Continuous synthesis of high-entropy alloy nanoparticles by in-flight alloying of elemental metals.

High-entropy alloy (HEA) nanoparticles (NPs) exhibit unusual combinations of functional properties. However, their scalable synthesis remains a significant challenge requiring extreme fabrication conditions. Metal salts are often employed as precursors because of their low decomposition temperatures, yet contain potential impurities. Here, we propose an ultrafast (< 100 ms), one-step method that enables the continuous synthesis of HEA NPs directly from elemental metal powders via in-flight alloying. A high-temperature plasma jet ( > 5000 K) is employed for rapid heating/cooling (103 - 105 K s-1), and demonstrates the synthesis of CrFeCoNiMo HEA NPs ( ~ 50 nm) at a high rate approaching 35 g h-1 with a conversion efficiency of 42%. Our thermofluid simulation reveals that the properties of HEA NPs can be tailored by the plasma gas which affects the thermal history of NPs. The HEA NPs demonstrate an excellent light absorption of > 96% over a wide spectrum, representing great potential for photothermal conversion of solar energy at large scales. Our work shows that the thermal plasma process developed could provide a promising route towards industrial scale production of HEA NPs.

Performance Investigations of InAs/InP Quantum-Dash Semiconductor Optical Amplifiers with Different Numbers of Dash Layers.

We present here a performance comparison of quantum-dash (Qdash) semiconductor amplifiers (SOAs) with three, five, eight, and twelve InAs dash layers grown on InP substrates. Other than the number of Qdash layers, the structures were identical. The eight-layer Qdash SOA gave the highest amplified spontaneous emission power (4.3 dBm) and chip gain (26.4 dB) at 1550 nm, with a 300 mA CW bias current and at 25 °C temperature, while SOAs with fewer Qdash layers (for example, three-layer Qdash SOA), had a wider ASE bandwidth (90 nm) and larger 3 dB gain saturated output power (18.2 dBm) in a shorter wavelength range. The noise figure (NF) of the SOAs increased nearly linearly with the number of Qdash layers. The longest gain peak wavelength of 1570 nm was observed for the 12-layer Qdash SOA. The most balanced performance was obtained with a five-layer Qdash SOA, with a 25.4 dB small-signal chip gain, 15.2 dBm 3 dB output saturated power, and 5.7 dB NF at 1532 nm, 300 mA and 25 °C. These results are better than those of quantum well SOAs reported in a recent review paper. The high performance of InAs/InP Qdash SOAs with different Qdash layers shown in this paper could be important for many applications with distinct requirements under uncooled scenarios.

2022 Optical Interference Coatings Conference: Manufacturing Problem Contest [Invited].

Participants in the 2022 Manufacturing Problem Contest were challenged to fabricate an optical filter with a specified stepped transmittance spanning three orders of magnitude from 400 to 1100 nm. The problem required that contestants be versed in the design, deposition, and measurement of optical filters to achieve good results. Nine samples from five institutions were submitted with total thicknesses between 5.9 and 53.5 µm with between 68 and 1743 layers. The filter spectra were measured by three independent laboratories. The results were presented in June 2022 at the Optical Interference Coatings Conference in Whistler, B.C., Canada.

2019 Topical Meeting on Optical Interference Coatings: Manufacturing Problem Contest [invited].

For the seventh Manufacturing Problem Contest, participants were challenged to fabricate an optical filter with transmittance specified for s-polarization at two incident angles: 10° and 50° from 400 nm to 1100 nm. The problem required that contestants be equally versed in the design, deposition, and measurement of optical filters in order to achieve good results. Eight teams from five different countries participated in the contest using various deposition techniques. The fabricated filters had a total thickness between 8.2 µm and 17.6 µm and a total number of layers from 74 to 255, which were deposited onto one or both sides of the substrate. The performances of the filters were measured by two independent laboratories. The evaluation results were presented at the Topical Meeting on Optical Interference Coatings conference held in Santa Ana Pueblo, New Mexico, in June 2019.

Pressure-dependent sensitivity of a single-pass methane detection system using a continuous-wave distributed feedback laser at 3270 nm.

A single-pass, compact, and low-power-consumption methane sensing system is presented, and its sensitivity is examined for a set of reference cells with pressures of 7.4, 74, and 740 Torr and the same methane concentration at laboratory temperature of 23°C. A GaSb-based continuous-wave distributed feedback tunable diode laser at 3270 nm and wavelength modulation spectroscopy were employed to collect 2f spectra in the ν3, R3 band of CH412. The collected 2f spectra were fitted to a modulated Voigt line profile model. An Allan-Werle variance analysis shows that the best detectivity, 4 ppbm, is obtained for the highest pressure cell. At pressures of 74 and 7.4 Torr the detectivities are 16 and 28 ppbm, respectively. For these low-pressure cells, further sensitivity improvements were achieved using a large modulation depth (m>2.2) at the expense of spectral resolution.

Bright Single InAsP Quantum Dots at Telecom Wavelengths in Position-Controlled InP Nanowires: The Role of the Photonic Waveguide.

We report on the site-selected growth of bright single InAsP quantum dots embedded within InP photonic nanowire waveguides emitting at telecom wavelengths. We demonstrate a dramatic dependence of the emission rate on both the emission wavelength and the nanowire diameter. With an appropriately designed waveguide, tailored to the emission wavelength of the dot, an increase in the count rate by nearly 2 orders of magnitude (0.4 to 35 kcps) is obtained for quantum dots emitting in the telecom O-band, showing high single-photon purity with multiphoton emission probabilities down to 2%. Using emission-wavelength-optimized waveguides, we demonstrate bright, narrow-line-width emission from single InAsP quantum dots with an unprecedented tuning range of 880 to 1550 nm. These results pave the way toward efficient single-photon sources at telecom wavelengths using deterministically grown InAsP/InP nanowire quantum dots.

Laser-induced plasmonic colours on metals.

Plasmonic resonances in metallic nanoparticles have been used since antiquity to colour glasses. The use of metal nanostructures for surface colourization has attracted considerable interest following recent developments in plasmonics. However, current top-down colourization methods are not ideally suited to large-scale industrial applications. Here we use a bottom-up approach where picosecond laser pulses can produce a full palette of non-iridescent colours on silver, gold, copper and aluminium. We demonstrate the process on silver coins weighing up to 5 kg and bearing large topographic variations (∼1.5 cm). We find that colours are related to a single parameter, the total accumulated fluence, making the process suitable for high-throughput industrial applications. Statistical image analyses of laser-irradiated surfaces reveal various nanoparticle size distributions. Large-scale finite-difference time-domain computations based on these nanoparticle distributions reproduce trends seen in reflectance measurements, and demonstrate the key role of plasmonic resonances in colour formation.

Manufacturing problem contest [invited].

For the Manufacturing Problem contest, participants were asked to fabricate on provided blank substrates a challenging filter with specific reflectance and transmittance targets covering a wavelength range from 400 nm to 1100 nm. The problem was selected such that to achieve a performance close to the targets, a submitted filter had to include at least one thin absorbing layer. Nine teams from six countries participated in the contest using different deposition techniques. The teams' designs had a number of layers varying from 36 to 235, and a total thickness from 2.0 µm to 14.6 µm. The performances of all submitted filters were measured by two independent laboratories, and the results were presented at the Optical Interference Coating meeting in June 2016.

Implementation of long-wavelength cut-off filters based on critical angle.

Our first attempts at the fabrication of long-wavelength infrared cut-off filters with extended transmission and rejection regions that are based on the use of the critical angle, the dispersion of refractive indices, and on thin-film interference were not very successful. The design of the filter consisted of layers placed at the interface between two high-index prisms. Using the available deposition equipment, the layers produced were porous and very rough. The pores adsorbed water vapor, which resulted in absorption. The roughness made the process of optical contacting very difficult. In this paper we describe the adjustments in the design and deposition processes that allowed us to obtain filters with a better and more stable performance.

Design of high-contrast OLEDs with microcavity effect.

There is a large demand for Organic Light-Emitting Displays (OLEDs) with higher contrast, particularly for outdoor applications. We show that lowering the reflectance of OLEDs, which is required for increasing the contrast, can also lead to a reduction of their efficiency when a small microcavity effect is not maintained in their structure. We describe in details the design of high-contrast bottom-emitting OLEDs that have low reflectance but still maintain a small cavity effect for efficient emission.

High-contrast organic light emitting diodes with a partially absorbing anode.

To be legible in high-ambient light conditions, organic light-emitting-diode displays should be optically designed to have a minimal reflectance without significantly affecting their overall efficiency. We demonstrate the use of an anode consisting of a partially absorbing metal layer and a multilayer distributed Bragg reflector to simultaneously absorb rather than reflect incoming light and to take advantage of a weak microcavity effect in the diode to improve light outcoupling.

Toward perfect antireflection coatings. 3. Experimental results obtained with the use of Reststrahlen materials.

The equipment and methods used to produce wide-angle antireflection coatings based on Reststrahlen materials are described. The optical constants of the coating materials used in the construction of the multilayers were determined by spectrophotometric ellipsometry and are compared with the literature values. The measured performance of an experimentally produced antireflection coating is compared with the expected calculated performance. The reflectance is low over a wide range of angles, but only in the narrow-wavelength region at which the refractive index of the Reststrahlen material is close to unity.

Femtosecond gain and index dynamics in an InAs/InGaAsP quantum dot amplifier operating at 1.55 microm.

We report on the characterization of the ultrafast gain and refractive index dynamics of an InAs/InGaAsP self-assembled quantum dot semiconductor optical amplifier (SOA) operating at 1.55 mum through heterodyne pump-probe measurements with 150 fs resolution. The measurements show a 15 ps gain recovery time at a wavelength of 1560 nm, promising for ultrafast switching at >40 GHz in the important telecommunications wavelength bands. Ultrafast dynamics with 0.2-1.5 ps lifetimes were also found consistent with carrier heating and spectral hole burning. Comparing with previous reports on quantum dot SOAs at 1.1-1.3 mum wavelengths, we conclude that the carrier heating is caused by a combination of free-carrier absorption and stimulated transition processes.

Toward perfect antireflection coatings. 2. Theory.

Recently we performed a numerical investigation of antireflection coatings that reduce significantly the reflection over a wide range of wavelengths and angles of incidence, and we proposed some experiments to demonstrate their feasibility. We provide a theoretical description of omnidirectional antireflection coatings that are effective over a wide range of wavelengths.

Ion-beam etching for the precise manufacture of optical coatings.

We propose using ion-beam etching as an additional tool for the accurate control of the thickness of thin films during the manufacture of sensitive optical multilayer coatings. We use a dual ion-beam sputtering system in the deposition and etch modes. In the deposition mode both the assist and sputtering ion beams are used to produce dense films at deposition rates in the range of 0.1-0.3 nm/s. In the etch mode, only the assist ion beam is used to remove material at a rate of less than 0.1 nm/s. A very high precision in the layer thicknesses can be obtained by alternating between deposition and etch modes. We observed that etching did not significantly affect the surface quality and the uniformity of the coatings. We introduced etching into our current manufacturing process and demonstrated its potential for the fabrication of several optical multilayer systems with performances that are very sensitive to the thickness of their layers.

Asymmetrical dual-cavity filters and their application to thickness uniformity monitoring.

The use of asymmetrical dual-cavity (ADC) fi lters with two transmittance peaks is proposed for a simultaneous monitoring of the thickness uniformities of high- and low-index materials. The method described works independently of the manufacturing technique used, and can detect non-uniformities of refractive indices. The properties of these ADC fi lters are studied using Smith's concept of effective interfaces, and admittance diagrams. An experimental application of this method is demonstrated, which can be used for increasing the yield of manufactured optical fi lters.

Design and plasma deposition of dispersion-corrected multiband rugate filters.

Inverse Fourier transform method has been commonly used for designing complex inhomogeneous optical coatings. Since it assumes dispersion-free optical constants, introducing real optical materials induces shifts in the position of reflectance bands in multiband inhomogeneous minus (rugate) filters. We propose a simple method for considering optical dispersion in the synthesis of multiband rugate filter designs. Model filters designed with this method were fabricated on glass and polycarbonate substrates by plasma-enhanced chemical vapor deposition of silicon oxynitrides and SiO2/TiO2 mixtures with precisely controlled composition gradients.

Admittance diagrams of accidental and premeditated optical inhomogeneities in coatings.

Intuitive interpretation of the effect of inhomogeneous layers in optical coatings is often difficult. I use admittance diagrams to interpret some properties of accidental and premeditated optical inhomogeneities. The effect of accidental homogeneous and inhomogeneous transition layers on the spectral properties of a single film is presented. I show that such layers affect the envelope of the reflectance spectra of thick films, effects that are extended to the shorter wavelength range. Using the same approach, I evaluate the properties and limitations of quintic antireflection coatings and show that the suppression of harmonics in rugate filters is due to an absentee-layer phenomenon and not solely to an antireflection effect as is often suggested.

Toward perfect antireflection coatings: numerical investigation.

A perfect antireflection (AR) coating would remove completely the reflection from an interface between two media for all wavelengths, polarizations, and angles of incidence. The degree to which this can be achieved is investigated numerically. It is shown that wideband solutions can be found provided that layers can be deposited with refractive indices that are close to that of the low-index medium. Thus realistic solutions exist for interfaces between two solid media. Narrow-band high-angle AR solutions are also possible for polarized light and for unpolarized light in the vicinity of certain reststrahlen bands.

Black layer coatings for the photolithographic manufacture of diffraction gratings.

A black layer coating for an aluminum-photoresist interface with a reflectance less than 0.1% for 413-nm, s-polarized light incident at 25 degrees is described. It is made of space-compatible materials, and its rms roughness is less than 15 A.