**Abstract**

Started by M. Stockman with his proposed idea of a nanoscale quantum generator of plasmons that he called surface plasmon amplification by stimulated emission of radiation (SPASER) in 2002, during the last two decades various devices have been proposed, fabricated, and tested for SPASERs or plasmonic nanolasers which have almost the same meaning. Despite all these efforts, there are still serious barriers in front of these devices to be an ideal nanoscale coherent source of surface plasmons. The main challenges are the difficulty of fabrication, over-heating, low output powers, high loss rates, lack of integration capability with commercial fabrication processes, inefficient performance in room temperature, and so on. In this chapter, governing principles of nanolaser operation are discussed. Important parameters, limitations, and design challenges are explained, and some of the proposed or fabricated structures are presented and their merits and demerits are expressed. Eventually, several novel structures resulting from our works are introduced, and their performances are compared to the state-of-the-art structures.

**Keywords:** nanoscale plasmon source, SPASER, plasmonic nanolaser, nanoplasmonics, stimulated emission

### **1. Introduction**

Theoretical postulation [1] and realization of laser [2] in the twentieth century changed both science and technology forever. Potential applications of lasers later have enormously expanded by the invention of the semiconductor diode laser in 1962 which brought them into the commercial market and in almost every device we know [3]. In the 1990s by the introduction of vertical-cavity surface-emitting laser (VCSEL) diodes, semiconductor lasers have pushed to their size limits [4]. The size of a dielectric cavity laser cannot be smaller than λ/2 in each dimension, and this limitation is known as the diffraction limit. With this in mind, modern VCSEL sizes are limited to a few microns [5]. On the other hand from the beginning of the millennia due to the rapid development of fabrication methods and tools [6], submicron manipulation of light using plasmonic devices has got lots of attention [7–10]. Plasmonic structures using metal/insulator interfaces broke the size limitation of the photonic devices and paved the way for integrating electronics, photonics, and optoelectronics on a single monolithic chip [11].

In order to bring benefits of the plasmonics into the field of laser research and fabrication, M. Stockman proposed the idea of a nanoscale quantum generator

called surface plasmon amplification by stimulated emission of radiation (SPASER) in 2002 [12]. This device utilizes a plasmonic feedback mechanism in a gain medium for exciting stimulated emission in local plasmon modes of metallic nanoparticles. In the next two decades, various mechanisms and devices have been proposed and fabricated for the realization of a nanoscale coherent plasmon source or plasmonic nanolaser [13]. These devices can be categorized in nanoparticles [12], waveguide-based nanolasers [15], nanowires [16], nanoresonators [17], nanopatches [18], nanodisks [19], plasmonic crystals [20], and so on. Although these devices have shown significant potentials, there are still serious problems with the nanoscale coherent sources of surface plasmons. For instance, the difficulty of fabrication, over-heating, low output powers, high loss rates, lack of integration capability with commercial fabrication processes like CMOS, inefficient performance in room temperature, and so on can be noted [12, 13].

In this chapter and in Section 2, we start with basic principles of nanoplasmonics like the definition of surface plasmon polariton (SPP) modes in classical and quantum mechanical pictures, different sources of plasmon loss, and specific properties of plasmons focusing on special characteristics of plasmons in metallic nanoresonators. Then, the interaction of plasmons with carriers in a cavity will be briefly discussed according to plasmonic cavity quantum electrodynamics. In Section 3, three different methods for analyzing plasmonic nanolasers are discussed, and in Section 4 several previously introduced nanolaser structures are briefly reviewed. In Section 5, the proposed nanolaser structures by the authors are introduced, and this chapter will be concluded in Section 6.

there are different types of plasmons like bulk plasmons and local surface plasmons (LSPs), SPP and LSP modes have significant roles in many plasmonic devices. There are several models for plasmons, and we are going to briefly overview them here. The most well-known and simple model is Drude's model which describes the metal as a free electron gas system and models the system using the classical spring-mass model with the external force exerted from the incident field "**E**" equals to "-q**E**" acting on the system. We are not going to derive the equations here and only use the final result as shown in Eq. (1) which can be derived as

*Interaction of an electromagnetic field with surface plasma waves and excitation of surface plasmon polariton*

*<sup>p</sup>* <sup>¼</sup> *ne*<sup>2</sup>

*<sup>ε</sup>*0*<sup>m</sup>* , *<sup>γ</sup>* <sup>¼</sup> <sup>1</sup>

*<sup>τ</sup>* (1)

mentioned in many related references like [22]:

in [11, 26].

**61**

**Figure 1.**

*waves.*

*<sup>ε</sup>r*ð Þ¼ *<sup>ω</sup>* <sup>1</sup> � *<sup>ω</sup>*<sup>2</sup>

*Nanoscale Plasmon Sources: Physical Principles and Novel Structures*

*DOI: http://dx.doi.org/10.5772/intechopen.90842*

of the electrons associated with all of the decay processes.

<sup>ϵ</sup>*Drude*�*Lorentz*ð Þ¼ *<sup>ω</sup>* <sup>1</sup> � *<sup>f</sup>* <sup>0</sup>*ω*<sup>2</sup>

and loss coefficient of the *j*'th oscillator, respectively.

*p <sup>ω</sup>*<sup>2</sup> <sup>þ</sup> *<sup>i</sup>γ ω*ð Þ*ω*, *<sup>ω</sup>*<sup>2</sup>

where "*n*" is number of electrons in the unit volume of the metal, "*e*" is the electron charge, "*m*" is the electron mass, "*ε*0" is permittivity of vacuum, "*ωp*" is the plasma frequency of the metal, "*γ*" is the total loss, and "*τ*" is the effective lifetime

According to Eq. (1), permittivity of a metal *εr*(*ω*) can be used in solving Helmholtz equations and finding the behavior of electromagnetic waves propagating at the metal/dielectric interface which are also known as SPP waves. However, Drude's model suffers from several shortcomings which leads to considerable errors especially near the plasma frequency of the metal. This is because in Drude's model, the effect of electrons in other energy bands (not just free electrons) is not included, and nonlocal effects are also not included [26]. To overcome these problems, Drude-Lorentz's model is introduced for the first problem which can be written in general multi-oscillator form as Eq. (2) and Landau damping correction according to Eq. (3) for the second problem. We are not going to further discuss these models either, and you can find details

> *p*,0 *<sup>ω</sup>*<sup>2</sup> <sup>þ</sup> *<sup>i</sup>γ*0*<sup>ω</sup>* <sup>þ</sup><sup>X</sup>

where the first sentence corresponds to Drude's model; "*fj*" is the power of the *j*'th oscillator; and "*ωp,j*," "*ωj*," and "*γj*" are plasma frequency, resonant frequency,

*j* max

*j*¼1

*ω*2

*fj ω*2 *p*,*j*

*<sup>j</sup>* � *<sup>ω</sup>*<sup>2</sup> � *<sup>i</sup>γj<sup>ω</sup>* (2)
