Shaping the emission of light with nanoantennas
It is well known that emission of light can be strongly modified when the emitters are in the proximity of a metallic particle. The emission efficiency can be reduced or enhanced depending on the emitter, the particle and their distance and orientation. The modification on the emission is caused by the strong interaction of the electric field of light with the electrons in the metal. This interaction leads to the excitation of the so-called localized surface plasmon resonances or the collective movement of the electrons in the metal driven by the light field. Researchers at the FOM Institute for Atomic and Molecular Physics AMOLF and at Philips Research have now demonstrated that by arranging the metallic particles in a periodic array, the emission can be enhanced and its directionality controlled. The results of this research were published online on the 10th of April in the prestigious journal Physical Review Letters.
The struggle to get more light out of emitters, such as fluorescent molecules, nanocrystals or quantum wells, is keeping busy many scientists working in the field of nanophotonics. Being able to control and increase the emission efficiency has a direct impact on widespread applications such as light emitting devices (LEDs) and lasers. However, not only the efficiency of the emission is relevant, but also the ability to simultaneously control its direction.
In recent years, several groups have investigated the optical properties of the so-called metallic nanoantennas. Metallic nanoantennas are particles of designable size and shape that can be fabricated by advanced nanofabrication techniques. Depending on the metal and, particularly, on the size and the shape of the nanoantennas, their optical response can be tuned. Light with the appropriate wavelength impinging on a nanoantenna will drive a collective motion of the electrons in the metal. This collective motion is called localized surface plasmon resonance.
Nanoantennas can be arranged in periodic structures (see figure 1). These structures form a grating on which incident light is scattered forming a diffraction pattern. Light of different wavelengths is scattered in different directions. The AMOLF-Philips group has shown that when light of a wavelength close to that of the localized surface plasmon resonance is scattered in the plane of the nanoantennas, a wave tightly bound to the surface is formed. More interestingly, it has been shown that light emitting molecules deposited on top of the array will preferentially emit into this surface wave. Because the surface wave has a larger spatial extension than the localized plasmon resonances of individual nanoantennas (see figure 2), more molecules will couple to this surface wave increasing the total emission of light. Improving the extraction of light from the structure is also crucial in the final emission efficiency. To this respect, the surface wave is advantageous because the probability for light to radiate out of the surface is high due to scattering with the grating. This light emission takes place at specific angles and wavelengths which closely match the diffraction pattern of the array (see figure 3). The ability of engineering the emission of molecules by arrays of nanoantennas is highly promising for future optical devices.
Reference: Shaping the Fluorescent Emission by Lattice Resonances in Plasmonic Crystals of Nanoantennas. Phys. Rev. Lett. 102, 146807
More information: Gabriele Vecchi, AMOLF c/o Philips Research, (0031)-(0)40 - 2742843; and Jaime Gómez Rivas AMOLF c/o Philips Research, Tel. (0031)-(0)40 - 2742349.
Figure 1: Photograph of a periodic array of gold nanoantennas taken with an electron microscope.
Figure 2: Calculation of the light field at (a) the wavelength of the localized plasmon resonance of the individual nanoantennas, and (b) at the wavelength of the surface wave which results from the coupling of the localized plasmon resonances and the diffracted wave on the plane of the array.
Figure 3: Emission enhancement (color scale)from light-emitting molecules on top of the array of nanoantennas as a function of the wavelength and the angle of emission. Light is preferentially emitted within a band which corresponds to the excitation of a surface wave in the array of nanoantennas.