Ultraviolet Spectral Regime Research Articles

Techniques for Constructing Ultra Low Cost Deep Ultraviolet Transmission Microscopes

This article presents the design, construction, and results obtained by our new compact, small and inexpensive deep UV microscope. Several methods are described for recording microscopic images of various types in the deep ultraviolet spectral regime (200 nm – 300 nm). The techniques employed utilize CCD cameras, de-bayered webcam sensors, photographic paper and cyanotype coated paper as detectors. In addition, we describe the techniques employed and results obtained by utilizing a cellphone camera to record deep ultraviolet microscopic images. The instrumentation techniques, presented in this paper are expected to be most useful in areas where compact, handheld systems, are needed for in field applications.

Improved gas-jet based extreme ultraviolet, soft X-ray laser plasma source.

We present a new nozzle design for an improved brilliance of laser-produced gas plasmas emitting in the soft X-ray and extreme ultraviolet spectral regime. A rotationally asymmetric gas jet is formed by employing two closely adjacent nozzles facing each other under the angle of 45°. The generated three-dimensional gas density distribution is tomographically analyzed using a Hartmann-Shack wavefront sensor. A comparison with numerical simulations accomplishes an optimization of the nozzle arrangement. The colliding gas jets create an optimized gas distribution with increased density, leading to a significant brilliance enhancement of the extreme ultraviolet, soft X-ray plasma.

Ultraviolet radiation impact on the efficiency of commercial crystalline silicon-based photovoltaics: a theoretical thermal-electrical study in realistic device architectures

Ultraviolet (UV) radiation has been identified as one of the most critical factors for the degradation of photovoltaics (PVs). Besides that, the UV spectral regime (∼0.28-0.4 µm) is less efficient for silicon-based PVs owing to the excess of the energy of the incident UV photons relative to the semiconductor’s bandgap; thus, a large part of the UV photon energy is transformed into heat, increasing the PV temperature and decreasing its efficiency. Therefore, it is crucial to investigate in detail and evaluate the UV radiation impact on the temperature and efficiency of realistic photovoltaic modules. Here we perform this investigation for crystalline silicon-based photovoltaics that operate outdoors. The investigation is performed by employing a thermal-electrical modeling approach, which takes into account all the major intrinsic processes affected by the temperature variation in the photovoltaic devices. We show that effectively reflecting UV radiation, i.e., up to a cut-off wavelength, which depends on the environmental conditions, results in a reduction of the overall operating temperature and enhancement of the PV cell’s efficiency. Additionally, blocking the high energy UV photons prolongs the lifetime of the PV and its performance in the long term.

AlGaN avalanche Schottky diodes with high Al-content

AlxGa1−xN-based avalanche photodiodes with a Schottky-contact grown on AlN bulk substrate with an Al-content of x = 0.68 have been examined with respect to their structural and electro-optical properties. The Schottky diodes suitable for the solar-blind ultraviolet spectral regime show avalanche gain for voltages in excess of 25 V reverse bias in linear gain mode under frontside illumination. For 60 V reverse bias, avalanche multiplication exceeding 104 was obtained. The devices were operated well below breakdown; the measured current for frontside illuminated conditions exceeds the dark-condition characteristics by more than two orders of magnitude at this voltage. The Schottky-barrier height and the ideality n were extracted to eV and at room temperature, respectively. Temperature dependent measurements indicate a temperature insensitive dark current mechanism with an enhanced multiplication process towards lower temperatures.

Avalanche multiplication in AlGaN-based heterostructures for the ultraviolet spectral range

AlxGa1-xN based avalanche photodiodes grown on sapphire substrate with Al-contents of x = 0.65 and x = 0.60 have been examined under back- and frontside illumination with respect to their avalanche gain properties. The photodetectors suitable for the solar-blind ultraviolet spectral regime show avalanche gain for voltages in excess of 30 V reverse bias in the linear gain mode. Devices with a mesa diameter of 100 μm exhibit stable avalanche gain below the break through threshold voltage, exceeding a multiplication gain of 5500 at 84 V reverse bias. A dark current below 1 pA can be found for reverse voltages up to 60 V.

Experimental evaluation of signal-to-noise in spectro-holography via modified uniformly redundant arrays in the soft x-ray and extreme ultraviolet spectral regime

We present dichroic x-ray lensless magnetic imaging by Fourier transform holography with an extended reference scheme via a modified uniformly redundant array (mURA). Holographic images of magnetic domains simultaneously generated by a single pinhole reference as well as by a mURA reference are compared with respect to the signal-to-noise ratio (SNR) as a function of exposure time. We apply this approach for spectro-holographic imaging of ferromagnetic domain patterns in Co/Pt multilayer films. Soft x-rays with wavelengths of 1.59 nm (Co L3 absorption edge) and 20.8 nm (Co M2,3 absorption edges) are used for image formation and to generate contrast via x-ray magnetic circular dichroism. For a given exposure time, the mURA-based holography allows to decouple the reconstruction SNR from the spatial resolution. For 1.59 nm wavelength, the reconstruction via the extended reference scheme shows no significant loss of spatial resolution compared to the single pinhole reference. In contrast, at 20.8 nm wavelength the single pinhole reveals some very intricate features which are lost in the image generated by the mURA, although overall a high-quality image is generated. The SNR-advantage of the mURA scheme is most notable when the hologram has to be encoded with few photons, while errors associated with the increased complexity of the reconstruction process reduce the advantage for high-photon-number experiments.

Absorption spectroscopy of xenon and ethylene–noble gas mixtures at high pressure: towards Bose–Einstein condensation of vacuum ultraviolet photons

Bose-Einstein condensation is a phenomenon well known for material particles as cold atomic gases, and this concept has in recent years been extended to photons confined in microscopic optical cavities. Essential for the operation of such a photon condensate is a thermalization mechanism that conserves the average particle number, as in the visible spectral regime can be realized by subsequent absorption re-emission processes in dye molecules. Here we report on the status of an experimental effort aiming at the extension of the concept of Bose-Einstein condensation of photons towards the vacuum ultraviolet spectral regime, with gases at high pressure conditions serving as a thermalization medium for the photon gas. We have recorded absorption spectra of xenon gas at up to 30 bar gas pressure of the $5p^6 - 5p^56s$ transition with a wavelength close to 147 nm. Moreover, spectra of ethylene noble gas mixtures between 155 and 180 nm wavelength are reported.

Insights into the optical and anti-bacterial properties of biogenic PbSe quantum rods

The detailed optical properties of lead selenide (PbSe) quantum rods biosynthesized in marine Aspergillus terreus were apprehended theoretically using ab initio calculations based on the experimental absorption spectrum. These studies indicate that the absorption coefficient of the biosynthesized PbSe quantum rods increases linearly with incident photon energies. The variation of other optical constants like extinction coefficient, refractive index and reflectance was comparable to that of the chemically synthesized counterparts. Further, the high dielectric constant and remarkable fluorescence of the biogenic PbSe quantum rods pronounce their application in opto-electronic devices in the Near Infra-Red and Ultraviolet spectral regime. The biosynthesized PbSe quantum rods were also found to possess appreciable anti-bacterial activity against various gram positive and gram negative bacterial species thus enhancing the relevance of the same for practical utility. Based on these results it can be concluded that biogenic PbSe quantum rods can be envisaged as potential candidates for bio-imaging, bio-sensing and other photo-voltaic applications.

Probing surface plasmons in individual Ag nanoparticles in the ultra-violet spectralregime

Previous investigations of surface plasmons in Ag largely focused on their excitations in thevisible spectral regime. Using scanning transmission electron microscopy with anelectron beam of 0.2 nm in conjunction with electron energy-loss spectroscopy, wespectrally and spatially probe the surface plasmons in individual Ag nanoparticles (∼30 nm), grown on Si, in the ultra-violet spectral regime. The nanomaterialsshow respective sharp and broad surface-plasmon resonances at∼3.5 eV(∼355 nm)and ∼7.0 eV (∼177 nm), and the correlated spectral calculations established their multipolar characteristics.The near-field distributions of the surface plasmons on the nanoparticles were alsomapped out, revealing the predominant dipolar nature of the 3.5 eV excitation withobvious near-field enhancements at one end of the nano-object. The unveilednear-field enhancements have potential applications in plasmonics and molecularsensing.

Multistep Ionization of Argon Clusters in Intense Femtosecond Extreme Ultraviolet Pulses

The interaction of intense extreme ultraviolet femtosecond laser pulses (lambda = 32.8 nm) from the FLASH free electron laser (FEL) with clusters has been investigated by means of photoelectron spectroscopy and modeled by Monte Carlo simulations. For laser intensities up to 5x10(13) W/cm(2), we find that the cluster ionization process is a sequence of direct electron emission events in a developing Coulomb field. A nanoplasma is formed only at the highest investigated power densities where ionization is frustrated due to the deep cluster potential. In contrast with earlier studies in the IR and vacuum ultraviolet spectral regime, we find no evidence for electron emission from plasma heating processes.

Identifying fragment vuv excitations with dissociation channels from the strong-field ionization ofN2

Along with attosecond pulses comes a growing need for broadband amplification in the extreme ultraviolet spectral regime. Ultrafast laser pulses interacting with high symmetry molecules produce electronic excitations that are approaching this energy regime. Although there is some theoretical work regarding such excitation, experimental progress had been hampered in a fundamental way. Ion time-of-flight spectroscopy measures dissociation pathways but provides no specific information regarding the final state of the fragments. Vacuum-ultraviolet spectroscopy provides complete information about excited fragments but says nothing of the dissociation channel. In this paper we measure the internal states of nitrogen ion fragments and we associate them with specific dissociation pathways.

Fast plasma mixing--A new excitation method for CW gas laser

Fast plasma mixing is proposed as a method to generate population inversions on electronic transitions for continuous lasers. Selective excitation is accomplished by preparing collision partners for excitation in two or more separate beams. Excitation in a well-defined reaction volume and a fast flow in combination with a suitable resonator will allow the fast removal of decay products from the active volume. This pumping method will allow the operation of new CW laser systems not possible with conventional discharges and CW operation of laser systems, which at this time can only be operated as pulsed lasers. A test device has been used to study the proposed excitation method. CW operation as a fluorine laser has demonstrated the general feasibility of this method. Of special interest are molecular lasers pumped by charge or energy transfer from atomic species or by chemical reactions and excimer lasers. These lasers may offer the capability of operating CW tunable gas lasers in the visible and ultraviolet spectral regime.