Direct pyroelectric detection of optical absorption in non-transparent materials

Infrared Phys. Vol. 24. No. 5. pp. 469%471, 1984 Printed m Great Bntain. All rights reserved

0020.0891/84

S3.00 fO.00

Copyright (f, 1984 Pergamm Press Ltd

DIRECT PYROELECTRIC DETECTION OF OPTICAL ABSORPTION IN NON-TRANSPARENT MATERIALS D. Institute

DADARLAT,

of Isotopic

M.

and Molecular

CHIRTOC,

R.

Technology, (Received

M.

CANDEA

3400 Cluj-Napoca

and I. BRATU 5, P.O. Box 700, Romania

27 April 1984)

Abstract-The optical absorption of some non-transparent materials (graphite plates-200 pm thick-and anodized Al plates-50 pm thick) was investigated by a direct method using pyroelectric sensing of the temperature rise of the sample. With a triglycine sulphate (TGS) crystal as a pyroelectric sensor, in close thermal contact with the sample, we found a uniform optical absorption for the graphite plates in the range 0.4-20 pm and for the anodized Al, a selective optical absorption, dependent on the anodization process conditions, in the range l-14pm. The simple technique developed in this paper prove to be advantageous for non-transparent samples of small area and low surface reflectivity.

INTRODUCTION

As a consequence of the rapid development of various IR techniques, the pyroelectric materials (triglycine sulphate, LiNbO,, LiTaO,, PZT etc.) are now widely used as IR radiation detectors. Due to its high sensitivity in thermal measurements, pyroelectric detection has become an transmission microscopy,“’ important tool for thermal-diffusivity investigations, (‘I thermal-wave investigations. On the magnetic resonance detectionc3’ and other thermal- or optical-properties other hand, the optical absorption in non-transparent materials is generally measured indirectly by the surface reflectivity method.‘4’ EXPERIMENTAL

METHOD

AND

RESULTS

In this paper we report on a simple direct method for measuring the optical absorption of non-transparent materials using pyroelectric detection. By gluing non-transparent thin samples of small area (i.e. 1 x 0.2 cm) on the active surface of a (250 pm thick) triglycine sulphate (TGS) sensor,(5) the pyroelectric crystal is isolated from the incident radiation, so the detector responsivity becomes a measure of the optical absorption of the material. Thermal contact between the pyroelectric crystal and the sample was assured by use of high thermal conductivity grease. Two types of samples were used to test the performances of this system: 200 pm thick graphite plates and 50 pm thick anodized Al plates. The optical absorption measurements were performed: in the 0.4-20 ,um range for graphite and in the l-14 pm range for anodized Al. The radiation sources were Ar and HeNe lasers in the 0.4-0.7 pm spectral range and the Globar source of a Perkin-Elmer 125 spectrophotometer in the 0.7-20 pm spectral range. The modulation frequency of the radiation was 7 Hz. The signal from the detector was processed with an Unipan 232B lock-in nanovoltmeter. The experimental results are presented in Figs 1 and 2, where R, is the voltage responsivity of the detector and R,/R,, is a non-dimensional responsivity normalized at its 2 pm value. The solid line in Fig. 2 represents the theoretical black-body radiation curve normalized at its maximum value. As one can see from Figs 1 and 2, the graphite has a uniform optical absorption (within + 14%) in the 0.4-20 pm range. The anodized Al has a selective optical absorption: strong absorption bands at 2.5-4 and 6-13 pm for Al anodized in H,SO,, and at 2.54. 5-8 and 9-13 pm for Al anodized in H, C, 0,. Similar absorption bands were found for anodized Al samples of high enough surface reflectivity, by using specular (20”) reflectivity measurements, performed with a UR-20 Carl Zeiss Jena spectrophotometer (Fig. 3). For the samples with low surface reflectivity, this method showed zero reflection over the whole spectral range (i.e. total absorption), whilst by our 469

470

D.

-

26

3 2

20

7>

I

0

o

D;ZD,~KLAT

rt

trl

0

o--o0

---

------

--

0

045

I

I

I

I

0 50

0 55

0 60

0.65

Fig. I. Spectral responslvity of the detector-graphite system in the 0.4&0.7pm range. R, = IOxCl. l

4

I

it 0

I 070809

I

I1 1

Fig. 2. Normalized spectral responsivity 0.7-20 and l-14 pm range, respectively.

I

I

I

I

2

3

4

5

of the detector-graphite The monochromator prism.

X

Graphite

o

Anodized

A( LH,SO,

d

Anodized

AL t H 2 CL

I 6

1 0,l

III1 7

8

910

I

I

15

20

and detector-anodized Al systems in the used has a NaCl (0 0 A) or a KBr ( x )

method we found the selective optical absorption bands described above. In this case at least, the direct pyroelectric detection method proves to be more sensitive than the reflectivity one. The different optical absorption bands of the two types of anodized Al are due to the substances chemisorbed during the anodization process (H,SO, and H2C204) and not to A120, (See Fig. 4 for the transmission spectrum of H2CZ04 and Ref. (6) for that of H2S04).

75

2 K

-

50-

25-

I

L

0

Fig. 3. Reflection spectra of two samples of Al anodized in H>SO,(O), and HZC204(0). respectively.

5

(0

15

X(prn) Fig. 4. The transmlsslon spectrum of the aqueous solution of H2C20, used for the anodization of the Al samples (obtained with a UR-20 Carl Zeiss Jena spectrophotometer).

Pyroelectric

detection

of optical

471

absorption

CONCLUSIONS

The direct method for optical absorption measurements reported in this paper is simple and rapid. This method gives no absolute values for the absorption coefficients of the materials but it is very useful in comparing the optical absorption of different non-transparent materials, e.g. those investigated as potential absorbent layers for IR detectors or laser energy meters. By this method one can also determine the presence of different substances, with selective optical absorption, in a given matrix. No large-area samples are needed for investigations using this method. Based on calorimetric considerations, we have estimated that, using this method, the minimum detectable rise in the sample temperature was less than 10e6 K. Acknowledgement-The

authors

are indebted

to Dr E. Palibroda

for preparation

of the anodized

Al samples.

REFERENCES 1. 2. 3. 4. 5. 6.

Yeack C. E., Melcher R. L. and Jha S. S., J. appl. Phys. 53, 3947 (1982). Baumann T., Dacol F. and Melcher R. L., Appl. Phys. Lert. 43, 71 (1983). Melcher R. L. and Arbach G. V., Appl. Phys. Left. 40, 910 (1982). Jacob J. H., Pugh E. R., Daugherty J. D. and Northam D. B., Rea. scienf. Insfrum. 44, 471 (1973). Chirtoc M., Cindea R. M. and Mercea V., Rev. Roum. Phys. 29, 99 (1984). Mangini A. (Ed.), Aduances in Molecular Spectroscopy, p. 963. Pergamon Press, Oxford (1962).