Precise detection of circular dichroism in a cluster of nano-helices by photoacoustic measurements

Abstract

Compact samples of nano-helices built by means of a focused ion beam technology with large bandwidth and high dichroism for circular polarization are promising for the construction of built-in-chip sensors, where the ideal transducer must be sufficiently confined without compromising its filtering ability. Direct all-optical measurements revealed the sample's dichroic character with insufficient details because of scattering and diffraction interference. On the other hand, photoacoustic measurements resulted to be a possible alternative investigation, since they directly deal with absorbed power and allow to get clear evidences of the differential selection for the two opposite polarization states. Multilevel numerical simulations confirmed the experimental results, proving once again the reliability of photoacoustic technique and the versatility of this class of dichroic artificial materials.In the last years, the ultimate nanofabrication frontier represented by 3D nanostructures is generating promising and versatile novel nano-photonics devices. In particular, 3D nanostructures with broken symmetry enable exciting complex interactions with chiral light, generating the typical forms of the optical activity, such as the optical rotation (OR), i.e., the polarization plane rotation of the incident light, and the circular dichroism (CD) 1, 2 , i.e., the different absorption levels for left and right circularly polarized (CP) waves 3 . Among the possible 3D chiral geometries, the helix architecture represents, because of its intrinsic chirality, an effective choice to manifest detectable chiral effects [4][5][6][7][8] . However, the engineering of the chiro-optical properties, such as the frequency as well as the absorbed and scattered portions of left-and right-handed circularly polarized light, requires the full control on the geometrical and spatial parameters and on material composition of the 3D nanostructures. Hence, great efforts have been made to develop flexible nanofabrication techniques for the realization of helical based structures with nanometer accuracy 9 . Recently, focused ion and electron beam induced deposition (FIBID/FEBID) have demonstrated the effective capability to tailor helical nanostructures as a function of application-driven chiro-optical properties 10-14 , leading to the nanometer scale controlled fabrication of helix-shaped metal and dielectric nanostructures, organized in dense and ordered arrays of few micron area. Moreover, the FIBID/FEBID approach is particularly suitable for the building and the spatial localization of compact nano-devices as parts of embedded systems, such as built-in-chip sensors and integrated optoelectronic filters, where it is of key importance the integration of nanostructured materials onto small areas.When dealing with 3D nanostructured samples with limited patterned area, commonly used all optical (AO) measurements, though requiring low power levels of inspecting light, are heavily altered by the electromagnetic (e.m.) field escaping from the un-patterned...

“Hot” in Plasmonics: Temperature‐Related Concepts and Applications of Metal Nanostructures

“…These measurements usually serve to derive the intensity of absorbed light. Recently, Belardini et al employed photoacoustic techniques to directly measure the intensity of absorbed light in gold nanowires, chiral platinum‐gallium‐carbon nanohelices, in self‐assembled GaAs nanowires and in chiral metal‐polystyrene metasurfaces . Upon absorption in the samples, light causes local heating, which is transferred to the small volume of air surrounding the sample, creating a pressure wave.…”

Circular Dichroism in the Second Harmonic Field Evidenced by Asymmetric Au Coated GaAs Nanowires

“…By breaking the symmetry of the nanostructure-light interaction, it is possible to observe a circular dichroism (CD) due to the so-called extrinsic chirality or pseudo chirality [6][7][8]. Chirality is the lack of mirror symmetry [9], and can be probed using photoacoustic techniques that are sensitive to the differential absorptions of opposite-handed light [10][11][12] or by techniques sensitive to symmetry breaking such as second harmonic generation-circular dichroism (SHG-CD) [13,14]. In the case of extrinsic chirality, the high sensitivity of SHG is related to the fact that SHG can only occur in systems with broken inversion symmetry, enabling background-free measurements and leading to higher CD responses with respect other measurement systems [15].…”

Optimization of a perfect absorber multilayer structure by genetic algorithms

“…This can be achieved with structured metallic surfaces [14], microcavities [15], subwavelength hole arrays and opals [16,17]. Alternatively structured phase change materials [18][19][20][21][22][23] and chiral metamaterials [24][25][26][27] have been recently used so to obtain an active switching of the absorption properties thanks to the metal-insulator transition in one case, or to intrinsic/extrinsic dichroism in the other case. However, the realisation of broadband absorber metamaterials requires long and expensive procedures with multiple steps of film deposition, photoresist coating, etching and photoresist removing [28].…”