>"Plasmonics refers to the generation, detection, and manipulation of signals at optical frequencies along metal-dielectric interfaces in the nanometer scale.[1]"

So, why is Plasmonics so important?

?

Well, here's a part answer to that question:

"Ten years of spasers and plasmonic nanolasers"

https://www.nature.com/articles/s41377-020-0319-7

>"Historically, lasers were heralded for their monochromaticity, high intensity, and low beam divergence; however, today, stimulated emission is utilized as a tool that offers

exquisite control to engineer light fields with well-defined frequencies, statistical properties, polarizations, and spatial profiles.

The range of applications is vast.

Miniaturization has always been a persistent topic of research in photonics; just 2 years after Maiman’s first laser [3], semiconductor lasers [6] emerged, which were orders of magnitude smaller.

Technologically, semiconductor lasers are naturally more compact, but with the advent of

heterostructures

[7], they also became able to operate at lower power under electrical injection, even under battery power. As transistor scaling and integration set in motion the microelectronics and computer revolutions, the integration of microelectronics with photonics was long perceived to be unavoidable [8].

When the smallest dimensions of lasers eventually reached wavelength scales

in the 1990s (see, e.g., ref. 9), they were still several orders of magnitude larger than transistors.

However, it was realized that micro- and nanoscale

optical resonators

could be utilized to control spontaneous emission. From this paradigm, spontaneous emission control emerged as a modern topic of research in the field of nanolasers.

By the turn of the millennium,

optically pumped photonic crystal (PC)

nanolasers were among the smallest devices [10]. Other forms of small lasers soon followed starting in the early 2000s [11], such as nanowire lasers [12,13,14,15,16].

However, the quest to reduce the resonator size made it extremely difficult to produce such small PC lasers operating under electrical injection. Metal contacts that closely approached the cavity inevitably introduced both scattering and absorption losses. Complications of poor heat conduction and low mechanical stability also arose as these devices were constructed from thin suspended membranes of semiconductor material. Meanwhile, many studies on metallic waveguides were indicating the possibility of confinement beyond the wavelength range of light [17,18,19,20,21,22,23].

As such approaches introduced metallic loss, integrated gain strategies were also devised to prolong the propagation of light [24,25,26]. The final ingredient, and the key to effectively scaling down the physical size of lasers, was the emergence of

metal-based cavities operating in dielectric modes

[27] or plasmonic modes in 2007 [28]. Such devices embraced the capability of metals to provide all of the requirements for any laser [29,30,31,32]: optical confinement, feedback, electrical contacts, and thermal management [33,34,35,36].

Such approaches relied on metals to support surface electromagnetic waves that used electron oscillations to promote optical confinement.

It was Bergman and Stockman [33] who first realized in 2003 that these

surface plasmon waves could also be amplified by stimulated emission.

Thus, the concept of the spaser

(an acronym for

surface plasmon amplification by stimulated emission of radiation

) was born.

This realization provoked considerable interest due to the predicted ability of such a new device to generate coherent plasmonic fields, along with the prospective applications of this capability.

Ten years ago, three teams [34,35,36] independently demonstrated the first plasmonic nanolasers, or spasers, from different perspectives and with different motivations (Fig. 1). These plasmonic nanolasers are extremely compact coherent light sources with ultrafast dynamics and a broad palette of promising applications [8,37]"