Optical coatings make every photon count

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Gemma Church finds out how a new generation of optical coatings can provide superior reflectivity in the deep UV range and beyond

The wavelength of light imposes a limit on the smallest feature that can be imaged or resolved. 

With sensor technology evolving rapidly as mobile phones and other consumer products offer cameras with greater than 10 megapixels (MP) image size, researchers are taking advantage of this rapid growth in resolution and sensitivity and extending their optical systems to shorter wavelengths, shifting away from the visible spectrum and into the short-wavelength ultraviolet (UV) region. 

With the commercialisation of UV LEDs, UV security features on banknotes and credit cards, advancements in DUV Raman and UV fluorescence, new applications are continually being discovered.

As sensor technology has evolved, it has also become smaller, increasing the need for higher throughput to increase photon sensitivity and maximise signal-to-noise ratios for benchtop and hand-held instruments that are being developed today. As part of this drive to increase throughput, high reflectivity optical coatings in the UV and especially the DUV have become increasingly important in scientific instrumentation.

The DUV region occupies wavelengths between 280 and ~200nm. This is an established range in which many instruments have been operating for decades. However, now growing in popularity with scientists and researchers are even shorter wavelengths in a region below 200nm known as vacuum UV (VUV). This VUV region gets its name from the fact that wavelengths below 190nm are strongly absorbed by oxygen in the atmosphere, and thus cannot be transmitted. A vacuum or nitrogen purge environment is required for these systems to operate. Technically speaking, the VUV region goes down to X-ray level energies, but practically speaking it is commonly recognised from 200 to 100nm.

Journey into space

Selective molecular imaging is now being used, not just on Earth, but to identify novel compounds and specific mineral elements in remote locations, including our Sun as well as far-flung stars and planets. Chemical analysis is another emerging application area, where optical systems must operate in the DUV and VUV regions and in potentially hazardous environments.

However, these DUV and VUV optical systems are prone to suffering from low overall throughput, which is a significant problem for end-users. The sensitivity of the optical systems must be improved to unlock these applications and successfully image in the DUV and VUV regions.

This is where the right optical coating can help to mitigate these losses and maximise the sensitivity of the system. Reflectivity and low scatter are critical factors for the end-user to take into account.

There are a limited number of potential coating materials capable of transmitting VUV wavelengths. When working above 200nm, metal oxide compounds are a popular choice, with SiO2 and Al2O3 both transmitting at around 225nm. But, as we move to shorter wavelengths, absorption limits the transparency and laser damage-resistance of oxide coatings, and various fluorides with good transmittance are chosen. 

When we move down to the sub-200nm VUV range, then the choice and capabilities of optical coatings further reduces.

Meeting challenges

This was the challenge that Spectrum Scientific, Inc (SSI) tackled head-on when a customer needed a system to operate at the 120nm wavelength a few years ago. David Cook, General Manager at SSI, explains: “We originally started looking outside to other suppliers to see if we could get some vacuum coatings for our optics in this range. But we quickly realised there were very few people out there capable of providing adequate vacuum UV coatings. Although we’re not a coating company per se, we started to develop VUV reflective optical coatings because of this gap in the market.”

SSI now provides a range of vacuum UV optical coatings, which work between 120 and 200nm, providing superior reflectivity, according to Cook, who adds: “In terms of reflectivity, our coatings are around 10 per cent higher than the other coatings that we’re aware of on the market.”

These optical coatings are used on a wide range of components, which SSI has been manufacturing since 2004. These include high-volume flat, aspheric and freeform reflective optics, hollow retroreflectors and holographic diffraction grating.

The company primarily uses the industry-established optical replication process to manufacture these parts, which allows them to supply high-fidelity, high-specification precision optics at a lower cost compared to traditional volume manufacturing. Cook adds: “We offer a range of optical coatings to our customers and this replication process provides us with the capability to produce large volumes of complex surfaces, very precisely.”

SSI now has the capabilities to produce aluminium, gold and custom optical coatings with high reflectivity. This is a significant achievement, according to Cook, especially given the rise in demand for silicon-based sensors, which can suffer when you reach the 200nm threshold. As the wavelength continues to shorten, these issues are exacerbated and the responsivity in these lower UV wavelengths diminishes greatly..

“Imagine these coatings are used for the mirrors in a spectrometer, for example. Such systems may have three, four or six mirrors between the light source and the sensor.”

Taking into account that throughput = Rn (where R is mirror reflectivity and n is the number of mirrors in the system), this results in a significant loss of photons by the time you reach the sensor.

The advancing miniaturisation of many of today’s optical systems also requires a smaller light source with fewer photons to work with. This has an impact where “every photon counts”, according to Cook.

Sensitivity is a primary capability for any optical system but especially when working in the DUV and VUV range where end users need to be able to confidently see the signal out of the noise. 

Instruments using silicon-based sensors need a boost in the deeper UV ranges. At wavelengths lower than 200nm, the sensitivity of silicon-based sensors, including CMOS, CCD and photodiode arrays, is greatly reduced. 

Grant Decastro, technical sales engineer at SSI, says: “The ultimate goal is to achieve high sensitivity for instrumentation operating in the DUV and VUV range and when you have highly reflective mirrors, signal strength increases and this translates to increased sensitivity.”

In the company’s latest white paper, SSI reveals how its range of VUV coatings are providing high reflectivity and helping customers achieve high sensitivity. Now, the company believes a wider range of DUV and VUV-based applications will be improved thanks to the superior reflectivity provided by its optical coatings.

“The difference we’re offering is a higher reflectivity than has been previously available. We’re pushing the boundaries of existing technology and we’ve only just finalised these optical coatings in the last six to 12 months,” Cook concludes. “We’re hoping that customers  will come to us with new and existing applications where their optical coatings require improvement.” 

Find out how Spectrum Scientific’s optical coatings can provide superior reflectivity to boost the sensitivity of optical systems operating in the DUV and VUV spectral regions by downloading the latest white paper.

Gemma Church