1-1 Optical properties of random lasers
Conventional laser versus a diffusive random laser(see Fig. 1.1) : While a conventional laser cavity (left) has highly reflecting mirrors that trap light long enough for amplification by the gain medium (light blue) to be efficient, trapping of light in a random laser (right) is not achieved by mirrors, but by multiple scattering between subwavelength-size particles (red dots). In such a random medium, the light emitted by an atom (yellow) can make a roundtrip in an infinite number of loops. In the special case of a diffusive random laser, the scattering is so weak that without gain most of the light would escape before returning to its starting point. Because of this strong damping (leakage), the oscillation frequencies of such a system are not well-defined until lasing sets in and a number of well-defined, sharp oscillation frequencies appear, bearing no straightforward relationship to the strongly damped natural oscillations of the system without gain. (Courtesy of Science Magazine and Robert Tandy).
Fig. 1.1 Schematic of the design of (a) a conventional laser resonator cavity with its two discrete end mirrors and (b) a random laser based on scattering. Due to their different geometries the lasers exhibit very different characteristics. Green arrows indicate the output laser beam from the devices; red spheres are scattering particles and blue arrows show optical paths.
Compared to traditional lasers which need fixed reflection mirrors to form a cavity, random lasers can be generated by using scattering materials to form cavity-like multiple scattering loop paths. There are several unique advantages of random lasers (see Fig. 1.2), such as simple fabrication process, small size, low cost, multiple lasing wavelengths, broad solid angle of lasing output. In recent years, random lasers have been employed in speckle-free imaging, medical diagnostics, liquid crystal display and illumination system.
Fig. 1.2 Advantages of random laser
Random lasers, unlike the conventional lasers that assemble rigid optical elements to construct the reflective resonant cavity for achieving an optical feedback, are one of the most unique light sources for which the lasing properties are mainly determined by the interaction between the light scattering and the gain material [1.1-1.5]. The emitted light is trapped and/or diffused into the gain medium through the disorder-induced multiple scatterings similar to the electron transport behavior observed in a defect-containing solid [1.6,1.7], leading to the formation of multiple closed loops for the emitted photons, which is the required optical feedback for lasing actions. In short, the occurrence of random lasing action could be regarded as a combined process of the diffusion of emitted photons and optical amplification. To probe the abundance of fundamental physics and possibly realize the Anderson localization of photons [1.8-1.11], random lasers have been intensely studied over the past decade along with the witness of random lasing
nanostructures/powders [1.12,1.13], dye-doped liquid crystals [1.14,1.15], dye-infiltrated composites [1.16,1.17], and even human tissue [1.18,1.19]. As reflective resonant cavity and additional optical elements are not involved in the lasing process, the random laser devices are of low cost, can easily be fabricated on a large area, and have high flexibility in operation; and the unique optical property of random lasers gives rise to an interesting potential for their applications in environment lighting, remote sensors, and medical diagnostics [1.20-1.22]. However, tunability is an important feature that profoundly affects and determines the application scope of laser devices. The tuning of conventional lasers can be easily achieved by adjusting the resonant frequency (or length) of the resonant cavity tocontrol both the wavelength and optical mode of lasing emissions. However, for a random laser, the lack of both a well-defined resonant cavity and the rigid alignment of optical elements results in a relatively more difficult tuning of random lasing emissions; therefore, considerable attempts have been made by researchers to overcome this issue [1.23-1.30].
To generate tunable random lasing emissions, a specific type of disordered medium, in which the degree/intensity of multiple random scatterings for the emitted photons can be varied to a certain extent, should be incorporated into the random laser system. Liquid crystal materials with the unique anisotropy of birefringence were first proposed and had been widely used as the disordered media to generate random lasers with high tenability [1.20,1.23,1.24]. Because the orientation of liquid crystal molecules can be controlled by applying an electric field or by changing the ambient temperature, it is therefore possible to obtain
different scattering intensities and diffusion constants for the emitted photons, and thus the physical properties of random lasers such as lasing threshold, emission wavelength, and even optical polarization are tunable.
Additionally, a novel three-dimensional (3D) disordered structure composed of self-assembled monodisperse polystyrene microspheres, which can spectrally modify the scattering coefficient of emitted photons through the so-called Mie resonances, was recently proposed [1.25-1.27].
The transmission spectrum of such 3D disordered structure can be controlled to match with the gain profile by changing the diameters or the refractive index of polystyrene microspheres. This gives rise to random lasers of preferential emission wavelengths. The utilization of a stretchable substrate is another feasible method to generate tunable random lasers. A disordered medium (e.g., ZnO nanobrushes [1.28,1.29]
and silver nanowires [1.30] ) is firs embedded into the stretchable substrate, and then the interplay between the disordered medium and the emitted photons, including the formation of coherent loops and the peak position of plasmonic resonance is then manipulated by mechanically stretching the deformable substrate. All the lasing parameters, for instance, the number of modes, the emission wavelength, and the degree of polarization, can be tunable by this method.
1-2 Bending strain of PET substrate
In this work, we developed a feasible and reliable approach to generate a random laser with a wide tunable range in the emission wavelength. We experimentally demonstrated that the lasing wavelength is correlated to the bending diameter of the curvature on the flexible PET substrate underneath, and hence, it exhibits a high tunability in accordance with the observed scattering spectrum of the silver nanoprisms (Ag NPRs). Although the generation of random lasing emissions has been well established on flexible substrates and widely reported in the literature [1.31], the aim of this work was to confirm that the optical properties of random lasers could be controlled simply by mechanical bending. Such phenomenon, to our knowledge, has been rarely observed or discussed in the past. However, the incorporation of regular or irregular shaped metallic nanoparticles into optical gain to stimulate random lasing emissions has been reported in the recent years [1.32-1.34]. A high degree of spectral overlap between the localized surface plasmon (LSP) resonance of metallic nanoparticles and the optical profile of gain medium facilitates the random lasing oscillations.
A similar mechanism that relies on the surface plasmon effect to stimulate the so-called plasmon nanolaser through the light-matter interactions has been intensively studied as well [1.35-1.37]. Such plasmon nanolaser outperforms the conventional laser because of its extremely small mode volume, large Purcell factor, and slow group velocity to ensure strong exciton/matter coupling.
Consequently, as compared to the previous work in which Ag
nanowires were used as the tunable medium [1.30], the proposed strategy in this study directly controls the scattering profile of the Ag NPRs to match with the Rhodamine 6G (R6G) gain by bending the flexible PET substrate that influences the coupling strength and the resonant frequency on the LSP of the Ag NPRs. Because the bending strain mainly affects the interparticle distance in Ag NPRs and barely causes breakages or damages to them, the resultant tuning of lasing wavelength is hence reversible and highly repeatable. It is very difficult to achieve by using the previously described method. Additionally, their claim of lasing emission is closer to the amplified spontaneous emission, rather than the coherent random lasing emission [1.30]. As a result, a maximum shift of
~15 nm was achieved in the lasing wavelength, which is higher than the previously reported value of ~7 nm. This means that we can stimulate a random lasing action with a preferential lasing wavelength within the gain profile of the gain medium, in turn providing a new important feature in the functionality of flexible optoelectronic devices. We expect that this flexible and tunable random laser would be used in a broad range of novel applications such as wearable gadgets for personal healthcare and structural health monitoring of civil infrastructure. Additionally, the fundamental of light-matter interaction and how it affects the transport mean free path of emitted photons at the nanoscale range can also be investigated using such novel structure.