Key words

1 Introduction

Laser technology research has expanded over the years as a result of its broad spectrum of applications in the biotechnology industry, office spaces, aviation, industries, surveillance, sensors, and other advanced technologies such as 3D imaging and fluorescence microscopy [1,2,3,4,5,6,7,8,9,10]. Laser is regarded as one of the twentieth century’s most innovative innovations due to its unique properties such as (a) monochromaticity, (b) coherence, (c) directionality, and (d) brightness [11, 12]. Einstein investigated Max Planck’s deduced relationship between energy and radiation frequency, and the resulting postulated theory of energy absorption and emission in discrete quanta of light, i.e., photons, led to the development of the concept and theory of stimulated light emission [13]. The theory of stimulated emission of radiation paved the foundations for the development of the first practical laser. The most fundamental process for laser action is stimulated emission of radiation [14]. When an electron is excited from its ground state to its excited state by an external source of radiation, a spontaneously emitted photon forces the excited electron to a lower energy state. When an electron returns to a lower energy level, it releases energy in the form of a photon. The process of photon emission precipitated by electron relaxation in an atom or molecule is referred to as “stimulated emission of radiation” [15, 16]. A laser is composed of three essential components that work together to produce lasing action via stimulated emission: (1) a pump source, (2) an active medium, and (3) an optical resonator. The pump energy source stimulates the atoms or molecules in the active medium, causing them to spontaneously emit radiation. The photons generated by spontaneous emission are trapped within the system by an optical resonator, resulting in stimulated radiation emission. Based on the theoretical work by Charles Townes and Arthur Leonard Schalow [17], Theodore H. Maiman created the first practical laser in 1960 by coating the ends of a ruby rod (aluminum oxide doped with chromium) with silver to make it reflective and serve as a resonator [18]. Since then, crystals, gases (argon, carbon dioxide, helium-neon, carbon monoxide, etc.), glasses (silicate or phosphate glasses) doped with lasing media, semiconductors (gallium nitride, gallium arsenide, etc.), and liquid dyes have all been introduced as potential active lasing media [19]. Nonetheless, despite its significance, the development of this powerful device was both complex and costly. As the number of applications increased, so did the demand for small-volume, compact, high-power, and high-efficiency lasers, prompting research into the development of smaller high-power lasers.

The idea of simulating and building small-volume lasers gave rise to the concept of nanolasers [20, 21]. Nanolasers operate on the same principle as the conventional lasers: stimulated emission of radiation [21, 22]. The dimensions of the lasing system distinguish a nanolaser from a conventional laser. The remarkable ability of nanolasers to confine light on a scale approaching the diffraction limit is due to their small size [20, 23]. The construction of a smaller lasing system was made possible by shrinking the laser dimension, but managing coherence remained a challenge. As a result, a new strategy for controlling coherence while also reducing the size of the laser system was required. When monochromatic coherent laser emission was observed, it was discovered that an optical resonator is required to maintain the laser emission’s directionality and monochromaticity [12, 19]. In a traditional laser setup, the resonator, which consists of an active medium sandwiched between two mirrors, is responsible for the lasing system’s bulkiness along with being an important laser component. The removal of the optical resonator from a laser was considered to be a means to lower the system’s overall size. A laser without an optical resonator is known as a “random laser.” Because of the spatial inhomogeneity of the medium, random lasers do not require an optical cavity to generate disorder-induced light scattering. The elimination of optical resonators from a laser system not only reduced manufacturing costs but also simplified the laser’s design. In 1967, Letokhov proposed the idea of producing laser-like emission using scattering particles with negative absorption [24]. Random lasing capabilities have since been demonstrated in a wide range of materials, including semiconductors, dielectrics, biological samples, and metal nanoparticles. Although, most dielectric-based random lasing cavities have experimentally recommended random lasers with a high threshold and a low-quality factor (Q-factor). Metal nanoparticles have recently been investigated to improve the performance of random lasers by integrating them into the active media, to overcome these disadvantages of high threshold low Q-factor random lasers [25, 26].

In this chapter, we explore plasmonic random lasing systems and their possible applications in addition to contemporary research developments in the field of random lasers. We also answer the following questions: (1) How does including metallic nanoparticles into a random lasing system improve random lasing emission despite metal-induced losses? (2) How can plasmons boost dye molecule fluorescence in random lasing media? (3) Why does a plasmonic-based random lasing media have a low threshold for lasing action?

2 Theoretical Background

2.1 Laser Fundamentals

A laser is a type of light source that generates an intense beam of coherent monochromatic light by optically amplifying photons released by excited atoms or molecules inside an active medium [12, 16, 19]. The three fundamental processes for laser action are (a) Absorption of radiation, (b) spontaneous emission, and (c) stimulated emission (Fig. 1).

Fig. 1
figure 1

Illustration of three fundamental processes for laser action—stimulated absorption, spontaneous emission, and stimulated emission

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When an incoming photon from an external energy source (pump source) matches the precise energy of the atomic system in the ground state, it excites the electron in the lower energy state (E1) to a higher energy level (E2). The process of excitation of an electron from lower to higher energy state is termed as “absorption of radiation.” The number of electrons in the excited state (N2) must be greater than the number of electrons in the lower energy state (N1) for stimulated emission to occur [11, 27]. The condition in which N2 exceeds N1 is known as population inversion. Radiation absorption contributes to an increase in the number of electrons in the excited state. The difference in energy between electrons in lower and higher energy states (E2 – E1) determines the amount of incident radiation absorbed. Because the electron has a finite lifetime, once excited to a higher energy level, it will relax back to its lower energy state and emit the energy in the form of a photon. The process of photon generation as a result of relaxation of an electron in the higher energy state to lower energy state is termed as “fluorescence” or “spontaneous emission.” As a result, spontaneous emission can be defined as a statistical phenomenon involving time, space, and an excited electron’s lifetime. The most important process of the three mechanisms leading to laser action is stimulated emission. When a photon interacts with an electron in an excited state, the electron is compelled to return to a lower energy level and emits a photon. The stimulated generation of the photon is proportional to the energy density of external radiation [12, 16]. The photon released as a result of stimulated emission has the same direction and phase as the photon that triggered it. These three critical steps—absorption of radiation, spontaneous emission, and stimulated emission, enable a laser to generate amplified coherent light.

2.2 Conventional Lasers

Lasers are recognized as a device that generate and amplify the monochromatic coherent light via stimulated emission of radiation. What are the individual components of a system that initiate the process of stimulated emission, and how does lasing occur? The three fundamental components of a laser are shown in Fig. 2. (i) gain medium, (ii) pump energy source, and (iii) optical resonator.

  1. (i)

    Active lasing media: The active lasing medium also known as gain media is a component of the lasing system that amplifies electromagnetic radiation to induce lasing action. The lasing media is contained within the optical resonator. The active lasing media generates optical gain by emission via molecular or electronic transitions of electrons from a higher to a lower energy state. It can be any material such as liquid dyes, semiconductors, crystals, and gases with quantum properties that amplifies the laser beam. When the gain in the cavity surpasses the loss, lasing occurs.

  2. (ii)

    Pump energy source: The pump energy source is a type of external energy source that is used to stimulate atoms or molecules into higher quantum mechanical energy levels in order to induce population inversion in an active medium. Pumping molecules in the active medium can be accomplished through optical pumping, electrical pumping, chemical pumping, and gas discharge pumping.

  3. (iii)

    Resonator: Lasing cannot be ensured only through stimulated radiation emission. Instead, we will only acquire the enhanced signals. As a result, an optical resonator is used to yield resonant eigenstates, which then lase as cavity modes. The most well-known and commonly utilized optical cavity in conventional lasing systems is the Fabry-Perot cavity (Fig. 2). It consists of two mirrors, completely and partially reflecting, positioned opposite to one another that bounce the generated radiation back and forth to create modes. As a result, an optical cavity or optical resonator can be defined as a standing light wave resonator and an important component surrounding the gain media, where standing wave patterns can be indexed by their modes. The mode’s quality is closely related to the cavity loss and can be evaluated using the Q-factor [22]. The photons produced by stimulated emission bounce back and forth within the resonator, generating temporal and spatial coherence.

Fig. 2
figure 2

Schematic of a conventional lasing system

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2.3 Random Lasers

2.3.1 Random Amplifying Media

After more than 60 years of growth in the direction of developing lasers with smaller volumes, greater power, higher efficiency, and faster modulation speeds, laser technology is rapidly advancing. As a result, research in the field of lasers has been restricted to engineering and technological advancements, leaving some scientific aspects of lasers untouched. Experimentally, controlling the monochromaticity and directionality of laser output was shown to be successful. However, as previously stated, the question of coherence control remained unresolved. The concept of resonator-less lasing arose as a result of concerns about coherence control. Random lasers, first proposed theoretically by Lethokov in 1968, are now one of the most intriguing and engaging areas of research in the field of laser technology. In his research, Lethokov theoretically developed the concept of producing laser-like emissions from scattering particles with negative absorption [24]. Following the theoretical studies of random lasers, other reports of laser-like emission in powdered luminophosphors emerged, introducing random lasing to the field of laser technology.

Fig. 3
figure 3

Illustration of a conventional random lasing system

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Random lasers are mirrorless lasing systems that operate on the principle of multiple light scattering (Fig. 3) [28]. Unlike conventional lasers, a random lasing system employs highly scattering disordered material in which the incident light undergoes recurrent scattering and is amplified before leaving the cavity. When the scatterers doped with dye molecules are pumped externally, the spontaneously produced photons are scattered. Photons have a long enough dwell time in the medium due to scattering to enhance the resultant random lasing radiation. This mechanism generates resonant structures with high-quality factors. Hence, we can infer that the gain medium is strongly dependent on the scattering strength as well as the gain volume within such systems [29]. The scattering strength is quantified using the scattering mean free path (ls), i.e., the distance travelled by a photon in between two successive collisions. According to the Mie scattering theory put forth by Gustav Mie, the scattering parameters (scattering mean free path \( {l}_s=\frac{1}{\rho {\sigma}_{\mathrm{sca}}} \), transport mean free path (lt), scattering cross sections (σsca) , and Mie scattering coefficients) are a function of size and shape of the scatterers and the refractive index contrast [30]. The mathematical formulation of the scattering parameters implies that the profile of the random lasing emission spectrum is influenced by the geometry of individual scatterers in the active random lasing media as well as the refractive index contrast between the scatterers and the surrounding media. As the scattering strength of the active random medium increases, so does the optical path length, or the transport mean free path of the emitted photon. Depending upon the optical path length or the transport mean free path (lt), there are three regimes of multiple scattering in a random media (L is the sample size, λ is the incident light wavelength) [31],

  1. (a)

    Localization regime (lt ≤ λ).

  2. (b)

    Diffusive regime (λ < lt < L)

  3. (c)

    Ballistic regime (lt ≥ L)

Furthermore, as illustrated in Fig. 4, two types of random lasing emission profiles are identified based on the photon travel path within the active random media: coherent emission and incoherent emission. In the case of incoherent emission, the feedback provided by multiple light scattering does not return the photon to its place of origin. Random lasers with coherent feedback, on the other hand, drive the photon to return to its origin, resulting in a closed loop. The coherent emission spectra are made up of several narrow spikes as a result of the interference created by the photon closed loop generation. The profile of the emission spectra can be changed from coherent to incoherent by altering the scatterer concentration in a random amplifying medium.

Fig. 4
figure 4

(a) Incoherent emission and (b) coherent emission random lasing profiles

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2.3.2 Types of Random Lasing Media

2.3.2.1 Dielectric Random Lasing System

Intriguing optical phenomena emerge when dye molecules or active centers are introduced into a disordered media. The optical amplification of spontaneous radiation emission caused by an increase in photon mean free path due to multiple light scattering has particularly been the optical phenomena of interest to researchers. Dielectrics are one of the most commonly used materials in active random lasing media due to their ability to generate strong modes within individual scatterers or resonators. As a result, the dielectric modes created by individual resonators combine to greatly boost the Q-factor. Dye molecules combined with dielectric systems can produce random lasing because dielectric resonators can provide efficient scattering for optical feedback, which is vital for efficient lasing activity [26]. The scatterer form (shape, size, dimension, aspect ratio) [32] and geometry (inter-scatterer distance and positioning) [33] will determine the permissible modes generated within the individual dielectric resonators, as well as the mode coupling. The coupling of dielectric-resonator modes (Mie mode, Fabry-Perot mode, Whisper gallery mode, etc.) will increase the Q-factor depending on the phase difference between the modes generated in the individual resonators, resulting in quantum mechanical phenomena such as Fano resonance [34], supercavity mode, and bound states in continuum (BIC) [32, 35]. Recently, a magnetic nanofluid was investigated as a passive system with a Fano-like profile [36]. Fano resonance is a phenomenon resulting in the appearance of asymmetric sharp-like shape because of the interference between the resonator eigenmodes and the background disturbance in a system. The dielectric medium in use exhibiting Fano-like profile in the emission spectra was a magnetic nanofluid. The profile of the peak in the emission spectra changed as the shape, size, and arrangement of the individual scatterers changed. The emission spectra with change in the shape and size of the scatterers in the disordered media exhibited Fano-like peaks implying that the dynamical changes within the passive random system can be tuned and adjusted to result in a change in the emission profile. The dynamics of the scatterers in the magnetic nanofluid were controlled using an external magnetic stimulus [37]. A similar study using a magnetic nanofluid doped with Rhodamine 6G was carried out to demonstrate random lasing phenomena. The emission spectrum shown in Fig. 5 was recorded at a constant external magnetic field of 100 Gauss as a function of varying time to observe the changes in the emission light intensity with change in the scatterer configuration. Fano-like emission profiles are observed at 573 nm and 579 nm wavelengths in the emission spectrum suggesting interference between resonant states and background disturbance.

Fig. 5
figure 5

Random lasing emission spectra of an active magnetic nanofluid (unpublished)

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2.3.2.2 Semiconductor Random Lasing Cavity

Stimulated emission in semiconductors was originally reported in 1981 using ZnO powder [38], which was then investigated further by Cao et al. to determine if the observed emission spectra were caused by an individual particle or an ensemble of ZnO particles [39]. In the photonic examination of ZnO powder, the emission spectra of 200–350 nm thick ZnO films composed of 50–150 nm sized particles randomly organized over an amorphous fused silica substrate was studied. The ZnO sample was pumped with 15 ps pulses at 355 nm using a frequency tripled Nd:YAG laser. To explain the spikes in the random lasing peaks in the emission spectra, it was stated that closed loops were created as a result of strong scattering. These loops can hold the generated photon for long enough to operate as a laser light ring resonator, allowing the frequencies of narrow stimulated emission lines to be identified. Because different resonators have different losses and support varied oscillation frequencies, discrete stimulated emission lines appear in the spectrum at variable thresholds. Furthermore, as the size of a pump spot on the sample grows, so does the number of ring cavities, resulting in a greater number of narrow emission lines in the spectrum. Qualitatively similar emission properties were observed in various semiconductor systems predominantly in the commercially most available semiconductor laser material GaAs [40].

2.3.2.3 Biological Random Lasing Cavity

Various complex biological structures including bones, human tissues, organs, nanoimprinted DNA, and a variety of animal biological membranes have been systematically studied and modeled to realize that biological samples can serve as scattering centers for random lasing purposes as well [41,42,43,44,45]. Coherent random lasing was demonstrated in biological tissues infiltrated with laser dye using the nanostructures on cicada wing as the scattering center [46]. Similarly, bone structures infused with dye molecules resulted in discrete peaks on optical excitation. Since the random lasing cavities are strongly dependent upon the geometrical and structural properties of the media in use, scattering in the biological samples have been used as an advantage to study the structural properties of biological sample from the obtained emission spectra [43, 45]. For instance, it is possible to identify the abnormal tissues from the healthy ones since experimentally it has been observed that the malignant human tissue in a gain medium yielded more laser lines when compared to that of the healthy ones. Plant species have also been observed to possess random lasing properties. Owing to that, recently, Li et al. have demonstrated random laser based on lotus leaves from planar liquid waveguide wherein they have achieved tunable random lasing for a wide wavelength range by changing the pump position [47].

2.3.2.4 Metallic Random Lasing System

Light-induced nanoscale optical fields on metallic nanoparticles strongly enhance optical intensities and have a wide range of applications. Recently, optical amplification and laser action by metallic nanostructures were extensively studied owing to their large charge density waves at nanoscale that resulted in some interesting optical phenomena such as localized surface plasmon resonance (LSPR). The laser systems exploiting this property of nanoscale metallic nanostructures are called plasmon lasers [48,49,50,51,52].

Plasmon random lasers in contrast to conventional random lasing cavity consisting of dielectric scatterers amplify light coupled to oscillating charge carriers which adds momentum to the light enabling their physical size and mode volume to shrink below the diffraction limit. The strong electromagnetic confinement in a plasmonic cavity modifies the laser action by enhancing spontaneous emission causing drastic spatial redistribution of spontaneous emission. Plasmons therefore significantly reduce the pump conditions for the onset of laser action hence modifying the threshold behavior. Initially, Dice et al. observed incoherent random lasing enhanced by surface plasmons after suspending silver nanoparticles in a laser dye solution [53]. Following the successful demonstration of incoherent plasmonic random lasing Meng et al. experimentally illustrated coherent random lasing in polymer films with silver nanoparticles. The metallic nanoparticles concentrate the incident optical field on the nanoparticle surface generating localized surface plasmons. From the observations, Meng suggested that the scattering systems embodying metal nanoparticles provide stronger absorption of excitation light and a larger amplification of fluorescence that the dielectric scatterers [54]. This happens because of the enhancement in the localized electric field as well as the scattering effect resulting from the large scattering cross section than the dielectric counterparts at the same dimension. However, addition of metal nanoparticles not only increases the scattering strength within a random lasing cavity because of its large scattering cross section but also induces absorption losses hindering the overall performance of random lasers.

2.3.3 Advantages of Random Lasers over Conventional Lasers

In case of conventional lasers, the construction of lasing system requires meticulous alignment of resonators and accurate fabrication which makes the manufacturing of conventional lasers both expensive and complicated. Additionally, conventional lasers are subjected to the diffraction limit, wherein their size cannot be less than half the wavelength restricting the miniaturization of lasers. In contrast, since the resonant random lasing is enabled by randomly formed cavities of high refractive index dielectric or metallic nanostructures inside a gain medium, it eliminates the need for an optical resonator within the system permitting an easy and cheap fabrication method making them extremely attractive for a manifold of potential applications, starting from displays and lighting devices [55], to smart sensors [56,57,58] and medical diagnostics [42, 44, 59]. The realization of this technology catalyzed research towards demonstrating various materials exhibiting random lasing. Many scattering systems including semiconductor materials [40, 60, 61], biological samples [41,42,43,44,45,46, 62, 63], dielectric systems, metallic nanoparticles [25, 26, 54, 64,65,66,67,68,69,70], liquid crystals [71, 72], polymer thin films, and many other materials have been studied to demonstrate random lasing phenomena.

2.3.4 Drawbacks of Random Lasers

Random lasers have certain drawbacks such as control, tunability, and flexibility of random lasing emission [64]. However, certain systems have been studied to control the geometry of the scattering particles such as controlling the dynamics of scatterers using external electric or magnetic field [36, 72]. Another limitation of random lasers is that a high laser threshold is required to initiate lasing in disordered media. For random lasing media that use dielectrics, the resonator cavity is observed to have a low Q-factor, i.e., decreased lasing efficiency is observed in random and disordered cavities. To maximize the available gain volume, it is essential to use scattering particles that have large geometrical cross sections. It is a challenge to achieve such a criterion with dielectric scatterers [53]. Therefore, the two main challenges in random lasers are (a) high lasing threshold and (b) low laser efficiency and Q-factor. To eliminate these issues one of the methods that has been extensively adopted is to additionally introduce metal nanoparticles instead of using dielectric disordered systems alone. The ability of metal nanoparticles to confine light into the subwavelength optical mode, due to their inherent property of localized surface plasmon resonance (LSPR) can be exploited to overcome the abovementioned shortcomings. In the following section, we discuss the theory of plasmonics in detail and how it can be used to increase the efficiency of a random lasing system by increasing its Q-factor and lowering the lasing threshold.

3 Theory of Plasmonics

Plasmonics research has gained increased attention over the last two decades because of their unique optical properties. The pronounced interest in the application of plasmonics is because of three major reasons:

  1. (i)

    Plasmons, the collective oscillations of electrons, can resonate with light at a particular frequency to a great degree of freedom.

  2. (ii)

    Plasmons are capable of producing enormous enhancement in light intensity.

  3. (iii)

    Plasmons have the ability to confine the photon energy in deep subwavelength region.

3.1 Bulk Plasmons and Surface Plasmons (SPs)

Metals, rich with a sea of free electrons, which on being stimulated by an external optical energy source are capable of inducing collective oscillation of these valence electrons. The quantized radiation induced by collective electron oscillation in a metallic system are known as plasmons [73,74,75]. In case of macro metallic systems, the bulk plasmons are charge density oscillations which do not have the capability to confine the electromagnetic radiation. Plasmons in a bulk metallic system oscillate above a plasmon frequency (ωp) given as,

$$ {\omega}_p=\sqrt{\frac{N{e}^2}{m{\varepsilon}_0}} $$
(1)

where N is the valence electron density, e is the charge of the electron, m is the effective mass of the electron, and ε0 is the dielectric constant of free space. The incident electromagnetic wave propagates through the volume of the bulk metal and induces collective oscillation of loosely bound valence electrons or conducting electrons within the bulk metallic system. The bulk plasmon energy depends only on the electron density of the metallic system (Eq. 1). In case of bulk metallic systems, the plasmons cannot couple with the electromagnetic field and hence there is no field confinement observed (Fig. 6).

Fig. 6
figure 6

(a) Bulk plasmon (b) Surface plasmon induced in a metallic system

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However, when an electromagnetic wave is incident on a metal-dielectric interface, the wave travels through the surface of the material with a frequency value that is below the plasma frequency, \( {\omega}_{\mathrm{surface}}=\frac{\omega_p}{\sqrt{2}} \). The electromagnetic radiation traveling through the surface of the metal results in the formation of photo-induced plasmon called “surface plasmon polariton (SPP).” In a metal-dielectric system, the electromagnetic field strongly couples with the electron plasma of the metal confining the plasmons to the surface that induces photon-plasmon interaction to produce SPP [73, 76, 77]. In summary, SPP are electromagnetic excitations propagating at the metal-dielectric interface. “Surface plasmons” on the other hand are photo-induced oscillations of charge carriers on the surface of the metallic system. Each collective oscillation of surface plasmons or polaritons associated with different surface charge distribution is known as surface plasmon resonance (SPR). SPR is exclusively observed in metallic systems because of its high charge density. The point at which the oscillation amplitude reaches a maximum at a particular incident radiation wavelength is the SPR peak.

3.2 Localized Surface Plasmons (LSPs)

As the size of the metallic system is decreased, particularly size reduction along one or two dimensions limits the movement of the charge carriers hence appearing to be localized on the surface of the metallic nanosystem. As a result, localized surface plasmons are defined as the non-propagating excitations of the valence electrons of metallic nanoparticles [73]. The resonance resulting from the oscillation of these restricted electrons is called “Localized surface plasmon resonance (LSPR) [78, 79] (Fig. 7).”

Fig. 7
figure 7

Illustration of localized surface plasmon resonance (LSPR)

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The oscillatory motion of the charge carriers or free electrons in metal nanoparticles under the influence of the external electric field component of the incident electromagnetic wave is given by the following equation:

$$ m\frac{d^2x}{d{t}^2}+ m\gamma \frac{dx}{dt}=-e{E}_0\exp \left(- i\varepsilon t\right) $$
(2)

where m, γ, ω, and E0 are the electron mass, damping constant, frequency, and amplitude of the external electric field, respectively [80].

3.3 Factors Affecting Localized Surface Plasmon Resonance (LSPR) Peak

The striking feature of metallic nanosystems is that the LSPR wavelength can be significantly altered by tuning their shape, size, and dielectric constant contrast with the surrounding media. The variation in the LSPR peak can be understood in detail using the Mie scattering theory. The extinction cross section coefficient can quantitatively explain SPR for metal nanoparticles smaller than 20 nm, which is mathematically given as follows,

$$ {C}_{\mathrm{ext}}=\frac{24{\pi}^2{R}^3{\varepsilon}_m^{3\slash 2}}{\lambda }\ \frac{\varepsilon_i}{{\left({\varepsilon}_r+{\varepsilon}_m\right)}^2+{\varepsilon}_i^2} $$
(3)

where R is the radius of the metal nanoparticle, εm is the permittivity of the surrounding media, and εr and εi are the real and imaginary parts of the permittivity of the metal nanoparticle, respectively. The LSPR intensity and wavelength depend upon various factors that affect the electron charge density on the particle surface such as the type of metal, particle shape, size, composition and structure, and the dielectric constants of both the metallic particles and the surrounding media. In case of metal nanoparticles with size less than 20 nm, the extinction efficiency spectra are mainly influenced by absorption efficiency, given that small metal particles absorb large amounts of electromagnetic radiation. In contrast, when the size of the particle exceeds 20 nm scattering is the dominant phenomena occurring in the system and therefore governs the extinction output. The real part of the dielectric constant determines the LSPR position and the imaginary part determines the resonance peak width. The LSPR resonances occur when εr(ω) =  − 2εm and the peak broadens with an increase in the dielectric constant in the surrounding media. From the expression of scattering cross section of a metallic sphere (Eq. 2), we can infer that the LSPR phenomena are dependent mainly on the dielectric constants of both the surrounding medium and the metal nanoparticle. The dielectric constants of the metal nanosystems are much larger than the dielectric ones which is why metal nanoparticles have a larger scattering cross section than dielectrics of the same dimensions (Fig. 8).

Fig. 8
figure 8

COMSOL Simulation of (a) dielectric nano-scatterer (b) metal nanoparticle

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Therefore, the various applications of plasmonic nanoparticles are associated because of two important physical effects:

  1. 1.

    Optical extinction of the metallic nanoparticles has a maximum at visible near-infrared wavelengths which is much larger than their geometrical size (because of their large extinction cross section).

  2. 2.

    Localized enhancement of electromagnetic field near the particles.

Hence, metallic nanoparticles because of their unique optical properties are ideal for being used as scattering centers in an active random lasing system. Both the scattering and absorption cross sections will be greatly enhanced due to the resonant surface plasmon response caused by the negative real part of the metal’s permittivity. Plasmon-enhanced scattering is observed to be greatest for metallic nanoparticles which when are incorporated in an active random media, result in optimization of both the scattering strength as well as the gain volume of the random laser. Additionally, confinement of electromagnetic radiation near the particle’s surface will enhance the local field strength increasing the gain within the particle’s vicinity. This gain is accessed by colocalization of the pump light and laser emission, whose wavelengths fall within the surface plasmon resonance bandwidth.

3.4 Employing Metal Nanoparticles in Random Lasing

The study of the emerging field of nanophotonics now calls for lasing systems that are unimpeded from diffraction limitations to extend its horizons towards investigating the rich physics at the nanometer scale by developing high-performance devices with new capabilities. The use of plasmonics due to its fascinating properties as discussed in Sect. 3.2. allow us to break the diffraction limit reducing the size of the bulky coherent light sources. It has been experimentally observed over distributed studies that the incorporation of metal nanoparticles predominantly gold and silver with various configurations and dimensions in active random lasing media offers low threshold lasing because of their good optical confinement and strong plasmonic enhancement properties. Furthermore, the ability of plasmons to scatter light more effectively and efficiently as compared to its dielectric or semiconductor counterparts of similar dimensions and shape of higher refractive index allows us to model lasers with high lasing efficiency. Metal scattering nanosystems provide stronger absorption of excitation light and a larger amplification of fluorescence than conventional dielectric scatterers owing to the enhancement of both the scattering effect and the localized electric field. The use of metal nanoparticles also offers greater amplification of weak physical and chemical light-matter interactions at nanoscale.

Plasmonic random lasers consist of metallic nanostructures that support localized surface plasmon modes [22]. The dye molecules coupled with the metallic nanostructures and a resonant energy transfer can occur between stimulated emission of the gain medium and EM fields of the surface plasmons, resulting in population inversion. These plasmons undergo stimulated emission and result in plasmon laser emission [81]. Consequently, the electromagnetic fields of the metal nanostructures can enhance both the spontaneous and stimulated emission of the surrounding gain media.

4 How Do Plasmons Enhance Random Lasing Performance?

In case of random lasing systems using dielectric particles as scatterers, light scattering is identified as the dominant phenomenon in the cavity. However, in the case of random lasers consisting of metal nanoparticles, the lasing mechanism becomes much more complicated given that plasmons not only scatter light but also absorb some amount of light. Therefore, in the case of plasmonic random lasers, the idea of tuning the lasing threshold and enhancing the random lasing output is based on controlling (a) scattering, (b) absorption, and (c) local field enhancement.

4.1 Increasing Random Lasing Efficiency

Lasing efficiency can be enhanced via two mechanisms in a metallic active disordered system,

  1. (a)

    Enhancement of scattering strength.

  2. (b)

    Enhancement of localized electromagnetic field in the vicinity of the metallic scatterer.

4.1.1 Increasing Lasing Efficiency Via Enhancement of Scattering Strength

In a random lasing system, the scattering strength is the decisive factor to enhance random lasing output. Physically, the scattering mean free path describes the distance traveled by a photon in between the two successive collisions the photon undergoes with the scattering centers. According to the Mie scattering theory, scattering strength of a random laser is quantified using the scattering mean free path (ls), which is given as [30],

$$ {l}_s=\frac{1}{\rho {\sigma}_{\mathrm{sca}}}, $$

where ρ and σsca are the scatterer number density and scattering cross section of a single scatterer, respectively (Fig. 9).

Fig. 9
figure 9

Illustration of scattering mean free path (ls) in a disordered system

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Therefore, the scattering strength of a disordered system can be controlled by either tuning the scatterer number density or by using scatterers with smaller scattering cross section. The number of particles in an optical gain volume can only be finite; therefore, the other method of increasing the scattering strength of a random lasing media is increasing the scattering cross section of the scattering particles. The scattering cross section of metal nanoparticle is larger than that of dielectric particles for a given size thereby making metal nanoparticle systems a better candidate to use in a random lasing media. In addition to controlling the scattering strength, light absorption needs to be tuned which again is dependent upon the cross section of the scatterers. Hence, metal nanoparticles have been widely embodied in a random laser medium.

4.1.2 Plasmonics-Induced Enhancement of Localized Electromagnetic Field

The spectral position of the plasmon resonance peak strongly depends on the material type, shape, size, and surrounding environment of the plasmonic nanoparticles. The variation in one or more of these parameters allows spectral tuning of the plasmon resonance to overlap with the absorption and emission spectrum of the active medium of interest. Plasmonic resonances change the local density of optical states in the close proximity of the nanoparticles, resulting in enhanced and highly confined optical fields close to the metal nanoparticle’s surface. Thus, plasmonic nanoparticles modify the non-radiative and radiative decay rates of nearby dye molecules by altering their electron decay lifetime and quantum yield. When metallic nanoparticles are excited resonantly, they scatter the energy of the emitters with enhanced scattering cross section therefore easily leading to spectral narrowing of the resonance peak. The radiation from the dye molecules is strongly scattered by metal nanoparticles due to the particle plasmon resonance. The enhancement in the fluorescence occurs when there is a spectral and spatial overlap between the metal particle plasmon resonance and the emission spectrum of the dye molecule [82]. In summary, depending upon the distance between the fluorophore and the metal nanoparticle the metal nanostructures have two main effects on the fluorescence of the dye molecule: (a) fluorescence enhancement and (b) fluorescence quenching. Here, we discuss how the presence of metallic nanoparticles contributes to increasing the dye molecule luminescence by affecting its radiative relaxation rate. When the separation distance between the dye molecule and the metal nanostructure is in the order of a few nanometers, a dramatic enhancement in fluorescence is observed due to the influence of localized surface plasmons (LSPs) excited in the metallic nanoparticles under the influence of external radiation. The fluorescence enhancement is possible due to the interaction between the near fields generated by LSPR and the fluorophores. The enhancement in the fluorescence of dye molecules is maximum when the plasmon resonance spectrum overlaps the absorption of that of the dye molecules. When the two spectra overlap, the pump radiation can be used completely for inducing the electronic or molecular transitions within the fluorophores, exciting a greater number of dye molecules thereby improving the overall absorption of pump light. When the external electromagnetic radiation induces electronic transitions within the dye molecules, the excited state fluorophores polarize the metal by interacting with the free electrons of the metal nanoparticle [83]. The LSPs generated as a result of the interaction, in turn, impose a reactive field on the fluorophore causing constructive interference between the molecular and metal nanoparticle dipole as shown in Fig. 10. Therefore, the metal nanoparticles serve as a source of enhanced local fields and amplified excitation.

Fig. 10
figure 10

Effect of localized surface plasmon resonance (LSPR) on fluorophore

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The presence of metal nanoparticle also increases the rate of energy transfer. The increase in resonance energy transfer within a random lasing system inclusive of plasmonic particles will increase the radiative decay rate. The fluorophore lifetime (τN) will decrease with an increase in the radiative decay (τN = Γ−1) [84]. As a result, in the presence of metal nanoparticles within a lasing system, the light absorption and scattering are enhanced by 5–6 times more than the emission of most strongly emitting fluorescent molecules. In contrast, the direct contact between the dye molecules and the metallic nanostructures when in close spatial proximity will introduce additional losses into the lasing system [68]. However, due to the fluorescence enhancement caused due to the presence of plasmon resonance, the optical gain can compensate for intrinsic losses in the metal nanocavities [85].

Since the metal nanoparticles scatter the energy of emitters with larger scattering cross section when resonantly excited, they lead to spectral narrowing. The Q-factor of the lasing cavity (Qc) is a function of the spectral width (Δλ) and the emission wavelength (λ). Therefore, spectral narrowing corresponds to a high Q-factor. The mathematical formulation of Q-factor of metal nanoparticles (QMNP) is given as follows [85],

$$ {Q}_{\mathrm{MNP}}=-\frac{\operatorname{Re}\left[\varepsilon \left(\omega \right)\right]}{\operatorname{Im}\left[\varepsilon \left(\omega \right)\right]} $$
(4)

A higher Q-factor of the metal nanoparticles corresponds to stronger local field enhancement. Similar to dielectric scattering centers, when metal nanoparticles are spatially close to each other, a variety of enhanced optical modes are generated as a function of the phase difference between the interfering modes. Depending upon the type of interference, constructive or destructive interference, asymmetric LSPR emission profiles can be observed in the resulting spectra which are known as Fano resonance. In conclusion, we can use metal nanoparticles to not only intensify the fluorescence but also increase the quality factor of the random lasing media for constructing high-power lasers.

4.2 Lowering Random Lasing Threshold

According to the mathematical definition of lasing threshold, it is a ratio of total cavity loss to spontaneous emission factor (β). The use of dielectric nanosystems for random lasing required a comparatively high threshold to initiate lasing action. To unravel the issue of high threshold random lasers, metal nanoparticles were tested in an active disordered media since the emission factor of metal nanoparticles is high. Metallic nanoparticles generate modes at discrete wavelengths. The most confined modes in the metallic nanostructures exhibit faster emission coupling rates as compared to less well confined modes, leading to preferential coupling of the most confined modes. The spontaneous emission factor (β) is extremely important in the threshold behavior of a laser. It is the proportion of emission that couples into a single mode. The emission factor for metal nanoparticles can be as high as 0.8 [86] suggesting low threshold lasing conditions in the presence of metallic nanoparticles. For instance, in their work, Sebastian et al. used rhodamine 6G doped polymer optical fibers as active media for random lasing. However, the sample showed no signs of lasing in the sample containing only the active media doped polymer fibers, i.e., in the absence of metal nanoparticles. As soon as the silver nanoparticles were added to the sample, the sample began to lase randomly at an optimum concentration of silver nanoparticles suggesting that the laser threshold can be significantly reduced with addition of metal nanoparticles as shown in Fig. 11 [54]. In Fig.11, R1, R2, R3, and R4 represent samples containing different concentrations of silver. R1 contains only the dye molecules, i.e., Rhodamine 6G, R2, R3, and R4 contained 9 × 10−6 mol L−1, 1.5 × 10−6 mol L−1, and 4.5 × 10−6 mol L−1 of Ag nanoparticles, respectively.

Fig. 11
figure 11

Laser-induced emission from the fibers as a function of pump energy. The inset shows the output intensity versus wavelength graph of all fibers R1, R2, R3, and R4 (different concentrations of silver nanoparticles—refer to Sect. 4.2) at pump energy of 26 mJ. Reprinted with permission from [54]. Copyright 2014, Institute of Physics

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We are aware that the addition of metallic nanosystems in random lasing media can induce losses; therefore, often silver nanoparticles are recognized as the best choice and are preferred over gold nanoparticles for random lasing purposes. Furthermore, since losses dominate over gain due to feedback from multiple scattering in case of gold nanoparticles, the local field enhancement is moderate as compared to silver nanoparticles. Silver nanoparticles are the best choice to realize a low threshold plasmon random laser because it has low surface plasmon loss in the visible and near-infrared spectral range. However, gold nanoparticles feature much lower chemical degradation and are hence more stable in ambient conditions.

5 Plasmonic Systems Employed in Random Lasing

Reports of random lasers received considerable attention and inspired vigorous efforts in research towards designing systems exhibiting random lasing. The substantial research in the field of random lasing included using dielectric particles, semiconductors, biological samples, polymers and also metal nanoparticles, specifically plasmonic nanoparticles [53, 64, 67, 70, 87, 88]. The incorporation of plasmonic nanoparticles in laser dye medium has been of particular interest owing to the extraordinary optical properties displayed by metal particles at nanoscale. Most of the research using metal nanostructures focuses on reducing the laser threshold by modifying the fluorescence yield of the gain molecules. The idea of using metal nanosystems in random lasing was set in motion by Dice et al. by mixing silver nanoparticles of diameter 55 nm to a dye solution of rhodamine 6G [53]. Spectral narrowing in the random lasing emission spectra was observed at threshold value. However, in this work, the plasmon resonance wavelength (~411 nm) weakly overlapped with the emission maximum of rhodamine 6G. Popov et al. suggested that for optimization of enhancement of random lasing using metal nanoparticles, a strong spectral overlap between the plasmon resonance and emission of gain media is a necessary condition [70]. This assertion was further backed by Popov by using a sample of polymer film with dye molecules and dispersed spherical gold nanoparticles. On examining the effect of metal nanoparticle size on the emission spectra, it was observed that with increase in the size of the plasmonic particles, the lasing threshold decreased thereby increasing the lasing efficiency. The optimum enhancement of the random lasing system was obtained when the plasmon wavelength overlapped with the emission of the dye molecule. Following these studies, many active random lasing systems were introduced incorporating metal nanoparticles coupled with various materials. For instance, Kang reported a silica nanosphere coated with gold producing strong random lasing with a low power continuous wave laser pump (200 mW) [87]. Coherent feedback random lasing from polymer optical fibres doped with silver and rhodamine 6G was observed with increased photostability of fluorophores and low threshold spectra narrowing was reported for the first time [54]. With different morphologies of metal nanoparticles tested to study the effect of varying size and dimension of metal nanoparticles on random lasing output, Ziegler et al. reported gold nanostars to demonstrate the enhanced performance of this configuration as compared to gold nanospheres and nanorods. Gold nanostars produced multiple plasmon resonances that overlapped with the emission spectra of the gain molecule (Rhodamine 6G) [65]. Studies concerning the tunability of plasmonic random lasers have also been conducted. One of the approaches that were used was mechanically tuning the polarization of the random lasers by stretching the silver nanowires embedded on a flexible substrate doped with Rhodamine 6G by Zhai et al. Using the mechanical tunability approach, Zhai and group could tune the random lasing emission wavelength from 558 nm to 565 nm. The wavelength-tunability within the sample was a result of narrowed plasmon resonance of silver nanowires on stretching the sample [64]. Recently, another cross-sectional study with a change in shape and size of the metal nanoparticle was carried out using gold nano-urchins in a dye doped polymer sample for random lasing. The study showed that gold nano-urchins had a higher scattering cross section as compared to gold nanospheres indicating two-fold local field enhancement [67]. Various similar experiments coupling semiconductors and dielectrics to metal nanoparticles are being carried out for testing enhanced random lasing [25, 68, 89, 90].

6 Applications of Plasmonic Random Lasers

Plasmonics when incorporated into a random lasing system allow us to reduce the size of the lasers increasing the laser efficacy which in turn could be integrated into photonic circuits to increase the speed of the optical communications opening up new applications in optics and spectroscopy. The potential applications for these unique plasmon-driven lasers are displays [55], sensors [58, 91, 92], and tunable narrowband coherent light sources [93]. Since the random lasing emission depends upon the configurational and geometrical changes within the cavity, they can be used in monitoring structural changes in biological samples [41]. For instance, random lasing emission spectra can assist in examining the structural deformations and material damage in bone structures [43, 45] allowing us to carry out detailed analysis of bone composition at nanoscale. Furthermore, as discussed (Sect. 2.3.2.3), there have been studies concentrated on using plasmonic random lasers for distinguishing tumor cells from a system of mammalian cells by monitoring the changes in the scattering pattern observed in the form of multiple spectral lines for cancerous cells [42, 44]. The addition of metal nanoparticles to enhance the lasing effects in a biological system has been of great interest, ranging from adding metal nanoparticles predominately gold and silver nanoparticles to amplify the effects in photodynamic and photothermal therapy [94, 95] to biological applications such as nanomedicine and drug delivery [96, 97], biosensing [57, 98,99,100], imaging and spectral analysis [57, 59, 101, 102]. Similarly, in order to increase the efficacy of tumor cell analysis and structural deformation studies, one can incorporate plasmonics to amplify the random lasing output simplifying analysis.

7 Conclusion and Future Scope

In this chapter, we have discussed a method of using plasmonic nanosystems for random lasing applications by addressing concerns of high threshold and low Q-factor that limited the significance and usefulness of random lasers till date. We have highlighted all the elements of research directed towards overcoming the shortcomings of random lasers by using metallic nanosystems that harness plasmon resonance. Over a period of time, the field of plasmonic random lasers has grown and displayed unexpected futuristic applications and hence has now become a rich field for investigation. High-efficiency lasers without cutoff sizes can be manufactured by integrating plasmonic nanosystems in the random lasing media. Additionally, the study of plasmonic nanosystems can be extended towards real-time tunable lasing emission. There are many other possibilities of future work in plasmonic random lasers such as the emission directionality control and also coherence studies for determining the concentration of plasmonic nanoparticles to observe spatial and temporal coherence [22]. The engineering studies to integrate plasmonic nanolasers to model and construct powerful small volumed devices can open up a new branch of technology for newfangled futuristic applications.