# Photosensitivity of the Er/Yb-Codoped Schott IOG1  Phosphate Glass Using 248 nm, Femtosecond, and  Picosecond Laser Radiation

Authors: Stavros Pissadakis; Irini Michelakaki
Journal: Laser Chemistry
 (2008)

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## Abstract

The effect of 248 nm laser radiation, with pulse duration
of 5 picoseconds, 500 femtoseconds, and 120 femtoseconds, on the optical properties and the Knoop hardness of a commercial Er/Yb-codoped phosphate glass is presented here. Refractive index changes of the order of few parts of10-4 are correlated with optical absorption centers induced in the glass volume, using Kramers-Kroning relationship. Accordingly, substantially lower refractive index changes are measured in volume Bragg gratings inscribed in the glass, indicating that, in addition to the optical density changes, volume dilation changes of negative sign may also be associated with the 248 nm ultrafast irradiation. The Knoop hardness experimental results reveal that the glass matrix undergoes an observable initial hardening and then a reversing softening and volume dilation process for modest accumulated energy doses, where the Knoop hardness follows a nonmonotonic trend. Comparative results on the Knoop hardness trend are also presented for the case of 193 nm excimer laser radiation. The above findings denote that the positive or negative evolution of refractive index changes induced by the 248 nm ultrafast radiation in the glass is dominated by the counteraction of the color center formation and the volume modification effects.


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## Body

## 1. Introduction

Phosphate  glasses are
promising host matrixes for the development  of free-space and waveguiding optical
devices, serving a diverse number of photonic applications. The pentavalent
phosphorus can build flexible tetrahedra, where a double oxygen bond occupies one coordination corner, allowing the easy accommodation of other network modifiers, without affecting the glass polymerization chains. That
microcoordination structure of the fundamental phosphoric glass matrix leads to
specific macroscopic and spectroscopic properties, such as high solubility of
rare earths without increasing quenching problems for laser excitation, low
transformation temperatures, and potentially high transmissions in the deep
ultraviolet wavelengths. Phosphate glasses have been employed as hosts for efficient
waveguide [1]
and fiber amplifiers [2]
and lasers [3],
biosensing devices [4],
while recently, they have been used as platforms for inscribing into 2D photonic
crystal structures [5]. Even though the above unique
optical properties that phosphate glasses exhibit and the numerous applications
serve, their photosensitivity has been examined in depth, only recently. The first
studies on the phosphate glasses photosensitivity were carried out using
femtosecond infrared [6]
and near ultraviolet laser sources [7],
in pristine or silver-doped matrixes. Later, other studies were presented including deep
ultraviolet lasers, by means of 248 nm [8] and 193 nm [9] nanosecond excimer and 213 nm, 150 picoseconds
Nd:YAG laser radiation [10]. In general, phosphate glasses
appear to be of low photosensitivity with the index changes induced in those
within the range of 10-4 and 10-5, while utilizing high
intensities and rather prolonged exposures. Addition of codopand ions, such as
Ag [8] or Ge [11],
can substantially increase the photosensitivity and refractive index changes
obtained by more than one order of magnitude. Moreover, the index changes
induced in pristine glasses can be either of positive or negative sign, depending
upon the exact composition of the exposed glass, the intensities, and energy
densities used, as well as the accumulated energy doses dissipated in the glass
volume [9, 12].
The origin of that interesting index engineering behavior is possibly
associated with counteracting, radiation triggered, physical mechanisms. In
particular, recent studies have revealed that in addition to the strong
photochromic effects that occur in phosphate glasses under deep ultraviolet
laser irradiation, significant volume modifications also take place, promoted
by significant microstructural rearrangements undergone in the glass coordination [5, 9].Herein, we investigate the photosensitivity of the high-performance rare earth doped IOG1 phosphate glass (manufactured by Schott, NY, USA)
[13], after exposure to 248 nm, 5-picosecond,
500-femtosecond, and 120-femtosecond laser radiation. The IOG1
glass is optimized for being ion-exchangeable using K+ and Ag+ ions, for developing low loss, high-power lasers, and amplifiers [14].
Our target is to provide
and correlate the laser-induced changes in the optical and structural data, for
gaining a better insight in the underlying photosensitivity behavior of a commercial
ultraphosphate glass matrix. The first part of the study presented herein investigates
the effect of the exposure to such high photon energy and extreme intensity
radiation on fundamental optical quantities of the glass such as that of the optical
absorption and the refractive index. Moreover, the last part of the current study
will include additional results on the effect of the irradiation on the Knoop
hardness of the pristine and exposed glass.


## 2. Experimental

The IOG1 samples exposed
here had a thickness of 1 mm and contained 2.3% wt. Er2O3 and 3.6% wt. Yb2O3. Energy dispersive x-ray (EDX) microanalysis
measurements performed in similar IOG1 glass batches revealed the existence of
other oxides such as Al2O3, La2O3,
and Na2O [8]. The laser used for the exposures was 248 nm, hybrid Lambda-Physik laser
emitting 5-picosecond and 500-femtoseconds pulses, at a repetition rate of 10 Hz.
The flux contrast ratio between the emitted picosecond/femtosecond pulse
and the nanosecond tail is greater than 95%. For obtaining 120-femtosecond pulses, the 500-femtosecond
output was compressed using two reflection gratings which were positioned in
parallel, with a spacing of approximately 6 cm between them [15].
All the exposures performed using 36 000 pulses. In the nongrating exposures, the beam was focused onto the
sample using a spherical fused silica lens of 20 cm focal length. A circular
aperture was used for selecting the optimum part of the beam, and an
oscillatory CaF2 plate was placed in front of the sample to
spatially scan the beam for averaging irregularities. The glass samples were
exposed to energy densities varying between 25 mJ/cm2 and 258 mJ/cm2.
The spectrophotometric data were obtained by employing an UV-Visible
spectrophotometer, scanning from 190 to 1500 nm.The spectral data obtained for each sample were normalized to the damage
depth induced by the radiation. This volume damage depth was estimated using
optical microscopy imaging of the endface of polished samples and subsequent
suitable image processing (see Figure1).Figure 1
Optical microscope picture obtained from a
side polished IOG1 phosphate glass sample, exposed using 248 nm, 500-femtosecond
laser radiation and 36 000
pulses of 258 mJ/cm2 energy density. The red line is a normalized gray
scale absorption line obtained using image processing.The inscription of Bragg
gratings was carried out by employing a 1070 nm period fused silica, phase mask
in “almost” contact mode. The phase mask was of ±1 order design, while it was
optimized for the 248 nm wavelength, exhibiting 0th order less than 1%. For
assuring preferential inscription of the 535 nm period, the phase mask was
thoroughly aligned with respect to the planar sample using an external He-Ne
red beam. A rectangular aperture was used for selecting the optimum part of the
beam and a fused silica 20 cm cylindrical lens for beam focusing. The samples
were exposed to different energy densities which varied between 15 mJ/cm2 and  ≈50 mJ/cm2,
for the case of 500-femto second pulse duration. The volume gratings inscribed
in the IOG1 glass were measured using a simple diffraction efficiency setup,
employing a 632 nm He-Ne laser, while the gratings probed at Bragg angle. The
refractive index changes induced in the glass were evaluated using standard
coupled-mode theory for thick, phase-absorption gratings [16],
considering a spatial coherence length of the order of the laser beam of  ≈75μm [17].
Knoop hardness measurements were performed using Matsuzawa, MXT70, digital
microhardness indentation microscope, and applying a 50 gf load for 20-second
indentation time. For reducing statistical errors, more than 15 indentations
were applied on each sample examined. The exposed sample area was in general
wider than 2 mm2, allowing easy accommodation of the indentation
traces. The Knoop hardness measurements performed in the pristine sample leaded
to a hardness value of 381, a figure that is very close to the value that is
provided by the manufacturer [13]. All spectral, refractive index, and hardness measurements were obtained a week after the laser exposures performed, for allowing defect and structural relaxations to reach a plateau state.


## 3. Results and Discussion

The changes in the
optical absorption changesΔα  of the exposed glass
slabs for exposures of different energy densities using 500-femtosecond pulse
duration, normalized to the penetration depth of the laser radiation at the 1/e
target point, are presented in Figure 2. The exposures were performed using 36 000 pulses, while real time
probing of the defect 540 nm band by employing a 2 mW 541 nm He-Ne revealed
that color center saturation was easily reached shortly after the first 20 minutes
of the exposure time, for all the pulse durations and energy densities used.Figure 2
Optical absorption changesΔα  for IOG1
glass samples exposed to different energy densities using 36 000 pulses of 500-femtosecond
pulse duration. The absorption data have been normalized to coloration-damage
depth as that was measured using optical microscopy imaging.Also, the optical
absorption changesΔα  of the exposed glass
for fixed energy density and variable pulse duration are presented in Figure 3.
A significant absorption band related to PO bond hole center defects is formed between 220 nm and 700 nm. The spectral
strength of the data presented (see Figure 1) denotes the occurrence of significant
electronic changes induced into the exposed glass, by means of
cleaving/deforming of the characteristic PO bond and the generation of
PO-related centers by electron or hole retrapping
[18].Figure 3
Optical absorption changesΔα for IOG1
glass samples exposed with different pulse durations using 36 000 pulses and similar
energy density.The changes in optical
absorption induced by 248 nm, 120-femtosecond, and 500-femtosecond laser
radiation are substantially greater than that obtained using nanosecond pulses
of similar energy density [8]; and on the same magnitude with those induced by 193 nm excimer laser radiation [9]. A point of disagreement with previous reports
refers to the bleaching band located at the spectral vicinity of 240 nm [8]. In the spectral data presented in Figure 2, such
bleaching feature does not exist. This may be attributed either to
stoichiometric variations of the composition between the IOG1 samples exposed
in different studies or to fundamental differences emerge from the underlying
photosensitivity mechanism. We speculate that a bleaching band may be formed at
that spectral location due to the annihilation of the PO4 electron center;
however, its strength can be screened by extensive electron retrapping
generated  by the longer wavelength bands [18]. Moreover, the accumulated energy density figures
needed for inducing such optical density changes, using ultrafast 248 nm
radiation, are significantly lower compared to that for corresponding exposures
using 193 nm or 248 nm nanosecond radiation [9]. Another comment related to the high durability is that
the phosphate glasses exhibit to the ultraviolet radiation. Extreme intensities
of the order of  ≈0.87 TW/cm2 (104 mJ/cm2 at 120 femtoseconds) can be dissipated inside the glass
matrix without triggering surface ablation or filamentation in the glass volume,
while the corresponding figure for high-purity silicate glass is  ≈0.2 TW/cm2 (24 mJ/cm2 at 120 femtoseconds).The dependence of the
radiation-induced optical density changesΔα  upon the energy
density of the exposure is presented in Figure 4. The data presented refer to
two characteristic defect peaks that of PO hole center (HC) located at 540 nm and that of the PO4 electron center
(EC) located at 242 nm [18].Figure 4
Spectral strength of the PO hole center (HC) located at 540 nm and that of the PO4 electron center (EC)
located at 242 nm versus energy density, for exposures using 248 nm, 500-femtosecond
laser radiation.Both peaks increase with
respect to the energy density of the exposure, while by fitting them to a
simple exponential law of the formΔα∝Fb,
where F is the exposure energy density, the b factor is evaluated to be
slightly greater than unity (see b-data in Figure 4). The last indicates that
the underlying color center generation process is linearly dependent upon the
number of photons per area provided in the glass matrix.Further, refractive index
changes induced by the femtosecond radiation into the glass were estimated by
using Kramers-Kronig transformation, from relaxed spectrophotometric data, for
the wavelength of 633 nm. The refractive index differencesΔnKK, which are solely associated with color-centers
formation (see Figure 5), were estimated to be of the order of few parts of 10-4,
for all the pulse durations and the energy densities employed. These refractive
index changes were calculated using the volume damage depths that were measured
using optical microscopy imaging. For reference, the FWHM of these volume
damage depths was varied between 300μm for the 26 mJ/cm2 exposure and 135μm for the 258 mJ/cm2 exposure, for the case of the
500-femtosecond radiation (see Figure 1). Accordingly, longer penetration
depths up to 1 mm were measured for the case of the 5-picosecond exposures,
while these depth figures
become smaller for the case of 120-femtosecond irradiation (≈100μm). Similarly, with the spectral strength data presented in
Figure 4, the refractive index changes ΔnKK of Figure 5 exhibit a linear dependence upon the
laser energy density, confirming that the color center generation process and
the mutually-related refractive index changes are possible single photon
absorption products.Figure 5
Refractive
index changesΔnKK induced in
the IOG1 phosphate glass using 248 nm, 5-picosecond, 500-femtosecond, and 120-femtosecond
laser radiation versus energy density, estimated using Kramers-Kronig
transformation.Also, the intensities
dissipated inside the glass volume affect the absorption correlated refractive
index changesΔnKK, where the 120-femtosecond exposures progress
these refractive index changes in a faster rate. However, this effect may be
related with a reduction of the penetration depth due to extensive damage
accumulation close to the glass surface vicinity, for greater exposure
intensities applied.In such a case, the
photosensitivity products (optical defect concentration or refractive index
changes) obtained will exhibit a sublinear dependence upon intensity, reducing
the order of interaction below that of a single quantum for the 248 nm
wavelength (see Figure6).Figure 6
Refractive
index changesΔnKK induced in
the IOG1 phosphate glass versus pulse duration for fixed energy density dose of
248 nm laser radiation.Additional refractive
index change measurements were carried out for Bragg gratings recorded in the
Er/Yb-codoped IOG1 glass matrix (see Figure7). The refractive index changes ΔnBG obtained from diffraction efficiency
measurements (Figure 7) are substantially lower than those estimated using
Kramers-Kronig transformation, for exposures of the same energy density, while
they exhibit a clear saturation trend. The above observation related to the
absolute refractive index change magnitude indicates that an additional
structural mechanism also contributes to the overall index changes measured in
the grating structures. Yliniemi et al.
reported that negative refractive index changes dominate the photosensitivity
of the same glass using 193 nm excimer radiation [9], while we have found that similar negative
refractive index changes may also occur for exposures using 213 nm, 150-picosecond
radiation [10]. We believe that a negative refractive index change
component also exists in the photosensitivity of the IOG1 phosphate glass using
248 nm picosecond and femtosecond-laser radiation. This negative sign refractive
index component, that was revealed using the above correlated measurements (see
Figures 5 and 7), may be associated with either localized stresses in the
grating fringes [19]
or absolute volume dilation.Figure 7
Refractive
index changesΔnBG measured in
Bragg gratings inscribed in the IOG1 glass using diffraction efficiency
measurements.For further identifying
volume changes induced by the 248 nm laser irradiation into the IOG1 glass, we
have performed Knoop hardness measurements on the exposed samples (see Figures8(a)
and 8(b)). The data of Figures 8(a) and 8(b) exhibit a nonmonotonous trend, for
the three pulse durations employed.Knoop
hardness measurements in pristine and exposed IOG1 phosphate glass versus (a)
energy density and (b) energy dose for exposures using 5-picosecond, 500-femtosecond,
and 120-femtosecond 248 nm laser radiation.F: energy density; τ: pulse duration; N: number
of pulses.
(a)(b)In particular, for the
cases of 500-femtosecond and 120-femtosecond irradiations, the Knoop hardness of
the exposed samples undergoes an initial increase reaching a maximum value that
is≈6.8% greater than
that of the pristine glass; while for increased doses [20]
(see Figure 8(b)), the absolute hardness follows a declining trend, restoring
to normal values. While the Knoop hardness changes induced using 5-picosecond
pulses are rather noisy without following a clear trend for the exposures
conditions applied, the results referring to the 500-femtosecond and 120-femtosecond
pulse durations reach maximum for the same energy dose, indicating that the
mechanisms triggered are of the same nature and strength. Changes in the
hardness of the glass matrix are directly associated with elastic modulus
changes and, therefore, radiation induced volume modifications [21, 22].
The nonmonotonous increase of Knoop hardness observed here can be initially
attributed to volume compaction effects, which may related with the exhausting
of a specific type of defect or with the passivation of surface singularities [23].
Accordingly, we believe that the decrease of hardness is related with the glass
depolymerization proposed by Yliniemi et
al. and observed in exposures using 193 nm nanosecond excimer laser
radiation [9]. We have also performed micro-Raman spectroscopy
using a 473 nm Ar-ion laser, in glass samples exposed using 120 femtoseconds,
100 mJ/cm2, where the measurements show a  ≈2.5% broadening of the two fundamental peaks located at 704 cm-1 (POP symmetric bond)
and 1184 cm-1 (PO2 symmetric bond), compared to that of the pristine
glass; denoting the generation of a broad band of structural defects.
Therefore, a glass depolymerization model based on the extensive cleaving of
the fundamental PO bond may be used for
explaining the volume dilation effects observed in the case of this
ultraphosphate glass, when exposed to 248 nm picosecond and femtosecond radiations.We believe that longer
exposures employing 248 nm femtosecond radiation will succeed in lowering the
Knoop hardness below the levels of the pristine sample. Such massive reduction
of the surface hardness and the glass density has been observed in exposures
using 193 nm excimer laser radiation (see Figures9(a) and 9(b)), where the
greater energy per photon progresses such reverse sign, structural, and volume
modification effects much faster than the 248 nm radiation. These significant
structural changes are induced for the case of 193 nm radiation without
employing extreme sub-TW/cm2 intensities, but by providing a photon
energy that is close or slightly higher than the bandgap of the exposed
material, while utilizing MW/cm2 intensities. However, the massive
decrease of the Knoop hardness and the accompanying strong volume dilation
effects induced by the 193 nm exposures are observed in expense of increased
accumulated energy densities; which in turn are manifold larger than those
utilized for the 248 nm, 120-femtosecond irradiations (see Figure 9(b)). Finally,
in Figure 9(b), we see that the Knoop hardness modifications induced by both
wavelengths follow rather similar trends, while peaking at quite similar accumulated energy density value.
Further work on the estimation of the elastic modulus of the glass exposed using 193 nm laser radiation and its correlation with the Knoop hardness and the exposure conditions are presented in detail elsewhere [24].Knoop
hardness measurements for IOG1 phosphate glass exposed using 120-femtosecond,
248 nm and 10 nanoseconds, 193 nm laser radiation versus (a) the energy density; and (b) the accumulated energy density of
the exposure.
(a)(b)


## 4. Conclusions

The photosensitivity of
the Er/Yb-codoped IOG1 phosphate glass using 248 nm, 5-picosecond, 500-femtosecond,
and 120-femtosecond laser radiation is presented here. We employed
spectrophotometric measurements for identifying the absorption bands induced by
the radiation; while later on, these spectral measurements were interpreted
using Kramers-Kroning relationship for obtaining refractive
index changes associated with this specific mechanism. The refractive
index changes correlated with optical absorption centers induced in the glass
volume are of the order of few parts of 10-4. Moreover, we employed
Bragg grating recording in the glass volume for estimating the overall
refractive index changes induced by the ultrafast radiation exposures. These
measurements revealed that the overall refractive index changes formed are of substantially
lower magnitude than those estimated using the Kramers-Kroning approach,
indicating that, in addition to the optical density changes, volume dilation
changes of negative sign are also associated with the 248 nm ultrafast irradiation.
The Knoop hardness microindentation experimental results obtained show that the
exposed glass matrix undergoes an observable initial hardening and then a
reversing softening and volume dilation process for modest accumulated energy
doses, wherein the Knoop hardness follows a nonmonotonic trend. Such
observation justifies the lower refractive index changes gratings probed in
volume Bragg grating reflectors, where the positive refractive index changes, due
to color centers, are competed by the negative index changes induced by the
volume dilation effects. We
are working on a deeper investigation of the photosensitivity of phosphate
glasses using ultraviolet nanosecond and femtosecond laser radiation, and the
correlation of the results with those obtained for other well-studied glasses
(i.e., Silicate and Germanosilicate amorphous matrices), using the same
wavelengths and pulse durations. Also, gaining a better understanding on the
volume damage and modification effects can be further exploited in the high
yield selective chemical etching micro/nanoprocessing of the specific glass [5].



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