Plasmonic structures

Plasmon resonances represent another method of confining light. Compared to dielectric waveguides and feedback structures, the volume to which they confine light (mode volume) is often much smaller and this can result in a local enhancement of the electric field. Plasmonic field enhancement is widely used in sensing applications, e.g. for surface plasmon resonance sensors or surface-enhanced Raman spectroscopy. There is also interest in using the plasmon-mediated concentration of the electric field in the vicinity of metal surfaces or metallic nanostructures to design better lasers. In the following we will review some of the developments in this field. References [11,224,225] provide more detailed reviews on small lasers and optical amplifiers that use metallic or plasmonic structures in combination with organic materials.

Local field enhancement is often considered beneficial for designing more compact or potentially even more efficient lasers. However, surface plasmon modes generally also show much larger losses than dielectric modes, i.e. the modal absorption coefficient α that was introduced in section 1.2 can be drastically increased. As any loss has to be compensated by optical gain for lasing or amplification to occur, the development of plasmonic lasers based on organic gain materials has been challenging so far.

One strategy towards organic lasers and amplifiers based on plasmon resonances is to make use of so called long-range surface plasmon polariton (LRSPP) modes. These modes propagate on both sides of thin metal films (thickness usually < 30 nm) and show much lower losses than conventional surface plasmon modes, which propagate on a bulk metal surface. Waveguides for LRSPP modes have recently

been combined with organic gain media to compensate the residual propagation loss present in these. Using this approach, net amplification, i.e. complete compensation of all losses plus additional gain, has been demonstrated for gold LRSPP waveguides. These waveguides were either embedded in a polymer blend based on a PPV based gain material226 or combined with an optofluidic device that passed a solution of an IR emitting organic dye (IR140) across the waveguide227. In both cases the ability to adjust the refractive index of the gain medium turned out to be important to control the losses of the LRSPP mode. More recent work demonstrated LRSPP waveguide amplifiers based on small molecule materials and showed the integration with grating-based input and output couplers. This work also reported detailed analysis of the achievable gain at different pump powers and for different samples.228 The propagation loss of LRSPP waveguides critically depends on the smoothness of the metal film.229 Tuning of the surface energy between the metal and underlying substrate material enabled fabrication of extremely smooth gold and silver films and enabled fabrication of continuous metal films that were thinner than the traditionally assumed percolation limit for these materials (approximately 10 to 15 nm).230,231 Despite these advances, to the best of our knowledge, no organic LRSPP laser has been demonstrated so far. However, designs and models for such a device — with DBR or DFB gratings embedded in the waveguide — have been reported.232

Recently, loss compensation has also been attempted for silver nanowires deposited on Rhodamine doped PMMA layers but only partial compensation of the losses was achieved.233

An alternative to propagating surface plasmons is the use of localized non-propagating modes, which are supported for instance by metallic nanoparticles. Stimulated emission of surface plasmons is expected in these structures if sufficient optical gain is provided by the medium surrounding the nanoparticles. Such a device is often referred to as spaser (surface plasmon amplification and stimulated emission of radiation) although this term has also been used for other plasmon-based lasers. Due to the large losses induced by metallic particles, the Q-factors expected for these localized plasmon modes are generally

well below 100, which means that the required material gain is much higher than in other optical feedback structures. To our knowledge, there have been only been very few demonstrations of true spasers. These devices were based either on dye-coated gold nanoparticles234 or silver nanorods embedded in a dye-loaded polymer matrix235. In both cases, the experiments involved a large ensemble of particles (>106 particles) and demonstration of spaser action from a single isolated particle has yet to be shown. In related work based on a similar sample structure, strong coupling of the gain material to the plasmon mode was demonstrated by a reduction in excited state lifetime but here no spaser action was claimed.236

A device configuration that makes use of localized plasmon modes but relaxes the gain requirements are ordered or random arrays of plasmonic nanocavities and there are now a number of reports on organic lasers based on such arrarys.235,237,238 By using a periodic subwavelength hole array perforated in a metal film, directional emission was recently achieved with the plasmonic array approach.239

3.9.Conclusion

A large variety of resonators are compatible with organic gain materials and this has allowed the development of a large number of different organic lasers with different characteristics and advantages. Some geometries, such as DFB gratings and WGM resonators, offer direct access to the organic gain materials and are therefore of interest for applications in sensing and detection (see section 7.2 for outlook on applications of organic lasers). Other resonators are particularly attractive due to their high Q-factors or facile preparation routes (see section 4 for fabrication strategies). Plasmonic structures can supply feedback on scales much smaller than the typical resonator geometries, with footprints much smaller than the wavelength of visible light. Therefore, it is unlikely that an ideal resonator design for organic lasers will emerge. Instead, the different geometries allow researchers to select the most suitable design for each application or scenario.