Near-ultraviolet Spectral Range Research Articles

High-Q Whispering-Gallery-Modes Microresonators for laser frequency locking in the Near-Ultraviolet Spectral Range

High-Q Whispering-Gallery-Modes Microresonators for laser frequency locking in the Near-Ultraviolet Spectral Range

Impact of Mg ii interstellar medium absorption on near-ultraviolet exoplanet transit measurements

ABSTRACT Ultraviolet (UV) transmission spectroscopy probes atmospheric escape, which has a significant impact on planetary atmospheric evolution. If unaccounted for, interstellar medium absorption (ISM) at the position of specific UV lines might bias transit depth measurements, and thus potentially affect the (non-)detection of features in transmission spectra. Ultimately, this is connected to the so called ‘resolution-linked bias’ effect. We present a parametric study quantifying the impact of unresolved or unconsidered ISM absorption in transit depth measurements at the position of the Mg ii h&k resonance lines (i.e. 2802.705 Å and 2795.528 Å, respectively) in the near-ultraviolet spectral range. We consider main-sequence stars of different spectral types and vary the shape and amount of chromospheric emission, ISM absorption, and planetary absorption, as well as their relative velocities. We also evaluate the role played by integration bin and spectral resolution. We present an open-source tool enabling one to quantify the impact of unresolved or unconsidered Mg ii ISM absorption in transit depth measurements. We further apply this tool to a few already or soon to be observed systems. On average, we find that ignoring ISM absorption leads to biases in the Mg ii transit depth measurements comparable to the uncertainties obtained from the observations published to date. However, considering the bias induced by ISM absorption might become necessary when analysing observations obtained with the next generation space telescopes with UV coverage (e.g. LUVOIR, HABEX), which will provide transmission spectra with significantly smaller uncertainties compared to what obtained with current facilities (e.g. HST).

Quiet-Sun Mg ii h and k Line Profiles Derived from IRIS Full-Sun Mosaics. I. Reference Profiles and Center-to-limb Variation

Abstract We derived high-precision reference profiles of the Mg ii h and k lines that represent the quiet Sun during a minimum of the solar activity. To do so, we used the broad catalog of full-Sun mosaics obtained by the Interface Region Imaging Spectrograph (IRIS). To minimize the influence of the local variations due to the on-disk solar features and to achieve low levels of uncertainties, we used 12 IRIS full-Sun mosaics without sunspots or other significant signs of solar activity. These mosaics were obtained between 2019 April and 2020 September in the near-ultraviolet spectral range. In this paper, we present the disk-averaged reference profiles of Mg ii h and Mg ii k lines, together with a series of reference profiles spanning the distance between the disk center and the solar limb. These series of profiles offer a detailed representation of the center-to-limb variation of both Mg ii h and Mg ii k lines. The reference Mg ii h and k line profiles provided in this paper can be used as the incident radiation boundary condition for radiative-transfer modeling of prominences, spicules, and other coronal and chromospheric structures.

Broadband femtosecond spectroscopic ellipsometry.

We present a setup for time-resolved spectroscopic ellipsometry in a pump-probe scheme using femtosecond laser pulses. As a probe, the system deploys supercontinuum white light pulses that are delayed with respect to single-wavelength pump pulses. A polarizer-sample-compensator-analyzer configuration allows ellipsometric measurements by scanning the compensator azimuthal angle. The transient ellipsometric parameters are obtained from a series of reflectance-difference spectra that are measured for various pump-probe delays and polarization (compensator) settings. The setup is capable of performing time-resolved spectroscopic ellipsometry from the near-infrared through the visible to the near-ultraviolet spectral range at 1.3 eV-3.6 eV. The temporal resolution is on the order of 100 fs within a delay range of more than 5 ns. We analyze and discuss critical aspects such as fluctuations of the probe pulses and imperfections of the polarization optics and present strategies deployed for circumventing related issues.

Size-dependent optical properties of periodic arrays of semiconducting nanolines.

We study the size-dependent optical properties of periodic arrays of semiconducting nanolines in the near-infrared to near-ultraviolet spectral range, where the absorption of the semiconductor increases. Using band structure calculations, we demonstrate that specific dimensions allow the slow down of the light, resulting in an enhanced absorption as compared to bulk material once the extinction coefficient of the semiconductor becomes comparable to its refractive index. Further, the refractive properties of the arrays can be tailored beyond the values of the constituting materials when the extinction coefficient of the semiconductor exceeds its refractive index. To confirm our theoretical findings, we propose a simple semi-analytical model for the light interactions with such structures and validate it with experimental reflectance spectra collected on arrays for the next-generation transistors.

Intensive measures of luminescence in GaN/InGaN heterostructures.

The intensive measures of luminescence in a GaN/InGaN multiple quantum well system are used to examine the thermodynamics and phenomenological structure. The radiative /nonradiative transitions along with absorbed or emitted phonons that occur between the different quantum states of the electrons and holes associated with these processes make the quantum efficiency of a semiconductor nanosystem in an equilibrium state an extensive property. It has long been recognized that tuning of the indium (In) composition in InGaN interlayers gives the potential to obtain a spectrum in the near-infrared to near-ultraviolet spectral range. The thermodynamic intensive properties, including the Debye temperature, carrier temperature, and junction temperature, are the most appropriate metrics to describe the optical-related interactions inherent in a given heterostructure and so can be used as the state variables for understanding the quantum exchange behaviors. The energetic features of the quantum processes are characterized based on analysis of the intensive parameters as determined by means of electroluminescence (EL) and photoluminescence (PL) spectroscopy and current-voltage measurement and then correlated with the designed InGaN/GaN microstructures. According to the McCumber-Sturge theory, the EL and PL Debye temperatures obtained experimentally signal the strength of the electron-phonon and photon-phonon interaction, respectively, while the EL and PL carrier/junction temperatures correspond to the carrier localization. Higher EL Debye temperatures and lower EL carrier/junction temperatures reflect significantly higher luminescence quantum yields, indicative of electron-phonon coupling in the transfer of thermal energy between the confined electrons and the enhancement by excited phonons of heat-assisted emissions. On the other hand, the observation of low luminescence efficiency, corresponding to the lower PL Debye temperatures and higher PL carrier/junction temperatures, is attributed to photon-phonon coupling. These findings are in good accordance to the dependence of the EL and PL quantum efficiency on the In-content of the InGaN/GaN barriers, suggesting that the characteristic Debye and carrier/junction temperatures are intensive parameters useful for assessing the optical properties of a nano-engineered semiconductor heterostructure.

Airglow monitoring by one-pixel detector

The night time airglow is a dynamic phenomenon that acts as a background for the detection of the extensive air shower (EAS) fluorescence. It is a mandatory task to monitor this background for the ground-based telescopes and for the planned space-based telescopes, dedicated to observe the EAS events induced by the ultra-high energy cosmic rays. To perform this task, we have developed a one-pixel Airglow MONitor (AMON) instrument. This instrument provides the absolute intensities of the measured night sky background in the near-ultraviolet spectral range and in the one second temporal resolution. It is designed to be easily operated in different locations and so to offer required spatial resolution. The first results demonstrate that AMON data might be useful not only for the high-energy astrophysics purposes, but also for the studies of the airglow dynamics.

Strong carrier localization and diminished quantum-confined Stark effect in ultra-thin high-indium-content InGaN quantum wells with violet light emission

Here, we report on the optical and structural characteristics of violet-light-emitting, ultra-thin, high-Indium-content (UTHI) InGaN/GaN multiple quantum wells (MQWs), and of conventional low-In-content MQWs, which both emit at similar emission energies though having different well thicknesses and In compositions. The spatial inhomogeneity of In content, and the potential fluctuation in high-efficiency UTHI MQWs were compared to those in the conventional low-In-content MQWs. We conclude that the UTHI InGaN MQWs are a promising structure for achieving better quantum efficiency in the visible and near-ultraviolet spectral range, owing to their strong carrier localization and reduced quantum-confined Stark effect.

QUIET-SUN INTENSITY CONTRASTS IN THE NEAR-ULTRAVIOLET AS MEASURED FROM SUNRISE

We present high-resolution images of the Sun in the near-ultraviolet spectral range between 214 nm and 397 nm as obtained from the first science flight of the 1 m SUNRISE balloon-borne solar telescope. The quiet-Sun rms intensity contrasts found in this wavelength range are among the highest values ever obtained for quiet-Sun solar surface structures—up to 32.8% at a wavelength of 214 nm. We compare the rms contrasts obtained from the observational data with theoretical intensity contrasts obtained from numerical magnetohydrodynamic simulations. For 388 nm and 312 nm the observations agree well with the numerical simulations whereas at shorter wavelengths discrepancies between observed and simulated contrasts remain.

Theoretical and experimental studies of the Nd^3+4f^3↔4f^25d transitions in monoclinic Nd:BaY_2F_8 crystal

Experimental spectroscopic results related to Nd3+-doped BaY2F8, are presented that include vacuum-ultraviolet ground-state absorption and excitation spectra as well as polarized emission and excited-state absorption spectra recorded in the near-ultraviolet spectral range at room and low temperatures. Calculations were performed to determine the positions of the 4f25d sublevels and the intensities and polarizations of the 4f3↔4f25d optical transitions of the Nd3+ ions in the C2 symmetry sites of the biaxial host crystal. The simulated spectra agree well with the experimental spectra; in particular, the model that was used successfully reproduced the differences between the polarized spectra on one hand and between the spectra recorded at low and room temperatures on the other hand.

Nonlinear optical response of silver and copper nanoparticles in the near-ultraviolet spectral range

The nonlinear optical response of silver and copper nanoparticles synthesized by ion implantation in silica glasses is studied in the near-ultraviolet spectral range at a wavelength of 354.7 nm. The real and imaginary parts of the third-order nonlinear susceptibility χ(3) of composite materials are measured. It is shown that the quantity Imχ(3) is due to saturated absorption, while Reχ(3) is due to the self-defocusing effect in composite materials.

Tunable femtosecond pulses in the near-ultraviolet from ultrabroadband parametric amplification

Using an ultrabroadband amplification technique in a β-barium borate noncollinear optical parametric amplifier, pumped by the third harmonic of a 1 kHz Ti:sapphire laser, we generate tunable femtosecond pulses in the range of 335–480 nm with energies of a few hundred nJ. The developed setup is an amplification source with a bandwidth of more than 200 THz and provides femtosecond pulses in the near-ultraviolet spectral range using coherent amplification. Parts of the amplified white-light continuum spectrum were compressed to 24–35 fs using a prism pair. Further improvements could make it possible to generate tunable ultraviolet pulses as short as 4–5 fs.

Relation between photoreflectance excitation and absorption spectra for GaAs and GaN films

Photoreflectance excitation (PRE) spectroscopy is shown to be a suitable method to investigate semiconductor materials in the near-infrared to near-ultraviolet spectral range. The investigation of a GaAs and a GaN bulk film demonstrates the applicability of this method even up to room temperature. For both materials, the line shape analysis suggest a nonlinear relation between the PRE and the absorption line shapes, leading to a reduction of the excitonic peak height in the PRE spectra compared to the absorption spectra. To keep this reduction as small as possible, the pump power should be on the order of the probe power or smaller.

Sub-6-fs blue pulses generated by quasi-phase-matching second-harmonic generation pulse compression

We demonstrate a novel scalable and engineerable approach for the frequency-doubling of ultrashort pulses. Our technique is based on quasi-phase-matching and simultaneously provides tailored dispersion and nonlinear frequency conversion of few-cycle optical pulses. The method makes use of the spatial localization of the conversion process and the group velocity mismatch in a chirped grating structure. The total group delay of the nonlinear device can be designed to generate nearly arbitrarily chirped second-harmonic pulses from positively or negatively chirped input pulses. In particular, compressed second-harmonic pulses can be obtained. A brief summary of the underlying theory is presented, followed by a detailed discussion of our experimental results. We experimentally demonstrate quasi-phase-matching pulse compression in the sub-10-fs regime by generating few-cycle pulses in the blue to near-ultraviolet spectral range. Using this new frequency conversion technique, we generate sub-6-fs pulses centered at 405 nm by second-harmonic generation from a 8.6 fs Ti:sapphire laser pulse. The generated spectrum spans a bandwidth of 220 THz. To our knowledge, these are the shortest pulses ever obtained by second-harmonic generation.

Zn(Mg)BeSe-based p-i-n photodiodes operating in the blue-violet and near-ultraviolet spectral range

We present the growth and characterization of p-i-n photodiodes based on ZnBeSe and ZnMgBeSe compounds. High-quality diodes exhibiting dark current as low as 12 nA/cm2 at −2 V bias have been fabricated. The spectral response shows a high responsivity of 0.17 A/W at 450 nm, with a rejection of ∼104 at longer wavelengths. Our results thus demonstrate the potential of ZnSe-based heterostructures for efficient detection in the visible-ultraviolet region.

High detectivity ZnSe-based Schottky barrier photodetectors for blue and near-ultraviolet spectral range

A characterisation of ZnSe-based Schottky barrier photodetectors grown by molecular beam epitaxy is presented. High quality diodes exhibiting a high responsivity of 0.10 A/W at 455 nm and a sharp cut-off of three to four orders of magnitude have been fabricated. A detectivity of 1.4 × 1010 mHz1/2W-1 has been obtained for a structure with a 5 mm2 area operating at –3.5 V bias. These results highlight the potential of ZnSe in blue and near-ultraviolet light detection.

The magneto-optical polar Kerr effect and birefringence spectra in PrFeO3between 2 and 5.6 eV

The complex polar Kerr effect and the effect of birefringence in PrFeO3 are reported between 2 and 5.6 eV. It is shown that the birefringence observed in transmission has its origin in the electronic transitions involving 3d levels of Fe3+ which are situated in the near-ultraviolet spectral range. The low-energy Kerr ellipticity and the associated Faraday rotation in PrFeO3 are of opposite sign compared to the other orthoferrites studied so far. This situation is analogous to that observed in Pr3+ and Nd3+ substituted iron garnets. It is concluded that the mechanism responsible for the anomalous Faraday rotation in Pr3+ and Nd3+ iron garnets does not require the presence of tetrahedral iron ions.