Channel spaser: Coherent excitation of one-dimensional plasmons from quantum dots located along a linear channel

Recently, a new branch of quantum optics—quantum nanoplasmonics—has arisen.1,2 Advantageous plasmon properties such as small wavelength and a high-energy concentration open new perspectives for constructing nanodevices such as waveguides, cavities, and antennas. In this regard, studies of plasmons propagating along 1D objects such as wires,3 wedges,4,5 and channels6,7 are of great interest. Losses in metals are the main obstacle to practical applications of plasmonics. It has been suggested that this problem can be overcome by compensating loss in a gain medium.8–11 This relates nanoplasmonics to quantum optics.1,2 In particular, a generator of plasmons propagating along a flat surface has been suggested in Refs. 12–14. In this system, periodic surface corrugations produce Bragg mirrors of the resonator cavity. The first quantum nanoplasmonic device that was referred to as surface plasmon amplification by stimulated emission of radiation (spaser) was proposed in Ref. 1. The spaser consists of a quantum dot (QD) located near a metal nanoparticle (NP). The plasmonic oscillations in the NP play the role of photons in a laser and the cavity effect is provided by the plasmon localization in the vicinity of the NP.1,15–17 Thus, a pumped QD nonradiatively transfers its excitation to surface plasmons localized at the NP. As a result, one observes an increase of intensity of the surface plasmon field. Thus, the spaser does not radiate an energy beam but generates a near field. The spaser has recently been realized.18–21 In this paper, we study surface plasmons propagating along the bottom of a groove (channel) in the metal surface22–24 (see schematic in Fig. 1). The surface plasmons are coherently excited by a linear chain of QDs at the bottom of the channel. We show that for realistic values of the system parameters, gain can exceed loss and plasmonic lasing in ring or linear channels can occur.