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.
This paper experimentally demonstrates a crossed reaction of pure and hybrid graphene oxide (GO)/tantalum dioxide (TaO2) as a volatile organic compound (VOC) absorber in a guided mode resonance (GMR) sensing platform. The proposed GMR platform has a porous TaO2 film as the main guiding layer, allowing for more molecular adsorption and enhanced sensitivity. GO is applied on top as an additional VOC absorber to increase the selectivity. The hybrid sensing mechanism is introduced by varying the concentration of the GO aqueous solution. The experimental results show that the pure TaO2-GMR has a high tendency to adsorb most of the tested VOC molecules, with the resonance wavelength shifting accordingly to the physical properties of the VOCs (molecular weight, vapor pressure, etc). The largest signal appears in the large molecule such as toluene, and its sensitivity is gradually reduced in the hybrid sensors. At the optimum GO concentration of 3 mg/mL, the hybrid GO/TaO2 -GMR is more sensitive to methanol, while the pure GO sensor coated with GO at 5 mg/mL is highly selective to ammonia. The sensing mechanisms are verified using the distribution function theory (DFT) to simulate the molecular absorption, along with the measured functional groups measured on the sensor surface by the Fourier transform infrared spectroscopy (FTIR). The crossed reaction of these sensors is further analyzed by means of machine learning, specifically the principal component analysis (PCA) method and decision tree algorithm. The results show that this sensor is a promising candidate for quantitative and qualitative VOCs detection in sensor array platform.
Surface enhanced Raman scattering substrate was an alternative analytical tool with ultrahigh sensitivity and rapid response for chemistry, medicine and forensics science. In this work, the surface enhanced Raman scattering (SERS) substrate based on PDMS grating structure created by laser interference lithography using excimer laser modification and further 80 nm-thick Au thin film deposition by dc magnetron sputtering was proposed. We investigated the effect of the grating depth on SERS performance. The methylene blue solution of different concentration was employed to test the SERS performance using the portable Raman spectrometer. The optimal SERS performance can be optimized by fabricating the PDMS grating structure with 850 nm-period, 310 nm-depth and 0.5-filling factor.
Here, we report an efficient approach to optimize the performance of surface enhanced Raman scattering (SERS) substrate. The geometry of the SERS substrate consists of the Au film deposited on polymer grating. The SERS substrate were fabricated by the laser interference lithography (LIL) and magnetron sputtering of Au thin films. The effect of the Au thin film prepared by magnetron sputtering at different deposition time (5-180 s) and operated pressure (3-5 mTorr). The morphology of the obtained samples was observed by field-emission scanning electron microscopy (FE-SEM). The results indicated that optimal SERS substrate with deposition time of 180 s and 3 mTorr-operated pressure was obtained. The limit of detection for methylene blue (MB) and methyl parathion were evaluated at 10-4 M and 10-2 M, respectively. Moreover, our SERS substrate shows the application of a portable Raman spectrophotometer which also promising for on-site pesticide substance detection.
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