Tunable Ultraviolet Research Articles

Ferroelectrically modulated and enhanced photoresponse of a Ag/PZT/NSTO self-powered photodetector in the ultraviolet range

The photoelectric response of Ag/PZT/NSTO ultraviolet (UV) photodetectors is modulated through the manipulation of the polarization state of a PZT ultrathin film.

Tuning the UV absorbing ability of CeO2 nanoparticles with F− doping

Ultraviolet (UV) absorbing material is the key to achieving high performance UV shielding device. Because of the large absorption ability on UV light and excellent long-term stability under UV irradiation, CeO2 nanoparticle is considered potential UV absorber for practical applications. However, the CeO2 related materials still face challenges since its UV absorbing capability is non-adjustable, which is unable to meet the changing requirement. In this work, we obtain serial CeO2 nanoparticles with tunable UV absorbing ability through F− doping. Using un-doped CeO2 nanoparticles with size of about 6 nm as starting material and trifluoroacetic acid as F− source, serial F− doped CeO2 marked as CeO2:F1, CeO2:F2, CeO2:F3 and CeO2:F4 are obtained through a post- fluorination strategy. The F− doping rarely changes the size of CeO2, but obviously changes the optical absorbing ability. As the F− content increases, the optical absorption band of the F− doped nanoparticles gradually shifts to short wavelength. Because of the highest F− content, CeO2:F4 sample displays the greatest blue shift with an absorption band of 345 nm. Calculation result reveals that the introduction of F− enlarges the energy band gap of CeO2, leading to the blue shift of the UV absorption band. Finally, serial coated glasses with tunable UV shielding ability are prepared. Using CeO2 nanoparticles or F− doped CeO2 nanoparticles as UV absorber, the optical absorption band of coated glasses could be tuned between 380 nm and 345 nm.

UVB upconversion of LiYO2:Ho3+,Gd3+ for application in luminescence thermometry.

Development of novel ultraviolet (UV) upconversion materials has been emerging as a hot research topic for application in tunable UV lasers, photocatalysis, sterilization, tagging, and most recently luminescence thermometry. We readily synthesized a series of Ho3+/Gd3+ co-doped LiYO2 upconversion phosphors by a traditional high-temperature reaction. Under excitation from a blue ∼445 nm laser, LiYO2:Ho3+,Gd3+ polycrystalline powders yield intense sharp ultraviolet B (UVB) upconversion luminescence from Gd3+ 6Pj (j = 7/2, 5/2, 3/2) excited states. By means of steady and dynamic photoluminescence spectra, we systematically investigated the involved two-photon absorption upconversion as well as the accompanying energy transfer processes between Ho3+ and Gd3+ ions in the LiYO2 host lattice. Interestingly, the distinguishable UVB luminescence constituents from Gd3+ 6Pj excited states exhibit sensitive temperature dependence in a 353-673 K range. Shedding light on thermal equilibria between Gd3+ 6Pj UV-emitting levels, their luminescence intensity ratios follow Boltzmann statistics for the application of new luminescence thermometry. For the scheme of 6P7/2-6P3/2 thermally coupled levels, it works over a temperature range of 373-673 K with a maximum relative sensitivity (Sr) of about 1.07% K-1 at 373 K, and its 6P7/2-6P5/2 counterpart works over 353-533 K with a maximum Sr of about 0.83% K-1 at 353 K. Overall, our study provides a new pathway to develop UV upconversion materials, and promotes the application of Gd3+-related UV luminescence constituents in sensitive temperature sensing over a wide temperature range.

Tunable and high-performance self-powered ultraviolet detectors using leaf-like nanostructural arrays in ternary tin zinc sulfide system

Recently, self-powered ultraviolet (UV) detectors have received increasing attention for developing the future demands for optoelectronic devices due to their distinguished photoresponse properties. In this study, an emerging type of self-powered UV detectors using the ternary SnxZn1-xS alloy thin films (0.20 ≤ x ≤ 0.40 mol) were fabricated using the solvothermal process. The structural study showed the formation of two distinct phases of ZnS and SnS for the samples with high content Sn2+ ions. According to the ideal crystal-growth mechanism proposed by periodic bond chain theory, a morphological evolution from nanorods to nanoflakes and unique aligned three-dimensional leaf-like nanostructures was achieved by enhancing the Sn2+ content up to the maximum value of 0.40 mol. The optical properties featured the less influence of band tailing on the band gap energy at the high concentration of Sn2+ ions, resulting in the widening of the band gap energy up to 4.56 eV thanks to the Burstein-Moss effect. From the photosensing measurements, the Schottky barrier heights of the devices were found to significantly reduce under UVB exposure as the Sn2+ content increases, confirming a high Ion/Ioff ratio (>103) for the devices. The UV detectors exhibited superior self-powered characteristics under UVB illumination, having high photosensitivity and fast photoswitching response times (τr = 1.9 and τd = 2.6 ms) at zero voltage. Interestingly, the photoresponse performance of UV detectors can be adjusted by varying the content of Sn2+ ions, which signifies that tunable self-powered UV detectors can be designed by varying the Sn2+ concentration in the ternary SnxZn1-xS system.

Electrically Excited Plasmonic Ultraviolet Light Sources.

The emission of photons from metal-insulator-metal (MIM) nanojunctions through inelastic tunneling of electrically driven electrons is a well-acknowledged approach to develop miniaturized light sources and ultradense photonic instruments. Generally, the existing research in the optimization of electromigrated tunneling junctions is principally centered on the generation of visible and near-infrared lights. This study reports on the near-ultraviolet (NUV, λ ≈ 355nm) light emission from enhanced tunneling of electrons using aluminum nanoelectrodes. Compared to conventional noble metals, the high electron density and low screening of aluminum enable supporting of pronounced local fields at high energies (i.e, ultraviolet (UV)). As the color of light can be straightforwardly determined by the properties of tunneling structures, the exquisite features of aluminum have empowered the fashioning of tunneling devices that are able to effectively sustain plasmons at short wavelengths and emit UV light with high photon yield. This demonstration is a breakthrough in the generation of high-energy beams using electrically excited aluminum tunneling platforms, which promisingly accelerates the implementation of electrically tunable and ultradense UV light sources.

Design and Application of a Nitrate Measurement System Based on a Narrowband Tunable Ultraviolet Light Source

The nitrate concentration in seawater is an important parameter for marine environment monitoring, which is of great significance for the analysis of seawater ecosystems. In this paper, a novel nitrate measurement system based on a narrowband tunable ultraviolet light source and SVM-based algorithm is proposed and demonstrated. The system is composed of a tunable light source module, an optical fiber splitter, a temperature control module, a spectrometer and a data processing module. By controlling the motorized rotation stage and motorized filter wheel, a continuous and automatic narrowband tunable deep ultraviolet (DUV) light source module is developed. Different seawater samples from Aoshan Bay (Qingdao, China), the Western Pacific, Sanggou Bay (Weihai, China) and the South China Sea with nitrate concentrations from 0 to 102 μmol/L are measured by this system based on a narrowband tunable DUV light source and the results are compared with a broadband DUV light source system. The calculation model is established based on the support vector machine (SVM) algorithm with/without temperature and salinity correction (TSC), and the prediction results are compared with those of the in situ ultraviolet spectrophotometry (ISUS) algorithm. The results show that the method based on a narrowband tunable light source system and the TSC-SVM algorithm has the best performance in predicting the nitrate concentration, with the root mean squared error (RMSE) reduced by 29.16% (from 1.20 to 0.85 μmol/L) compared with the results of the broadband light source system, and the error range is from -3.01 to 2.99 μmol/L. The method proposed in this paper has the potential to provide more accurate measurements in marine environment monitoring.

Suppression of Fragmentation in Mass Spectrometry.

Suppressing fragmentation is a constant challenge in mass spectrometry because a molecular ion can readily be identified and provides information concerning the molecular weight of an analyte. Several techniques such as charge exchange chemical ionization (CECI) and vacuum ultraviolet emission ionization (VUVEI) have been developed to date for achieving this purpose. In this study, we report on the use of tunable ultraviolet (UV) and near-infrared (NIR) femtosecond (fs) lasers (35 fs) for the multiphoton ionization (MPI) of cis- and trans-4-methylcyclohexanols, the reference molecules that are currently used to examine fragmentation suppression. The results obtained here were compared with those obtained by CECI and VUVEI because they were reported as the best techniques for suppressing fragmentation. A molecular ion was strongly enhanced by carefully minimizing the excess energy in the ionic state using tunable UV and NIR fs-lasers. The ratio of the intensities for molecular and fragment ions, [M]+/[M-H2O]+, increased significantly (9.5-fold and 8.5-fold for cis- and trans-isomers, respectively, in UV fs-MPI) compared to the values obtained by CECI and VUVEI, respectively.

Highly Sensitive and Tunable Self-Powered UV Photodetectors Driven Jointly by p-n Junction and Ferroelectric Polarization.

Ferroelectric (FE) materials are thought to be promising materials for self-powered ultraviolet (UV) photodetector applications because of their photovoltaic effects. However, FE-based photodetectors exhibited poor performance because of the weak photovoltaic effect of FE depolarization field (Edp) on the separation of photo-generated carriers. In this work, self-powered photodetectors based on both Edp and built-in electric field at the p-n junction (Ep-n) were designed to obtain enhanced device performance. A NiO/Pb0.95La0.05Zr0.54Ti0.46O3 (PLZT) heterojunction-based device is constructed to take advantage of energy level alignments that favor electron extraction. The device exhibits a tunable performance upon varying the polarization direction of PLZT. The NiO/PLZT heterojunction-based device with the PLZT layer in the poling down state shows a higher responsivity [R = (1.8 ± 0.12) × 10-4 A/W] and detectivity [D* = (3.69 ± 0.2) × 109 Jones], a faster response speed (τr = 0.34 ± 0.03 s, τd = 0.36 ± 0.02 s), and a lower dark current [Idark = (1.3 ± 0.19) × 10-12 A] under zero bias than the PLZT-based device because of the synergistic effects of Edp and Ep-n. Moreover, under weak-light illumination (0.1 mW/cm2), it exhibits even higher R [(6.3 ± 1.2) × 10-4 A/W] and D* [(1.29 ± 0.26) × 1010 Jones] values, which surpass those of most previously reported FE-based self-powered photodetectors. Our work emphasizes the role of the coupling effect between Ep-n and Edp in the photovoltaic process of NiO/PLZT heterojunction-based devices and provides an effective way to promote the self-powered UV photodetector applications.

Cavity ringdown spectroscopy of nitric oxide in the ultraviolet region for human breath test

We report the spectrum of nitric oxide (NO) in the ultraviolet (UV) (225.4–227.0 nm) region based on cavity ringdown spectroscopy (CRDS). A cavity ringdown system, which consisted of a tunable UV laser source and a vacuum-pumped ringdown cavity, was constructed to measure NO at room temperature and atmospheric or reduced pressure. The measured spectra were validated using LIFBase simulations. The absorption cross-section of NO at the strongest absorption peak at 226.255 nm was measured to be 7.64 × 10−18 cm2 molecule−1. Using the measured mirror reflectivity of 99.55% at 226.255 nm, the detection limit of NO was determined to be 7.4 ppb (parts per billion) based on the standard 3-σ criteria. The stability and reproducibility of this CRDS system were also tested. Furthermore, exhaled gas samples from 203 human subjects (105 healthy people and 98 lung cancer patients) were measured using the system. Results demonstrated that the cavity ringdown spectroscopy in the deep-UV region has potential for breath NO test.

Tunable Ultraviolet Photorefractive Effect in LiNbO3/TiO2‐X (X = Pd or N): Theoretically Prediction and Experimental Validation

In this study, the introduction of unoccupied or partially occupied surface states energy levels into TiO2 upon Pd or N modification, respectively are theoretically predicted, suggesting a tunable carriers behavior in LiNbO3/TiO2‐X (X = Pd or N) composites. Inspired by the theoretical simulation, a new low‐cost method of compositing LiNbO3 with TiO2‐X (X = Pd or N) to mediate the ultraviolet photorefraction of LiNbO3 is proposed. The experimental results indicate that LiNbO3/TiO2 and LiNbO3/TiO2‐Pd show enhanced photorefraction due to reduced recombination of photoinduced carriers or increased free holes in LiNbO3. While LiNbO3/TiO2‐N exhibits photorefraction resistance since the photoinduced holes in LiNbO3 would get trapped by NOX species at the interface. This study will not only provide a simple way to fabricate tailored photorefractive material, but also offer new insights into the mechanism of photorefraction and photorefractive material design method.

INVESTIGATING THE IMPLEMENTATION OF ZnO NANOPARTICLES AS A TUNABLE UV DETECTOR FOR DIFFERENT SKIN TYPES

A facile chemical reduction method was used to synthesize ZnO nanoparticles (NPs) in ethylene glycol solvent at two different calcination temperatures. As a result of variation in the calcination temperature, ZnO NPs with two different sizes were achieved. The NPs were investigated for their structural and optical characteristics using X-ray diffraction and ultraviolet (UV)–Vis spectroscopy. The synthesized ZnO NPs exhibited a hexagonal structure with sizes of 46 and 65[Formula: see text]nm. The synthesized NPs were then used to investigate dye photocatalytic behavior of products as a tunable UV detector for different skin types. The dye degradation and decolorization of methylene blue in the presence of ZnO NP, following UV radiation as a function of time, were studied at different pH levels. The optical absorption spectra were then taken every 15[Formula: see text]min for all samples. The UV–Vis spectroscopy spectra revealed that optical absorption of solution was decreased upon UV exposure as a function of time. Photocatalytic reaction indicated that the dye degradation and decolorization rate were accelerated with the increase of pH level. Therefore, a tunable UV detector for different skin types could be engineered by varying the pH level of solution to avoid human skin burning.

Electronic sideband locking of a broadly tunable 318.6 nm ultraviolet laser to an ultra-stable optical cavity

We demonstrate frequency stabilization of a tunable 318.6 nm ultraviolet (UV) laser system using electronic sideband locking. By indirectly changing the frequency of a broadband electro-optic phase modulator, the laser can be continuously tuned over 4 GHz, while a 637.2 nm laser is directly stabilized to a high-finesse ultra-stable optical cavity. The doubling cavity also remains locked to the 637.2 nm light. We show that the tuning range depends mainly on the gain-flattening region of the modulator and the piezo-tunable range of the seed laser. The frequency-stabilized tunable UV laser system is able to compensate for the offset between reference and target frequencies, and has potential applications in precision spectroscopy of cold atoms.

Development and characterization of a 22 W narrow-linewidth 3186 nm ultraviolet laser

We demonstrate a high-power narrow-linewidth ultraviolet (UV) laser system at 318.6 nm for direct 6S1/2-nP (n = 70 to 100) Rydberg excitation of cesium atoms. Based on commercial fiber lasers and efficient nonlinear frequency conversion technology, 2.26 W of tunable UV laser power is obtained from cavity-enhanced second harmonic generation following sum-frequency generation of two infrared lasers at 1560.5 nm and 1076.9 nm to 637.2 nm. The maximum doubling efficiency is 57.3%. The typical UV laser power root-mean-square fluctuation is less than 0.87% over 30 minutes, and the continuously tunable range of the UV laser frequency is more than 6 GHz. Its beam quality factors M2 X and M2 Y are 1.16 and 1.48, respectively. This high-performance UV laser has significant potential use in quantum optics and cold atom physics.

Extended femtosecond laser wavelength range to 330 nm in a high power LBO based optical parametric oscillator.

We experimentally demonstrate a compact tunable, high average power femtosecond laser source in the ultraviolet (UV) regime. The laser source is based on intra-cavity frequency doubling of a temperature-tuned lithium tribotate (LBO) optical parametric oscillator (OPO), synchronously pumped at 520 nm by a frequency-doubled, Yb-fiber femtosecond laser amplifier system. By adjusting crystal temperature, the OPO can provide tunable visible to near-infrared (NIR) signal pulse, which have a wide spectral tuning range from 660 to 884 nm. Using a β-barium borate (BBO) crystal for intra-cavity frequency doubling, tunable femtosecond UV pulse are generated across 330~442 nm with up to 364 mW at 402 nm.

High power tunable femtosecond ultraviolet laser source based on an Yb-fiber-laser pumped optical parametric oscillator.

We report a high average power tunable 51 MHz femtosecond ultraviolet (UV) laser source based on an intra-cavity sum frequency mixing optical parametric oscillator (OPO) pumped by a fiber laser. The UV laser is generated by sum frequency generation (SFG) between the second harmonic of a mode-locked Yb-fiber laser and the signal of the OPO. A non-collinear configuration is used in the SFG to compensate the group velocity mismatch, and to increase the SFG conversion efficiency dramatically. Tunable ultraviolet pulses within the wavelength range from 385 to 400 nm have been produced with a maximum average power of 402 mW and a pulse width of 286 fs at 2 W Yb-fiber laser pump, corresponding to 20.1% near-infrared to UV conversion efficiency at 387 nm. To our knowledge, this is the first demonstration of tunable femtosecond UV pulse generation from a fiber laser pumped OPO, and is also the highest average power tunable UV femtosecond pulses from an OPO.

All-periodically poled, high-power, continuous-wave, single-frequency tunable UV source.

We report on experimental demonstration of an all-periodically poled, continuous-wave (CW), high-power, single-frequency, ultra-violet (UV) source. Based on internal second-harmonic-generation (SHG) of a CW singly resonant optical parametric oscillator (OPO) pumped in the green, the UV source provides tunable radiation across 398.94-417.08 nm. The compact source comprising of a 25-mm-long MgO-doped periodically poled stoichiometric lithium tantalate (MgO:sPPLT) crystal of period Λ(SLT)=8.5 μm for OPO and a 5-mm-long, multi-grating (Λ(KTP)=3.3, 3.4, 3.6 and 3.8 μm), periodically poled potassium titanium phosphate (PPKTP) for intra-cavity SHG, provides as much as 336 mW of UV power at 398.94 nm, corresponding to a green-to-UV conversion efficiency of ∼6.7%. In addition, the singly resonant OPO (SRO) provides 840 mW of idler at 1541.61 nm and substantial signal power of 108 mW at 812.33 nm transmitted through the high reflective cavity mirrors. UV source provides single-frequency radiation with instantaneous line-width of ∼18.3 MHz and power >100 mW in Gaussian beam profile (ellipticity >92%) across the entire tuning range. Access to lower UV wavelengths requires smaller grating periods to compensate high phase-mismatch resulting from high material dispersion in the UV wavelength range. Additionally, we have measured the normalized temperature and spectral acceptance bandwidth of PPKTP crystal in the UV wavelength range to be ∼2.25°C·cm and ∼0.15 nm·cm, respectively.

Two-dimensional Fourier transform spectroscopy in the ultraviolet with sub-20 fs pump pulses and 250–720 nm supercontinuum probe

Experimental realizations of two-dimensional (2D) electronic spectroscopy in the ultraviolet (UV) must so far contend with a limited bandwidth in both the excitation and particularly the probe frequency. The pump bandwidth is at best 1500 cm−1 (full width at half maximum) at a fixed wavelength of 267 nm or 400 cm−1 for tunable pulses. The use of a replica of the pump pulse as a probe limits the observation of photochemical processes to the excitation region and makes the disentanglement of overlapping signal contributions difficult. We show that 2D Fourier transform spectroscopy can be conducted in a shaper-assisted collinear setup comprising fully tunable UV pulse pairs and supercontinuum probe spanning 250–720 nm. The pump pulses are broadened up to a useable spectral coverage of 2000 cm−1 (25 nm at 316 nm) by self-phase modulation in bulk CaF2 and compressed to 18 fs. By referencing the white light probe and eliminating pump stray light contributions, high signal-to-noise ratios even for weak probe intensities are achieved. Data acquisition times as short as 4 min for a selected population time allow the rapid recording of 2D spectra for photolabile biological samples even with the employed 1 kHz laser system. The potential of the setup is demonstrated on two representative molecules: pyrene and 2,2-diphenyl-5,6-benzo(2H)chromene. Well-resolved cross-peaks are observed and the excitation energy dependence of the relaxation processes is revealed.

Surfactant Free, Non-aqueous Method, for the Deposition of ZnO Nanoparticle Thin Films on Si(100) Substrate with Tunable Ultraviolet (UV) Emission

We report, strong ultraviolet (UV) emission from ZnO nanoparticle thin film obtained by a green synthesis, where the film is formed by the microwave irradiation of the alcohol solution of the precursor. The deposition is carried out in non-aqueous medium without the use of any surfactant, and the film formation is quick (5 min). The film is uniform comprising of mono-disperse nanoparticles having a narrow size distribution (15-22 nm), and that cover over an entire area (625 mm2) of the substrate. The growth rate is comparatively high (30-70 nm/min). It is possible to tune the morphology of the films and the UV emission by varying the process parameters. The growth mechanism is discussed precisely and schematic of the growth process is provided. Keywords: Nanostructures, chemical synthesis, electron microscopy, optical properties.

Water-Based Route to Ligand-Selective Synthesis of ZnSe and Cd-Doped ZnSe Quantum Dots with Tunable Ultraviolet A to Blue Photoluminescence

A water-based route has been demonstrated for synthesizing ZnSe and Cd-doped ZnSe (Zn(x)Cd(1-x)Se, 0 < x < 1) quantum dots (QDs) that have tunable and narrow photoluminescence (PL) peaks from the ultraviolet A (UVA) to the blue range (350-490 nm) with full-width at half-maximum (fwhm) values of 24-36 nm. Hydrazine (N(2)H(4)) was used to maintain oxygen-free conditions, allowing the reaction vessel to be open to air. The properties of the QDs were controlled using the thiol ligands, 3-mercaptopropionic acid (MPA), thiolglycolic acid (TGA), and l-glutathione (GSH). On the basis of optical spectra, linear three-carbon MPA attenuated nucleation and growth, yielding small ZnSe QDs with a high density of surface defects. In contrast, TGA and GSH produced larger ZnSe QDs with lower surface defect densities. The absorption spectra show that growth was more uniform and better controlled with linear two-carbon TGA than branched bifunctional GSH. After 20 min of growth TGA-capped ZnSe had an average diameter of 2.5 nm based on high-resolution transmission electron microscopy images; these nanocrystals had an absorbance peak maximum of approximately 340 nm (3.65 eV) and a band gap PL emission peak at 372 nm (3.34 eV). Highly fluorescent Zn(x)Cd(1-x)Se QDs were fabricated by adding a Cd-thiol complex directly to ZnSe QD solutions; PL peaks were tuned in the blue range (400-490 nm) by changing the Zn to Cd ratio. The Cd-bearing nanocrystals contained proportionally more Se based on X-ray photoelectron spectroscopy, and Cd-Se bonds had ionic character, in contrast to primarily covalent Zn-Se bonds.

Cerium-doped fluoride lasers

In the 30 years since tunable ultraviolet (UV) lasers based on 5d /spl rarr/ 4f transition of trivalent lanthanides doped into solid-state hosts were first demonstrated, tremendous progress has been made in these unique laser systems. Today, cerium-doped fluoride lasers offer wide tunability (280-333 nm), high efficiency (up to 62%) and narrow-band output. These lasers can also be used for femtosecond pulse amplification in the UV. Cerium lasers represent a logical route to generation of tunable UV in all-solid-state systems. In this paper, we review the current state-of-the-art cerium laser crystal development and cerium laser systems.