uv-induced two-photon absorption in bib3o6 single crystals

Optics Communications 250 (2005) 334–343

www.elsevier.com/locate/optcom

UV-induced two-photon absorption in BiB3O6 single crystals

A. Majchrowski a, J. Kisielewski b, E. Michalski a, K. Ozga c,I.V. Kityk d,*, T. Lukasiewicz b

a Institute of Applied Physics, Military University of Technology, 2 Kaliskiego Str., 00-908 Warsaw, Polandb Institute of Electronic Materials Technology, 133 Wolczanska Str., 01-919 Warsaw, Poland

c Institute of Biology and Biophysics, Technical University of Czestochowa, Al. Armii Krajowej 36, Czestochowa, Polandd Institute of Physics AJD Czestochowa, Solid State Department, J. Dlugosz University of Czestochowa, Al. Armii Krajowej 1315, 42201

Czestochowa, Poland

Received 6 November 2004; received in revised form 14 February 2005; accepted 15 February 2005

Abstract

We show that BiB3O6 (BiBO) crystals, well known for their excellent second harmonic generation (SHG) properties,

may also be of interest for third-order optical phenomena, particularly for two-photon absorption (TPA). Photoin-

duced TPA measurements were performed under illumination of excimer Xe–F laser (k = 217 nm) as a photoinducing

(pumping) beam. It created a thin surface layer (about 85 nm) that was a source of the observed photoinduced TPA.

Raman shifted Nd-YAG laser radiation (k = 1.9 lm) as well as its second and fourth harmonics (k = 950 and

k = 475 nm, respectively) were used as fundamental (probing) beams of the TPA. The highest values of the TPA b coef-

ficient were achieved for a polarization of the pumping light directed along crystallographic axis b. Quantum chemical

simulations indicate on substantial contribution of UV-induced electron–phonon anharmonicity to the observed TPA.

The obtained values of TPA coefficients indicate a possibility of using BiBO crystals as UV-operated optical limiters in

a wide spectral range.

� 2005 Elsevier B.V. All rights reserved.

PACS: 42.70.Mp; 42.50.Hz

Keywords: Two-photon absorption; Non-linear optics materials

0030-4018/$ - see front matter � 2005 Elsevier B.V. All rights reserv

doi:10.1016/j.optcom.2005.02.053

* Corresponding author. Tel.: 48 601 50 42 68; fax: 48 223 61

22 28.

E-mail address: [email protected] (I.V. Kityk).

1. Introduction

First report on BiB3O6 (BiBO) single crystalgrowth was done by Liebertz [1]. The paper was

devoted to crystallization of oxide single crystals

ed.

mailto:[email protected]

A. Majchrowski et al. / Optics Communications 250 (2005) 334–343 335

from viscous melts and at that time BiBO was not

recognized as a promising non-linear optical

(NLO) material. Not before 1999 it was found that

the crystal is an excellent NLO material for second

harmonic generation (SHG) applications with higheffective second-order optical susceptibility

(deff = 3.2 pm/V) [2]. From that time a number of

works on BiBO crystallization [3–7] and its NLO

properties [8–11] was published. The material is a

good alternative for well-known and broadly ap-

plied LBO and BBO crystals in such applications

as SHG, third harmonic generation (THG), opti-

cal parametric amplifiers (OPA) and oscillators(OPO). It is transparent in the range 286–

2500 nm, non-hygroscopic, free of inclusions and

has high optical homogeneity. Its effective non-

linear coefficient is 4 times higher than that of

LBO and 1.5 times higher than that of BBO.

SHG and THG in this material give red, green

or blue radiation, depending on the laser source

used. The latest achievement with use of BiBOcrystal, that may lead to more efficient and less

expensive blue lasers, is generation of CW 2.8 W

at 473 nm blue radiation after frequency-doubling

of 946 nm line of 4.6 W diode-pumped YAG:Nd

laser [11]. Recently, Kaminskii et al. [12] reported

also on third-order (v3) properties of BiBO.

In this paper, we describe investigations of the

third-order optical phenomenon of a two-photonabsorption (TPA) in BiBO crystals under condi-

tions of excimer laser�s UV inducing illumination.

We used the photoinduced methods earlier to

enhance the second-order NLO properties of par-

tially crystallized BiBO glass [13] and fibres [14],

and obtained several improvement of NLO sus-

ceptibilities. In case of bulk BiBO glass we in-

creased deff up to 5 pm/V, compared to 3.2 pm/Vfor pure BiBO crystals at k = 1.06 lm [13]. The

obtained results encouraged us to investigate pho-

toinduced TPA phenomenon in BiBO crystals.

2. Crystal growth

Monoclinic BiBO (space group C2) melts con-gruently at 726 �C [15] and no phase transitions

occur during cooling of the crystal to room tem-

perature; therefore, it can be crystallized from a

stoichiometric melt. Two-zone resistance furnace,

described elsewhere [16] allowed shaping of the

temperature conditions in a wide range. After dip-

ping the seed into the melt the temperature of the

heating zones was decreased linearly at rates from0.006 to 0.01 K/h range. Due to small temperature

gradients BiBO single crystals grew into the melt

and were confined with flat crystallographic faces.

X-ray diffraction patterns of BiBO single crys-

tals grown under described conditions confirmed

crystallization of pure BiB3O6 phase [17]. How-

ever, when slower cooling was used, to ensure full

annealing of the as-grown BiBO crystals, traces ofBi3B5O12 phase [18] appeared in the BiBO diffrac-

tion patterns.

3. Third-order optical properties of BiBO

3.1. General formalism of two-photon absorption

In this work, we will show that the BiBO crys-

tals may be also of interest because of the third-

order optical applications, particularly for the

two-photon absorption (TPA) described by the

imaginary part of fourth rank optical susceptibil-

ity tensors. It may be very important due to the

large energy gap of the BiBO (about 4.6 eV),

which allows to realize the optical limiters basedon the TPA in the blue and UV spectral ranges.

Non-linear optical effects are generally de-

scribed phenomenologically, by the optical re-

sponse P, of a material to an effective electric

field, E [19]:

P ið~r; tÞ ¼ vh1iij � Ejð~r; tÞ þ vh2iijk � EjEkð~r; tÞ þ vh3iijkl

� EjEkElð~r; tÞ þ � � � : ð1Þ

The vh1iij term is responsible for linear optics phe-

nomena like light absorption and refraction. The

terms vh2iijk and vh3iijkl are responsible for non-linearoptical phenomena and describe the second- and

third-order optical effects, respectively. The Ek,

El are the electric field strength components.

When the charges in a material are bound by a

harmonic potential, the induced dipole moment is

a linear function of the applied electric field

strength. The response of a molecule is �non-linear�

336 A. Majchrowski et al. / Optics Communications 250 (2005) 334–343

(acentric) if the charges are bound to the molecule

by a non-harmonic potential. In this case, the di-

pole moment of the molecule is a non-linear func-

tion of the applied electric field strength. More

generally, if a �non-linear� molecule is exposed tolight, the time-dependent induced dipole moment

is a non-linear function of the time-dependent elec-

tric field. The second-order and third-order optical

susceptibilities of the crystals are related with bijkand cijkl renormalized by Lorenz factors corre-

sponding to first- and second-order optical hyper-

polarizabilities, respectively.

Generally the TPA does not require a chargedensity non-centrosymmetry. However, it is deter-

mined by dipole moments both of ground and

excited states. To create additional possibilities of

their using as third-order optical materials it is

necessary to achieve substantial enhancement of

the corresponding dipole moments. For clarifica-

tion let us consider a simplified two-band model,

relating appropriate third-order hyperpolarizabili-ties with microscopic dipole momentums, de-

scribed within a framework of quantum chemical

and band structure calculations.

The following three-level model was used to cal-

culate second-order optical hyperpolarizability

[20]:

cijkl ffil2ij;ðegÞ � l2

kl;ðehÞ

E�eg

Eeh

2

� �2

� C; ð2Þ

where lij,(eg) is the first-order variation of the dipole

moment, i.e., the difference between first excited (e)

and ground (g) states; lkl(eh) is the second-order var-

iation in the dipole moment, i.e., the difference be-

tween first excited (e) and higher (h) states; Eeg is

the energy of transition between ground (g) and first

excited (e) states; Eeh is the energy of transition be-tween first (e) and higher (h) excited states;

This model was introduced for non-conjugated

dimers possessing highly covalent chemical bonds.

However, due to the crucial role played by the

highly covalent chemical bonds 2pB–2pO one

Table 1

Influence of k-samplings on convergence of cijkl

Number of special points in the BZ 1 2 3 4

Arbitrary values of the hyperpolarizabilities cijkl 1.2 8.9 3.4 7

can say about a similarity in the charge density dis-

tribution between carbon sp3 chemical bond

configuration and 2pB–2pO charge density distri-

bution. Earlier this equation was verified for the

other borate crystals, particularly YAB, andshowed good agreement with experimental data.

All the calculations were performed over the

effective Brillouine zone for the probing beam of

the YAG:Nd laser. During the calculations the in-

duced dipole moments stimulated by the UV-laser

were included. We have found that it is sufficient

to take 12 special points within the BZ to achieve

a proper convergence of cijkl values (see Table 1).The simulations were done with and without

taking into account of electron–phonon anharmo-

nicity. The general conception of the calculation

approach and several computational details are gi-

ven in [21].

3.2. Simulation of the UV-induced changes

For simulations of picosecond photoinduced

charge density non-centrosymmetry we have used

the method of norm-conserving pseudopotential

renormalized by EPI together with appropriate

molecular dynamics simulations similarly to that

one described in [21].

The band energy structure of BiB3O6 for the

perfect crystals was estimated first. Startingpseudo-wavefunctions were built using a proce-

dure proposed in [21] for a non-local self-consis-

tent norm-conserving pseudo-potential. Actual

6s,p Bi and 2s,p B and O occupied and diffused

(excited) 7s,p Bi together with the 3s,p B and 4d

Bi orbitals were considered during the calculation.

Through the work BHS basis set was applied [22].

The total energy was expressed within a localdensity functional approximation with respect to

the charge density q(r) and all the analytical

expressions used for calculations of secular matrix

elements are described in [21].

Afterwards a following secular equation corre-

sponding to a pseudopotential was solved:

5 6 7 8 9 10 11 12 13 14

.6 4.2 6.9 4.8 6.3 5.2 5.6 5.3 5.3 5.29 5.30


k½h2ðkþGnÞ2=2m� EðkÞ�dn;n0þX

aV aðG0n �GnÞSaðG0

n �GnÞk ¼ 0; ð3Þ

where E(k) is the eigenenergy for a k-point in the

Brillouin zone; G0n, Gn are wavevectors of interact-

ing plane waves. A structural form-factor which

takes into account positions of atoms sb was eval-

uated by an expression:

SbðG0n �GÞ ¼ gðbÞ=XN a

�X

exp �i G0n �G

� �sb

� �: ð4Þ

Here, g(b) is a weighting factor taking into account

UV-induced disturbances of symmetry.

To achieve a sufficient convergence of the self-

consistent procedure we needed up to 120 plane

waves corresponding to energy cut-off about

12 Ryd and eigenenergy convergence up to

0.027 eV.Electron screening effects were taken into ac-

count using a parameterized Perdew–Zunger for-

malism [23,24]. The special Chadhi–Cohen point

method was applied for numerical calculation of

the electron charge density distribution. Diagonal-

ization procedure was carried out at special

weighting points for each structural cluster.

Acceleration of the iteration convergence wasachieved by transferring 75% of the (m � 1)th iter-

ation result to the mth iteration. A level of calcula-

tion error (e) was assumed to be better than 0.14%.

The level of agreement with previously performed

calculations of electron charge density of states

within a framework of LMTO or pseudopotential

methods was equal to about 0.23 eV. However, the

main drawback of all one-electron calculationsconsists in an underestimation of the band energy

gap values. For this reason self-energy correction

renormalization was carried out during the band

structure calculations. As a result, the total energy

deviation in relation to the energy cutoff and the

Perdew–Alder screening parameter was stabilized

to 0.22 eV.

We included a linear absorption as a back-ground independent on the external light, and

assumed that trapping from the EPI sublevels

was very fast and complete. At the first stage we

considered pure electronic and EP states as simply

superimposed, and only afterwards took into

account contribution of the secondary piezoelec-

tric effects.

In order to investigate the possible role played

by electron–phonon interaction (EPI) contribu-

tions, we carried out calculations for electronic ba-sis set renormalized by phonon subsystems.

Second derivatives of the electrostatic cluster po-

tential with respect to given normal coordinates

were calculated numerically with precision up to

0.9 eV for the main chemical bonds BO3, BO4

and BiO. These calculations were used to deter-

mine the force constants.

The electron–phonon anharmonic interactionpotential together with UV-induced level kinetics

were calculated in a non-linear approximation [21].

The performed calculations have shown that

during UV-induced polarized excitation one can

expect appearance of anharmonic phonon modes

originating from Bi–O stretching phonons at

190 cm�1 and antisymmetric B–O bonds within

BO3 clusters at 531 cm�1. Their displace phononvectors are directed along the crystallographic axis

b and they should possess high polarizability to

external polarized UV-light. From the performed

evaluations one can expect appearance of anhar-

monic phonon modes at total frequency about

726 cm�1.

We considered the contributions of both har-

monic, as well as anharmonic EPI.To include the UV-induced lattice deformations

within the Green function, we took into account

optically induced deformation of potentials allow-

ing to apply the Dyson relations like in [21].

The potential operator U was determined by the

UV-induced potential deformation and corre-

sponding disturbance caused by the charge defect.

Thus:

Gcc0

DD0 ð1Þ ¼ Gcc0

DD0 ð0Þ þ Gcc0

DD0 ð0ÞUGcc0

DD0 ð1Þ; ð5Þ

where Gcc0

DD0 ð0Þ is the Green function for harmonic

subsystems, and Gcc0

DD0 ð1Þ is the Green function for

subsystems disturbed by the UV-induced anhar-

monic electric field potential U.

The performed calculations allowed to deter-

mine the parameters of the dipole moments which

were used for calculations of corresponding hyper-

polarizabilities (see Eq. (2)).

Fig. 2. Charge density distribution within the BO3 cluster with

taking into account of anharmonic electron–phonon interac-

tions. All the indications are the same as in Fig. 1.


4. Results and discussions

In Figs. 1–4 there are presented calculated dis-

tributions of charge density for the two basic

B–O clusters without and with inclusion of theUV-induced electron–phonon anharmonicity.

The calculations were done by a method described

above. During the calculations it was assumed that

the UV-induced nanolayer is about 85 nm thick,

which is caused by absorption coefficient at

214 nm of about 5 · 104 cm�1. The latter value

was obtained from measurements of transparency

for ultra-thin films of the BiBO. Within this layerthe effective electric strength is formed.

One can clearly see that inclusion of electron–

phonon anharmonicity favors substantial charge

density re-distribution, which should be immedi-

ately manifested in the corresponding optical sus-

ceptibilities. Earlier it was mentioned that major

contribution should give anharmonic phonon

modes at about 726 cm�1. Contribution of othermodes is substantially smaller. Totally they give

less than 12% to the output susceptibility. A sub-

stantial difference in charge density distribution


taking into account of harmonic electron–phonon interactions.

The grid of the surface lines corresponds to the 0.1 e/X. Blackcolor corresponds to the charge density distribution within the

BO3 cluster. Green color corresponds to influence of Bi ions. B

atoms are indicated by the black color. Oxygen have cyan color.


taking into account of harmonic electron–phonon interactions.

All the indications are the same as in Fig. 1.

for BO4 clusters compared to the BO3 ones (see

Figs. 1–4) is striking. In the latter case UV-induced

polarized light favors substantial charge density

redistribution, which also deals with variation of

the corresponding dipole moments. Because due


taking into account of anharmonic electron–phonon interac-

tions. All the indications are the same as in Fig. 1.

0

0.5

1

1.5

480 550 620 690 760 830

f[cm-1]

Ram

an in

t.[a

rb.u

n.]

Fig. 5. UV-induced fragment of Raman spectra for perpendic-

ular scattered Raman modes: blue, UV-induced intensity

0.2 GW/cm2; red, 0.8 GW/cm2; black, 1.2 GW/cm2.


to Eq. (2) the dipole moments play cruciall role

one can expect that in the UV-induced TPA the

dominant role will not belong to the BO4 clustersbut to the BO3 ones.

To confirm independently this conception, we

have done additional experimental investigations

of variation of the oscillator strengths of the men-

tioned phonon bands under influence of the exci-

mer laser UV-pumping power densities using

Raman scattering with Ar* laser (k = 521 nm) as

a source. In Fig. 5 there are presented the obtaineddata. The measurements are performed for the

perpendicular direction of the scattering light with

respect to the propagating light in the crystallo-

graphic X direction.

One can clearly see substantial enhancement of

the anharmonic phonon modes at about 24% dur-

ing increasing of the UV-induced power (see Fig.

5). This fact indicates on a creation of photoin-duced phonons of third-order polar symmetry

which give substantial contribution to the non-

linear optical susceptibility.

To avoid possible fluorescence background

which may be overlapped with the FTIR spectra

additional angle-dependent measurements were

done which have shown that the nearest fluores-

cent line is not overlapped with the observed FTIR

spectra.

Contrary to the usual harmonic mode situated

at lower frequencies which are not sensitive to

the UV-induction the anharmonic mode at about726 cm�1 shows very critical dependence of the

corresponding Raman bands. So this fact indicates

the occurrence of the photoinduced anharmonic

modes within the bulk crystals, propagating from

the thin UV-induced layers through the crystal.

The calculations of the time-dependent re-

sponse have shown that the optimal 1output

third-order susceptibility was achieved for pump–probe delaying times equal to about 15 ps (see

Fig. 6) corresponding to the EPI contribution

and about 2 ps in the case of the harmonic elec-

tron–phonon interactions. Moreover, the obtained

spectra show substantial dependences versus the

angle between directions of polarizations of pump-

ing and probing beams.

4.1. TPA set-up

We have performed measurements of photoin-

duced TPA using as a source laser beams genera-

tion of the Raman shifted Nd-YAG laser with

k = 1.9 lm with pulse duration about 22 ps, fre-

quency repetition 11 Hz and maximal peak power

about 22 MW. Additionally we have used the sec-ond and fourth harmonic of this laser with wave-

lengths 0.95 and 0.475 lm, respectively.

Principal experimental set-up for the perform-

ing of the photoinduced TPA is shown in the

Fig. 7. The polarized light from the excimer laser

with k = 217 nm creates strong effective electric

0

10

20

0.5 4.5 8.5 12.5 16.5

t[ps]

TP

A[a

rb.u

n.]

Fig. 6. Simulated pump–probe dependence of the TPA with

inclusion only of the harmonic electron–phonon interactions

(left part) and with inclusion of the anharmonic electron–

phonon interactions (right maximum).


strength within the nano-layer of BiBO. The sys-

tem of polarisers and mirrors allowed to vary an

angle between the pumping UV-induced polarized

light beam and the fundamental ones. The moni-

toring system consisting of the photomutiplier to-

gether with the electronic boxcar registrationsystem allowed to observe the kinetics of the out-

put transparency versus power density of the prob-

ing Nd-YAG laser beams for each UV-induced

pulse.

Because the thickness of the UV-induced layer

is equal to about 85 nm due to the BiBO absorp-

tion coefficient of 5 · 104 cm�1 the effective thin

layer creating the bulk-like third-order susceptibil-ity through the sample�s bulk becomes a source of

the mentioned anharmonic phonon waves. Vary-

Fig. 7. Principal experimental set-up for t

ing polarization of the incident and output beams

we have detected different fourth rank tensor com-

ponents of the optical susceptibility corresponding

to the TPA.

The TPA coefficient was evaluated fromintensity-dependent transparency T using an

expression:

T ¼ 1� bdI : ð6ÞHere, d is the sample�s thickness; I is the intensity

of fundamental beam; b is the two-photon absorp-

tion coefficient.For reliable control, we have done all the mea-

surements at different sample�s thicknesses and at

varying angles between directions of polarization

between the pump UV light and probe Nd-YAG

laser beams.

4.2. Results of the TPA measurements and

discussion

Typical measured intensity-dependent transpar-

ency is shown in Fig. 8. For convenience the

dependences obtained from the modelling are de-

picted by green and red lines. Because the real

pulses are of the relatively complicated form we

have presented the effective pulses in the square-

like form as showing effective time distributionfor convenient presentation of the temporary

process.

One can see an occurrence of a bending in the

corresponding dependences of probing YAG-Nd

laser transparency versus intensity. The system of

polarizers allows obtaining angular dependences

he UV-induced TPA measurements.

60

70

80

90

0.01 0.21 0.41 0.61

I[GW/cm2]

T[%

]

Fig. 8. Typical dependences of the transparency versus funda-

mental power density: squares, without UV-pumping; triangles,

for 45� between the angle of polarization for pumping UV and

fundamental light beams; crises, for the parallel directions

between polarizations of pumping and fundamental beams. All

the measurements are done for the pump–probe delaying time

about 18 ps. The TPA coefficients are evaluated from the bends

of the corresponding dependences with respect to the non-

illuminated case. By red and green are given theoretically

modeled curves for the corresponding dependences.


of the TPA. The time resolution of the detected

apparatus is equal to about 3 ps.

The probing and pumping beams are temporary

synchronized. Shifting the beginning of the mea-

sured probing beam with respect to the achieved

Fig. 9. Time kinetics of the UV-pumping (first line), fundamental (sec

we present the averaged square-like form of the pulses.

probe beam maximum we have revealed that the

transparency is saturated at about 18 ps beginning

from about 200 ps after the starting of the pump

pulse (see Fig. 9). Finally the output transparency

is saturated. A crucial role may play here a compe-tition between creation of electron–phonon anhar-

monic polarization and free carrier diffusion due to

the defect presence.

The performed investigations have shown that

the TPA coefficient b is maximal for the parallel

polarizations of the pumping and probing beams,

and delaying time between the pumping and prob-

ing laser beams equal to 18 ps. The better resultswere achieved for the polarization of the pump

light directed along the crystallographic axis b,

corresponding to the v2222 tensor component. This

fact additionally confirms substantial role played

by the anharmonic electron–phonon interactions

for the mentioned third-order optical susceptibili-

ties. The thin UV-induced layer is a source of

anharmonic phonons responsible for the observedTPA. Varying the sample�s thickness within the

0.1–1 mm we have revealed that the effect is similar

to the bulk-like one. Because free carriers are not

able to penetrate more than 85 nm one can exclude

possible manifestation of fluorescent signal.

From Fig. 10, one can see that at pumping UV

excimer laser beam power densities higher than

1.6 GW/cm2 there appears a saturation of the

ond line) and output transparency (third line). For convenience,

0

1

2

3

0.25 1.5 2.75 4

p(GPa)

TP

A(c

m/G

W)

Fig. 11. Pressure dependence of the TPA for different wave-

lengths: n, 0.475 lm; h, 1.90 lm.


TPA coefficient. This may also reflect a possibility

of inclusion also the cascading non-linear optical

processes of higher order. The values of the ob-

tained TPA coefficients indicate on possibility to

use the mentioned crystals as optically operatedlimiters in a wide spectral range. Changes of sam-

ple thickness do not change substantially the fea-

tures of the TPA confirming the bulk-like origin

of the phenomenon with the nanolayer UV-

induced source.

To explore additionally role played by anhar-

monic electron–phonon contributions we have

done the measurements at high hydrostatic (pos-sessing scalar symmetry) pressures. The pressure

chamber with sapphire windows allowed varying

the pressure up to 5 GP within methane atmo-

sphere. The parallel direction of the incident and

output beam was used. The incident beam angles

did not exceed several degrees. One can see (Fig.

11) a substantial enhancement of the TPA at

hydrostatic pressures about 1.8–2.9 GP range.The technique of calculations for the effective pho-

non displacements contributing to the third-order

susceptibilities is similar to the observed UV-

induced effect; however, in this case role of the

phonons is not only near-the surface, but possess

more bulk-like feature. So one can say about inde-

pendent confirmation of electron–phonon anhar-

monicity caused by external mechanical field.Generally the substantial dipole momentum

contribution to the second-order optical suscepti-

bilities also may play crucial role in the higher or-

der optical effects, particularly in the third-order

0

0.5

1

1.5

2

2.5

0.1 0.6 1.1 1.6

Ip(GW/cm2)

β(cm

/GW

)

Fig. 10. Dependence of the TPA versus UV-induced power

density at different fundamental wavelengths: n, 0.475 lm; ·,0.950 lm; h, 1.90 lm. The results are given for delaying times

equal about 18 ps.

susceptibilities. The distribution of the incident

and output light intensities seems to be in agree-

ment with general phenomenological symmetry as-

pects typical for the crystals under investigations.The bends observed in the TPA dependences ver-

sus UV power densities can serve as a confirmation

of the prediction. The latter may be favored by

external UV laser polarized light. Simultaneously

the direct application of the mechanical field even

of the scalar symmetry (hydrostatic) may cause

analogous effect. Using different parameters of

the incident light (power density, polarization, la-ser wavelength) one can achieve sufficiently good

third-order optical parameters that together with

excellent second-order susceptibilities put BiBO

crystals among the promising multi-functional

non-linear crystals.

Finally, it should be also emphasized that the

description of the effect observed may be given

within a framework of excited state absorption.However, it will be a subject of a separate work

in a future.

5. Conclusions

In this paper, we show that BiBO crystals may

also be of interest due to the third-order optical

applications, particularly for the UV-operated

TPA. We performed photoinduced TPA measure-

ments using excimer Xe–F laser (k = 217 nm) as a

source of the photoinducing beam. The illumina-tion created a thin near-the-surface layer (85 nm)

that was a source of bulk TPA. The Raman shifted


Nd–YAG laser radiation (k = 1.9 lm) as well as its

second and fourth harmonics (k = 950 nm and

k = 475 nm, respectively) were used as fundamen-

tal beams. The highest values of the TPA b coeffi-

cient were achieved for the polarization of thepumping light directed along the crystallographic

axis b. The obtained values of the TPA coefficients

indicate on a possibility of using the BiBO crystals

as optically operated limiters in the wide spectral

range. The performed quantum chemical simula-

tions and Raman spectra confirm substantial role

played by UV-induced electron–phonon anharmo-

nicity in the observed effects.

Acknowledgement

This work was partly supported by the Polish

Committee for Scientific Research Grant No.

4T11B 051 25.

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uv-induced two-photon absorption in bib3o6 single crystals

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