The United States Naval Research Laboratory (NRL) faced a challenge. It wanted to conduct photoemission spectroscopy in the extreme low UV range using a tunable light source. It’s a difficult application. No off-the-shelf instruments existed to achieve its goals.
The lab’s end game was to study high band gap materials in semiconductors. In an organization that ultimately develops warships, the specific reasons for the research remain a mystery. But one can deduce the aim was to increase the efficiency of semiconductor materials.
Photoemission spectroscopy (PES) is one of the most extensively and efficiently used methods to explore the electronic structure of atoms, molecules and solids. If a molecule is excited by a high-energy photon in the ultraviolet region of the spectrum that has sufficient energy to ionize the molecule, the excited species will eject electrons. PES is the analysis of the kinetic energies of the ejected electrons, according to John Holmes in the Encyclopedia of Spectroscopy.
The NRL identified a laser light source that would get it to these low frequencies of 177 nm to 2100 nm. The lab approached HORIBA near the end of 2018. It was looking for a spectrometer it could adapt to the light source and its sample chamber. The NRL planned to build its own system.
HORIBA is a high-end supplier of spectrometers, so it was a logical step for the NRL. But HORIBA offered something more. As a systems integrator, HORIBA builds custom systems to meet researchers’ unique needs.
“They were looking for a company that can supply the spectrometer, as well as the coupling optics that would be compatible in this UV range,” Francis Ndi, Ph.D. and Product Line Manager for HORIBA’s Optical Spectroscopy Division said. “But when they contacted us, they found out that we've done a similar light source before.”
The light source was a demanding component to integrate. It’s a laser-driven xenon lamp with special technology that enables it to go down to low wavelengths. UV light sources are finicky and fragile in this range. These need special cooling and other physical requirements.
But this particular light source is not fragile, Ndi said. That is the novelty of light source itself.
“Not only are we able to provide the light source, we were able to provide all the relevant UV coupling uptakes, both input and output,” he said.
As a system integrator, HORIBA could both design custom components and assemble them.
“We were able to leverage some components that we made here in the US,” Ndi said. “We leveraged some technologies in the VUV that are developed in our French office, HORIBA France SAS, to put together a unique tunable light source solution.”
HORIBA’s French office played a key role. It is HORIBA’s Center of Excellence for vacuum UV (VUV) components. These components are challenging. That's because there are few UV light sources. And most materials absorb the wavelengths.
The system design had complicated needs. HORIBA had to design optics to optimize reflection or transmission of the UV light. For a reflective system, it needed special coatings optimized for this wavelength. That includes the input optics, the coupling optics, the output optics and the actual spectrometer optics, the gratings and the mirrors in the spectrometer.
HORIBA used UV optimized MgF2 coatings - magnesium fluoride - MgF2 - provides minimum reflectance with only a single layer film. MgF2 becomes a hard durable coating when applied at a high temperature.
The result was a tunable user/software controlled tunable UV light source.
HORIBA used its popular IHR 320 Spectrometer as the heart of the customized system.
The NRL first approached HORIBA in the fourth quarter of 2018. Within a month, HORIBA had generated a system design and created a CAD model of the system. That gave NRL scientists a visualization of the instrument before HORIBA built the system. The deal closed in 30 days, and the instrument shipped early in 2019.
HORIBA had to address some unknown parameters along the way. The system had to integrate a third-party light source. The light source sent to HORIBA deviated from the drawing from the lamp vendor. HORIBA had to adapt its design to accommodate a different light source orientation.
“Customization and system integration is a very dynamic process,” Ndi said.
The system had to take into account the lab’s existing sample chamber. It also had to fit within a designated space.
“We have a history of doing these kinds of thing because we have developed similar light sources in the past,” Ndi said. “That's what gave them the customer confidence now to say, ‘Why should I try to do this as a first-timer, versus having HORIBA do it for me with the level of experience that they have doing these things.’”
The instrument aims to separate specific wavelengths and pass it along to its specialized application. That process of separation through a monochromator is challenging because, in the UV, almost everything is absorbed. To have a spectrometer do this, you have to capture as much light as feasible. It needs special optics to collect the maximum possible light from the light source.
The optics in the spectrometer need specialty coatings. These could not absorb or lose light as the spectrometer separates different wavelengths. It needs the same kind of optics when the light exits. When it does, the same optimized optics are necessary to send most of this light to the sample.
“That’s what we are bringing to the table,” Ndi said. “We didn't make the light source, but we can bring up all the connective tissue to take light from the source it and send it to the sample.”
HORIBA packaged a spectrometer, a specialized light source, and all the components that interface with the light source. That included the software and computer to operate the instrument. The collaboration went from a component sale to a full-blown system.
“We have to show the customer that not only can we put this together, but we can answer all the uncertainties as to how this will fit into whatever they are trying to do,” he said.
The United States Naval Research Laboratory provides advanced scientific capabilities to bolster the country’s global naval leadership. It is the corporate research laboratory for the United States Navy and the United States Marine Corps. It handles basic scientific research, applied research, technological development, and prototyping.
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