Nebular™ Technology: Nano-Structured Anti-Reflective Surfaces

Nebular™ Technology is an alternative to traditional thin-film anti-reflection (AR) coatings for high-power laser applications developed by Edmund Optics®. Subwavelength surface structures are etched into the optic, offering numerous advantages over traditional coatings including high broadband transmission and near-bulk laser-induced damage threshold (LIDT) (Figure 1). Laser system integrators can maximize system throughput and reduce the likelihood of laser-induced damage by incorporating optical components featuring Nebular™ Technology into their systems.

Figure 1: Nano-structured anti-reflective (AR) surfaces are a valuable alternative to thin film AR coatings for high-power laser applications.

LIDT of Nebular™ Technology

Applying coatings to a laser optic also typically decreases its LIDT because of the potential for coating defects across multiple layer interfaces. Nebular Technology™, on the other hand, does not involve depositing any additional materials onto the optic. The LIDT of nano-structured AR surfaces can approach that of the uncoated substrate, which is significantly higher than most traditional coatings. This is advantageous for high-power laser applications of all pulse durations, from continuous wave (CW) to femtosecond pulses (Figure 2).

Figure 2: Sample LIDT report from Lidaris of a window with Nebular™ Technology showing a LIDT >40 J/cm² @ 532nm, 5.6ns, 100Hz, which is significantly higher than most thin film coatings.

When thin-film coatings experience laser-induced damage, the damage can propagate and cause system failure. When Nebular™ Technology experiences laser-induced damage it does not propagate and effectively “melts” the nanostructures back into the bulk material (Figure 3). This has a minimal effect on system throughput and leads to systems with high laser durability.¹

Optic with Thin-Film Coating	Optic with Nebular™ Technology

Spectral Properties of Nebular™ Technology

The broad waveband of Nebular™ Technology compared to typical thin-film laser AR V-coatings is shown in Figure 4.

Figure 4: When properly-designed, the nano-structured surfaces of Nebular™ Technology minimize reflectivity and scatter, resulting in maximum throughput over a wider range of wavelengths than typical thin-film anti-reflective V-coats.

Additionally, Nebular™ Technology is typically less sensitive to angle of incidence and polarization than standard thin-film AR coatings. For a standard AR coating, a wavelength shift can be seen as the angle of incidence either increases or decreases from the design angle, typically 0 or 45 degrees (Figure 5). This can limit an optical system where scanning of the laser beam is required, such as changing the incidence angle through a window through the scan and thus the transmitted intensity. It has been shown that nano-structured AR surfaces can produce equal transmission for both polarized and nonpolarized light over an angular range of at least 0 +/- 30 degrees.²

Figure 5: When properly-designed, the nano-structured surfaces of Nebular™ Technology minimize reflectivity.

Formation of Sub-Wavelength Surface Structures

The nano-structured AR surfaces are formed through reactive ion etching (RIE), where an inductively-coupled plasma accelerates ions towards the substrate and an applied metal mask defines the shape and spacing of the structures (Figure 6). Each parameter in the manufacturing process is finely controlled; the surfaces can be modeled using rigorous coupled wave analysis, resulting in highly-repeatable and predictable surface specifications and spectral performance.

Figure 6: During reactive ion etching, subwavelength surface structures are etched into an optical substrate by ions accelerated towards the surface by an inductively-coupled plasma.

While the sub-wavelength surface structures of Nebular™ Technology have high laser durability, they have low mechanical durability. Abrasion can easily damage the structures, making them difficult to clean and requiring special handing and assembly techniques. However, surface contaminants and abrasion will cause premature damage in any optical coating for a laser system, and a well-designed system should include provisions to maintain a protected, clean beam line. Nebular™ Technology is ideal for intra-cavity optics and other components for high-power laser systems.

More information about standard optics with Nebular™ Technology available and custom nanostructured components from Edmund Optics can be found below:

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References

1.  L.E. Busse et al. (2014). Anti-reflective surface structures for spinel ceramics and fused silica windows, lenses and optical fibers. Opt Mater Express, Vol. 4, Issue 12, pp. 2504-2515
2. C. D. Taylor, L. E. Busse, J. Frantz, J. S. Sanghera, I. D. Aggarwal, and M. K. Poutous, "Angle-of-incidence performance of random anti-reflection structures on curved surfaces," Appl. Opt. 55, 2203-2213 (2016)

More Resources

• Nano-Structured Anti-Reflective Surfaces Webinar Recording
• Bulk Laser Damage in Glass
• Understanding and Specifying LIDT of Laser Components
• Laser Damage Threshold Testing
• Different Types of LIDT Specifications
• Uncertainty in LIDT Specifications
• Importance of Beam Diameter on Laser Damage Threshold
• Laser Optics Lab Video Series
• Nano-Structured Anti-Reflective Surfaces for Materials Processing" by Becca Silver and Cory Boone - Novus Light Technologies