Figure 2

Nanorods for nano-antenna dual focus fluorescence correlation spectroscopy. (a) A sketch of the nanorod based geometry for nano-antenna 2fFCS. Two 40 × 70 nm nanorods (30 nm height) with 120 nm center to center distance form a T-shape. When placed on glass and in water, their near field response is resonant at 676 nm. (b,c) The near-field intensity in a plane 30 nm above the rods, upon plane wave illumination at 690 nm (targeted emission wavelength) for polarization along the horizontal (b) and vertical (c) rod. Through reciprocity, this intensity is proportional to the collection efficiency function. The scale bar is 200 nm. (d,e) The molecular detection efficiency reconstructed from simulations for a λ = 676 nm pump wavelength (w ₀ ≈ 300 nm Gaussian beam width) and λ = 690 nm emission wavelength (i.e., panels b,c). As a global scaling factor does not affect the correlation curves, both MDFs are normalized to the maximum in panel d). (f) Predicted autocorrelation functions for the intensity on the x- and y-polarized detector (blue and red curves), predicted cross-correlation between polarization channels (yellow and dashed purple), and autocorrelation of the polarization contrast (green), constructed as (ACF _x  + ACF _y  − 2CCF)/2. Circles indicate the time at which the correlation has dropped to 50% of its maximum value at τ = 0.