Optical Biochips - Biophotonics - page 219

The resulting enzymatic chemical reaction leads to the emission of (visible)
light, without any need for an optical excitation, and this light can be detected
by a sensitive camera. Since it is often easier to label an analyte with a fluo-
rescent molecule than to label it with a suitable chemiluminescent molecule,
luminescence is not very commonly used in optical biochips.
12.3.5 Raman Scattering
Raman scattering describes an optical process in which incident light of a
certain energy is absorbed by a molecule, and it is re-emitted with a slightly
different energy, where the energy difference is a characteristic function of the
chemical structure of the molecule. Although Raman scattering is very specific
and rich in the information it carries, its e ciency is very low: Typically, only
about one in 10 5 photons is Raman scattered [4]. For this reason, Raman
scattering is currently not used in optical biochips.
12.3.6 Nonlinear Optical (NLO) Effects
If the electrical field strength is not too high, the optical polarization of a
molecule is linearly proportional to the amplitude of the applied electric field.
Under illumination with an intense light source, however, the response of the
molecule is not any more su ciently described by a linear effect, and nonlinear
optical (NLO) effects come into play.
Two NLO effects are of particular importance for biochemical sensing:
higher harmonic generation and intensity-dependent multiphoton absorption.
Higher harmonic generation involves the mixing of two or more light beams
with frequency ν i to produce a light beam with frequency ν =Σ ν i . Multipho-
ton absorption describes the simultaneous absorption of two or more photons
to reach a particular excited energy state [4].
Since the e ciency of both processes is a sensitive function of a molecule's
environment, these NLO effects can effectively be used for biochemical sensing.
However, because intense laser light is required, NLO effects are rarely used
in optical biochips.
12.4 Preferred Sensing Principles for Optical Biochips
Because of the simplicity of the practical realization and the sensitivity of
the methods, two basic sensing principles are preferred in optical biochips.
(1) Evanescent wave sensing, where an electromagnetic wave is propagating
along an optical interface, and a part of its energy is detecting changes in
the refractive index in the sample volume. (2) Fluorescence sensing, where
incident light is exciting the fluorescence labels on the analytes whose light
emission is detected.