Characterisation of Deep Layers of Tissue and Powders: Spatially Offset Raman and Transmission Raman Spectroscopy - Emerging Raman Applications and Techniques in Biomedical and Pharmaceutical Fields - page 56

to further improve sensitivity in a given application. In practical terms, the ad-
ditional time required may not be available. Recently Matousek [46] reported
a new approach to enhance signal in Raman spectroscopy using dielectric mir-
rors. This is achieved by limiting the large photon loss which occurs within
the immediate vicinity of the laser illumination zone; this dominant loss mech-
anism leads to severely reduced Raman signals, in particular in transmission
Raman. The technique is straightforward to implement and applicable to both
Raman spectroscopy (conventional, SORS and transmission) and fluorescence
measurements [47].
The enhancement is facilitated by a multilayer dielectric optical element
placed on the sample surface within the laser illumination zone. The method
is based on the angular dependence of dielectric filters on impacting photon
direction, which exhibit a spectral profile shifted to shorter wavelengths with
increasing angle of incidence. This property is used to facilitate a 'unidirec-
tional' mirror (photon diode) through which a semi-collimated laser beam
passes from one side at normal incidence while, at the other side, laser pho-
tons emerging from the sample at angles away from normal incidence are
reflected back into the sample (see Fig. 3.8).
This leads to a substantial increase in the coupling of laser radiation into
the sample with a corresponding boost to the overall Raman signal. In prac-
tical terms, the method requires that the sample surface is not excessively
curved as dielectric filters are typically only readily available as flat optical
elements, although this is not a fundamental obstacle.
Although conventional mirrors have long been used to redirect transmitted
laser light back into the sample as a way of increasing the intensity of Raman
signal [48] and to reduce photon loss near the laser radiation coupling zone
[49] such elements do not prevent photon loss at what is often the most critical
area, the delivery zone of laser radiation into the sample. This loss becomes
more marked in applications where safety or other limits prevent the laser
radiation from being concentrated onto a small area. Examples include the
illumination of human skin or applications in explosive powder environments
in the pharmaceutical industry. The solution presented here is fully compatible
with the defocused laser beams used in such conditions.
Figure 3.9 shows the results of a feasibility study performed in the trans-
mission Raman geometry on a standard paracetamol tablet with and with-
out the 'unidirectional' mirror (3.2 nm bandwidth dielectric band-pass filter
centred at 830 nm at normal incidence). Raman signal in transmission mode
was enhanced using this method by an order of magnitude . The measurement
was carried out using a compact spectrometer equipped with a detector ar-
ray cooled to moderate levels (-15 ◦ C). Under such conditions the spectral
noise was dominated by the detector dark count (thermal noise) [1] and the
Raman spectrum signal-to-noise improvement was linearly proportional to the
enhancement factor (i.e. it improved by a factor of 10). Achievement of the
same signal-to-noise ratio would require an increase in the acquisition time
of a factor of 100 (signal to noise improves as the square root of the acqui-