# Characterization of Chirality in Diffractive Metasurfaces by Photothermal Deflection Technique
## 1 Introduction  
A chiral object cannot be superimposed onto its mirror image; and thus has a left and right version, like our hands. Even though the physical properties of the two versions are equal, their interactions with the outside world depend on the \"handedness\". For example, by looking at the components of many commercial weed killers, some of them can act as a herbicide when in the proper handedness; while the other handedness is inactive [1]. Chirality governs the biochemical reactions of many important molecules, defining their pharmacology and toxicity; more than 50% of pharmaceuticals and 40% of agrochemicals are chiral [2, 3]. The characterization of chirality involves an interaction of the chiral material with circularly polarized beams of the opposite handedness. The pioneering work was done by [PERSON], by studying the rotation of the polarization plane of linear polarized light travelling in chiral media, i.e., the optical rotatory power or optical activity (OA) [4]. This effect is due to the difference in the propagation velocity of two circularly polarized beams of opposite handedness, in which a linear polarized beam can be decomposed. Another aspect of interaction between chiral light and chiral substances is the differential absorption of the left and right circular polarized light (LCP from RCP, respectively), and this differential absorption is usually defined as circular dichroism (CD). Unfortunately, the CD of most molecules is extremely low and lies in the ultraviolet range; the absorption vanishes at small molecular concentrations [5, 6]. One way to enhance CD is to exploit self-assembly processes in supramolecular chiral arrangements, as demonstrated in [7, 8, 9, 10, 11]. Another possibility is the coupling of molecules with chiral nanostructured substrates, as shown in [12]. Over the past few years, nanotechnology developments have enabled the fabrication of specially engineered nanostructured materials, which can confine and enhance chiro-optical effects in the near- and far-field [13; 14; 15; 16]. In parallel, scientists have been developing new methods to characterize diro-optical effects at the nanoscale. Interestingly, unconventional measurements of chiral detection by means of photothermal phenomena are once again becoming topical. This is because of the definition of CD, which would optimally require direct measurements of absorption to occur during the diro-optical process. Such absorption, without the influence of scattering, was measured in nanostructures by means of a photo-acoustic technique [17; 18; 19; 20; 21]. On the other hand, the combination of a thermal lens and circular dichroism has led to very sensitive chiral analyses on microchips [22], which has further led to remarkable works on the CD microscopy of a single nanostructure [23; 24]. Chiral photothermal effects in planar metamaterials have been numerically proposed for chiral bolometers [25], while photothermal chirality in Au helical nanoparticles has been measured by means of luminescence ratio thermometry [26]. Here, we show that the photothermal deflection technique (PDT) can be applied to study chiral photothermal effects in the diffraction range of metasurfaces, and that PDT results can be used to reconstruct thermal profiles, which depend on the excitation handedness. PDT is usually used for scattering-free and non-destructive characterization of different solid materials [27; 28; 29; 30]. It was not until recently that scientist realized the new possibilities of using PDT to study thermal effects at the nanoscale, e.g., for plasmon-enhanced solar cells [31], photon quantum yield in quantum dots [32], and phosphor with high photoluminescence efficiency [33]. We recently proposed a PDT setup as a cheaper and simpler alternative to diffraction spectroscopy: we measured chirality-dependent diffraction orders in asymmetric metasurfaces [34]. In the present work, we go deeper into this new methodology with respect to the work in [34].