RESUMEN
We report on fabricated titanium dioxide (TiO2) thin films along with a transimpedance amplifier (TIA) test setup as a photoconductivity detector (sensor) in the ultraviolet-C (UV-C) wavelength region, particularly at 260 nm. TiO2 thin films deposited on high-resistivity undoped silicon-substrate at thicknesses of 100, 500, and 1000 nm exhibited photoresponsivities of 81.6, 55.6, and 19.6 mA/W, respectively, at 30 V bias voltage. Despite improvements in the crystallinity of the thicker films, the decrease in photocurrent, photoconductivity, photoconductance, and photoresponsivity in thicker films is attributed to an increased number of defects. Varying the thickness of the film can, however, be leveraged to control the wavelength response of the detector. Future development of a chip-based portable UV-C detector using TiO2 thin films will open new opportunities for a wide range of applications.
RESUMEN
We propose a scheme for imaging mid-infrared (MIR) wavelengths via pre-excitation-assisted up-conversion luminescence in lanthanide ion (Ln3+)-doped Self-organizing Optical FIber Array (SOFIA) crystal. First, near-infrared pre-excitation wavelength excites an electron from the ground state to an excited state of Ln3+. Next, the MIR wavelength to be imaged promotes this excited electron to a higher-lying energy state. Finally, relaxation of the electron from the higher-lying energy state to the ground state emits the up-conversion luminescence in the visible region, completing the MIR-to-visible wavelength conversion. An analysis of the 4f to 4f intra-configurational energy level transitions in Ln3+, together with an appropriate selection of the pre-excitation wavelength and the visible luminescence constrained within the 500-700 nm wavelength range, reveals that trivalent erbium (Er3+), thulium (Tm3+), holmium (Ho3+), and neodymium (Nd3+) can be used to image MIR wavelengths. Our proposed scheme, called MIR imAging through up-Conversion LuminEscence in a SOFIA crystal, will enable the imaging of MIR wavelengths using low-cost optics and readily available silicon-based detectors in the visible spectral region and will open up new possibilities for MIR wavelength detection and imaging.
RESUMEN
A photoconductive detector (PCD) responding only to vacuum ultraviolet (VUV) radiations below 180 nm without any filter was fabricated using an yttrium fluoride (YF3) thin film grown by femtosecond (fs) laser pulsed laser deposition (PLD). The structural morphology (particle size and surface roughness) of the thin film was improved using a low laser fluence and a high substrate temperature during the fabrication. The smallest average particle size achieved was 159 nm with a roughness of 37 nm at a laser fluence of 13.5 J cm-2 and a substrate temperature of 400 °C. The resistances for the dark current of the PCD increased from 10 TΩ to 680 TΩ using YF3 thin films with a smaller average nanoparticle diameter of 159 nm rather than 330 nm. The time response of the PCD to a VUV flash lamp emitting at 170 nm showed that a small average nanoparticle diameter results to a fast response time. By covering the Al electrode pairs with another fs PLD-grown YF3 film, the influence of external photoelectric effect was suppressed and the response wavelength edge decreased from 280 nm to 180 nm without any filter. The filterless PCD is expected to enhance the use of fluoride thin films in conjunction with VUV light sources for various scientific and industrial applications.
RESUMEN
We report on the potential of the potassium magnesium fluoride (KMgF) crystal as a fast-response scintillator with tunable cross-luminescence (CL) emission wavelength through high-pressure applications. By performing first-principles density functional theory calculations using the Perdew-Burke-Ernzerhof (PBE) hybrid functional including exact exchange (PBE0) and Green's function and screened Coulomb interaction approximation as implemented in the Vienna Ab initio Simulation Package using plane-wave basis sets within the projector-augmented wave method, we identify the specific valence-to-core band transition that results in the experimentally observed CL emission at 148 nm (8.38 eV) and 170 nm (7.29 eV) wavelengths with intrinsically fast decay times of 290 ps and 210 ps, respectively. Uniform volume compression through hydrostatic high-pressure applications could decrease the energy gap between the valence and core bands, potentially shifting the CL emission wavelength to the ultraviolet (UV) region from 200 nm (6.2 eV) to 300 nm (4.1 eV). The ability to tune and shift the CL emission to UV wavelengths allows for the detection of the CL emission using UV-sensitive photodetectors in ambient atmosphere instead of highly specialized vacuum UV detectors operating in vacuum while maintaining the intrinsically fast CL decay times, thereby opening up new possibilities for KMgF as a fast-response scintillator.
RESUMEN
Metastability of Aln/12Ga1-n/12N (n= 2-10: integer) with the 1-2 monolayer (ML) in-plane configuration towards thec[0001] direction has been demonstrated recently. To theoretically explain the existence of these metastable structures, relatively large calculation cells are needed. However, previous calculations were limited to the use of small calculation cell sizes to estimate the local potential depth (Δσ) of ordered Al1/2Ga1/2N models. In this work, we were able to evaluate large calculation cells based on the interaction energies between proximate Al atoms (δEAl-Al) in AlGaN alloys. To do this,δEAl-Alvalues were estimated by first-principles calculations (FPCs) using a (5a1× 5a2× 5c) cell. Next, a survey of the possible ordered configurations using various large calculation cell models was performed using the estimatedδEAl-Alvalues and the Monte-Carlo method. Then, various Δσvalues were estimated by FPCs and compared with the configurations previously reported by other research groups. We found that the ordered configuration obtained from the (4a1× 2a2× 1c) calculation cell (C42) has the lowest Δσof -9.3 meV/cation and exhibited an in-plane configuration at thec(0001) plane having (-Al-Al-Ga-Ga-) and (-Al-Ga-) sequence arrangements observed along them11-00planes. Hence, we found consistencies between the morphology obtained from experiment and the shape of the primitive cell based on our numerical calculations.
RESUMEN
Vacuum ultraviolet radiation (VUV, from 100 nm to 200 nm wavelength) is indispensable in many applications, but its detection is still challenging. We report the development of a VUV photoconductive detector, based on titanium dioxide (TiO2) nanoparticle thin films. The effect of crystallinity, optical quality, and crystallite size due to film thickness (80 nm, 500 nm, 1000 nm) and type of substrate (silicon Si, quartz SiO2, soda lime glass SLG) was investigated to explore ways of enhancing the photoconductivity of the detector. The TiO2 film deposited on SiO2 substrate with a film thickness of 80 nm exhibited the best photoconductivity, with a photocurrent of 5.35 milli-Amperes and a photosensitivity of 99.99% for a bias voltage of 70 V. The wavelength response of the detector can be adjusted by changing the thickness of the film as the cut-off shifts to a longer wavelength, as the film becomes thicker. The response time of the TiO2 detector is about 5.8 µs and is comparable to the 5.4 µs response time of a diamond UV sensor. The development of the TiO2 nanoparticle thin film detector is expected to contribute to the enhancement of the use of VUV radiation in an increasing number of important technological and scientific applications.