In
this work, a nanocolumnar tantalum oxide waveguide is introduced
in a guided-mode resonance-based sensor for volatile organic compound
detection. The presence of a nanocolumnar structure where optical
resonance is localized allows for molecular diffusion and adsorption
and hence the enhancement of the sensor’s sensitivity. Here,
the nanocolumnar tantalum oxide film is fabricated using a pulsed
direct current reactive magnetron sputtering system at low kinetic
energy deposition. By optimizing the operation pressure, both the
size and density of the nanocolumnar film can be controlled. The results
show that the tantalum oxide film deposited at a higher pressure (30
mTorr) forms a more discrete nanocolumnar structure (refractive index
of 1.93 and 20.7% porosity). As a result, the sensor’s sensitivity
is remarkably increased up to 15-fold in comparison to the deposition
at a lower pressure (10 mTorr, higher refractive index, n = 2.1, and 6.4% porosity). The sensor exhibits good stability and
reusability over 25 measurements of isopropanol vapor within a duration
of 60 days with 6.4% coefficient of variance at a lower concentration
(5%). The selectivity experiment shows good potential of using the
proposed sensor for toluene and formaldehyde detection.
A frequency-stabilized diode laser is widely used for applications in laser cooling and high-resolution spectroscopy. In this work, the 780-nm external cavity diode laser was constructed and subsequently frequency-controlled by three parameters, i.e., temperature, injection current and optical feedback. The laser frequency was measured with respect to the 5S1/2 → 5P3/2 (D2-lines) transition of Rubidium, while the laser mode was characterized by a Fabry-Perot interferometer. The laser temperature was passively controlled to a single value between 20 ̊C and 25 ̊C while the injection current was investigated in combination with course and fine adjustments of optical feedback. Only data relevant to a single-mode laser operation was collected. It was found that as the current increased, the laser frequency shifted linearly with slopes approximately 0.5-0.8 GHz/mA. Optical feedback from the external cavity was tuned by the voltage applied to the piezoelectric transducer, yielding a linear frequency response of approximately 0.2 GHz/V. The measured parameters were rearranged to represent the island of stability of the laser, suggesting suitable conditions that yielded single-mode operation, at a desirable laser frequency. The results were important for a design of an active feedback, in order to further reduce the frequency linewidth and intensity noise of the laser.
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