Self-organised materials offer route to scalable optical filters

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Optomel's Damien Gardiner, a semi-finalist for SPIE's Startup Challenge at Photonics West, explains how the firm is making optical filters based on self-organised materials

Optical filters are everywhere; almost any system detecting or creating light, such as digital cameras, LCD TVs and lighting, through to microscopy and point-of-care diagnostics, for example, use these crucial components. They are often taken for granted but are an essential factor in determining a system’s performance and cost.

But the traditional approaches to filtering, which have changed little in recent times, are likely to be not fit for the next generation of high-volume photonic systems. For high-end interference filters processing is complex, typically taking place in a vacuum. The recent trend is to bring more sophisticated systems out of specialist laboratories into high-volume, user-friendly devices, and this complex processing environment does not scale cost effectively to high volumes.

However, recent developments in self-organised materials – where the optical filtering effect happens spontaneously – have the potential to revolutionise traditional approaches, combining cost-effective processing and optical characteristics that can be fully customised to demanding, high-volume applications. These include advanced LED lighting applications, point-of-care applications, laser protection and augmented reality displays.

Current technology

Designers are faced with two broad choices when it comes to selecting a filter. Absorbing filters offer very low cost and large areas which are critical for many applications. The optical effect typically arises from an absorbing species, such as an organic dye or inorganic pigment, dispersed in a polymer binder. Extinction ratios can be high, since more material can be added. However, key optical properties, like the absorption position and bandwidth, cannot be readily controlled, limiting more sophisticated applications, such as fluorescence filtering.

Interference filters offer the most sophisticated optical properties; and can be customised to a high degree, with high extinction ratios (greater than OD3 at 99.9 per cent reflectivity is standard) in narrow bands with high out-of-band transmission (>90 per cent).

This performance comes at a price; the complex processing required restricts the size and throughput of such filters. Their fabrication uses a vacuum environment, allowing accurate deposition of high- and low-refractive index metal oxides on the 100nm-scale and with perhaps ten to hundreds of such layers making up the total stack. Capital equipment and running costs are high, which ultimately limits their application to high-end, low-volume applications.

A new approach

But what if we could combine the best of both approaches? Would this be a route to enabling the next generation of optical systems scalable to address new volume applications in healthcare, augmented reality, sensing and safety?

A recent development could unlock the potential of such a future: self-organised optical materials. Self-organisation is a property where, owing to the chemistry of certain materials, molecules aggregate into structures. An important class of these materials is liquid crystals, the simplest of which – nematic liquid crystals – are the building block of the LCD TV industry. These materials have orientational ordering, as the molecules point in a common direction. Chiral nematic materials are similar in that, on a local level, they possess this common direction but on a macroscopic scale, the direction rotates in space. This leads to a periodic refractive index variation, creating Bragg-like scattering for light of certain wavelengths and the optical filtering effect.

Cut-through section of liquid crystal film

These materials can be readily processed using simple roll-to-roll techniques and, crucially, avoiding vacuum processing. Volume costs are significantly less than traditional interference filters. A wide variety of polymer substrates can be used and tailored to a given application or for conformable processing, for example. Furthermore, individual filter sizes can be much larger, because the coating area is no longer limited by the size of the deposition chamber.

The filter characteristics are determined by the material formulation and processing characteristics. Control of the material birefringence leads to a broader bandwidth; controlling the periodicity and layer thickness positions the centre of the reflection band anywhere in the UV, visible and NIR to within nanometre accuracy. Multiple layers can be added to give more complex filter profiles or improve notch reflectivity. Current optical densities are around OD3 with further improvement ongoing.

 

Example reflection notch centre at 532nm

Applications

The new approach to optical filtering shows significant promise in existing and emerging applications. Specific examples where the combination of precise optical specificity, high out-of-band transmission (>80 per cent), and cost-effective processing, are shared by the likes of: laser eye and sensor protection in aerospace and defence, optical point-of-care diagnostics, precision LED lighting for health and horticultural applications and augmented reality display technology. Often, in these applications, high-performance aspects of interference filters are not required; for example in lighting, filter extinction and transmission levels of 1 per cent and 85 per cent, respectively, are adequate.

Optomel is commercialising breakthrough photonic technology for OEMs, integrators and end-users

Optical filters -the latest commercial products

 

ALLUXA - Featured product

Alluxa offers and manufactures high-performance optical thin films that are used in wide ranging applications including life sciences, research, semiconductor and lidar. All of Alluxa’s thin-film optical filters and mirrors are hard-coated using a proprietary plasma deposition process on equipment that was designed and built by our team. This allows us to repeatably produce the same high-performance optical thin films in all of our coating chambers.

Alluxa is an ISO 9001:2008 certified, ITAR registered, optical coating manufacturer located in Santa Rosa, California. Founded in 2007 by a team of thin-film deposition veterans, Alluxa’s core team brings together decades of expertise and diverse backgrounds in deposition, automation, metrology, and optics.

www.alluxa.com

ADMESY - Featured product

Admesy offers a broad range of test and measurement instruments focused on colour and light measurements in inline production process environments. Our compact and robust devices are designed for accurate high-speed measurements with a low maintenance need. 

The Rhea is Admesy’s high-end spectrometer with a cooled CCD detector and neutral density filter wheel, combining great sensitivity and dynamic range.

It offers a selection of (fixed) grating and slit choices to fit your specific wavelength and FHWM requirements. Combined with an Admesy light source, it is the perfect solution for transmissive filter measurements.

The Hera is a compact cost effective spectroradiometer based on a transmissive grating design. It is available in three fixed wavelength ranges.

The Cronus combines a transmissive-grating based spectrophotometer with a tri-stimulus colorimeter. This unique combination offers both accurate spectral measurements and fast colorimeter measurements in one device.

All devices are available in various optical configurations, varying from a fixed lens or cosine corrector, to fibre attached accessories.

www.admesy.com/products-1 

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Companies that have launched optical filters recently include Acton Optics & Coatings, which has introduced a series of UV-NIR neutral density filters that can deliver broadband performance down to 190nm. The new ND filters, which are ideal for use with broadband sources like xenon, deuterium and tungsten halogen, have been designed to optimise the utility of precision optical systems, spectrometers and medical systems. 

The filters offer constant transmission from 190 to 1,700nm and an optical transmitted quality of 1/10 wave. Various standard densities are available from OD0.3 to OD2.5 and the filters can be stacked to create additional or deeper densities. They can also be deposited on custom-sized substrates for OEM applications and, if required, can be designed for other optical density values. 

Infrared dual bandpass filters from Deposition Sciences (DSI) deliver a combination of optical performance and environmental stability for demanding military, security, and imaging applications, such as infrared search and track systems (IRST) and targeting. 

In particular, these filters are typically configured to pass a wide band of infrared wavelengths while blocking the regions associated with atmospheric water (5-8µm) and CO2 (4.2µm) absorption. This results in improved system range and noise characteristics, and makes detection relatively insensitive to variations in ambient environmental conditions, such as rain or fog. 

 DSI dual bandpass filters can be provided on a variety of IR transmitting substrates, including Si, Ge and InAs, and are available in diameters up to 100mm. DSI can also provide a variety of custom dual bandpass filter designs for other applications, optimising various optical performance characteristics, such as in-band transmission and out-of-band blocking, to meet specific technical requirements and budgets. 

In response to increasing demand for flat-top narrow bands for laser applications, Laser Components has added to its range of optical filters a new series of four-cavity Fabry-Perot narrowband filters, for demanding lidar, range-finding and free space optical communications applications. 

The filter coatings are fabricated from hard oxide materials, so as to minimise wavelength shift due to temperature and angle of incidence variations, and ensure high durability. Spectral stability remains over a temperature range of -60°C to +80°C. The range features multi-cavity designs with flat transmission peaks and a steep transition region to deep blocking of greater than OD4 from 1 to 2.5µm. Custom sizing is also available, along with narrow bandwidths of 1 to 5nm with customer defined centre wavelength values. 

Viavi Solutions has developed a suite of bandpass filters specially designed for lidar systems, demonstrating how optical filters are moving into new and demanding areas of the automotive manufacturing industry.

As advanced driver-assistance systems (ADAS) continue to evolve, manufacturers are faced with critical technical challenges associated with adapting optical filter technology that operate in lidar systems. Challenges include inoperability in high ambient light conditions, insufficient light capture and stringent durability standards. Overcoming these challenges, Viavi optical technology filters light so that the signal-to-noise ratio can be improved, to ensure the most accurate measurements in challenging light conditions. Additionally, the optical filters pass rigorous testing for heat soaking, temperature cycling, manufacturability and MIL-C hardness and adhesion tests.

Another company targeting lidar applications is Iridian Spectral Technologies. Regardless of the wavelength employed, Iridian offers customised optical filter solutions to provide more signals with less background, from design and manufacture of initial prototypes, through to high-volume production at its ISO9001:2015-certified Canadian operation. 

The company offers: high transmission at the laser wavelength (>90 per cent); narrow bandwidths (<1nm to 20+nm dependent on the system requirements); deep blocking (OD3-5 or better over detector range); no maintenance/calibration needed; highly stable, environmentally robust and reliable low sensitivity to angles of incidence and temperature variation; and low cost, high volume manufacturing capabilities.