Optical spectral filters based on plasmonic metamaterials

Abstract

Laser light can be a hazard for eyes and sensors and there is a need to protect sensitive devices from high intensity laser beams. At the same time, it is essential in many cases that a protective filter does not hinder the collection of low intensity signals. To this end, passive (linear) spectral filters and active filters with intensity dependent transmission, are of importance for numerous applications.While numerous approaches for spectral filtering exist, many suffer from strong angular dependence of the transmission properties, including notch filters and narrow-band pass filters, and require high intensities of light needed to trigger nonlinear optical response. In this work, we show that by incorporating various plasmonic metamaterials in the spectral filter design, i) the spectral filters with broad acceptance angle can be achieved and ii) Kerr-type nonlinear response can be triggered at lower light intensities providing new opportunities for realisation of active, intensity limiting filters.

Plasmonic metamaterials are artificial structures that display features that either in nature don’t occur or negligible. In order to achieve these features, a plasmonic material (usually, but not limited to, a noble metal such as gold) is arranged in a predetermined geometrical pattern, for example columns in a dielectric host. The dimensions of these plasmonic elements are smaller than the wavelength of the incoming light, which thus sees an average response by the structure. Such structures are referred to as metamaterials. One of the features than can be engineered is the control over the refractive index, which can be engineered to be very high, and this is a basis for achieving angular independent filters. The other is the enhancement of third-order nonlinearities such as the Kerr effect, which leads to a variety of self-induced effects in which the incoming light, due to its high intensity,modifies the environment response to the light itself. Since the optical properties of a plasmonic metamaterial depend on its geometrical configuration rather than just the optical properties of the materials on which they are based, the different types of spectral filters can be engineered by geometrical tailoring of metamaterial using the same material platform and also selecting the spectral response of the filter. This is of particular importance since different types of lasers have different wavelength, pulse duration and energy.

With these concepts in mind, we engineered various types of linear filters, in the visible range of the electromagnetic spectrum as well as in the infrared range.We have demonstrated angular independent spectral band-pass filters, band-stop filters using different metamaterial platforms based on nanorods, nanoparticles and nanotubes. In all the configurations, the interplay between resonant features of meta-atoms and Fabry-Perot modes of the slab play a crucial role in the determining performance. We have shown that for active intensity limiters, suspensions of nanotubes or especially spherical nanoparticles are particularly advantageous, providing high damage threshold for nanosecond pulses at visible (532 nm) and infrared (1064 nm) wavelengths. While the studied nanotube suspensions provide relatively low linear transmission, the metaparticle suspensions are found to have excellent performance parameters, showing a high low-intensity transmission (around 70%)and at the same time limiting the output peak fluence at less than 1 J/cm² over four orders range of magnitude of incoming fluence: from 10¹ to 10⁵ J/cm² for laser light at 532 nm wavelength and 10 ns pulse duration. These results provide practical route for using plasmonic metamaterials in spectral filtering and intensity limiters, important for many real life applications.

Date of Award	1 Sept 2020
Original language	English
Awarding Institution	King's College London
Supervisor	Anatoly Zayats (Supervisor) & Wayne Dickson (Supervisor)