Photoacoustic investigation of low optical absorption in several nonlinearopticalcrystalsGuifen Wang, Genyuan Ma, Guangyin Zhang, Gefeng Zhang, and Xue Qian Li Citation: Journal of Applied Physics 71, 586 (1992); doi: 10.1063/1.350410 View online: http://dx.doi.org/10.1063/1.350410 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/71/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Simultaneous optical and photoacoustic measurement of nonlinear absorption Appl. Phys. Lett. 102, 041116 (2013); 10.1063/1.4789870 Nonlinear-optical probing of nanosecond ferroelectric switching Appl. Phys. Lett. 83, 2402 (2003); 10.1063/1.1612905 Dry-etching method for fabricating photonic-crystal waveguides in nonlinear-optical polymers Appl. Phys. Lett. 82, 2966 (2003); 10.1063/1.1572962 Resonant photoacoustic measurements of very low optical absorption in piezoelectric and dielectriccrystals J. Acoust. Soc. Am. 89, 1910 (1991); 10.1121/1.2029473 Crystal structure of piezoelectric nonlinearoptic AgGaS2 J. Chem. Phys. 59, 1625 (1973); 10.1063/1.1680242

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

129.22.67.107 On: Mon, 24 Nov 2014 05:06:06

Photoacoustic investigation of low optical absorption in several nonlinear-optical crystals

Guifen Wang, Genyuan Ma, Guangyin Zhang, and Gefeng Zhang Department of Physics, Nankai University, Tianjin 300071, Peoples Republic of China

Xue Qian Li CCAST (World Laboratory), P.O. Box 8730, Beijing 100080. PeopIes Republic of China and Department of Physics, Nankai University, Tianjin 300071, Peoples Republic of China

(Received 18 April 1991; accepted for publication 4 October 1991)

We use developed high-sensitivity photoacoustic cell to determine the absorption coefficient in crystals of pure LiNbOs, Mg-doped LiNbOs, Fe-doped LiNb03, KH2P04, and KD,P04 at the wavelengths of 1060.0, 632.8, 514.5, 488.0 nm. The experimental results are in good agreement with those measured by the laser calorimetric method.

I. INTRODUCTION

Nonlinear-optical crystals are an important prerequi- site for improving laser technics and optical information processing. The absorption coefficient is an important pa- rameter of these materials, so we turn to adopt the photo- acoustic method to measure absorptions in LiNbOs. The LiNbOs crystal is a high-transparency and weakly absorb- ing material in the spectral region of visible and near-in- frared, so it is difficult to study its low optical absorption property by conventional spectroscopy method. In addi- tion, owing to the photorefractive effect in LiNb03, there is strong scattering of light at higher intensities, so it is dif- ficult to determine the accurate absorption value. There- fore, the search for new methods is necessary.

The photoacoustic (PA) method is based on the fact that the optical energy absorbed by the sample can be transferred into heat through the process of nonradiative transition, and the subsequent acoustic signal will be pro- duced due to the thermal expansion of the sample. Since the PA signal can only be produced by the absorbed optical energy, the results measured by the PA method are not affected by the scattered light. Thus this method can mea- sure not only the absorption of a high transparent sample but also that of a strongly scattering sample, as well as powder and amorphous samples. Formerly we have mea- sured the low absorption coefficients in some infrared ma- terials such as Ge, GaAs, NaCl, etc., by the PA method’ and the photothermal deflection (PD) method.” It was proven by experiment that PA and PD methods are good for studying the weak absorption property in those mate- rials.

In this paper we use a high-sensitivity PA cell3 devel- oped by us to determine the absorption coefficients of pure LiNb03, LiNbO,:Mg, LiNbO,:Fe, and KH2P04 (KDP), KD,P04 (DKDP) at the wavelengths of 1060.0, 632.8, 514.5, and 488.0 nm, and compared the results with the result by the laser calorimetric (LC) method.4

II. PRINCIPLE

As an audio-modulated light beam is incident to the sample, the sample absorbs the light energy, and some en- ergy levels in the sample will be excited. The excited energy

levels will produce heat in the sample through nonradiative transitions, and cause thermal expansion. A part of the heat energy is transferred into the air nearby, thus a ther- mal atmosphere boundary layer is formed. As the air in this boundary layer is heated periodically, it expands and contracts periodically, just like an audio piston.

In our developed high-sensitivity PA cell, the polished crystal sample is put at the position of the window of the PA cell, and an aluminum-plating reflecting mirror is put at the position at which a sample is located for the case of a common PA cell, in order that the ret&ted light beam from the reflecting mirror can also be absorbed by the sample. In this way, the signal sensitivity of PA can be enhanced and the polished crystal sample can be measured directly. The profile of this developed high-sensitivity PA cell is shown in Fig. 1. In this PA cell, two surfaces of the sample can be irradiated by light and a fraction of the light will be converted into heat. According to Rosencwaig and Gersho’s theory,5 the equations of heat diffusion are

a’f$ 1 af$ -- Z%, at - A&( 1 + e’“‘) _ A’e - (x + l)B( 1 + e i”f)

( - kx<O), (1)

a? 1 a+ -Q =<- ~O<-Mg), (2)

where qS(x,t) is the spatial-time distribution of tempera- ture, and A = f l107Q2K, p is the absorption coefficient, q is the energy conversion coefficient, I, is the light flux of the incident beam, Ki is the thermal conductivity of mate- rial i, ai = Ki/plCi is the thermal diffusivity, ci is the ~pe~ifk

heat, pi is the specific density, and ai = (~/2cr~)“~ is the thermal expansion coefficient (the subscript i refers to solid, gas, and substrate denoted by s, g, and b, respec- tively). o is the modulating angular frequency, whereas A’ = pI&7/2K, with I,!, being the light flux of the reflected light beam, Ic = v’l&$ is a ratio of light flux incident from the left over that from the right. Solving Eqs. ( 1) and (2), the amplitude of the PA signal can be given as

Q=-&(e+;) 9 (35

586 J. Appl. Phys. 71 (2), 15 January 1992 0021-6979/92/140566-04$04.00 @ 1992 American Institute of Physics 586

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

129.22.67.107 On: Mon, 24 Nov 2014 05:06:06

.

-1 0 z&4 11

FIG. 1. The profile of the PA cell. 1, incident light; 2, sample, 3; boundary layer; 4, gas; 5, mirror. FIG. 2. The experimental apparatus. 1, power; 2, laser; 3, diaphragm; 4,

chopper: 5, beam splitter; 6, power meter: 7, mirror; 8, PA cell; 9, mi- crophone; 10, reference signal; 11, lock-in amplifier.

where Pc is the pressure of atmosphere, r, is the temper- ature at the surface of the solid, y is the ratio conventional specific heats, and 8 and W are the amplitudes of temper- ature changes in the air of the right side of the sample as 0

corresponds to the case where the sample is irradiated by 1, from the left, whereas W to the It, from the right, re- spectively:

i-l NO (r- l)(b+ l)eOJ-- (Y+ l)(b- l>e 4 + 2(b - r)e 8z u=2Ks($ - 0;) (gf l)(b+ l)e%‘-- (gTbll)e~-rrJ~

W= - D-10 [(g+ l)(~+ l)e++ (Y- l)(g- 1)e-“Jle-pr+2(g+r)

=W2 - d) (gf l)(b+ l)eUs’-- (g- l)(b- l)e-qJ ’

where b = g = KgdKps r = (1 - j)/V2a,, of = (1 + j>ak In our developed PA cell, where the substrate is

gas as well, we use g for b. The expression of Q in Eq. (3) is very complicated. But in some special situations, the expression of Q is quite simple. According to the partition of optical diaphaneity in solid determined by the ratio of the light absorption length ip= l/p to its thickness, the five materials measured in our experiments are all optically transparent solid, i.e., 1~ > 1. Since we use a chopping fre- quency of 86-Hz, the thermal expansion length p (in LiNb03 it is about 0.072 mm) is small enough to meet ,u<l, p ( Z,, therefore they are all thick solids for the thermal diffusion. Thus, one can assume that e -@ = 1 - fll,e-“‘s 0 and 1 r1 (1. Then Eq. (3) can be simplified

as

Q=zgf$$(l+;)==*(l+$)- (6)

From Eq. (6) it can be determined that the PA signal Q is directly proportional to f - 3’2 (for a9 is proportional to f”2). Define C = Pd4 $%-&a~To, in a fixed PA cell, once f is fixed, C is a constant. The expression (1 + l/r]‘) in this formula indicates that the detection sensitivity of our developed high-sensitivity PA cell is increased by ( 1 + l/ 71’) times over common PA cell. B is defined to be the piezoelectric strain constant of the microphone, so the out- put voltage signal of the microphone is

(7)

As the microphone is defined, then B is a constant, and in our experiment p, c, 3/, v’ are all parameters of material itself. In our experiment, the chopping frequency is de- lined, and the power of input laser beam can be deter- mined. From 1, = P/(d2/4)a we can obtain the light flux, where d is the beam diameter. The PA signal can be de- termined by a lock-in amplifier. Finally, the absorption coefficient fl can be obtained from formula (7).

Ill. EXPERIMENT, RESULTS, AND DISCUSSION

Our experimental apparatus is shown in Fig. 2. The continuous wave (CW) light sources used in this experi- ment are, in turn, YAG (1060.0 nm), H*Ne (632.8 nm), and Ar + (514.5 and 488.0 run) laser beams, the available corresponding power values are 150, 30, 150, and 150 mw, and the chopping frequency is 86 Hz. A powermeter is used to detect the power change, so that errors caused by the power change are toriected accordingly. The sample is a 2-mm-thick circular disk which is cut along the direction vertical to c axis. All surfaces of the samples were polished to the same smoothness (except for KDP, DKDP) .

It should be pointed out that power P in formula (7) should be the power entering into the sample and Pt = P( 1 - R). Here P is the power of the laser. R is the reflectance of the sample at the selected wavelength. Fur- thermore, since we use an aluminium-plating reflecting mirror to lead the laser beam into the PA cell, formula (7)

587 J. Appl. Phys., Vol. 71, No. 2, 15 January 1992 Wang ef al. 587 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

129.22.67.107 On: Mon, 24 Nov 2014 05:06:06

TABLE I. The absorption coefficients of several materials measured by the PA method.

Materials p lo-‘cm-’

wavelength nm

1060.0 632.8 514.5 488.0

pure LiNbOs Mg :LiNb03 Fe :LiNbOs KDP DKDP

a b c a b 0.01% 0.03% 0.07% a b a

0.192 0.243 0.200 0.254 0.266 0.258 0.278 0.287 0.127 0.148 0.207 2.14 2.56 2.29 2.30 2.42 2.19 2.68 2.45 1.03 1.03 1.59 2.69 3.01 2.92 2.75 2.83 3.08 3.40 3.86 1.78 1.78 2.05 3.19 3.26 3.16 3.10 3.14 3.37 3.77 4.10 1.92 1.92 2.31

should be multiplied by a reflectance c, of the mirror at the selected wavelength; thus formula (7) becomes

(8)

The absorption coefficient of several materials mea- sured by the PA method and calculated in formula (8) is listed in Table I.

According to Table I, the absorption coefficient of LiNbOs:Mg at ;1= 488.0 nm is smaller than that of pure LiNbOs, and those of KDP and DKDP crystals are even smaller. This indicates the high transparency of KDP, DKDP in violet region and the consistency of our results with the well-known behavior of KDP confirms the reli- ability of the PA method. Owing to the high humidity of air in summer, the sample becomes deliquescent, so the measured absorption coefficient is slightly larger than that of a nondeliquescent sample. With the increase of Fe in the Fe-doped LiNbOs, the absorption coefficient increases slightly, but the change isn’t obvious.

We also compared our results of pure LiNbOs with those measured by the LC method listed in Table II.

From Table II it can be seen that the results that we obtained by the PA method agree with the results obtained by the LC method. The value measured by the PA method is slightly larger than that obtained by the LC method at 514.5 nm. Because our samples are prepared in our own laboratory, the impurity contents in the samples may be a bit higher than that used by authors of Ref. 4. It is noted that the effects of impurity are not obvious at longer wave- lengths, whereas at the violet region, then give rise to larger measurement values, so the results shown in our table de- viate from the real values at 514.5 nm; this is not the sake of the PA method itself but due to the impurity contents of our samples. The feasibility of the PA method is illus-

TABLE II. The comparison between results of the PA and LC methods.

Method /3 lo-” cm-’ wavelength nm

1060.0 El c 514.5 El c

PA method LC method”

0 b c a b

0.192 0.243 0.201 0.19 0.23 2.64 3.01 2.92 1.9 2.5

aFrom Ref. 4.

588 J. Appl. Phys., Vol. 71, No. 2, 15 January 1992

trated. Furthermore, the experimental apparatus of the LC method is complex and the experimental conditions are rather strict.

The error affecting the survey accuracy of this system is the error of the measuring system. In the experiment we use a power meter to monitor the change of laser power to eliminate the influence caused by power drifts. Besides the influence caused by power drift there is a noise of the in- strument itself. The values listed in the tables are the av- erage results of several repeated measurements. It is found in the experiment that the greatest deviation from the av- erage is less than 5%. The chopping frequency of 86 Hz is used which is far away from commercial frequency (50 Hz) and its harmonic frequencies, so that one can elimi- nate the influence from the civil power supply.

In the experiment we use a 150-mW YAG laser to determine the absorption coefficient of several kinds of ma- terials at wavelength 1.06 ,um to order of magnitude 10 - 3 cm-‘. The range of PA signal measured is 50-100 ,uV. The background noise is 2-3 ,uV- Since the greatest FS sensitivity of the 5205 lock-in amplifier is 100 nV (the internal noise is 5 nV rms) even though the absorption coefficient decreases by l-2 orders of magnitude at the same chopping frequency, the PA signal will also decrease by l-2 orders of magnitude, it still can be measured reli- ably by our apparatus. Furthermore, since the PA signal is directly proportional to f - 3’2, the PA signal will increase by one order of magnitude as f decreases from 86 to 20 Hz, so when the absorption coefficient decreases again by one order of magnitude, this apparatus has no difficulty mea- suring it. From this it can be found that the sensitivity of our system is as high as 10-5-10-6 cm-i.

Because the thickness, dimension, shape, and smooth- ness of the sample and the beam position all influence the PA signal, these conditions must be kept constant as far as possible. In order to improve the sensitivity, the thickness, dimension, shape, and smoothness of the sample should be appropriate, the incident laser beam should be vertical to the surface at the center of the sample, and the chopping frequency should be as low as possible, so the PA signal can be increased.

In brief, in this paper we use a developed high-sensi- tivity PA cell to determine the absorption coefficient of some nonlinear-optical material. The results agree with those obtained by the LC method very well. This indicates the feasibility of the PA method.

Wang et al. 588

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

129.22.67.107 On: Mon, 24 Nov 2014 05:06:06

r Wang Guifen, Zhang Guangyin, and Zhang Chunpin, Chinese J. Semi- conductor, 3,230 (1982).

s Wang Guifen, Ma Genyuan, and Zhang Guangyin, Chinese J. Infrared and Millimeter Waves 6, 333 (1987).

Guangyin, Chinese J. Lasers, 11, 491 (1984).

s Wang Guifen, Wang Jinxiong, Yang Fuhua, Ma Genyuan, and Zhang

4D. J. Gettemy, W. C. Harker, G. Lindholm, and N. P. Barnes, IEEE J. Quantum Electron. QE-24, 2231 (1988).

‘A. Rosencwaig and A. Gersho, J. Appl. Phys. 47, 64 ( 1986).

589 J. Appl. Phys., Vol. 71, No. 2, 15 January 1992 Wang et al. 589

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

129.22.67.107 On: Mon, 24 Nov 2014 05:06:06