Local field enhancement and thermoplasmonics in multimodal aluminum structures

Aluminum nanostructures have recently been at the focus of numerous studies due to their properties including oxidation stability and surface plasmon resonances covering the ultraviolet and visible spectral windows. In this article, we reveal a facet of this metal relevant for both plasmonic purposes and photothermal conversion. The field distribution of high-order plasmonic resonances existing in two-dimensional Al structures is studied by nonlinear photoluminescence microscopy in a spectral region where electronic interband transitions occur. The polarization sensitivity of the field intensity maps shows that the electric field concentration can be addressed and controlled on demand. We use a numerical tool based on the Green dyadic method to analyze our results and to simulate the absorbed energy that is locally converted into heat. The polarization-dependent temperature increase of the Al structures is experimentally quantitatively measured, and is in an excellent agreement with theoretical predictions. Our work highlights Al as a promising candidate for designing thermal nanosources integrated in coplanar geometries for thermally assisted nanomanipulation or biophysical applications.

Figure 1

(a) SEM image of an aluminum square. Scale bar represents 250 nm. (b) SP density of states computed for the same metallic pad at $\lambda =750$ nm. (c) Sketch of the nonlinear photoluminescence microscope. (d) Nonlinear emission spectrum acquired from the aluminum structure shown in (a) under pulsed excitation at $\lambda =750$ nm.

Figure 2

(a), (b) Dark-field spectra acquired on an aluminum square structure with (a) 400 nm and (b) 2 µm side length for an excitation linearly polarized at 0 and ${45}^{\circ }$. (c), (d) Same as (a) and (b) for hexagonal pads for polarization at 0 and ${90}^{\circ }$, respectively. SEM and DF images (with scale bars) are systematically shown in insets. The experimental excitation wavelength at 750 nm used for the nonlinear photoluminescence measurements is indicated by the gray arrows.

Figure 3

(a) SEM image of a hexagonal structure with 600 nm long sides. (b)–(e) nPL maps acquired with incident polarization oriented at (b) $0^{\circ }$, (c) ${30}^{\circ }$, (d) ${60}^{\circ }$, and (e) ${90}^{\circ }$. Polarization is indicated by the white arrows. (f)–(i) Corresponding simulated nPL maps. (j) SEM image of a hexagonal structure with 900 nm long sides. (k)–(n) nPL maps acquired with polarizations similar to (b)–(e). (o)–(r) Corresponding simulated nPL maps. The scale bars are 300 nm.

Figure 4

(a) Partial SP-LDOS for polarization along $X$ and (b) total heat deposited in an 800 nm Al square on glass substrate, placed in water, as a function of the wavelength and the position of the focal spot. $X=0$ corresponds to the center of the aluminum pad and its borders are indicated by white dashed lines. (c) shows the imaginary part of the dielectric function of aluminum. The angle of the incident linear polarization (${\mathbf{E}}_{\text{in}}$ along $X$) and the positions where the data are calculated (green line, 100 nm above the lower edge of the square) are depicted in the inset in (c).

Figure 5

Reconstructed maps of the total heat deposited in 800 nm (a) Al and (b) Au squares at $\lambda =750$ nm. Each map is the sum of two simulated maps with orthogonal polarizations (sum symbol). Simulated (c) crosscuts taken along the blue solid line shown in (b) for the aluminum structure in (a) (red curve) and the gold structure in (b) (black curve).

Figure 6

(a) SEM image of an 800 nm aluminum square structure. Scale bar is 300 nm. (b), (c) nPL images recorded on this structure for excitation polarization at (b) ${90}^{\circ }$ and (c) ${45}^{\circ }$. (d), (e) Corresponding simulated nPL maps. (f), (g) Simulated maps of the temperature variation ($\mathrm{\Delta }T$) 150 nm above the metallic square for the two different polarizations. (h), (i) are the experimentally reconstructed $\mathrm{\Delta }T$ maps deduced from the measurement of the resistance of a Au nanowire located at the vicinity of the Al square (not shown) followed by a numerically assisted calibration step. Polarization orientations are indicated by the white arrows.