research

optical plasmonics

In traditional light sources such as incandescent lamps, about 90 % of the energy used is wasted producing heat. Fluorescent lamps are about four times more energy efficient, but still, a significant fraction of the energy is wasted. Not much progress has been made in the energy efficiency of conventional light sources within the past decades. Therefore, new lighting technologies are necessary. One of these technologies that has emerged in the past two decades is solid state lighting technology, i.e., organic and inorganic light emitting diodes LEDs. Light emitting diodes are thus of central industrial and social relevance. Low cost and high efficiency LEDs are intensively sought for displays and lighting applications. LEDs still face great challenges before large scale commercialization of white-light sources can be considered. Due to internal losses, only a fraction of charge carriers injected in a LED recombine emitting photons. This fraction, called internal quantum efficiency, depends on the material and on the structure and dimensions of the LED active region. LEDs have already achieved efficiencies close to those needed for energy efficient operation, but not in the whole visible spectrum.

The spontaneous emission rate of light sources, and consequently their efficiency, can be modified by placing them in the proximity of metallic structures. The emitter can experience a strong quantum electrodynamic coupling to the structure due to the modification of the optical local density of states (OLDOS). Not only the efficiency but also the degree of polarization and the direction of the emission can be tailored by coupling emitters to these structures. Control on the polarization and the direction of the spontaneous emission of light sources is of great technological relevance and it may lead to low threshold lasers, bright displays and efficient light management.

In the group Nanowire Photonics we investigate novel methods to increase the efficiency of light emitters, such as semiconductor nanowires, quantum wells and phosphorus materials, and to modify their emission characteristics, such as the degree of polarization and the direction of the emission. In particular we investigate how the emission characteristics are modified when an emitter is located close to complex structures such as metallic surfaces, gratings and nanostructures. The proximity of an emitter to a metallic surface leads to a coupling between the structure and the emitter due to the excitation of surface plasmon polaritons SPPs. SPPs are electromagnetic waves coupled to the coherent oscillation of free carriers in the metal surface. Due to this coupling, the spontaneous emission rate of light sources may be increased and its efficiency improved: The total decay rate G of the emitter is the sum of the radiative G_rad and non-radiative G_non-rad decay rates, as well as the decay rate due to excitation of SPPs

G = G_rad + G_non-rad + G_SPP.

When the emitter is very close to the metal (a few tens of nanometers), the coupling to SPPs may be very efficient and G_SPP may dominate over G_rad and G_non-rad. If the surface of the metal is structured to form a grating, SPPs can couple into free space electromagnetic radiation. In this way the internal quantum efficiency, defined as

h =	Grad + GSPP
	Grad + Gnon-rad + GSPP

can be increased. Moreover, the coupling of SPPs into free space electromagnetic radiation using a grating is polarization and direction dependent. Therefore, it is possible to modify the emission characteristics of light sources.

A similar method to enhance the emission efficiency is using metallic nanoparticles or nanoantennas. When a metallic nanoantenna is illuminated, resonances called particle plasmon can be excited. These resonances produce a local electromagnetic field enhancement close to the nanoantenna. The local field enhancement is the largest in regions close to sharp features and in between particles. A light emitter placed within this local field can couple very efficiently to the nanoantenna exciting particle plasmons that can radiate into the far field. This mechanism can lead to large improvements in the efficiency of light sources. Not only the efficiency can be modified with nanoantennas but also the direction and polarization of the emission can be tailored by structuring nanoantennas with different geometries. Depending on the geometry of the nanoparticle the dipole moment will be preferentially oriented on a particular direction determining the emission characteristics of the emitter.