UV Pulsed Laser Irradiation Effect on Spectral Properties of

Borosilicate and Phosphate Glasses with CuCl Nanocrystals

Anastasiia Babkina, Ksenya Trots, Elena Kolobkova and Nikolay Nikonorov

Department of Optical Information Technologies and Materials, ITMO University,

49 Kronverksky Pr., St. Petersburg, Russia

Keywords: Phosphate Glass, Borosilicate Glass, Copper Chloride, Nanocrystals, Photochromism, Exciton Absorption.

Abstract: The results of the study of the pulsed UV laser radiation effect on the spectral properties of the borosilicate

and the phosphate glasses doped with the copper chloride nanocrystals with the mean size of 26-70 Å are

discussed. The changes of the exciton absorption spectra of the CuCl nanocrystals with various mean sizes

induced by different duration of the laser exposure are studied. The effect of the phosphate glass

transmission reduction in the visible region upon pulsed UV laser irradiation is obtained for the first time.

The nature of the transmission reduction is discussed. The assumption is made that the transmission

reduction is carried out through the formation of the color centers consisted of the Cu

(n>13) clusters which

have the absorption bands in the visible region. In conclusion the presence of the irreversible

photochromism in the phosphate glass is stated.

1 INTRODUCTION

In the semiconductor crystal field the study of

properties of the copper chloride crystals is of a

great interest for the development of photonics

nowadays. Copper chloride is a wide-gap

semiconductor with allowed direct interband

electron transitions. Macrocrystals of CuCl have

been well studied (Cardona, 1963) and are known to

demonstrate the intense exciton absorption at the

edge of the band gap and the negative spin-orbit

splitting (Goldmann 1977).

Borosilicate (BS) glasses, activated by the CuCl

nanocrystals with the mean size of more than 70 Å,

are known to exhibit reversible photochromism

(Dotsenko et al., 1998): continuous exposure by

ultraviolet radiation leads to the reduction of the

glass transmission in the visible region, when the

activating irradiation is switched off the glass

transmission returns back to its initial level.

Composite materials with the reversible

photochromism are widely used for the protection of

the human eye and the detectors of the

optoelectronic systems from UV and stray visible

radiation. In case of the nucleation of the CuCl

crystals having the mean size of over 100 Å the

glass transmission reduction under UV irradiation

occurs due to the presence of the surface plasmon

resonance absorption band of metallic Cu

nanoparticles centred at 560-580 nm (Sheng et al.,

2009; Morse, 1981). With the presence of the CuCl

crystals having the mean size between 70 and 100 Å

the transmission of the silicate glass during UV

irradiation is reduced due to the nucleation of the

clusters (n> 13), the absorption bands of which

occupy the region of 360-460 nm (Vázquez-

Vázquez et al., 2009). It is noteworthy that the

copper nanoparticles are formed only in the presence

of the interface between the nanocrystalline phase

(NCP) and the “vacuum pore” (Golubkov, 1986).

The certain interface appears only when the large

size droplets of the copper halide phase at

temperatures above glass transition temperature and,

consequently, the large size crystals at room

temperature exist. When the size of the phase droplet

is small, the difference between the volumes of

liquid and crystal phases will be too small for the

formation the vacuum pore of full value (Golubkov,

1982), on the boundary of which the nucleation of

the copper particles or clusters can take place under

the UV irradiation.

Phosphate glasses doped with CuCl nanocrystals,

which have recently been discussed for the first time

in works (Shirshnev et al., 2015; Babkina et al.,

2015), show the effect of nonlinear optical limiting

due to the two-photon absorption under the pulsed

298

Babkina A., Trots K., Kolobkova E. and Nikonorov N.

UV Pulsed Laser Irradiation Effect on Spectral Properties of Borosilicate and Phosphate Glasses with CuCl Nanocrystals.

DOI: 10.5220/0006221102980303

In Proceedings of the 5th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2017), pages 298-303

ISBN: 978-989-758-223-3

laser irradiation of 532 nm. Hitherto there are no

papers dedicated to the interaction of these glasses

and the laser irradiation of UV range.

Therefore, in the present paper we carry out the

comparison study on the pulsed UV laser irradiation

effect on the spectral properties of the borosilicate

and the phosphate glasses activated by the CuCl

nanocrystals.

2 MATERIALS AND METHODS

The following glass systems are used as the objects

of this study: borosilicate and phosphate. The

composition of the BS glass is similar to the glass

matrix described in (Golubkov, 1982) and comprises

: 56.3 SiO

– 28.3 B

– 12.1 Na

O – 3.3 Al

–

1.2 Cu

O – 1.2 P

– 1.7 Cl

- 0.7 F

(wt. %). The

phosphate glass of the following composition:

45 P

–19 BaO - 12 Na

O – 7 Al

– 8 F

1Cu

O - is used as a constant with a variable value

of Cl

from 8 to 11.6 (wt. %). All glasses were

prepared in the high temperature furnace Gmp

(Gero) using a platinum and a carbon crucible for

the BS and the phosphate glass respectively and a

platinum stirrer for the glass melt homogenization.

After the synthesis the phosphate glass was

quenched to room temperature and the BS glass was

annealed in a stepwise regime. The final glass

chemical composition determination was obtained

by X-ray fluorescence spectrometer ARL

PERFORM'X 4200 (Thermo Scientific). Glass

transition temperatures were determined by

differential scanning calorimeter STA 449F1 Jupiter

(Netzsch) and were found to be 757 K and 673 K for

the BS and the phosphate glass respectively.

The nanocrystalline phase was precipitated in the

BS glass bulk by isothermal treating the samples at

temperatures exceeding glass transition temperature

and subsequent quenching to room temperature.

Because the concentrations of both copper and

halogen exceeded the solubility limit for the matrix,

the system obtained might be considered to be the

supersaturated solid solution. During the high-

temperature heat treatment of this glass, the phase

separation of the supersaturated solid solution and

subsequent fluctuation nucleation of a new phase

occurred (Onushchenko, 1996; Ekimov, 1996).

The

usage of the hydrochloride acid during the phosphate

glass synthesis created the hard reducing conditions

so that the nanocrystalline phase nucleation was

conducted during quenching after it. In former case

the average size and concentration of the

nanocrystalline phase were defined by heat

treatment options, in latter one they were derived

from the hydrochloride acid concentration.

The glass samples irradiation was carried out by

the third harmonic of the Nd

:YAG laser with a

wavelength of 355 nm, a pulse length of 9 ns, a peak

power of 13.2 MW/cm

and frequency of 10 Hz. The

absorption spectra of the samples were recorded at

room temperature before and after the irradiation

process. The deuterium lamp AvaLight-DH-S-BAL

(Avantes) was used as a light source and the fiber

optic spectrometer AvaSpec-2048L (Avantes) as a

detector.

3 RESULTS

Fig. 1 shows the absorption spectra of the initial BS

glass, the BS glass with the CuCl nanocrystals,

precipitated after the heat treatment at 823 K, and

the BS glass with the CuCl nanocrystals after

irradiation of various duration. The BS glass heat

treatment at temperature of 823 K leads to the

precipitation of the CuCl nanocrystals with the mean

size of 70 Å. The strong exciton absorption bands

occur near the edge of the crystal band gap after the

heat treatment at temperatures exceeding glass

transition temperature. The mean crystal size

calculations are produced from the optical spectra

according to the method described in (Efros, 1982).

According to the work (Dotsenko et al., 1998) such

treatment leads to the arising of the glass sensitivity

to the UV radiation, i.e. the photochromic effect

occurrence. UV laser irradiation of the BS glass

promotes the occurrence of the strong absorption

band centered at 340 nm (3.65 eV) and the weak

absorption band at 450 nm (2.76 eV). The increase

in the duration of the exposure leads to the increase

in the intensity of the laser-induced absorption and

the CuCl exciton absorption bands. The difference in

the absorption intensity between the non-radiated

and irradiated during 10 minutes the BS glass

samples at the wavelength of 340 nm is 35.5 cm

-1

, at

367 nm (maximum of Z

1,2

CuCl absorption band) is

18 cm

-1

and at 450 nm is 5.7 cm

-1

. The transmission

decrease in the visible region is about 53%. In case

of the precipitation of the CuCl nanocrystals having

the mean size 26 Å the BS glass does not become

photochromic, and the radiation-induced increase of

absorption at the same wavelengths is about two

times less than in the first sample.

UV Pulsed Laser Irradiation Effect on Spectral Properties of Borosilicate and Phosphate Glasses with CuCl Nanocrystals

299

Figure 1: The absorption spectra of the BS glass before

heat treatment (1), after heat treatment and before

irradiation (2) and after irradiation for 100 sec (3), 300 sec

(4), 400 sec (5), 600 sec (6).

Fig. 2 shows the absorption spectra of phosphate

glass doped with CuCl nanocrystals with the mean

size of 70 Å before and after irradiation by pulsed

UV laser. The irradiation for 10 minutes leads to the

absorption increment at 340 nm (3.65 eV) and

450 nm (2.76 eV) of 23.87 and 4.25 cm

-1

respectively. The transmission decrease in the

visible region after irradiation is turned out to be

45%. If the CuCl nanocrystals having the mean size

of 34 Å have been precipitated in the phosphate

glass the radiation-induced increase of absorption in

the visible and UV regions is almost the same as in

case of the bigger nanocrystals.

Figure 2: The absorption spectra of the phosphate glass

before irradiation (1) and after irradiation for 100 sec (2),

300 sec (3), 600 sec (4).

The additional absorption spectra of the color

centers formed by 20-minutes UV-laser irradiation

of the BS and the phosphate glasses are shown in

Fig.3. General view of the additional absorption

band of different glasses is almost identical.

Figure 4 demonstrates the time dependence of

the optical density at 500 nm of the glass samples

under study during I-irradiation, II-relaxation (with

switched off excitation) and III-re-irradiation. The

samples were chosen so that the level of their initial

optical density is the same. The formed color centers

show the temporal stability in the phosphate glass,

while in the BS glass the laser-induced absorption

relaxes for 5 days.

Figure 3: The additional absorption spectra of the color

centers formed in glasses with CuCl NC after pulsed laser

irradiation. The inset: photos of the non-radiated (left) and

the irradiated (right) phosphate glass.

4 DISCUSSION

Despite the difference in the glass matrices

compositions, the location of the radiation-induced

absorption bands is the same. As to the reasons

responsible for this situation, some tentative

assumptions can only be proposed for discussing.

According to the works (El-Batal, 2008; ElBatal

et al., 2013; Barkatt et al., 1981; Bishay, 1970;

Möncke et al., 2014; Narayanan, 2015b; Narayanan,

2015a) the phosphate glass structure comprises

[PO

] tetrahedra. In the case of the pure amorphous

phosphorus (V) oxide the [PO

] groups are

connected to the neighbour units by three out of the

four oxygen atoms, and the latter one is connected to

the phosphorus by the double bond (the terminal

oxygen). Although the phosphate glass structure

changes take place during the introduction of the

alkaline oxides, the phosphorus remains the four

coordinated state in the entire range of compositions

from the pure P

to the orthophosphate MPO

saturated by alkaline oxides. When the alkaline

oxides are added to the amorphous P

the

phosphate structural units change from Q

to Q

then to Q

, and, finally, to Q

, together with a molar

ratio of the alkali metal oxide to the phosphorus

oxide M

O/P

=R, where R varies from 0 to 3

PHOTOPTICS 2017 - 5th International Conference on Photonics, Optics and Laser Technology

300

Figure 4: Optical density time kinetics of the glasses during I-irradiation, II-subsequent relaxation and III-second

irradiation.

(Narayanan and Shashikala, 2015a). A similar

transformation of the structure with the alkali

addition occurs in the silicate glass. With the

addition of M

O a number of the terminal oxygen

remains, while the number of non-bridging oxygen

changes. At first, the addition of M

O or MO (e.g.,

O or CaO) leads to the transformation of the

three-dimensional amorphous P

into the linear

phosphate chain. These structures lead to the

destruction of the P-O-P bonds and the formation of

the terminal oxygen atoms. Due to a decrease in the

average length of the phosphate chain, the increasing

concentration of Cu

O enhances the covalent

character of the P-O-O bonds thus leading to the

glass depolymerization. The depolymerization of the

ring and chain structures with the addition of the

monovalent metal oxides occurs also in the silicate

glasses.

In several sources (Ruller, 1991; Tsai et al.,

1990) have been shown that the exposure by the

ionizing radiation or by optical radiation with high

energy leads to the compression of the amorphous

silica due to the breaking of the Si-O-Si bonds and

the formation of the ≡Si type defects (E’ centers).

The strained Si-O bonds in the rings consisting of 6

or more SiO

tetrahedra capture the radiolytic

charge, which breaks them and allows the ring

structure to relax and become more compact, for

example, to be transformed into the ring with a small

number of the tetrahedra and a more dense packing.

X-ray irradiation of the multicomponent silicate

glass (Tsai et al., 1989; Tsai et al., 1987) promotes

the formation of the oxygen hole centers, which

have the absorption in the region of 440-460 nm

(Bishay, 1970). In this study the laser radiation with

energy lower than the optical band gap of the

material is used, therefore, the change of the glass

structure, namely the Si-O bonds reflow, is not

plausible. On the other hand, if a transmittance

reduction of the BS glass depends only on the

concentration of the radiation-induced E' centers, the

connection of this effect with the mean crystal size

remains unclear.

According to (El-Batal, 2008; ElBatal et al.,

2013) the high-energy irradiation of the phosphate

glasses results in the formation of several types of

the glass network defects, namely: the oxygen

vacancy in the в [PO

] tetrahedra with two trapped

electrons; the trapped electron centre; the trapped

hole centre near non-bridging oxygen; the trapped

hole centre near the monovalent metal ion covalently

bonded with oxygen, - which have the absorption

bands centred at 200, 225, 420 and 540 nm,

respectively.

Due to a similar behavior of the monovalent

copper ions in the structure of different glasses, the

similar nature of the defects initiated by pulsed UV

irradiation can be assumed. As it has been

mentioned above, in all the glasses under study a

possibility of the formation of the defects such as

trapped electrons and holes (Bishay, 1970), and free

electrons and holes does exist. The defects in a

similar environment, for example, near the copper

ions and the non-bridging oxygen, would have the

absorption bands in the same range. The free

electrons created due to the inflow of the additional

energy from the laser beam cause the reduction of

the divalent copper ions to the monovalent state and

the monovalent copper ions to the copper atoms.

Assuming the high localization of the copper atoms

the opportunity of the Cu

molecular clusters

formation arises. The molecular copper clusters with

n <10 demonstrate luminescence under the

excitation by UV emission (Vázquez-Vázquez et al.,

2009). In this study the irradiation of samples by

340 nm does not initiate any emission. However, the

UV Pulsed Laser Irradiation Effect on Spectral Properties of Borosilicate and Phosphate Glasses with CuCl Nanocrystals

301

Stokes shift between the excitation and the emission

of the Cu

(n=3-5) molecular clusters is about 4000-

5000 cm

-1

, so that the initiated luminescence bands

would be located in the region of the exciton

absorption of the CuCl nanocrystals. The excitation

by longer wavelengths has not yielded the expected

emission either. This is why, the nature of the color

centers can be attributed to two possible situations:

first, the glass network defects, originated from the

powerful laser irradiation, and, second, the Cu

(n>13) clusters, which demonstrate no fluorescence,

but can be characterized by the absorption bands in

the visible region. The phosphate glass network

defects mentioned above also do not reveal the

fluorescent properties.

Let us consider a model of the nanocrystalline

phase proposed in the works (Golubkov, 1998;

Golubkov, 1982). During the thermal treatment of

the glass doped with the copper and chlorine ions at

temperatures above glass transition temperature due

to the presence of spatially fluctuations in the

chemical composition of the glass the regions with

an increased content of the future nanocrystalline

phase components are formed. The presence of the

inhibitor defects in the glass allows the

concentration increase of such areas. In regard of the

multi-component glass the composition of these

areas is not uniform: besides the copper and chlorine

molecular compounds it may include some

components of the glass matrix. During the cooling

process the thermal contraction of the copper halide

droplet phase and the glass matrix does occur in

accordance with their linear thermal expansion

coefficients. In the BS glass the linear thermal

expansion coefficient of the glass matrix is three

times smaller than the one of the CuCl (Golubkov et

al., 2012; Dotsenko et al., 1998). Therefore, the

contraction of the nanocrystalline phase goes faster

than the contraction of the matrix, thus in the

interlayer between the nanocrystalline phase and the

glass matrix the “vacuum pore” is formed, whose

presence has been shown in the works (Golubkov,

1986). The mechanism of the radiation-induced

darkening of the BS glass is the following. UV

irradiation leads to ionization on the surface of the

CuCl nanocrystalline phase, to the occurrence of the

so-called "halide gas" consisting of Cl

in the

“vacuum pore” and to the formation of the metallic

copper film on the nanocrystalline phase surface

with the absorption in the visible region.

The formation of the areas with high

concentration of the components of the future

nanocrystalline phase in phosphate glasses occurs

during the glass synthesis. It is known (Ehrt, 1992)

that the introduction of a large amount of fluorine

(as in our case) to the phosphate glass leads to the

incorporation of the fluorine into the glass structure

and, therefore, to the depolymerization of the

phosphate chains and the formation of the defect end

structures, next to which the components of the

future nanocrystalline phase can be accumulated.

The fluorine content in all phosphate glasses under

study was the same, so, the probability of the

formation of these defects was equal. The thermal

expansion coefficients of the phosphate matrix and

the CuCl are approximately equal, therefore, during

quenching after the synthesis the matrix and the

nanocrystalline phase are compressed in the same

way. The nanocrystalline phase droplets detachment

from the glass does not seem plausible, so, it is

impossible to talk about the “vacuum pore”

formation. Surface tension between the copper

halide droplet phase and the matrix always

compresses drop so that a part of its composition

falls into the matrix. As a result the matrix is

enriched with nanocrystalline phase components

around the droplets (Golubkov, 1998). The copper

ions trapped in this transition layer would be highly

localized. This is a favorable condition for the

creation of the Cu

(n>13) molecular clusters.

The difference in sensitivity to UV radiation

between the BS and the phosphate glasses can be

assigned to, firstly, the presence of a clear interface

between the matrix and the nanocrystalline phase in

the BS glasses providing greater localization of the

copper ions in the transition layer unlike the

phosphate glasses, where the interface does not

appear; secondly, as the nanocrystalline phase in the

phosphate glasses nucleates during quenching after

the glass synthesis a large amount of the copper ions

is distributed in the matrix in the free form, during

the laser irradiation they can be reduced to the

atomic form, but they concentration is too small to

form the particles or the molecular clusters.

5 CONCLUSIONS

As a conclusion, it can be stated that the irreversible

photochromism occurs in the phosphate glass

activated by CuCl nanocrystals. The differences in

sensitivity to UV irradiation between the borosilicate

and the phosphate glasses are mostly associated with

the methods of the nanocrystalline phase formation

in different glass matrices and with the difference in

thermal expansion coefficients of the glasses under

study. Materials possessing the irreversible

photochromic effect, unlike reversible one, can be

PHOTOPTICS 2017 - 5th International Conference on Photonics, Optics and Laser Technology

302

used for the amplitude image recording and for

storing information in the form of the Bragg

gratings.

ACKNOWLEDGEMENTS

Research was funded by Russian Science

Foundation (Agreement #14-23-00136).

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