bk-1979-0102.ch012

12

Spectroscopy with the Evanescent W a v e in the Visible Region of the Spectrum

Downloaded by FUDAN UNIV on December 17, 2016 | http://pubs.acs.org Publication Date: June 21, 1979 | doi: 10.1021/bk-1979-0102.ch012

GERHARD J. MÜLLER Zentralinstitut fur Biomedizinische Technik;UniversitätErlangen Nürnberg; D 8520 Erlangen, Turnstr. 5; West-Germany This article shows how the evanescent wave can be used with advantage for spectroscopic purposes in the field of biomedical engineering. Three types of spectroscopy can be done with the evanescent wave in the UV-VIS range of the spectrum: (a) attenuated total reflection (ATR) spectroscopy, which is well known in the infrared; (b) the excitation of Raman scattering with the evanescent wave; and (c) the excitation of fluorescence with the evanescent wave. The first two types will be discussed in this article; the third is discussed for example by Hirschfeld (1) and more recently by Watkins and Robertson (2). But before going into details a historical review may be of some interest. Historical Review Total internal reflection (TIR) at the interface of an optically dense medium to an optically rare one has been of continuous interest over the years. Closely related to this phenomenon is the existence of an evanescent wave in the second (rare) medium, a fact first mentioned and even proved by Newton (3). More than 150 years later Mach and co-workers (4) used TIR to demonstrate the existence of anomalous dispersion. They reported a method for projecting the dispersion curve n = η(λ) directly. To the authors knowledge this is the first time that TIR was used for spectroscopic purposes. After that the behaviour of the evanescent field rather than possible applications was of primary interest. In 1902 Hall (5) succeeded in photographing the evanescent wave and soon after wards fluorescence (6) and scattering (7) excited by such waves were observed. In 1910 Schaefer and Gross (8) measured quanti tatively the exponential decay of the amplitude of the field with microwaves. A f i r s t r a t h e r comprehensive t h e o r e t i c a l treatment o f TIR, based on Maxwell's theory, was p u b l i s h e d by Schaefer (9) i n 1 9 3 2 . In t h i s treatment a displacement o f the incoming and the t o t a l l y r e f l e c t e d ray, as shown i n F i g u r e 1, was p r e d i c t e d . But i t was 0-8412-0504-3/79/47-102-239$06.00/0 © 1979 American Chemical Society Talmi; Multichannel Image Detectors ACS Symposium Series; American Chemical Society: Washington, DC, 1979.


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Figure 1. Total internal reflection (TIR). n : refractive index; 6 : critical angle, for TIR the angle of incidence must be larger than 6 ; d : penetration; depth; D: Goos-Hanchen shift. 1>2

C

C

p

Talmi; Multichannel Image Detectors ACS Symposium Series; American Chemical Society: Washington, DC, 1979.


12.

MULLER

The Evanescent Wave in the Visible Spectrum

241

not u n t i l 1947 t h a t Goos and Hânchen (10) could r e p o r t on an experimental proof o f t h a t displacement D, which i s now known as Goos-Hânchen s h i f t . And i t took another 15 years before TIR was again used as a s p e c t r o s c o p i c t o o l . I t was r e d i s c o v e r e d more o r l e s s at the same time by Fahrenfort (11,12) and H a r r i c k ( 1 3 , 1 4 , 15), p r e f e r e n t i a l l y f o r the i n f r a r e d region o f the spectrum. In 1964 Hansen (16) reported, f o r the f i r s t time, on an app l i c a t i o n o f t h i s technique i n the v i s i b l e r e g i o n , i . e . the det e r m i n a t i o n o f the r e f r a c t i v e index η and the absorbance k o f E o s i n Β s o l u t i o n s . A f u r t h e r attempt o f applying TIR spectroscopy i n the UV-VIS region was p u b l i s h e d by H i r s c h f e l d i n 1966 (17). And i t was i n 1967 t h a t a d e t a i l e d review and d e s c r i p t i o n o f the s t a t e o f the a r t o f i n t e r n a l r e f l e c t i o n spectroscopy was given i n a short form by H i r s c h f e l d and Wilks (18) and more compre h e n s i v e l y i n the w e l l known book o f H a r r i c k (19) . In the f o l l o w i n g years up t i l l now there were many papers on t h i s t o p i c , but with very few exceptions (which w i l l be mentioned l a t e r ) a l l o f them deal with a p p l i c a t i o n s i n the i n f r a r e d r e g i o n . T h i s h i s t o r i c a l review cannot be completed without mention of some o f the many t h e o r e t i c a l papers, which are recommended t o the i n t e r e s t e d reader f o r a deeper i n s i g h t i n t o the p h y s i c a l process o f TIR and the behaviour o f the evanescent wave (20 - 2 5 ) . Total Internal Reflection Since there are v a r i o u s comprehensive papers and monographs on the p h y s i c a l p r o p e r t i e s o f TIR, t h i s s e c t i o n i s r e s t r i c t e d t o a b r i e f summary o f the most important equations and f a c t s . As seen i n the l i t e r a t u r e c i t e d , i n TIR the incoming l i g h t forms a standing wave p a t t e r n at the i n t e r f a c e w i t h i n the dense medium, whereas i n the rare medium the amplitude o f the e l e c t r i c f i e l d f a l l s o f f e x p o n e n t i a l l y with the distance from the phase boundary according to

£ = £ -exp|^p-]

(D

0

The depth o f p e n e t r a t i o n d (compare F i g u r e 1) which i s d e f i n e d by the d i s t a n c e w i t h i n whiêh Ε decreases to 1/e, i s given by

d

p

=

η 2π(ήη 0 - ( f i / n , ) ) 2

1

2

2

1 / 2

;

Θ

>

θ

°

^

According to H i r s c h f e l d (260 there i s a d e f i n i t e r e l a t i o n s h i p be tween the p e n e t r a t i o n depth d and the Goos-Hânchen s h i f t D, which i s reduced near t o the C r i t i c a l angle to

Talmi; Multichannel Image Detectors ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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M U L T I C H A N N E L I M A G E DETECTORS

d /D

=

p

1/COS0

(3)

I f the r a r e medium e x h i b i t s a b s o r p t i o n , the p e n e t r a t i n g wave be comes attenuated. In the case o f transmittance measurements a t weak absorption the law o f Lambert-Beer i s v a l i d i n a l i n e a r approximation a l s o


τ

= -!- = ~ ° = e

d

bk-1979-0102.ch012

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