Surface Plasmon-Coupled Ultraviolet Emission of 2,5-Diphenyl-1,3,4-oxadiazole

Abstract

We studied surface plasmon-coupled emission (SPCE) of 2,5-diphenyl-1,3,4-oxadiazole (PPD) using a 20 nm aluminum film deposited on a quartz substrate. The directional SPCE UV fluorescence occurs within a narrow angle at 57° from the normal to the coupling hemicylindrical prism. This radiation is almost completely p-polarized, consistent with its origin from surface plasmons. These surface plasmons are induced by excited PPD molecules. The coupling of excited fluorophore dipoles with the aluminum is highly efficient, exceeding 50%. Different fluorescence emission wavelengths are emitted at slightly different angles on the prism, providing intrinsic spectral resolution. SPCE fluorescence on thin aluminum films can be used with many UV absorbing and emitting fluorophores.

Figure 1

Comparison of SPR shifts for gold (left) and aluminum (right) due to the presence of a 15 nm thick dielectric layer with refractive index n_s = 1.5. (Top) Four-layer system used for shifts calculation. The dielectric constants were ε_Au⁶⁵⁰ = −11.55 + 1.08i and ε_Al³⁰⁰ = −11.03 + 1.67i.

Figure 2

Calculated (solid line) and measured (open circles) reflectances of the 295 nm line from a frequency-doubled rhodamine 6 G dye laser for a quartz slide coated with 20 nm of aluminum (d_m) and 10 nm of SiO₂ (d_s). The dielectric constant for aluminum at 295 nm was ε_Al²⁹⁵ = −10.67 + 1.60i.

Figure 3

Schematic of sample-coupling prism configuration for SPCE fluorescence of PPD. The sample can be excited directly (RK configuration) or through the prism at θ_I angle (KR configuration). SPCE radiation appears as a hollow cone of p-polarized light.

Figure 4

(Top) Angular distribution of PPD SPCE. (Bottom) Calculated (solid line) and measured (open circles) reflectances at the 350 nm line from pyridine 1 dye laser. Calculations were done for combined thickness of the SiO₂ protective layer (10 nm) and PPD-doped PMMA layer (19.5 nm) with refractive index n_s = 1.5. The dielectric constant for aluminum was ε_Al³⁵⁰ = −15.24 + 2.55i.

Figure 5

(Top) Fluorescence spectrum of PPD spin-coated on aluminized quartz slide observed in free space with reverse Kretschmann configuration (RK/FS). (Bottom) SPCE spectrum observed through the coupling prism (RK/SPCE). The angles are determined in Figure 4, top. The dashed lines are signals recorded for reference slide without PPD.

Figure 6

Observation angle dependent SPCE spectra of PPD spin coated from PMMA solution on aluminized quartz slide. The sample was excited directly (295 nm) in reverse Kretschmann configuration (RK/SPCE).

Figure 7

Frequency-domain intensity decays of PPD spin coated from a solution of PMMA on an aluminized quartz slide measured with RK/FS (top) and RK/SPCE (bottom) configuration.

Figure 8

SPCE spectrum of PPD spin coated from a PMMA solution on an aluminized quartz slide. The excitation was with Kretschmann configuration through the prism (Figure 3). The KR/SPCE excitation is many-fold stronger than direct (RK) excitation, and attention must be paid to the photobleaching.