia, V. Leca, L. Neagu, G. Lanzalone, V. Meot, S.R. Mirfayzi, I.O. Mitu, P. Morel,

C. Murphy, C. Petcu, H. Petrascu, C. Petrone, P. Raczka, M. Risca, F. Rotaru, J.J. Santos,

D. Schumacher, D. Stutman, M. Tarisien, M. Tataru, B. Tatulea, I.C.E. Turcu, M. Versteegen,

D. Ursescu, S. Gales, N.V. Zamfir, Laser driven nuclear physics at ELI-NP, Romanian Reports in

Physics 68, Supplement, S37–S144 (2016).

7. T. Asavei, M. Tomut, M. Bobeica, S. Aogaki, M.O. Cernaianu, M. Ganciu, S. Kar, G. Manda,

N. Mocanu, L. Neagu, C. Postolache, D. Savu, D. Stutman, D. Vizman, D. Ursescu, S. Gales,

N.V. Zamfir, Materials in extreme environments for energy, accelerators and space applications

at ELI-NP, Romanian Reports in Physics 68, S275–S347 (2016).

8. T. Tolenis, L. Grineviciute, R. Buzelis, L. Smalakys, E. Pupka, S. Melnikas, A. Selskis,

R. Drazdys, A. Melninkaitis, Sculptured anti-reflection coatings for high power lasers, Optical

Materials Express 7, 1249–1258 (2017).

9. J. Tauc, Amorphous and liquid semiconductors, J. Tauc (ed.), Plenum, Springer US, New-York,

1974.

10. D. Ristau, M. Jupé, K. Starke, Laser damage thresholds of optical coatings, Thin Solid Films 518

(5) 1607–1613 (2009).

11. P. Baumeister, O. Arnon, Use of hafnium dioxide in multilayer dielectric reflectors for the near

UV, Applied Optics 16 (2), 439–444 (1977).

12. J.D. Traylor-Kruschwitz, W.T. Pawlewicz, Optical and durability properties of infrared

transmitting thin films, Applied Optics 36 (10) 2157–2159 (1997).

13. Y. R. Tu, C. C. Lin, W. S. Wang, S. L. Huang, Characterization of Reactively Rf-Sputtered

Tantalum Oxide Waveguides, SPIE Proc. 836, 40–47 (1987).

14. M. Ritala, H. Saloniemi, M. Leskelä, T. Prohaska, G. Friedbacher, M. Grasserbauer, Studies on

the morphology of Al2O3 thin films grown by atomic layer epitaxy, Thin Solid Films 286 (1–2)

54–58 (1996).

15. M.K. Smit, G.A. Acket, and C.J. Vanderlaan, Al2O3 films for integrated optics, Thin Solid Films

138 (2) 171–181 (1986).

16. F. Compoint, D. Fall, H. Piombini, Ph Belleville, Y. Montouillout, M. Duquennoy, M. Ouaftouh,

F. Jenot, B. Piwakowski, C. Sanchez, Sol–gel-processed hybrid silica-PDMS layers for the optics

of high-power laser flux systems, Journal of Materials Science 51, 5031–5045 (2016).

17. B. Mangote, L. Gallais, M. Commandré, M. Mende, L. Jensen, H. Ehlers, M. Jupé, D. Ristau, A.

Melninkaitis, J. Mirauskas, V. Sirutkaitis, S. Kičas, T. Tolenis, R. Drazdys, Femtosecond laser

damage resistance of oxide and mixture oxide optical coatings, Optics Letters 37, 1478–1480

(2012).

18. T. Rajagopalan, X. Wang, B. Lahlouh, C. Ramkumar, Low temperature deposition of

nanocrystalline silicon carbide films by plasma enhanced chemical vapor deposition and their

structural and optical characterization, Journal of Applied Physics 94, 5252 (2003).

19. R. Szczęsny, I. Szymańska, P. Piszczek, L. Dobrzańska, E. Szłyk, Chemical Vapour Deposition

(CVD) of metallic layers prepared from silver carboxylates complexes with tertiary phosphines,

Material Science Poland 23 (3) 671-667 (2005).

20. D. Bae, S. Kwon, J. Oh, W. K. Kim, H. Park, Investigation of Al2O3 diffusion barrier layer

fabricated by atomic layer deposition for flexible Cu(In,Ga)Se2 solar cells, Renewable Energy 55,

62–68 (2013).

Article no. 606 A. Bercea et al. 14

21. H. Tomaszewski, J.Haemers, J. Denul, N. De Roo, Roger De Gryse, Yttria-stabilized zirconia thin

films grown by reactive r.f. magnetron sputtering, Thin Solid Films 287 (1–2) 104–109 (1996).

22. D. B. Chrisey and G. K. Hubler, Pulsed Laser Deposition of Thin Films (Wiley), (ed) 1994.

23. M. I. Serna, S. H. Yoo, S. Moreno, Y. Xi, J. P. Oviedo, H. Choi, H. N. Alshareef, M. J. Kim,

M. Minary-Jolanda, M. A. Quevedo-Lopez, Large-Area Deposition of MoS2 by Pulsed Laser

Deposition with In Situ Thickness Control, ACS Nano 10 (6) 6054–6061 (2016).

24. C.M. Herzinger, B. Johs, W.A. McGahan, J.A. Woollam,W. Paulson, Ellipsometric determination

of optical constants for silicon and thermally grown silicon dioxide via a multi-sample, multi-

wavelength, multi-angle investigation, Journal of Applied Physics 83 (615) 3323–3336 (1998).

25. H. Fujiwara, Spectroscopic Ellipsometry Principles and Applications, Maruzen Co. Ltd, Tokyo,

Japan, 2007.

26. L. Gallais, M. Commandré, Laser-induced damage thresholds of bulk and coating optical

materials at 1030 nm, 500 fs, Applied Optics 53 (4/1) (2014).

27. S. Chen, P. Gao, Y. Zhao, Y. Wang, Z. Fang, Y. Leng, J. Shao, Thermal-dynamical analysis of

blister formation in chirped mirror irradiated by single femtosecond lasers, Applied Optics 53

(15) 3347–3354 (2014).

28. K. Starke, D. Ristau, S., Martin, A. Hertwig, J. Krueger, P. Allenspacher, W. Riede, S. Meister, C.

Theiss, A. J. Sabbah, W. Rudolph, V. Raab, R. Grigonis, T. Rikickas, V. Sirutkaitis, Results of a

round-robin experiment in multiple-pulse LIDT measurement with ultrashort pulses, Proc. SPIE

5273, 388–395 (1994).

29. K. Starke, T. Gross, D. Ristau, Laser-induced damage investigation in chirped mirrors for

ultrashortpulse laser systems, Proc. SPIE 4347, 528–534 (2001).

30. J. Jasapara, A. V. Nampoothiri, W. Rudolph, D. Ristau, K. Starke, Femtosecond laser pulse

induced breakdown in dielectric thin films, Physics Review B 63, 045117 (2001).