Plasmonic Nanostars as Efficient Broadband Scatterers for Random Lasing

Time sequence variation of incoherent and coherent random laser based on positive replica of abalone shell

Besides the scattering structures, the energy transfer (ET) process in the gain medium plays a significant role in the competition between coherent (comprising strongly coherent components) and incoherent (consisting of weakly coherent or "hidden" coherent components) modes of random lasers. In this study, bichromatic emission random lasers were successfully created using polydimethylsiloxane (PDMS) replicas with grooved structures that imitate the inner surface of abalone shells as scattering substrates. The influence mechanism of the ET process from the monomer to dimer in the Rhodamine 640 dye on the competition of random laser modes was thoroughly investigated from both spectral and temporal dimensions. It was confirmed that the ET process can reduce the gain of monomers while amplifying the gain of dimers. By considering the dominant high-efficiency ET processes, an energy transfer factor associated with the pump energy density was determined. Notably, for the first time, it was validated that the statistical distribution characteristics of the time sequence variations in the coherent random laser generated by dimers closely resemble a normal distribution. This finding demonstrates the feasibility of producing high-quality random number sequences.

Wave amplitude gain within wedge waveguides through scattering by simple obstacles

Wave confinement, e.g., in waveguides, gives rise to a huge number of distinct phenomena. Among them, amplitude gain is a recurrent and relevant effect in undulatory processes. Using a general purpose protocol to solve wave equations, the boundary wall method, we demonstrate that for relatively simple geometries, namely, a few leaky or opaque obstacles inside a θ wedge waveguide (described by the Helmholtz equation), one can obtain a considerable wave amplification in certain spatially localized regions of the system. The approach relies on an expression for the wedge waveguide exact Green's function in the case of θ=π/M (M=1,2,...), derived through the method of images allied to group theory concepts. The formula is particularly amenable to numerical calculations, greatly facilitating simulations. As an interesting by-product of the present framework, we are able to obtain the eigenstates of certain closed shapes (billiards) placed within the waveguide, as demonstrated for triangular structures. Finally, we briefly discuss possible concrete realizations for our setups in the context of matter and electromagnetic (for some particular modes and conditions) waves.

Temperature-controlled spectral tuning of a single wavelength polymer-based solid-state random laser

We demonstrate temperature-controlled spectral tunability of a partially-pumped single-wavelength random laser in a solid-state random laser based on DCM [4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran] doped PMMA (polymethyl methacrylate) dye. By carefully shaping the spatial profile of the pump, we first achieve a low-threshold, single-mode random lasing with an excellent side lobe rejection. Notably, we show how temperature-induced changes in the refractive index of the PMMA-DCM layer result in a blue shift of this single lasing mode. We demonstrate spectral tunability over an 8nm-wide bandwidth.

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.

Plasmonic distributed feedback lasing in an anodic aluminum oxide/silver/polymer hybrid membrane

A hybrid membrane is employed as a high-order plasmonic distributed feedback (DFB) cavity to reduce the lasing threshold of polymer lasers. The hybrid membrane consists of an anodic aluminum oxide (AAO) membrane, a 25 nm thick silver layer and a free-standing polymer membrane. The AAO membrane is fabricated by a low-cost, single chemical etching method. Then, a layer of silver with a thickness of 25 nm is sputtered on the surface of the AAO. Subsequently, a polymer membrane is directly attached to the silver-plated AAO membrane, forming an AAO/silver/polymer hybrid membrane. Under optical pumping conditions, low-threshold, three-order DFB lasing is observed. The proposed laser device exhibited a dual-threshold characteristic because of the evolution from amplified spontaneous emission to DFB lasing. And a significant shift from omnidirectional emission to directional emission lasing can be observed while the pump energy density is beyond the second threshold. Furthermore, the plasmonic enhancement sourced from silver corrugation reveals important improvement effects to the DFB lasing of AAO/silver/polymer hybrid membrane for decreasing threshold, narrowing full width at half maximum (FWHM), and an increasing Q factor. This work may promote the design and production of low-cost and large-area high-order plasmonic DFB polymer lasers.

Controlling the lasing modes in random lasers operating in the Anderson localization regime

Random lasers, which rely on random scattering events unlike traditional Fabry-Pérot cavities, are much simpler and cost-effective to fabricate. However, because of the chaotic fluctuations and instability of the lasing modes, controlling the lasing properties is challenging. In this study, we use random InP nanowire (NW) arrays that operate in the Anderson localization regime with stable modes as the random lasers. We show that by changing the design parameters of the NW arrays, such as filling factor, dimensions of the NWs, degree of randomness, and the size of the array, the properties of the lasing modes including the number of modes, lasing wavelengths, and lasing threshold can be controlled.

Tailoring the Random Lasing Properties by Controlled Phase Separation Process in PMMA:PVK Dye-Doped Polymeric Blends

This article describes the random lasing (RL) phenomenon obtained in a dye-doped, polymeric double-phase system composed of PMMA and PVK polymers. It shows how relative concentrations between mentioned macromolecules can influence lasing parameters of the resulting blends, including obtained emission spectra and threshold conditions. We describe the influence of lasers' composition on their morphologies and link them with particular RL properties. Our studies reveal that the disorder caused by phase separation can support the RL phenomenon both in the waveguiding and quasi-waveguiding regimes. Changing the relative concentration of polymers enables one to switch between both regimes, which significantly influences threshold conditions, spectral shift, number of lasing modes, and ability to support extended and/or localized modes. Finally, we show that a simple phase separation technique can be used to fabricate efficient materials for RL. Moreover, it enables the tailoring of lasing properties of materials in a relatively wide range at the stage of the laser material fabrication process in a simple way. Therefore, this technique can be seen as a fast, cheap, and easy to perform way of random lasers fabrication.

Random Lasing via Plasmon-Induced Cavitation of Microbubbles

Numerous laboratories have observed random lasing from optically pumped solutions of plasmonic nanoparticles (NPs) suspended with organic dye molecules. The underlying mechanism is typically attributed to the formation of closed-loop optical cavities enabled by the large local field and scattering enhancements in the vicinity of plasmonic NPs. In this manuscript, we propose an alternative mechanism that does not directly require the plasmon resonance. We used high-speed confocal microspectroscopy to observe the photophysical dynamics of NPs in solution. Laser pulses induce the formation of microbubbles that surround and encapsulate the NPs, then sharp peaks <1.0 nm are observed that match the spectral signature of random lasing. Electromagnetic simulations indicate that ensembles of microbubbles may form optical corral containing standing wave patterns that are sufficient to sustain coherent optical feedback in a gain medium. Collectively, these results show that ensembles of plasmonic-induced bubbles can generate optical feedback and random lasing.

Dual-color plasmonic random lasers for speckle-free imaging

A dual-color plasmonic random laser under single-excitation is achieved in an ultrathin membrane doped with binary quantum dots and gold nanorods. The gold nanorods tune the luminescence lifetime and emission efficiency of quantum dots. Under single excitation, low-threshold random lasing is observed. Green random lasing at 547 nm is 'turned on' and red random lasing at 630 nm is greatly enhanced by the transversal and longitudinal surface plasmon resonance of the gold nanorods, respectively. Speckle-free color imaging is achieved by using the proposed dual-color random laser source. These properties would facilitate the development of random lasers in fields of illumination and imaging.

Microfluidic Synthesis of Hybrid TiO2-Anisotropic Gold Nanoparticles with Visible and Near-Infrared Activity

Anisotropic gold nanoparticles (AuNPs), with their unique physical and optical properties, are emerging as smart and key nanomaterials and are being exploited in many crucial fields. To further improve their range of action, anisotropic AuNPs have been coupled with semiconductors, mainly TiO₂ (titania), receiving great interest as powerful platforms both in biomedicine and in catalytic applications. Such hybrid nanoparticles show new properties that arise from the synergic action of the components and rely on NP size, morphology, and arrangement. Therefore, continuous advances in design and fabrication of new hybrid titania@gold NPs (TiO₂@AuNPs) are urgent and highly desirable. Here, we propose an effective protocol to produce multibranched AuNPs covered by a controlled TiO₂ thin layer, exploiting a one-pot microfluidic process. The proposed method allows the in-flow and reliable synthesis of titania-functionalized-anisotropic gold nanoparticles by avoiding the use of toxic surfactants and controlling the titania shell formation. TiO₂@AuNPs have been fully characterized in terms of morphology, stability, and biocompatibility, and their activity in photocatalysis has been tested and verified.

Small mode volume plasmonic film-coupled nanostar resonators

Confining and controlling light in extreme subwavelength scales are tantalizing tasks. In this work, we report a study of individual plasmonic film-coupled nanostar resonators where polarized plasmonic optical modes are trapped in ultrasmall volumes. Individual gold nanostars, separated from a flat gold film by a thin dielectric spacer layer, exhibit a strong light confinement between the sub-10 nm volume of the nanostar's tips and the film. Through dark field scattering measurements of many individual nanostars, a statistical observation of the scattered spectra is obtained and compared with extensive simulation data to reveal the origins of the resonant peaks. We observe that an individual nanostar on a flat gold film can result in a resonant spectrum with single, double or multiple peaks. Further, these resonant peaks are strongly polarized under white light illumination. Our simulation data revealed that the resonant spectrum of an individual film-coupled nanostar resonator is related to the symmetry of the nanostar, as well as the orientation of the nanostar relative to its placement on the gold substrate. Our results demonstrate a simple new method to create an ultrasmall mode volume and polarization sensitive plasmonic platform which could be useful for applications in sensing or enhanced light-matter interactions.

Plasmon-assisted random lasing from a single-mode fiber tip

Random lasing occurs as the result of a coherent optical feedback from multiple scattering centers. Here, we demonstrate that plasmonic gold nanostars are efficient light scattering centers, exhibiting strong field enhancement at their nanotips, which assists a very narrow bandwidth and highly amplified coherent random lasing with a low lasing threshold. First, by embedding plasmonic gold nanostars in a rhodamine 6G dye gain medium, we observe a series of very narrow random lasing peaks with full-width at half-maximum ∼ 0.8 nm. In contrast, free rhodamine 6G dye molecules exhibit only a single amplified spontaneous emission peak with a broader linewidth of 6 nm. The lasing threshold for the dye with gold nanostars is two times lower than that for a free dye. Furthermore, by coating the tip of a single-mode optical fiber with gold nanostars, we demonstrate a collection of random lasing signal through the fiber that can be easily guided and analyzed. Time-resolved measurements show a significant increase in the emission rate above the lasing threshold, indicating a stimulated emission process. Our study provides a method for generating random lasing in the nanoscale with low threshold values that can be easily collected and guided, which promise a range of potential applications in remote sensing, information processing, and on-chip coherent light sources.

Chromaticity-tunable white random lasing based on a microfluidic channel

The color and/or chromaticity controllability of random lasing is a key factor to promote practical applications of random lasers as high luminance sources for speckle-free imaging. Here, white coherent random lasing with tunable chromaticity is obtained by using broadband enhancement Au-Ag nanowires as scatterers and the resonance energy transfer process between different dyes in the capillary microfluidic channel. Red, green and blue random lasers are separately fabricated with low thresholds, benefiting from the plasmonic resonance of the nanogaps and/or nanotips with random distribution and sizes within Au-Ag nanowires and positive optical feedback provided by the capillary wall. A white random laser system is then designed through reorganizing the three random lasers. And, the chromaticity of the white random laser is flexibly tunable by adjusting pump power density. In addition, the white random laser has anisotropic spectra due to the coupling role between the lasers. This characteristic is then utilized to obtain different random lasing with different chromaticity over a broad visible range. The results may provide a basis for applying random laser in the field of high brightness illumination, biomedical imaging, and sensors.

Low-cost biosensors based on a plasmonic random laser on fiber facet

Low-cost and miniaturized biosensors are key factors leading to the possibility of portable and integrated biomedical system, which play an important role in clinical medicine and life sciences. Random lasers with simple structures provide opportunities for detecting biomolecules. Here, low-cost biosensors on fiber facet for label-free detecting biomolecules are demonstrated based on a plasmonic random laser. The random laser is achieved resorting to a self-assembled plasmonic scattering structure of Ag nanoparticles and polymer film on fiber facet. Refractive index sensitivity and near-surface sensitivity of the biosensor are systematically studied. Furthermore, the biosensor is used to detect IgG through specific binding to protein A, exhibiting the detecting limit of 0.68 nM. It is believed that this work may promote the applications of a plasmonic random laser bio-probe in portable or integrated medical diagnostic platforms, and provide fundamental understanding for the life science.

Dynamically wavelength-tunable random lasers based on metal-organic framework particles

We propose a strategy to construct dynamically tunable random lasers by continuously adjusting the excited state of gain molecules spatially confined in the nanoporous channels of metal-organic framework particles. Wavelength-tunable random lasers are achieved by thermally manipulating the intramolecular charge transfer process of gain molecules. The wavelength-tunable response to thermal stimuli exhibits excellent reversible behavior. We envisage that such random lasers based on metal-organic frameworks will raise new fundamental issues regarding light-matter interactions in complex photonic media and open up a new avenue toward highly efficient light-emitting devices.

A ring-shaped random laser in momentum space

A ring-shaped random laser in momentum space is designed by directly coupling a random laser with a commercial optical fiber. By using a simple approach of selectively coating the random gain layer on the surface of the fiber, red and yellow random lasers are respectively achieved with low threshold values and a good emission direction due to the guiding role of optical fibers. The unique coupling mechanism leads to a random laser with a ring shape in momentum space, which is an excellent illuminating source for high-quality imaging with an extremely low speckle noise. More importantly, a triple-state color-switchable random laser with yellow, red and yellow-red dual-colors can be flexible, and is obtained by simply moving the pump position. The results may promote the practical applications of random lasers in the fields of sensing, in vivo biological imaging, and high brightness full-field illumination.

Boosting the analytical properties of gold nanostars by single particle confinement into yolk porous silica shells

Herein we illustrate an effective protocol to boost the optical enhancing properties of gold nanostars. By coating single nanostars with a mesoporous silica layer of the appropriate size (yolk capsules), to localize them under optical microscopy, it is possible to enumerate single particles and design SERS quantitative methods with minute amounts of metallic particles.

Plasmonically enhanced random lasing and weak localization of light in powdered Nd3+ doped lithium niobate and its spectral transformation

We report surface plasmon enhanced random lasing action and weak localization owing to charge accumulation on the grain boundaries and its spectral transformation in powdered Nd³⁺ doped lithium niobate (Nd: LN) specimens. Accumulative charge density resulting from screening of electric field of spontaneous polarization was estimated, proving that surface plasmons (SPs) can be excited on the grain boundaries of powdered Nd: LN. The SP based scattering is believed responsible for random lasing to occur, which was further confirmed by the estimation of the scattering mean free path and the scattering cross section and intriguing three step-like backscattering reduction observed in the process of monitoring the variation of reflection spectrum with increasing pumping power. Under a certain pumping power, the powdered Nd: LN specimen was melted locally and this resulted in great changes in random lasing wavelengths. To delve into the reason behind these changes, photoluminescence spectra of the specimens were measured before and after melting. By taking a close look at their dynamics and slopes, it was found that spectral transformation of random lasing occurred owing to the change of lattice structure in powdered Nd: LN.

Plasmonic Nanoparticles Driven Enhanced Light Amplification in a Local 2D and 3D Self-Assembly

We present fluorescence and a random lasing enhancement effect due to the interaction between gold nanoparticles (AuNPs) and Rhodamine 6G (Rh6G) dye. Non-covalently bounded dyes in the proximity of nanoparticles are studied in three systems of varying dimensionality: from (i) three-dimensional freely distributed suspensions, through (ii) quasi-two-dimensional multilamellar liposomes, to (iii) solid two-dimensional thin layers. Liposomes facilitate the formation of stable AuNPs/Rh6G composition showing enhanced fluorescence, while solid thin films exhibit plasmon-assisted random lasing.

Flexible random lasers with tunable lasing emissions

In this study, we experimentally demonstrated a flexible random laser fabricated on a polyethylene terephthalate (PET) substrate with a high degree of tunability in lasing emissions. Random lasing oscillation arises mainly from the resonance coupling between the emitted photons of gain medium (Rhodamine 6G, R6G) and the localized surface plasmon (LSP) of silver nanoprisms (Ag NPRs), which increases the effective cross-section for multiple light scattering, thus stimulating the lasing emissions. More importantly, it was found that the random lasing wavelength is blue-shifted monolithically with the increase in bending strains exerted on the PET substrate, and a maximum shift of ∼15 nm was achieved in the lasing wavelength, when a 50% bending strain was exerted on the PET substrate. Such observation is highly repeatable and reversible, and this validates that we can control the lasing wavelength by simply bending the flexible substrate decorated with the Ag NPRs. The scattering spectrum of the Ag NPRs was obtained using a dark-field microscope to understand the mechanism for the dependence of the wavelength shift on the exerted bending strains. As a result, we believe that the experimental demonstration of tunable lasing emissions based on the revealed structure is expected to open up a new application field of random lasers.

Femtosecond laser-pumped plasmonically enhanced near-infrared random laser based on engineered scatterers

In this Letter, we report on the design, fabrication, and implementation of a novel plasmon-mode-driven low-threshold near-infrared (NIR) random laser (RL) in the 850-900 nm range based on plasmonic ZnS@Au core-shell scatterers. Plasmon modes in the NIR region are used for nanoscale scatterer engineering of ZnS@Au core-shell particles to enhance scattering, as against pristine ZnS. This plasmonic scattering enhancement coupled with femtosecond (fs) laser pumping is shown to cause a three-fold lasing threshold reduction from 325  μJ/cm² to 100  μJ/cm² and a mode Q-factor enhancement from 200 to 540 for ZnS@Au-based RL, as compared to pristine ZnS-based RL. Local field enhancement due to plasmonic ZnS@Au scatterers, as evidenced in the finite-difference time-domain simulation, further adds to this enhancement. This work demonstrates a novel scheme of plasmonic mode coupling in the NIR region and fs excitation in a random laser photonic system, overcoming the inherent deficiencies of weak absorption of gain media and poor scattering cross sections of dielectric scatterers for random lasing in the NIR spectrum.

Nanoplasmonically-engineered random lasing in organic semiconductor thin films

We demonstrate plasmonically nano-engineered coherent random lasing and stimulated emission enhancement in a hybrid gain medium of organic semiconductors doped with core-shell plasmonic nanoparticles. The gain medium is composed of a 300 ± 2 nm thin waveguide of an organic semiconductor, doped with 53 nm gold nanoparticle cores, isolated within silica shells. Upon loading the nanoparticles, the threshold of amplified spontaneous emission is reduced from 1.75 μJ cm^-2× 10² for an undoped gain medium, to 0.35 μJ cm^-2× 10² for a highly concentrated gain medium, and lasing spikes narrower than 0.1 nm are obtained. Most importantly, selection of silica shells with thicknesses of 10, 17 and 21 nm enables engineering of the plasmon-exciton energy coupling and consequently tuning of the laser slope efficiency. With this approach, the slope efficiency is increased by two times by decreasing the silica shell from 21 nm down to 10 nm, due to the enhancement of the localized electric field.