Holographic pH Sensors - Holographic Sensors - page 62

Fig. 3.10 The optical setup for angular-resolved measurements of the diffracted light from the
photonic structure. Supercontinuum white light illumination, a normal, b angle with respect to the
hologram surface. Diffraction measurements at, c 0 ° , d 45 ° , e specular re fl ection measurements

from 0 60 ° angle of incidence

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3.3.4 Diffraction Ef ciency Measurements

The power output of the pulsed Nd:YAG laser (350 mJ speci ed by the manu-

facturer) was measured using a powermeter (Fieldmaster, 30 V). The beam exiting
the laser was directed by dichroic mirrors and passed through a beam expander and
a collimating lens before entering the powermeter receiver elevated 8 cm above the
optical table. The measurements were made by producing single pulses at Q-switch
values from 450 to 258

S. Holograms comprising pHEMA + Ag 0 NP were

µ

prepared using different laser pulse energies. The samples were exposed to

subsequent (1 - 20) Nd:YAG laser (6 ns, 532 nm) pulses. Diffraction ef ciency

measurements were conducted to find out the effect of laser light in the organisation

of the Ag 0 NPs. These measurements were performed at the same laser wavelength,
and the diffracted and refracted lights were measured using a powermeter. First, the
laser power output of the laser was recorded. As the quality factor decreased, the
power output of the Nd:YAG pulsed laser increased exponentially (Fig. 3.11 a). The
results show that the highest diffraction ef

ciency was obtained after three laser
pulses (Fig. 3.11 b). This may be attributed to the ability to gradually organise Ag 0
NPs within the polymer matrix. However, multiple high-energy laser pulses may
damage the polymer matrix.