The role of polymers in random lasing

Speckle-Free, Angle-Free, Cavity-Free White Laser with a High Color Rendering Index

The freedom from efficiency droop motivates monochromatic lasers to progress in general lighting applications due to the demand for more efficient and sustainable light sources. Still, a white light based on monochromatic lasers with high lighting quality, such as a high color rendering ability, an angle-independent output, and a speckle-free illumination, has not yet been fabricated nor demonstrated. Random lasers, with the special mechanism caused by multiple scattering, the angle-free emission, and the uncomplicated fabrication processes, inspire us to investigate the feasibility of utilizing them in general lighting. In this work, a white random laser with a high color rendering index (CRI) value, regardless of pumping energy and observing direction, was performed and discussed. We also investigated the stability of white RL as its CIE chromaticity coordinates exhibit negligible differences with increasing pump energy density, retaining its high-CRI measurement. Also, it exhibits angle-independent emission while having a high color rendering ability. After passing through a scattering film, it generated no speckles compared to the conventional laser. We demonstrated the advances in white laser illumination, showing that a white random laser is promising to be applied for high-brightness illumination, biological-friendly lighting, accurate color selections, and medical sensing.

Lasing from Micro- and Nano-Scale Photonic Disordered Structures for Biomedical Applications

A disordered photonic medium is one in which scatterers are distributed randomly. Light entering such media experiences multiple scattering events, resulting in a "random walk"-like propagation. Micro- and nano-scale structured disordered photonic media offer platforms for enhanced light-matter interaction, and in the presence of an appropriate gain medium, coherence-tunable, quasi-monochromatic lasing emission known as random lasing can be obtained. This paper discusses the fundamental physics of light propagation in micro- and nano-scale disordered structures leading to the random lasing phenomenon and related aspects. It then provides a state-of-the-art review of this topic, with special attention to recent advancements of such random lasers and their potential biomedical imaging and biosensing applications.

2D tunable all-solid-state random laser in the visible

A two-dimensional (2D) solid-state random laser emitting in the visible is demonstrated, in which optical feedback is provided by a controlled disordered arrangement of air-holes in a dye-doped polymer film. We find an optimal scatterer density for which threshold is minimum and scattering is the strongest. We show that the laser emission can be red-shifted by either decreasing scatterer density or increasing pump area. We show that spatial coherence is easily controlled by varying pump area. Such a 2D random laser provides with a compact on-chip tunable laser source and a unique platform to explore non-Hermitian photonics in the visible.

Properties and Applications of Random Lasers as Emerging Light Sources and Optical Sensors: A Review

In a random laser (RL), optical feedback arises from multiple scattering instead of conventional mirrors. RLs generate a laser-like emission, and meanwhile take advantage of a simpler and more flexible laser configuration. The applicability of RLs as light sources and optical sensors has been proved. These applications have been extended to the biological field, with tissues as natural scattering materials. Herein, the current state of the RL properties and applications was reviewed.

Simultaneous evaluation of intermittency effects, replica symmetry breaking and modes dynamics correlations in a Nd:YAG random laser

Random lasers (RLs) are remarkable experimental platforms to advance the understanding of complex systems phenomena, such as the replica-symmetry-breaking (RSB) spin glass phase, dynamics modes correlations, and turbulence. Here we study these three phenomena jointly in a Nd:YAG based RL synthesized for the first time using a spray pyrolysis method. We propose a couple of modified Pearson correlation coefficients that are simultaneously sensitive to the emergence and fading out of photonic intermittency turbulent-like effects, dynamics evolution of modes correlations, and onset of RSB behavior. Our results show how intertwined these phenomena are in RLs, and suggest that they might share some common underlying mechanisms, possibly approached in future theoretical models under a unified treatment.

Bio-inspired wrinkle microstructures for random lasing governed by surface roughness

A method for fabricating bio-inspired scattering substrates based on polydimethylsiloxane (PDMS) for spatially incoherent random lasing is presented. The leaves of monstera and piper sarmentosum plants are used to mold PDMS polymer to form wrinkle-like scattering substrates, which are then used with a liquid gain medium for random lasing. Scattering is attributed to the surface roughness (S_a) of the samples. The rougher sample with 5.2 µm S_a shows a two-mode stable lasing with a 2 nm linewidth and a lower threshold fluence of 0.2mJ/cm² compared to the sample with smaller S_a (3.6 µm) with a linewidth of 5 nm and a threshold fluence of 0.5mJ/cm². The waveguide theory substantiates the results of incoherent random lasing through a relation between the microstructure feature size and the mean free path. Power Fourier transform analysis is used to deduce the resonant cavity length of 180 µm in the rougher sample, and the observed variations in cavity length with S_a validate the optical feedback. PDMS being hydrophobic, the scattering substrate can be reused by wiping off the gain medium. This Letter paves the way for facile fabrication methods of bio-inspired random lasers for sensing and imaging applications.

Hybrid electronically addressable random fiber laser

We report here a novel architecture for a random fiber laser exploiting the combination of a semiconductor optical amplifier (SOA) and an erbium doped fiber (EDF). The EDF was optically biased by a continuous wave pump laser, whereas the SOA was arranged in a fiber loop-mirror and driven by nanosecond duration current pulses. Laser pulses were obtained by synchronizing the SOA driver to the returning amplified Rayleigh back-scattered light from a selected short section of the EDF. By tuning the SOA pulse rate, random lasing was achieved by addressing selected meter-long sections of the 81-m long EDF, which was open-ended. Laser oscillation can be potentially obtained with SOA modulation frequencies from several kHz to the MHz regime. We discuss the mechanism leading to the hybrid random laser emission, connecting with phase sensitive optical time domain reflectometry and envision potential applications of this electronically addressable random laser.

Monolayer 2D ZrTe2 transition metal dichalcogenide as nanoscatter for random laser action

We demonstrate random laser emission from Rhodamine 6G with ZrTe2 transition metal dichalcogenide (TMD) as nanoscatters, both in powder and 2D nanoflakes liquid suspension. The 2D semimetal ZrTe2 was synthesized by a modified redox exfoliation method to provide single layer TMD, which was employed for the first time as the scatter medium to provide feedback in an organic gain medium random laser. In order to exploit random laser emission and its threshold value, replica symmetry breaking leading to a photonic paramagnetic to photonic spin glass transition in both 2D and 3D (powder) ZrTe2 was demonstrated. One important aspect of mixing organic dyes with ZrTe2 is that there is no chemical reaction leading to dye degradation, demonstrated by operating over more than 2 hours of pulsed (5 Hz) random laser emission.

Determination of the Best Empiric Method to Quantify the Amplified Spontaneous Emission Threshold in Polymeric Active Waveguides

Amplified Spontaneous Emission (ASE) threshold represents a crucial parameter often used to establish if a material is a good candidate for applications to lasers. Even if the ASE properties of conjugated polymers have been widely investigated, the specific literature is characterized by several methods to determine the ASE threshold, making comparison among the obtained values impossible. We quantitatively compare 9 different methods employed in literature to determine the ASE threshold, in order to find out the best candidate to determine the most accurate estimate of it. The experiment has been performed on thin films of an homopolymer, a copolymer and a host:guest polymer blend, namely poly(9,9-dioctylfluorene) (PFO), poly(9,9-dioctylfluorene-cobenzothiadiazole) (F8BT) and F8BT:poly(3- hexylthiophene) (F8BT:rrP3HT), applying the Variable Pump Intensity (VPI) and the Variable Stripe Length (VSL) methods. We demonstrate that, among all the spectral features affected by the presence of ASE, the most sensitive is the spectral linewidth and that the best way to estimate the ASE threshold is to determine the excitation density at the beginning of the line narrowing. We also show that the methods most frequently used in literature always overestimate the threshold up to more than one order of magnitude.

Two-dimensional Wrinkle Resonators for Random Lasing in Organic Glasses

Random lasers consisting of slab waveguides with two-dimensional disordered wrinkling patterns that act as scattering resonators are reported. As active material 2,2',7,7'-tetraphenyl-9,9'-spirobifluorene is used which is sandwiched between an oxidized silicon wafer and a cladding with higher glass transition temperature. Wrinkles with tailorable periodicity have been induced by thermal annealing. Photopumping experiments show the transition from amplified spontaneous emission to a multiple peak laser spectrum with linewidths as low as 0.1 nm, demonstrating the applicability of this approach for random laser design.

Random Laser Action in Dye-Doped Polymer Media with Inhomogeneously Distributed Particles and Gain

The properties of random lasing are investigated for bubble-structure (BS) dye-doped polymer random media in which non-scattering and no-gain regions are distributed. Experimental results demonstrate that, for BS random media, spectral narrowing and a decrease in the number of spectral spikes occur for incoherent and coherent random lasing, respectively, resulting in an increase in the spectral peak intensity in both cases. These features were observed owing to the differences in the diffusion properties of the pumping and emitted lights.

Natural Born Laser Dyes: Excited-State Intramolecular Proton Transfer (ESIPT) Emitters and Their Use in Random Lasing Studies

A series of five excited-state intramolecular proton transfer (ESIPT) emitters based on a 2-(2′-hydroxyphenyl) benzoxazole (HBO) scaffold, functionalized with a mono-or bis-(trialkylsilyl) acetylene extended spacer are presented. Investigation of their photophysical properties in solution and in the solid-state in different matrix, along with ab initio calculations gave useful insights into their optical behavior. Random lasing studies were conducted on a series of PMMA doped thin films, showing the presence of stimulated emission above the threshold of pumping energy density (ρth ≈ 0.5–2.6 mJ cm⁻²). In this work, the similarity of four level laser systems is discussed in light of the ESIPT photocycle.

Demonstration of a White Laser with V2 C MXene-Based Quantum Dots

Multicolor photoluminescence over the full visible color spectrum is critical in many modern science and techniques, such as full-color lighting, displays, biological and chemical monitoring, multiband communication, etc., but the ultimate white lasing especially on the nanoscale is still a challenge due to its exacting requirements in the balance of the gain and optical feedback at different wavelengths. Recently, 2D transition metal carbides (MXenes) have emerged, with some superior chemical, physical, and environmental properties distinguishing them from traditional 2D materials. Here, a white laser with V₂ C MXene quantum dots (MQDs) is originally demonstrated by constructing a broadband nonlinear random scattering system with enhanced gain. The excitation-dependent photoluminescence of V₂ C MQDs is enhanced by passivation and characterized, and their localized nonlinear random scattering is realized by the generation of excitation-power-dependent solvent bubbles. With the optimized excitation, the blue, green, yellow, and red light is amplified and simultaneously lased. This work not only provides a kind of promising material for white lasers, but also a design strategy of novel photonics for further applications.

A random laser based on electrospun polymeric composite nanofibers with dual-size distribution

Electrospun fiber-based random lasers are environment-friendly flexible systems in which waveguiding/scattering processes provided by their structure with a broad distribution of diameters are essential elements to generate a suitable lasing mechanism. In this work, we prepared electrospun fibers with dual-size diameter distribution (above and below the critical value for waveguiding), allowing that both optical processes can be established in the polymer network. As a result, random laser emission was observed for the electrospun fibers presenting dual-size diameters with rhodamine 6G as the gain medium, characterizing the combination of waveguiding/scattering as an adequate condition for development of organic nanofibrous random lasers. Degradation assays were also performed in order to evaluate the prolonged use of such random laser systems.

Random lasing emission tailored by femtosecond and picosecond pulsed polymer ablation

We report the realization of random lasers with spatially localized feedback in which the average number of lasing modes is tuned via the fabrication process. The scattering elements required for optical feedback are obtained by short-pulsed laser ablation. By varying the pulse parameters, we control the scattering properties of the induced defects and, thus, the emission spectra. We demonstrate a large variety of spectral signatures typical of resonant random lasing with sub-nanometer linewidths, low thresholds (about 40  pJ/μm²), and single-to-multimode emission. Our simple approach allows us to obtain optical resonators with sharp linewidths at frequencies covering the entire gain window for multiple applications.

Solid-state random microlasers fabricated via femtosecond laser writing

Here we demonstrate resonant random lasing in Rhodamine B-doped polymeric microstructures fabricated by means of femtosecond laser writing via two-photon polymerization. To the best of our knowledge, this is the first demonstration of random lasing action in on-chip microdevices. Their feedback mechanism relies on diffuse reflections at the structure sidewall surfaces, which is known as spatially localized feedback since the scattering centers lie over the edges of the gain medium. By exciting the structures with a pulsed laser at 532 nm, a multimode emission with randomly distributed narrow peaks was observed, in accordance with the random nature of the feedback mechanism. Interestingly, their lasing threshold was found to be on the order of tens of nanojoules, which is comparable to what had been achieved for usual microcavities, thereby demonstrating the potentiality of these devices as solid-state lasers for integrated optics applications.

An extended excited-state intramolecular proton transfer (ESIPT) emitter for random lasing applications

An original ESIPT emitter showing random lasing properties is presented in this article.

Nanoparticle-doped electrospun fiber random lasers with spatially extended light modes

Complex assemblies of light-emitting polymer nanofibers with molecular materials exhibiting optical gain can lead to important advance to amorphous photonics and to random laser science and devices. In disordered mats of nanofibers, multiple scattering and waveguiding might interplay to determine localization or spreading of optical modes as well as correlation effects. Here we study electrospun fibers embedding a lasing fluorene-carbazole-fluorene molecule and doped with titania nanoparticles, which exhibit random lasing with sub-nm spectral width and threshold of about 9 mJ cm^-2 for the absorbed excitation fluence. We focus on the spatial and spectral behavior of optical modes in the disordered and non-woven networks, finding evidence for the presence of modes with very large spatial extent, up to the 100 µm-scale. These findings suggest emission coupling into integrated nanofiber transmission channels as effective mechanism for enhancing spectral selectivity in random lasers and correlations of light modes in the complex and disordered material.

Emission regimes of random lasers with spatially localized feedback

We report the experimental results obtained with a novel architecture for random lasing, in which the active material, free of scatterers, is placed between two large scattering regions. Lasing emission is investigated as a function of the illuminated area of the scattering regions, obtaining typical "resonant" and "non-resonant" random lasing spectra, depending on the device geometry. We propose a theoretical approach for the understanding of the observed phenomena, modelling the scattering elements with arbitrary spectral profiles in amplitude and phase and considering strong coupling between lasing modes. Good agreement between experiments and simulation results is obtained.

Large area resonant feedback random lasers based on dye-doped biopolymer films

We report resonant feedback random lasing from dye-doped biopolymer films, consisting of a deoxyribonucleic acid-cetyltrimethylammonium (DNA-CTMA) complex doped with DCM dye. In the proposed devices, the optical feedback for random lasing is given by scattering centers randomly positioned along the edges of the active area. Scattering elements are either titanium dioxide nanoparticles or random defects at the interface between active polymer and air. Different emission spectra are observed, depending on the geometry of the excited area. A single random resonator with dimensions of 2.6 mm x 0.65 mm is fabricated and random emission with resonant feedback is obtained by uniformly pumping the full device.