Evaluation of | χ(3) | at the GTA of biexcitons in CuCl by high resolution polarization spectroscopy

OURNALO~

LUMINESCENCE Journal of Luminescence 60&6l

ELSEVIER

Evaluation of

1994) 672 67S

I

at the GTA of biexcitons in CuC1 by high resolution polarization spectroscopy M. Hasuo*, M. Nishino, N. Nagasawa

Depot blent at Phv.su

The L nit or sit at To1~ii

/ IIon~o Bunk a Au I oA

~

ii

Japan

Abstract High resolution polarization rotation spectroscopy is performed to evaluate the third-order nonlinear susceptibility. i’~, associated with the giant two-photon absorption (GTA) of biexcitons in CuCI. The true width of the biexciton at 1l at the resonant peak are obtained to be 24 + 2 beV and (3.0 + 0.9(x 10 4esu at the weakest excitation K 0 and i

density of

<

I kW cm2 for a high quality crystal, respectively.

I. Introduction The CuCl crystal is one of the model materials for developing new light sources of low noise or all-optical and fast-respondent data processing de vices. The main reason is due to its large optical nonlinearity associated with the GTA of biexcitons [1,2]. The estimation of y~ associated with the generation of the phase-conjugation at the GTA has been evaluated to be of the order of 10 ~esu for evaporated films at low temperature [3]. The value of ~ for single crystals has been measured to be 5.4 ~ 10 esu as a lower limit by polarization spectroscopy [4]. There have been two serious factors that make the correct evaluation for single crystals difficult: One is the large spectral width of the laser light from a conventional pulse dye laser system compared with the true width of the biexciton state of the order of lOj.teV. Since usual deconvolution

techniques cannot be applied to the analysis of the nonlinear spectra. it is difficult to evaluate the true width of the biexciton level that is essential to

estimate The other factor is the saturation effect of the relevant optical nonlinearity. As is well known, the width of the nonlinear optical spectrum shows a remarkable intensity-dependent broadening. Since the broadening is observed even under very low excitation intensity, one has had to analyze the data provisionally assuming the lowest order approximation. The purpose of this paper is to overcome these difficulties and to estimate the relevant ~ by applying high resolution polarization spectroscopy that has recently become possible with our new laser system [5]. ,‘~

.

2. Experimental Fig. I is a schematic diagram of the experimental setup for the high resolution polarization rotation

*

~pectroscopy. The part enclosed by a broken line is

Corresponding author

0022 2313 94 ~07 Of) C 1994

Elsevier Science B V All rights reserved

SSDJ 0022-2313(93)E0412 Q

M. 1-fasuo et a!.

Journal of Luminescence 60&61 (1994) 672 675

673

nyc

Ampiiflee ~ Ar

Laser

[~~r~l . Ring Laser

p1 n I ~

c~i~’~”~g

Polarizer multiplier

Polarizer A/4 Plate

(Analyzer)

Cryostat

Dye Laser

Polarizer

Air scanning inter fer ometer

XeCi Laser

Fig. I. Schematic diagram of the experimental setup for the polarizationspectroscopy.

the newly developed UV light source composed of a Ti-sapphire ring laser (Coherent 899-02) and an excimer laser (Lambda 53 EMC) pumped dyeamplifier and a frequency doubler (Li103 crystal) To improve the stability of the output intensity of this light source, we chose Oxazine 750 in DMSO instead of DOTC in DMSO as an active medium for the amplification of JR light. The pulse duration of the UV light and its repetition rate were iOns and 10 Hz, respectively. The spectral line width of the UV light was measured to be 1 j.teV by a pressure scanning air space Fabry Perot interferometer (Burleigh VS-25). The maximum intensity of the UV light on the sample was obtained of the 2. Thissurface UV light was circularly order of kW/cm polarized and used as the pump light for the polarization rotation spectroscopy [6]. Another UV light of broad spectral line width (—~170 jieV) was obtained from a conventional dye laser pumped by the excimer laser. After passing this UV light through an air space Fabry Perot interferometer (FSR = 225 j.teV, finess = 75), UV light of spectral linewidth, 3 j.teV, was obtained. This UV light was linear polarized and used as the probe light. The intensity of the probe light on the sample surface was about 0.18 kW/cm2. The photon energy of the probe light can be tuned by controlling the air pressure in the interferometer. The intensity of the probe light transmitted through the sample and [5].

a crossed polarizer (analyzer) was measured by a photomultiplier (Hamamatsu R654). Single crystal platelets of CuCI, typically 10 ~.Im thick were held in an immersion-type cryostat at 2K.

3. Results and discussion A solid line in Fig. 2 is an example of the polarization spectrum due to the two-photon resonance of the biexciton as a function of the sum of the photon energy of the probe and pump light. The photon andsurface the intensity the pump light on the energy sample were of3.18622eV and 1.28 kW/cm2, respectively. The sample thickness was 9.8 JIm. Here the signal intensity, J(Eprobe), is defined by the intensity of the signal light, Isig(Eprobe), normalized by the intensity of the probe light inside the sample, IO(Eprobe), at the photon energy of the probe light, Eprobe. Two peaks marked by arrows are due to the two-photon resonances. In the present excitation geometry shown in Fig. 1, the direct coupling between the counter-propagating pump and probe light occurs. Thus, the main peak is due to the excitation of biexcitons of K = kpump kprobe 3 x i0~cm ~, where kpump and kprob. are the wave numbers of the respective light inside the sample. A small peak is due to the —

—

674

14. J—Iasuo

C

u I

it

at

Jour no! at I umlnescence 60&6 /

1994) 672 6’

of the incident pump light and the pump light reflected back at the sample surface, respectively. They are approximately given as

.~k

Punrp Laser

2

2

Ern~

0 ( Epump) + I

~ __________________________________

6 77161)

6 77165

6.77170

2

n0( Epurnp) + 1

Eret 2

6 771~

exp(

~l)

fl

Probe Laser

.5I.

~

1

2

x

E0~1 2

exp (

flo(Epump)

I

(3)

n0( Epump) + I ~l)

exp(

2~/

(4)

6 77I7~

respectively [4], where E0~~ and ~ is the amplitude

TWO PHOlO~ ENERGY feVi

Fig 2 An example of a polarization spectrum. The photon intensity the 1 probe lightand is energy and2.therespectively. intensity ofThe the pump lightof are 18622eV 0.18kW cm2 The broken curve shows the result ofa theoretical 1.2 kW cm fit. Arrows mark resonances at K and K (for details, see texu

of the electric field of the pump light on the sample surface and the photon linear absorption the crystal for the energy of coefficient the pump oflight. Epump, respectively. We assume here that ,‘~ is given as 5)

coupling between the probe and the pump light that was reflected back at the rear surface of the sample. In this case, the wave number of the excited biexciton is K’ kpump + kprobe 8.85 x lO~cm The energy of each peak is consistent with the energy of the biexciton level predicted from the effective mass approximation for the dispersion curve of the biexciton. In order to analyze the signal spectrum, we adopted the following procedure. The signal intensity is given by 2 (1) J(EPCOb~) 2 ‘ —

Ank0l~

“i’

—

E,,

11K

5

2’

(5)

(Epump + Eprobe) +

where EK and T’K are the resonant energy and the phenomenological damping constant of biexciton state of wave number K. The same form is also assumed for i~ The broken curve in Fig. 2 is the result of a theoretical fit assuming C 5~= Fig. 3(a) shows the phenomenological damping constant of the K 3 x l0~cm biexciton state obtained by this fitting as a function of the pump light intensity, ‘pump’ As observed usually, the damping increasing However, constant it seems increases constant with at the intensity ‘pump level —

less than about 1 kW cm2. Fig. 3(b) shows the where An, k 0 and 1 are the nonlinear change of the complex refractive index between the circularly polarized components of the probe light of opposite sense, the wave number of the probe light outside the sample and the sample thickness, respectively earity, An is expressed by taking account of the [6]. According to the third-order optical nonlinreflection of the pump light at the sample surface as

An

signal intensity at the resonant peak of K 3 x iO~cm biexciton state as a function of According to Eqs. (1), (2) and (5), the signal intensity should be proportional to the square of ‘pump where the damping constant is independent 2, of this quadratic law is valid as shown by a1 kW solidcm line ‘mp At the intensity level less than in the figure. From this analysis, we conclude that ‘pump

(2)

the damping constant at the low intensity level corresponds to the true width of the biexciton level and that the signal is due to the third-order nonlin-

0 is the linear refractive index of the crystal. Eref are the amplitude of the electric fields

earity associated with the biexciton resonance without saturation effects.

2it flO(Eprobe)

X

Eirj2

+

~

where n E1~and

M. Hasuo et a!. 1Q00

,,,

Journal of Luminescence 60&61 (1994) 672 675

/~

101

,,,,,,,

I

675

(b) I

/

/aiH -~

>

z

l00~—

—

10~—

—

—

10’—

—

o

10 ‘5’’’’’’

0.1 Ir.’m

(

1 2) kW/cm

10

0.1

5,’,,’,,

~

1

( kW/cm2)

10

Fig. 3. Phenomenological damping constant of K 3 x io~cm biexciton state (a) and the signal intensity at the peak (b) as a function of the intensity of the pump light, ~ The solid line in (b) shows J x I~umpas a guide for the eye.

4. Conclusion

Ix13~associated with the hiexciton resonance at K 0 is evaluated by high resolution polarization rotation spectroscopy. The true width and x1311 at the resonant peak of the biexcitons of K = 3 x iO~cm 1 are obtained to be 24 ±2 peV and (3.0 ±0.9) x 10 4esu, respectively. It is also found that the effect of the intensity-dependent broadening namely saturation effect appears even in the very weak excitation range of 1 kW/cm 2 .

the Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture. References [1] E. Hanamura, Solid State Commun. 12(1973)951. [2] A.A. Gogolin, Fiz. Tverd. Tela 15 (1973) 2746; Soy. Phys.

Solid State 15(1974)1984.

[3] L.L. Claude, Phys.Chase, Rev. A ML. 28 (1983) 3696.D. Hulin and A. Mysyrowicz,

[4] N. Nagasawa, M. Kuwata, E. Hanamura, T. Itoh and A. Mysyrowicz, Appl. Phys. Lett. 55(1989)1999.

Acknowledgements We would like to thank Prof. T. Itoh for valuable discussions. This work was partially supported by

[5] M. Hasuo, N. Nagasawa and T. Itoh, Opt. Commun. 85

(1991) 219. [6] M. Kuwata, T. Mita and N. Nagasawa, Opt. Commun. 40 (1982) 208; M. Kuwata, J. Phys. Soc. Jap. 53 (1984) 4456.