Results of an interlaboratory study on the working curve in vat photopolymerization.

The working curve informs resin properties and print parameters for stereolithography, digital light processing, and other photopolymer additive manufacturing (PAM) technologies. First demonstrated in 1992, the working curve measurement of cure depth vs radiant exposure of light is now a foundational measurement in the field of PAM. Despite its widespread use in industry and academia, there is no formal method or procedure for performing the working curve measurement, raising questions about the utility of reported working curve parameters. Here, an interlaboratory study (ILS) is described in which 24 individual laboratories performed a working curve measurement on an aliquot from a single batch of PAM resin. The ILS reveals that there is enormous scatter in the working curve data and the key fit parameters derived from it. The measured depth of light penetration Dp varied by as much as 7x between participants, while the critical radiant exposure for gelation Ec varied by as much as 70x. This significant scatter is attributed to a lack of common procedure, variation in light engines, epistemic uncertainties from the Jacobs equation, and the use of measurement tools with insufficient precision. The ILS findings highlight an urgent need for procedural standardization and better hardware characterization in this rapidly growing field.

Characterizing light engine uniformity and its influence on liquid crystal display based vat photopolymerization printing.

Vat photopolymerization (VP) is a rapidly growing category of additive manufacturing. As VP methods mature the expectation is that the quality of printed parts will be highly reproducible. At present, detailed characterization of the light engines used in liquid crystal display (LCD)-based VP systems is lacking and so it is unclear if they are built to sufficiently tight tolerances to meet the current and/or future needs of additive manufacturing. Herein, we map the irradiance, spectral characteristics, and optical divergence of a nominally 405 nm LCD-based VP light engine. We find that there is notable variation in all of these properties as a function of position on the light engine that cause changes in extent of polymerization and surface texture. We further demonstrate through a derived photon absorption figure of merit and through printed test parts that the spatial heterogeneity observed in the light engine is significant enough to affect part fidelity. These findings help to explain several possible causes of variable part quality and also highlight the need for improved optical performance on LCD-based VP printers.

Traveling tunable laser projector for UV-blue disinfection dose determinations.

As the COVID-19 pandemic was overtaking the world in the spring of 2020, the National Institute of Standards and Technology (NIST) began collaborating with the National Biodefense Analysis and Countermeasures Center to study the inactivation of SARS-CoV-2 after exposure to different ultraviolet (UV) and blue light wavelengths. This paper describes a 1 kHz pulsed laser and projection system used to study the doses required to inactive SARS-CoV-2 over the wavelength range of 222 to 488 nm. This paper builds on NIST's previous work for water pathogen inactivation using UV laser irradiation. The design of the laser and projection system and its performance in a Biosafety Level 3 (BSL-3) laboratory are given. The SARS-CoV-2 inactivation results (published elsewhere by Schuit, M.A., et al., expected 2022) demonstrate that a tunable laser projection system is an invaluable tool for this research.

SARS-CoV-2 inactivation by ultraviolet radiation and visible light is dependent on wavelength and sample matrix.

Numerous studies have demonstrated that SARS-CoV-2 can be inactivated by ultraviolet (UV) radiation. However, there are few data available on the relative efficacy of different wavelengths of UV radiation and visible light, which complicates assessments of UV decontamination interventions. The present study evaluated the effects of monochromatic radiation at 16 wavelengths from 222 nm through 488 nm on SARS-CoV-2 in liquid aliquots and dried droplets of water and simulated saliva. The data were used to generate a set of action spectra which quantify the susceptibility of SARS-CoV-2 to genome damage and inactivation across the tested wavelengths. UVC wavelengths (≤280 nm) were most effective for inactivating SARS-CoV-2, although inactivation rates were dependent on sample type. Results from this study suggest that UV radiation can effectively inactivate SARS-CoV-2 in liquids and dried droplets, and provide a foundation for understanding the factors which affect the efficacy of different wavelengths in real-world settings.

Chip-Scale Droop-Free Fin Light-Emitting Diodes Using Facet-Selective Contacts.

Sub-micron-size light sources are currently extremely dim, achieving nanowatt output powers due to the current density and temperature droop. Recently, we reported a droop-free fin light-emitting diode (LED) pixel that at high current densities becomes a laser with record output power in the microwatt range. Here, we show a scalable method for selectively metallizing fins via their nonpolar side facet that allows electrical injection to sub-200 nm wide n-ZnO fins on p-GaN with at least 0.8 µm2 active area. Electrically addressable fin LEDs are fabricated in a linear array format using standard 2 µm resolution photolithography. Electroluminescence analysis across different pixels shows that the fin acts as the active region of the LED and generates a narrow-band ultraviolet emission between ≈368 and ≈390 nm. Investigating fins at high current densities, ranging from 100 to 2000 kA/cm2, shows that their emission increases without any decline even as the junction temperature reaches a range of 200-340 °C. The absence of electron leakage to p-GaN at high injection levels and an undetectable electron-hole escape from the fin at high temperatures indicate that the fin shape is highly efficient in controlling the nonradiative recombination pathways such as Auger recombination. The fin LED geometry is expected to enable the realization of high-brightness arrays of light sources at sub-micron-size regimes suitable for operation at high temperatures and high current densities.

High-brightness lasing at submicrometer enabled by droop-free fin light-emitting diodes (LEDs).

"Efficiency droop," i.e., a decline in brightness of light-emitting diodes (LEDs) at high electrical currents, limits the performance of all commercial LEDs and has limited the output power of submicrometer LEDs and lasers to nanowatts. We present a fin p-n junction LED pixel that eliminates efficiency droop, allowing LED brightness to increase linearly with current. With record current densities of 1000 kA/cm2, the LEDs transition to lasing, with brightness over 20 µW. Despite a light extraction efficiency of only 15%, these devices exceed the output power of any previous electrically driven submicrometer LED or laser pixel by 100 to 1000 times while showing comparable external quantum efficiencies. Modeling suggests that spreading of the electron-hole recombination region in fin LEDs at high injection levels suppresses the nonradiative Auger recombination processes. Further refinement of this design is expected to enable a new generation of high-brightness LED and laser pixels for macro- and microscale applications.

Invited Article: Advances in tunable laser-based radiometric calibration applications at the National Institute of Standards and Technology, USA.

Recent developments at the National Institute of Standards and Technology's facility for Spectral Irradiance and Radiance responsivity Calibrations using Uniform Sources (SIRCUS) are presented. The facility is predicated on the use of broadly tunable narrow-band lasers as light sources in two key radiometric calibration applications. In the first application, the tunable lasers are used to calibrate the spectral power responsivities of primary standard detectors against an absolute cryogenic radiometer (ACR). The second function is to calibrate the absolute radiance and irradiance responsivities of detectors with uniform light sources, typically generated by coupling the laser light into integrating spheres. The radiant flux from the uniform sources is determined by the ACR-calibrated primary standard detectors. Together these sources and detectors are used to transfer radiometric scales to a variety of optical instruments with low uncertainties. We describe methods for obtaining the stable, uniform light sources required for low uncertainty measurements along with advances in laser sources that facilitate tuning over broader wavelength ranges. Example applications include the development of a detector-based thermodynamic temperature scale, the calibration and characterization of spectrographs, and the use of a traveling version of SIRCUS (T-SIRCUS) to calibrate large aperture Earth observing instruments and astronomical telescopes.

Simple spectral stray light correction method for array spectroradiometers.

A simple, practical method has been developed to correct a spectroradiometer's response for measurement errors arising from the instrument's spectral stray light. By characterizing the instrument's response to a set of monochromatic laser sources that cover the instrument's spectral range, one obtains a spectral stray light signal distribution matrix that quantifies the magnitude of the spectral stray light signal within the instrument. By use of these data, a spectral stray light correction matrix is derived and the instrument's response can be corrected with a simple matrix multiplication. The method has been implemented and validated with a commercial CCD-array spectrograph. Spectral stray light errors after the correction was applied were reduced by 1-2 orders of magnitude to a level of approximately 10(-5) for a broadband source measurement, equivalent to less than one count of the 15-bit-resolution instrument. This method is fast enough to be integrated into an instrument's software to perform real-time corrections with minimal effect on acquisition speed. Using instruments that have been corrected for spectral stray light, we expect significant reductions in overall measurement uncertainties in many applications in which spectrometers are commonly used, including radiometry, colorimetry, photometry, and biotechnology.