Relative Transmission Spectrum Research Articles

Improved visible-blindness of AlGaN deep ultraviolet photodiode with monolithically integrated angle-insensitive Fabry-Perot filter.

Despite the rapidly increasing demand for accurate ultraviolet (UV) detection in various applications, conventional Si-based UV sensors are less accurate due to disruption by visible light. Recently, Ga(Al)N-based photodiodes have attracted great interest as viable platforms that can avoid such issues because their wide bandgap enables efficient detection of UV light and they are theoretically blind to visible and infrared light. However, the heteroepitaxy of a Ga(Al)N layer on sapphire substrates inevitably leads to defects, and the Ga(Al)N photodiode (PD) becomes not perfectly insensible to visible light. Employment of a dielectric stacked UV pass filter is possible to avoid unwanted absorption of visible light, but the angle-dependent pass band limits the detection angle. Here, we have demonstrated the Ag-Al2O3 Fabry-Perot UV pass filter-integrated AlGaN ultraviolet photodiode. The inherent optical extinction characteristics of Ag was utilized to design the fabrication-tolerant UV pass filter with a peak transmittance at ∼325 nm. As the angle of incidence increased, the peak transmission decreased from 45% to 10%, but the relative transmission spectrum remained almost unchanged. By integrating these filters, the visible light rejection ratio (responsivity for 315 nm light to responsivity for 405 nm light) was improved by a factor of 10, reaching a value of 106 at angles of up to 80 degrees.

Coupling Waveguide-Based Micro-Sensors and Spectral Multivariate Analysis to Improve Spray Deposit Characterization in Agriculture.

The leaf coverage surface is a key measurement of the spraying process to maximize spray efficiency. To determine leaf coverage surface, the development of optical micro-sensors that, coupled with a multivariate spectral analysis, will be able to measure the volume of the droplets deposited on their surface is proposed. Rib optical waveguides based on Ge-Se-Te chalcogenide films were manufactured and their light transmission was studied as a response to the deposition of demineralized water droplets on their surface. The measurements were performed using a dedicated spectrophotometric bench to record the transmission spectra at the output of the waveguides, before (reference) and after drop deposition, in the wavelength range between 1200 and 2000 nm. The presence of a hollow at 1450 nm in the relative transmission spectra has been recorded. This corresponds to the first overtone of the O–H stretching vibration in water. This result tends to show that the optical intensity decrease observed after droplet deposition is partly due to absorption by water of the light energy carried by the guided mode evanescent field. The probe based on Ge-Se-Te rib optical waveguides is thus sensitive throughout the whole range of volumes studied, i.e., from 0.1 to 2.5 μL. Principal Component Analysis and Partial Least Square as multivariate techniques then allowed the analysis of the statistics of the measurements and the predictive character of the transmission spectra. It confirmed the sensitivity of the measurement system to the water absorption, and the predictive model allowed the prediction of droplet volumes on an independent set of measurements, with a correlation of 66.5% and a precision of 0.39 μL.

Photoluminescent graphene oxide porous particles in solution under environmental conditions produced by hydrothermal treatment

Photoluminescent graphene oxide particles (PGOPs) were synthesized by hydrothermal treatment. The relative transmission spectra of the PGOPs in the infrared region between 800 cm−1 and 4000 cm−1 showed frequencies that characterize chemical species such as ethers (CO), ketones (CO), carboxylic acids (COOH), double bonds of carbon (CC) and water (H2O). The emission wavelengths were 568 nm for graphene oxide with green photoluminescence (GOGP) and 634 nm for graphene oxide with orange photoluminescence (GOOP) when excited with a light source of 375 nm. Moreover, the detection of light emission at 600 nm in the photoluminescent excitation (PLE) spectrum is detected with more intensity in GOGP than GOOP due to the presence of groups CC and carboxylates ions (COO−) linked to graphene (GN) and the photon effective emission time according to the decay profile for each of them was 9.65 ns and 10.85 ns respectively. PGOPs films made by drop casting at room conditions were observed by Atomic Force Microscopy (AFM) and Scanning Electronic Microscopy (SEM). These films acquire a wrinkled and porous shapes with a pore diameter of 51 nm and roughness (Ra) of Chemically Modified Graphene (CMG) sheets equal to 0.14 nm for GOOP and 0.21 nm for GOGP.

Spectral blueshift as a three-dimensional structure-ordering process

The transmission spectra of a TiO2-silicone oil suspension in an increasing external electric field are studied. As the electric field increases, the structure of the suspension changes from a disordered one to an ordered one. Interestingly, the transmission spectra blueshift in this structure-ordering process. Furthermore, the relative transmission spectra exhibit Fano-like asymmetric line shapes. The deviation ratio of each asymmetric line shape increases monotonously as the disorder of the suspension decreases. We suggest that this blueshift phenomenon can be used to characterize the disorder strength of three-dimensional systems.

A precise optical transmission spectrum of the inflated exoplanet WASP-52b

We have measured a precise optical transmission spectrum for WASP-52b, a highly inflated hot Jupiter with an equilibrium temperature of 1300 K. Two transits of the planet were observed spectroscopically at low resolution with the auxiliary-port camera (ACAM) on the William Herschel Telescope (WHT), covering a wide range of 4000-8750 \AA. We use a Gaussian process approach to model the correlated noise in the multi-wavelength light curves, resulting in a high precision relative transmission spectrum with errors on the order of a pressure scale height. We attempted to fit a variety of different representative model atmospheres to the transmission spectrum, but did not find a satisfactory match to the entire spectral range. For the majority of the covered wavelength range (4000-7750 \AA) the spectrum is flat, and can be explained by an optically thick and grey cloud layer at 0.1 mbar, but this is inconsistent with a slightly deeper transit at wavelengths $> 7750$ \AA. We were not able to find an obvious systematic source for this feature, so this opacity may be the result of an additional unknown absorber.

The propagation character of black body radiation in a uniform plasma layer

A modified radiation source model is built up to study the interaction between electromagnetic waves and partially ionized uniform plasma layer, by considering the incident power spectrum profile and introducing a perfect black body. The effect of black body radiation temperature, electron number density, and collision frequency on the relative power absorption and transmission spectra of black body terahertz radiation is studied under atmosphere condition. The relative transmission spectrum on the plasma–air interface is calculated and agrees with measured results under the experiment condition. The black body radiation character spectrum with different radiation temperature can be decreased and modified by a dense plasma layer, especially by increasing the electron number density and the collision frequency, being favourable to the plasma cloaking and the far-infrared countermeasure in terahertz frequency range.

Terahertz imaging: a new non-destructive technique for the quality control of plastic weld joints

We present the first investigation of plastic weld joints using terahertz waves. Terahertz time-domain spectroscopy clearly reveals contaminations like metal or sand within the weld joint of two high-density polyethylene sheets. Furthermore, areas can be identified where the welding process has failed and the parts to be joined are separated by a small air gap. We show that a three layer structure of polyethylene-air-polyethylene has a characteristic, frequency-dependent transmission behaviour. This allows for a distinction between welded and non-welded material as well as for the calculation of the air layer thickness from the relative transmission spectrum. Consequently, terahertz time-domain spectroscopy provides a promising new non-destructive and even contactless technique, which is desired by the plastics industry for detecting a variety of deviations from the ideal welding process.

Transmission spectrum method to measure the photonic bandgap of hollow-core photonic crystal fiber

A full-vector plane-wave method is introduced, particularly in calculating the photonic crystal in-plane photonic bandgap of a 2D triangular structure. Using numerical simulation, we can obtain the in-plane bandgap structure of a hollow-core photonic crystal fiber. In the experiments, we measured the transmission spectra of a hollow-core photonic crystal fiber and a silica rod. And using the numerical calculation we can get the relative transmission spectrum of a hollow-core photonic crystal fiber. Compared with the result of theoretical simulation, we found that they are in accordance with each other, i.e. they have the same bandgap in about the same normalized frequency.

Retardation effects on intra- and intersubband plasmons in quantum wells and their manifestations in grating-coupler-assisted optical transmission

The dispersion relations of intra- and intersubband plasmon polaritons in quantum wells are investigated in the small wave-vector region. It is shown that the intrasubband plasmon polariton is a normal mode of the layered semiconductor microstructure for all wave vectors. The intersubband plasmon polariton, however, appears in two branches. The higher-frequency branch is a radiative virtual mode and the lower-frequency branch is a normal mode. It is shown that the normal modes appear as maxima and the virtual mode as a minimum in the relative transmission spectra of samples with a grating coupler above the electron system. Thus, the well-known deformation of the transmission line shape of the intersubband plasmon mode from a maximum for smaller grating periods to a minimum for larger grating periods is explained by the excitation of the radiative virtual intersubband plasmon polariton above the onset of the Rayleigh anomaly.

A STUDY OF ULTRASONIC WAVE PROPAGATION THROUGH PARALLEL ARRAYS OF IMMERSED TUBES

Tubular array structures are a very common component in industrial heat exchanging plant and the non-destructive testing of these arrays is essential. Acoustic methods using microphones or ultrasound are attractive but require a thorough understanding of the acoustic properties of tube arrays. This paper details the development and testing of a small-scale physical model of a tube array to verify the predictions of a theoretical model for acoustic propagation through tube arrays developed by Heckl, Mulholland, and Huang [1–5] as a basis for the consideration of small-scale physical models in the development of non-destructive testing procedures for tube arrays. Their model predicts transmission spectra for plane waves incident on an array of tubes arranged in straight rows. Relative transmission is frequency dependent with bands of high and low attenuation caused by resonances within individual tubes and between tubes in the array. As the number of rows in the array increases the relative transmission spectrum becomes more complex, with increasingly well-defined bands of high and low attenuation. Diffraction of acoustic waves with wavelengths less than the tube spacing is predicted and appears as step reductions in the transmission spectrum at frequencies corresponding to integer multiples of the tube spacing. Experiments with the physical model confirm the principle features of the theoretical treatment.