Flattening the curve

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Left: Birefringence image of a flat lens and intensity patterns of 488nm laser beam with different handedness circular polarisations focused and defocused by the same lens. Right: The same lens corrects short -5 D and long +5 D sightedness.

Researchers have developed a new technique for producing flat optics with ultra-low loss and high damage threshold

Optical elements can be found in many of today’s systems, from smartphones in our pockets to lidar systems guiding vehicles autonomously on the roads. The continued trend towards technology miniturisation poses a challenge to the field of optics, however, as classical lenses and mirrors tend to be bulky, as highlighted by Engelberg and Levy in a recent Nature Communications article1.

One way of dealing with this challenge is the use of flat optics, and over the past decade promising new optical technologies known as geometric phase optics, based on metasurfaces and liquid crystals, have emerged that can be used to fabricate them. Rather than using differences in optical path – controlled by adjusting lens thickness, curvature or refractive index – to manipulate light, as is the case with classical optics, metasurfaces and liquid crystals instead manipulate light via sub-wavelength features induced in their optical substrate or anisotropy of molecular structure. This enables them to be fabricated without curvature and at dimensions much smaller than classical optics, which will be crucial if optical systems – and the technologies containing them – are to be miniaturised further.

Flat optics with metasurfaces and liquid crystals have already been fabricated successfully using polymers and metal thin films, via processes including UV photoalignment and photolithography. However, delivering them with low loss, high damage threshold, increased durability, and compatibility with the infrared and UV wavelengths, has so far proven to be a challenge.

However, thanks to the work of UK researchers at the University of Southampton’s Optoelectronics Research Centre (ORC), metasurfaces exhibiting such characteristics can now be made using a new type of ultrafast laser-induced modification in silica glass.

Described in Light, Science & Applications2, the scientists explain how laser pulses with durations between 300-500fs and energies between 500-1,000nj can be used to generate birefringence in silica glass. A material’s birefringence describes how its refractive index can vary depending on the polarisation and direction of the light travelling through it. The property can be observed in certain crystals, and can be induced in plastics via mechanical stress.

Femtosecond laser writing has previously been used by the researchers to create birefringence in silica glass, by producing self-assembled nanograting structures. The technique is promising for the fabrication of metasurfaces with high thermal and chemical durability, as well as high optical damage threshold. However, a drawback is that the created nanograting structures cause significant amounts of light scattering, which leads to high optical loss, especially for UV wavelengths, thus limiting their application.

Instead, the ORC researchers have demonstrated a new type of femtosecond laser writing with which nanopore structures with elongated anisotropic shape can be created throughout silica glass, aligned perpendicularly to the polarisation of the femtosecond pulses. The nanopores are fundamentally different to nanogratings, as rather than existing as a periodic structure, they are instead randomly distributed throughout the glass. On experimenting with the structures, the researchers found that they not only dramatically reduce light scattering compared to nanogratings – thus enabling ultra-low levels of optical loss – but also deliver transmittance as high as 99 per cent in the visible and near-infrared ranges, and more than 90 per cent in the UV range down to 330nm.

Illustration of the patterning of birefringence in silica glass by femtosecond laser direct writing.

One of the researchers, Professor Peter Kazansky, explained to Electro Optics that while the team has discovered and experimented with this creation of randomly distributed oblate nanopores, they do not yet fully understand how the structures are created. Regardless, the result is a birefringence modification that can now be used to fabricate ultra-low loss, spatially variant optical elements, which due to being made of silica glass – rather than metal films or polymers – also have a high optical damage threshold. The researchers say that the new technique overcomes the limitations of geometric phase and polarisation shaping using conventional materials and fabrication methods, including photo-aligned liquid crystals and metasurfaces.

Optical elements that can be produced using the new technique include geometric phase flat prisms and lenses, gratings, zero-order retarders and vector beam converters, which can be used for high-power lasers. While the ORC team has been able to create such elements for some time by modifying birefringence using nanogratings, it is only recently that they have been able to create them with ultra-low loss. Now, however, as the loss is comparable to that of conventional optics, the group can start to think about commercialising the technique.

Having patented the new technique, the group has been able to license it out to Lithuanian optics firm Altechna, which is now using it to produce vector beam convertors. Vector beam convertors can be used to produce light beams with space-variant polarisation. This includes radial vector beams, which can be sharply focussed for effective application in laser machining, as well as azimuthal vector beams, which have applications in spectroscopy and microscopy.

Flat birefringent optics imprinted with high-loss (left) and low-loss (right) modifications in silica glass plates.

The new technique is not perfect, however – as, according to Kazansky, flat lenses with numerical aperture as high as those created using metal films and photolithography are not yet achievable using it. He explained however that the main ‘selling point’ of the new technique is that it enables metasurfaces with high transmittance in the infrared and UV – which has previously been a challenge for liquid crystals and metal film metasurfaces – as well as the high optical damage threshold. Because of this high threshold, such optics show promise for use with beams of up to kilowatts in power, Kazansky continued, however this is yet to be tested by the ORC team. He added that another advantage of these metasurfaces is that they can be used to perform beam steering without the use of mechanical parts, as instead the beam can be manipulated by altering the polarisation of the incident beam. This capability will also be crucial for the miniaturisation of optical systems.

The challenge with using ultrafast lasers to produce these metasurfaces is that the technology is relatively slow at materials processing compared to more conventional laser technologies. However, in recent years, ultrafast lasers of higher repetition rate have emerged on the market. According to Kazansky show promise for increasing the throughput of materials processing, making the technology more plausible for use in mass-production processes. This could enable a reduction in the cost of fabricating the new metasurfaces when manufacturing them on an industrial scale, an important factor for ensuring their widespread adoption in modern technologies.

The laser used by the ORC team to modify birefringence was a Pharos Yb-doped potassium gadolinium tungstate (Yb:KGW)-based mode-locked regenerative amplified femtosecond laser system from Lithanian firm Light Conversion. The laser operates at a wavelength of 1,030nm with a variable repetition rate from 1kHz to 1MHz and a pulse duration from 190fs to 10ps. The laser beam was focused via a 0.16NA aspheric lens into silica glass plates mounted on a computer-controlled three-axial air-bearing translation stage. The diameter of the laser beam at the focus was approximately 5µm. For the writing of birefringent patterns, parallel birefringent lines were written by raster scanning with a line separation of 1µm by translating a silica glass plate perpendicular to the incident laser during laser irradiation at a repetition rate of 200kHz.

In summary, the researchers have developed a new technique for ultrafast laser-induced modification in silica glass that produces anisotropic nanopores. The modification enables the fabrication of ultralow-loss birefringent optical elements with high transmission from the UV to infrared, as well as high optical damage threshold and high thermal and chemical durability. Kazansky concluded by saying that the technique could be used to manufacture any type of optical element, and that in addition to laser-based applications, these could be used in any photonics applications featuring optics.


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[1] Nature Communications: The advantages of metalenses over diffractive lenses - Engelberg & Levy
[2] Light, Science & Applications: Ultralow-loss geometric phase and polarization shaping by ultrafast laser writing in silica glass - Sakakura, Lei, Wang, Yu & Kazansky