# Characterization of Chirality in Diffractive Metasurfaces by Photothermal Deflection Technique
## 3 Results  
### Experimental Results Using PDT  
The photothermal deflection technique has been applied by changing different parameters of the Ar pump beam: laser wavelengths (458, 496, or 514 nm); circular polarization orientations (LCP or RCP); angle of incidence (0 or 20 deg); and chopper modulation frequency (16 or 36 Hz). The typical photothermal deflection amplitude response, where the probe beam scans along the metasurface, is shown in Figure 3. The PDT scan contains a number of \"M\" shape signals, allowing the detection of heat sources found along the probe beam path (see Figure 4a). Each \"M\" shape corresponded to the local thermal gradients produced by the incident pump beam (i.e., the largest signal at \(d=0\)), and by several diffracted orders, which could be localized and identified with the integer \(m=1\), \(2\),..., \(6\), according to the anticlockwise numeration introduced in Figures 4 and 5, as described in the next section. Each \"M\" shape is due to the lateral deflection of the probe beam being proportional to the lateral thermal gradient; it is zero exactly above the center of the heat source, where a local maximum of the temperature rise occurs. On the contrary, it reaches its maximum amplitude at a distance of approximately one Gaussian beam width, from both sides of the beam center (see Figure 4b) [34]. Figure 3a,c shows the PDT amplitude signals for normal incidence excitation at 458 nm and 514 nm, respectively. In both cases, all six existing diffracted orders were clearly visible from scanning the surface, from \(-2.5\) mm to \(+2.5\) mm (with respect to the incident pump beam position at \(d=0\)). The position of the center of each diffracted order can be calculated from the zeroes of the lateral deflection reported in Table 1. Moreover, the chiral response  
Figure 2: Schematic of numerical simulations: (**a**) 3D geometry of the sample; (**b**) top view of the metasurface; (**c**) schematic of the two different processes during the numerical modeling. of the structure was tested, sending left or right circular polarized incident pump beams. In Figure 3, the LCP and RCP responses are in blue and red, respectively. By comparing Figure 3a,c, one may note that:  
(i) With a wavelength increase, the diffraction pattern enlarges due to the dispersion law of the glass refractive index, which spreads out the diffracted orders according to wavelength (see Figure 4c). This can be appreciated from the numerical simulations of the diffraction behavior of the different wavelengths, when the sample was excited at a normal incidence with an RCP beam (see Video S1 in the Supplementary Material). (ii) The chiral response and the circular dichroism (i.e., difference between red and blue curves) was negligible at 458 nm, though it became relevant for diffracted orders at 514 nm, as is shown in the next session in Figure 6. Numerical simulations of the diffraction for different angles of the quarter wave plate are shown in Videos S2 and S3 in the Supplementary Materials. Figure 3b,d shows the PDT amplitude signals at 458 nm and 514 nm, for excitation under an oblique incidence at 20 degrees with respect to the norm. The following considerations can be made in this case:  
(iii) The main heat source corresponding to the incident beam is always at \(d=0\). (iv) The secondary heat source, which is always visible at \(d=-0.47\) mm, is a specularly reflected (SR) beam, corresponding to the zeroth order of the pump beam; this beam also gets transmitted to the glass, reflects back from the mirror, and eventually gets absorbed at the surface. The distance of 0.47 mm between SR peak and the main peak is compatible with the pump inclination of 20 degrees (in air), which becomes \(\theta=13.2\) degrees in glass, producing a secondary heat spot at \(d=2L\text{tan}(\theta)=0.47\) mm, where \(L=1\) mm is the glass thickness (see Figure 4c). (v) Only two diffracted orders are clearly visible on the right side with respect to the pump beam.