Decoupling gain and feedback in coherent random lasers: experiments and simulations

Light propagation in two-dimensional and three-dimensional slabs of reflective colloidal particles in solution: The effect of interfaces and interparticle correlations

The propagation of light across 2D and 3D slabs of reflective colloidal particles in a fluidlike state has been investigated by simulation. The colloids are represented as hard spheres with and without an attractive square-well tail. Representative configurations of particles have been generated by Monte Carlo. The path of rays entering the slab normal to its planar surface has been determined by exact geometric scattering conditions, assuming that particles are macroscopic spheres fully reflective at the surface of their hard-core potential. The analysis of light paths provides the transmission and reflection coefficients, the mean-free path, the average length of transmitted and reflected paths, the distribution of scattering events across the slab, the angular spread of the outcoming rays as a function of dimensionality, and thermodynamic state. The results highlight the presence of a sizable population of very long paths, which play an important role in random lasing from solutions of metal particles in an optically active fluid. The output power spectrum resulting from the stimulated emission amplification decays asymptotically as an inverse power law. The present study goes beyond the standard approach based on a random walk confined between two planar interfaces and parametrized in terms of the mean-free path and scattering matrix. Here, instead, the mean-free path, the correlation among scattering events, and memory effects are not assumed a priori, but emerge from the underlying statistical mechanics model of interacting particles. In this way the dependence of properties on the thermodynamic state, the effect of particle-particle and particle-interface correlations and of spatial inhomogeneity, and memory effects are accounted for in a transparent way. Moreover, the approach joins smoothly the ballistic regime of light propagation at low density with the diffusive regime at high density of scattering centers. These properties are exploited to investigate the effect of weak polydispersivity and of large density fluctuations at the critical point of the model with the attractive potential tail.

Random lasers from photonic crystal wings of butterfly and moth for speckle-free imaging

Several biological membranes have been served as scattering materials of random lasers, but few of them include natural photonic crystals. Here, we propose and demonstrate a facile approach to fabricating high-performance biological photonic crystal random lasers, which is cost-effective and reproducible for mass production. As a benchmark, optical and lasing properties of dye-coated Lepidoptera wings, including Papilio ulysses butterfly and Chrysiridia rhipheus moth, are characterized and show a stable laser emission with a superior threshold of 0.016 mJ/cm², as compared to previous studies. To deploy the proposed devices in practical implementation, we have applied the as-fabricated biological devices to bright speckle-free imaging applications, which is a more sustainable and more accessible imaging strategy.

A Self-Consistent Quantum Field Theory for Random Lasing

The spatial formation of coherent random laser modes in strongly scattering disordered random media is a central feature in the understanding of the physics of random lasers. We derive a quantum field theoretical method for random lasing in disordered samples of complex amplifying Mie resonators which is able to provide self-consistently and free of any fit parameter the full set of transport characteristics at and above the laser phase transition. The coherence length and the correlation volume respectively is derived as an experimentally measurable scale of the phase transition at the laser threshold. We find that the process of stimulated emission in extended disordered arrangements of active Mie resonators is ultimately connected to time-reversal symmetric multiple scattering in the sense of photonic transport while the diffusion coefficient is finite. A power law is found for the random laser mode diameters in stationary state with increasing pump intensity.

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.

Wrinkled 2D Materials: A Versatile Platform for Low-Threshold Stretchable Random Lasers

A stretchable, flexible, and bendable random laser system capable of lasing in a wide range of spectrum will have many potential applications in next- generation technologies, such as visible-spectrum communication, superbright solid-state lighting, biomedical studies, fluorescence, etc. However, producing an appropriate cavity for such a wide spectral range remains a challenge owing to the rigidity of the resonator for the generation of coherent loops. 2D materials with wrinkled structures exhibit superior advantages of high stretchability and a suitable matrix for photon trapping in between the hill and valley geometries compared to their flat counterparts. Here, the intriguing functionalities of wrinkled reduced graphene oxide, single-layer graphene, and few-layer hexagonal boron nitride, respectively, are utilized to design highly stretchable and wearable random laser devices with ultralow threshold. Using methyl-ammonium lead bromide perovskite nanocrystals (PNC) to illustrate the working principle, the lasing threshold is found to be ≈10 µJ cm^-2 , about two times less than the lowest value ever reported. In addition to PNC, it is demonstrated that the output lasing wavelength can be tuned using different active materials such as semiconductor quantum dots. Thus, this study is very useful for the future development of high-performance wearable optoelectronic devices.

Random lasing in human tissues embedded with organic dyes for cancer diagnosis

Various nanostructures found in biological organisms are often complex and they exhibit unique optical functions. This study surprisingly found that typical random lasing occurs in cancerous human tissues embedded with the nanotextured organic dye 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7- tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB). Hematoxylin and eosin stain images show that there are more laser resonators in cancerous tissues, caused by a large number of disordered scatters. It is also noteworthy that the random lasing thresholds were found to relate to the tumor malignancy grade. Consequently, the resulting typical random lasing resonators differ between cancerous tissues in different malignancy grades. Further studies are warranted to investigate tissue optical spectroscopy in the field of cancer diagnostics.

Lasing optical cavities based on macroscopic scattering elements

Two major elements are required in a laser device: light confinement and light amplification. Light confinement is obtained in optical cavities by employing a pair of mirrors or by periodic spatial modulation of the refractive index as in photonic crystals and Bragg gratings. In random lasers, randomly placed nanoparticles embedded in the active material provide distributed optical feedback for lasing action. Recently, we demonstrated a novel architecture in which scattering nanoparticles and active element are spatially separated and random lasing is observed. Here we show that this approach can be extended to scattering media with macroscopic size, namely, a pair of sand grains, which act as feedback elements and output couplers, resulting in lasing emission. We demonstrate that the number of lasing modes depends on the surface roughness of the sand grains in use which affect the coherent feedback and thus the emission spectrum. Our findings offer a new perspective of material science and photonic structures, facilitating a novel and simple approach for the realization of new photonics devices based on natural scattering materials.

Quasi mode-locking of coherent feedback random fiber laser

Mode-locking is a milestone in the history of lasers that allows the generation of short light pulses and stabilization of lasers. This phenomenon is known to occur only in standard ordered lasers for long time and until recently it is found that it also occurs in disordered random lasers formed by nanoscale particles. Here, we report the realization of a so-called quasi mode-locking of coherent feedback random fiber laser which consists of a partially disordered linear cavity formed between a point reflector and a random distributed fiber Bragg grating array with an inserted graphene saturable absorber. We show that multi-groups of regular light pulses/sub-pulses with different repetition frequencies are generated within the quasi mode-locking regime through the so-called collective resonances phenomenon in such a random fiber laser. This work may provide a platform to study mode locking as well as pulse dynamic regulation of random lasing emission of coherent feedback disordered structures and pave the way to the development of novel multi-frequency pulse fiber lasers with potentially wide frequency tuning range.

Low-Threshold and High Intensity Random Lasing Enhanced by MnCl₂

Energy transfer is known to have a significant influence on random lasers. However, the study about the effect of energy transfer between metallic salt and dye molecules on random lasers is still lacking at present. Here, we investigate random lasing actions in Pyrromethene-597 (PM597), PM597-doped MnCl₂ (manganese (II) chloride), PM597-doped polymer-dispersed liquid crystal (PDLC) and PM597-doped PDLC with MnCl₂ capillary systems. We find that random lasing of the systems with MnCl₂ exhibits lower threshold, higher intensity, sharper peak and variable resonance wavelength in comparison with the systems without MnCl₂. This behavior is closely related to the decrease of fluorescence quenching effect and the enhancement of local field induced by energy transfer between MnCl₂ and PM597. Red-shift of wavelength is observed with increasing dosage concentration of MnCl₂ in the PM597-doped PDLC with MnCl₂ system. Through the analysis of single-shot emission spectra of PM597-doped PDLC without and with MnCl₂ systems, the role of MnCl₂ in the coupling of lasing modes is confirmed. Lengths of laser oscillation cavities of the PM597-doped PDLC without and with MnCl₂ systems are calculated by a power Fourier transform (PFT) analysis of their emission spectra. It well accounts for the effect of MnCl₂ on the variation of the oscillation cavity.

Fiber-Type Random Laser Based on a Cylindrical Waveguide with a Disordered Cladding Layer

This letter reports a fiber-type random laser (RL) which is made from a capillary coated with a disordered layer at its internal surface and filled with a gain (laser dye) solution in the core region. This fiber-type optical structure, with the disordered layer providing randomly scattered light into the gain region and the cylindrical waveguide providing confinement of light, assists the formation of random lasing modes and enables a flexible and efficient way of making random lasers. We found that the RL is sensitive to laser dye concentration in the core region and there exists a fine exponential relationship between the lasing intensity and particle concentration in the gain solution. The proposed structure could be a fine platform of realizing random lasing and random lasing based sensing.

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.