Random Lasing Threshold Research Articles

Origin of light scattering in dye doped polymeric waveguides and the dependence of excitation geometry on coherent random lasing

We present an experimental investigation on the origin of coherent random laser (RL) emission from 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminos-tyryl)-4H-pyran (DCM) doped polyvinyl alcohol (PVA) thin film (DCM-PVA) planar waveguide using various characterization techniques. Absorption and fluorescence spectra of the DCM-PVA thin film confirm the presence of aggregates at dye concentrations greater than 0.08 wt. %. Time correlated single photon counting measurements were perfomed to confirm the presence of dye aggregates in PVA matrix. x-ray diffraction studies reveal the semi-crystalline nature of the DCM-PVA thin film. The optical gain coefficient was determined by the variable stripe length method under 532 nm pulsed laser excitation and was found to be 2.1 cm−1 using the one-dimensional amplifier model for the 0.08 wt. % thin film. The RL emission of the planar waveguide depends on the geometry of the excitation spot. The emission spectrum consists of randomly positioned narrow spectral lines under stripe excitation geometry whereas, a smooth spectrum lacking the narrow peaks is observed under circular spot excitation, both above the lasing thresholds. The origin of weak scattering in DCM-PVA waveguide is attributed to the formation of dye aggregates and inhomogeneities created by the semi-crystalline nature of the film. The RL threshold decreases with an increase of the stripe length due to weak waveguiding in the planar film under stripe excitation. The high optical gain and low lasing thresholds attainable in DCM-PVA waveguides make them a promising candidate for the fabrication of polymer waveguide based photonic devices.

Replica Symmetry Breaking in a Weakly Scattering Optofluidic Random Laser

We report the observation of replica symmetry breaking (RSB) in a weakly scattering optofluidic random laser (ORL). Coherent random lasing is indicated by the presence of narrow peaks rising out of the spectral background. This coherence helps to identify a random laser threshold, which is expected to be gradual with weak scattering. We find that lasing action initiated using optical pulsed pumping coincides with the onset of both RSB and Lévy flight statistics. However, the transition from the photonic paramagnetic to photonic glass phase is more subtle in that the Parisi overlap function broadens instead of completely changing shape. This subtlety is balanced by an accompanying result of identical experimental conditions giving rise to lasing or no lasing depending on the shot. Additional statistical calculations and investigations into the fundamental physical mechanisms present in the ORL support this conclusion. Using simple numerical models, we study the critical spectral properties required for RSB to occur, as indicated by the Parisi overlap function. The simplicity of the models helps demonstrate the sensitive nature of this tool and the necessity of additional verification of the physical mechanisms present in the experiment.

Coherent Random Lasing in Colloidal Quantum Dot-Doped Polymer-Dispersed Liquid Crystal with Low Threshold and High Stability.

High-concentration (2-10 wt %) ZnCdSeS/ZnS alloyed quantum dot-doped polymer-dispersed liquid crystals (QD-PDLCs) were prepared via ultraviolet (UV) curing. The QD-PDLC morphology and resonance characteristics of a coherent random laser were investigated. The doping concentration of the liquid crystal and quantum dots was varied to investigate its effect on the lasing threshold, line width, and stability with respect to the density and grain size of the liquid crystal droplets inside the PDLC structure. Furthermore, the QD-PDLC laser performance was influenced by the pump position and area because of spatial localization of the random resonators. Moreover, the QD-PDLC showed good long-term stability; after 15 days of laser excitation (3 h/day), the laser output was maintained at 92% of the original emission intensity. The random laser threshold was as low as 50 μJ/cm2 with the optimized preparation process, which suggested strong potential for applications in polymer random fiber lasers, sensors, and displays.

Resonant energy transfer and light scattering enhancement of plasmonic random lasers embedded with silver nanoplates.

The resonant energy transfer enhancement from a plasmonic random laser (PRL) has been investigated by means of a dye-covered PVA film with embedded silver nanoplates (DC-PVA/AgNPs). Different sizes and morphologies of AgNPs were adopted to shift the localized surface plasmon resonance (LSPR) and intensify recurrent light scattering between the AgNPs. For better overlap between surface plasmon resonance and the photoluminescence of fluorescent molecules with appropriately-sized silver nanoprisms, the slope efficiency of the PRL was greatly enhanced and the lasing threshold was obviously reduced. In addition, the photon lifetime for the DC-PVA/AgNPs film reveals an apparent decline around 1.39 ns owing to better coupling with LSPR. The stronger light scattering of samples with bigger-sized silver nanoprisms has been demonstrated by coherent back scattering measurements, which reveals a smaller transport mean free path around 3.3 μm. With α-stable analysis, it has been successfully demonstrated that the tail exponent α can be regarded as an identifier of the threshold of random lasing.

Coherent Random Lasing Realized in Polymer Vesicles

We have demonstrated the realization of a coherent vesicle random lasing (VRL) from the dye doped azobenzene polymer vesicles self-assembled in the tetrahydrofuran-water system, which contains a double-walled structure: a hydrophilic and hydrophobic part. The effect of the dye and azobenzene polymer concentration on the threshold of random laser has been researched. The threshold of random laser decreases with an increase in the concentration of the pyrromethene 597 (PM597) laser and azobenzene polymer. Moreover, the scattering of small size group vesicles is attributed to providing a loop to boost the coherent random laser through the Fourier transform analysis. Due to the vesicles having the similar structure with the cell, the generation of coherent random lasers from vesicles expand random lasers to the biomedicine filed.

Pump-Controlled Plasmonic Random Lasers from Dye-Doped Nematic Liquid Crystals with TiN Nanoparticles in Non-Oriented Cells

Manipulation of the performance of the random lasers from dye-doped nematic liquid crystals with TiN nanoparticles in non-oriented cells is studied. The experimental results show that the introduction of TiN nanoparticles into dye-doped nematic liquid crystals significantly reduces the threshold of random lasing due to the localized surface plasmon resonance of TiN nanoparticles. The emission spectrum of random lasers can be controlled by the shape of the pump spot. The threshold of random lasers increases with the decrease of the length of pump stripe. In order to obtain the emission spectrum with fine discrete sharp peaks, the narrow pump stripe is more effective than the circular pump spot. When the pump area is more like a circle, the emission spectrum is more like an amplified spontaneous emission. The underlying mechanisms of these phenomena are discussed in detail. This study provides a promising platform for designing the high-quality and low-threshold random lasers which can be controlled by the shape of the pump spot.

Random lasing in brain tissues

Brain is arguably the most complex organ in biology. It shows a heterogeneous structure, in which different areas are responsible for different functions. New non-invasive techniques to monitor the structure and/or composition of the brain can help to understand this organ. In this paper, we show successful random lasing from a mouse brain tissue impregnated with a dye laser and demonstrate that it can be used to detect changes in structure and composition. We systematically observe that the lowest random lasing threshold and highest intensity are recorded in the corpus callosum anatomical region of the brain. This area is formed by a large number of axon fibers coated by myelin, which is a lipid rich substance. Both the fiber structure and the lipid rich composition contribute to increase the scattering strength, which improves random lasing. A thorough photoluminescence characterization of the tissues as a function of the staining concentrations is also included in the study.

Random Lasing in ZnO Nanopowders Based on Multiphoton Absorption for Ultrafast Upconversion Application

This study reports ultraviolet picosecond random lasing (RL) based on multiphoton absorption in zinc oxide (ZnO) nanopowders with a mean size of ∼100 nm under femtosecond optical excitation. Upconversion random lasing is achieved with an excitation wavelength increasing from 600 to 1800 nm, corresponding to a five-photon excitation. The RL threshold has a slight positive relationship with excitation wavelength, except for a significant increase with four-photon excitation at 1200–1300 nm, demonstrating promising upconversion lasing properties of ZnO nanopowders. A remarkably lower RL threshold is observed at long excitation wavelengths ranging from 1500 to 1800 nm than that for four-photon pumping (1200–1300 nm) processes. Defect states in ZnO are suggested to be the reason that results in the excitation wavelength dependence of random lasing. Transient RL characteristics are investigated using time-resolved photoluminescence measurements, with the RL pulse delay time and pulse width observed to decrease ...

光调控的偶氮染料掺杂聚合物分散液晶微管随机激光器

This study demonstrated the optically tunable random lasing from azo-dye-doped polymer dispersed liquid crystal (PDLC) in capillary tubes, in order to expand new applications for this device. The azo-dye-doped PDLC was prepared in glass capillaries with different core diameters, and the random lasing thresholds of these samples were measured respectively. The spectra of random lasers were measured with increasing the intensity of ultraviolet radiation. The experimental results show that the random laser threshold measurements of different core diameter capillary samples (100 μm, 300 μm, 500 μm) are 11.8 μJ/pulse, 8.6 μJ/pulse and 13.2 μJ/pulse. The experimental results also show that as the intensity of ultraviolet radiation increases (from 0 mW/cm2 to 150 mW/cm2), the random laser intensity decreases gradually, and the FWHM of the spectrum becomes wider. And the random lasing showed similar properties under the UV beam with different polarized directions during the optical tuning process. Our work verifies the possibility to fabricate miniature laser devices in cylindrical confinement structures, and extends the application scope of PDLC.

Random lasing emission from WO3 particles dispersed in Rhodamine 6G solution

We experimentally investigate the random lasing emission from tungsten oxide (WO3) particles dispersed in Rhodamine 6G solution. Due to the growing interest in tungsten oxide materials and also their importance and applications in gas sensors, electrochromic and optochromic materials, photoelectrodes and other optoelectrical devices, we choose this material as the scattering elements in a random laser system. Up to our knowledge, the use of WO3 materials as the scattering elements in a random laser system has not yet been reported. WO3 particles provide optical feedback by multiple light scattering. Optical gain is provided by Rhodamine 6G solution through stimulated emission process. Our results show that by adding WO3 particles to Rhodamine 6G solution, single peak random lasing emission is achieved. Furthermore, we investigate the effects of changing the concentration of WO3 particles on the random lasing characteristics. Our results confirm that the increase of WO3 particles leads to the decrease of random lasing threshold, blue-shift of the emission peak, enhancement of the laser emission peak and decrease of its line-width. Due to electrochromic and gasochromic properties of WO3 nanostructures this work can open an avenue for design and fabrication of new tunable and controllable random lasers.

Enhanced random laser by metal surface-plasmon channel waveguide

Compared with conventional lasers, the random laser is realized through strong multiple scatterings in disordered gain system. In this paper, random lasing (RL) in one-dimensional metal surface plasmon (SP) waveguide with gold-plated self-formed silicon pyramids was investigated comprehensively. Consequently, the emission intensity of RL was enhanced dramatically and the RL threshold was reduced significantly through Au-coated Si spiky tips. Meanwhile, one-dimensional metal SP channel waveguides confined the emitting light in a certain direction with a small divergence angle. Using FDTD simulations, it was found that the enhancement effect for RL is likely attributed to the localized surface plasmon (LSP) field. In addition, the LSP field nearby the spiky tips can enhance field-molecule interaction, which was benefit for lasing in small scale. The results in this letter supplied a feasible method to realize the application of RL in subwavelength optical elements.

Influence of fiber and MnCl_2 on mode and threshold of random lasing in random gain systems

A mode and threshold tunable random laser assisted by MnCl2 and fiber is investigated. This is constructed by doping MnCl2 with different concentrations and embedding fiber in the dye Pyrromethene-597(PM597)-doped negative liquid crystal (NLC) glass capillary samples. The excellent overlap of the absorption spectrum of MnCl2 with both the fluorescence and absorption spectra of dye PM597 provides the low threshold and tuning properties for the random lasing. The low-threshold and fewer-mode peculiarity of the random lasing for the systems with fiber is attributed to longer photon residence time provided by the fiber in the random gain-scattering systems. The simple and straightforward approach of random lasing control holds potential to enable optical device fabrication based on random lasers.

The controllable intensity and polarization degree of random laser from sheared dye-doped polymer-dispersed liquid crystal

AbstractThe random laser from sheared dye-doped polymer-dispersed liquid crystal (DDPDLC) is investigated. As the emission intensity weakens, the threshold of random laser from DDPDLC increases from 2.0 mJ/pulse to 4.0 mJ/pulse, and the degree of polarization (DOP) increases from 0.1 to 0.78, obviously when the shear distance increases from 0 mm to 4 mm. As the liquid crystal droplets are gradually oriented in the shear direction caused by alignment direction of polymer chain and anisotropy of droplet shape, the scattering intensity perpendicular to the shear direction gradually decreases and that parallel to the shear direction gradually increases. The anisotropic absorption of the laser dye also plays a certain role as the shear distance is 0 mm. The controllable intensity and polarization degree of random laser have a huge potential for sensing applications.

Temporal profiles for measuring threshold of random lasers pumped by ns pulses

The working threshold is an important parameter to assess the performance of cavity-free random lasers. Here, the temporal profile measurement is proposed as an alternative method to determine the thresholds of the surface plasmon based random lasers pumped by ns pulses based on analyzing the delay time (tDelay) and rising time (tR) of the emission signal. The obvious and slight inflection points of the curves of tDelay and tR varying with the pump power density are observed as indicators for the thresholds of random lasing and for the transition of lasing mode, respectively. The proposed method supplies consistent values to those supplied by traditional methods in frequency-domain for the random systems with different gain length. The demonstrated temporal profile approaches are free from the spectrometers and may be as a candidate for measuring the threshold of random lasers in ultrafast optics, nonlinear optics and bio-compatible optoelectronic probes.

Effect of photonic stop-band on the modes of a weakly scattering DCM-PVA waveguide random laser

We present an experimental study on the effect of the photonic stop-band (PSB) on the random laser (RL) emission characteristics of a 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM) doped polyvinyl alcohol (PVA) film (DCM-PVA). The film, having its refractive index greater than the substrate and density variations at the microscopic scale, acts as a disordered active planar waveguide. The propagation losses for the transverse magnetic (TM) and transverse electric (TE) modes of the waveguide are observed to be 0.50 and 0.74 dB/cm, respectively, at λ = 632.8 nm. The waveguiding DCM-PVA film is then sandwiched between two silica 3-D photonic crystals (opals). The overlap of the DCM-PVA photoluminescence with the PSB of the opals is controlled by the choice of the particle size used for opal fabrication. The random lasing threshold studies have been carried out for both TM and TE polarizations for opals with different particle sizes. A reduction in the threshold of RL emission, with respect to the DCM-PVA waveguide, by about 20 times (to 0.67 mJ/cm2) is observed when the photoluminescence of the DCM-PVA film overlaps with the PSB of the opal structure for TM polarization, showing that the embedding of an RL in an engineered PSB material is an effective way to reduce the thresholds of RLs.

Robustness of replica symmetry breaking phenomenology in random laser.

Random lasers are optical sources where light is amplified by stimulated emission along random paths through an amplifying scattering medium. Connections between their physics and the one of quenched disordered nonlinear systems, notably spin glasses, have been recently suggested. Here we report a first experimental study of correlations of spectral fluctuations intensity in a random laser medium where the scatterers displacement significantly changes among consecutive shots. Remarkably, our results reveal that the replica symmetry breaking (RSB) phenomenology is robust with respect to an averaging over different realizations of the disorder. Moreover, besides opening new intriguing questions about the understanding of such a phenomenon, this work aims to clarify the connection between the RSB with the onset of the Lévy regime, i.e. the fluctuations regime that is a peculiar feature of the random lasing under critical conditions. Our results suggest that the former occurs independently of the latter and then the RSB phenomenology is a generic feature linked to the random laser threshold.

Hybrid Multilayered Plasmonic Nanostars for Coherent Random Lasing.

Here, we report that hybrid multilayered plasmonic nanostars can be universally used as feedback agents for coherent random lasing in polar or nonpolar solutions containing gain material. We show that silver-enhancement of gold nanostars reduces the pumping threshold for coherent random lasing substantially for both a typical dye (R6G) and a typical fluorescent polymer (MEH-PPV). Further, we reveal that the lasing intensity and pumping threshold of random lasers based on silver-enhanced gold nanostars are not influenced by the silica coating, in contrast to gold nanostar-based random lasers, where silica-coated gold nanostars support only amplified spontaneous emission but no coherent random lasing.

Localized surface plasmon resonance improved lasing performance of Ag nanoparticles/organic dye random laser

Random lasing threshold reduction and emission enhancement were reported from a suspension of Ag nanoparticles (NPs) in a laser dye doped polymer film. To evaluate the effect of the localized surface plasmon resonance (LSPR) of metal NPs on random lasing from lasing dye, we carry out a comparative study of the random lasing performance of the Ag NPs and SiO2 NPs scattered dye doped polymer films. Random lasing emission was obtained from Ag NPs and SiO2 NPs scattered dye doped polymer films. The threshold was dramatically reduced from 0.01 to 0.003 mJ/pulse as Ag NPs was used as scatters instead of SiO2 NPs. Theoretical simulation indicates that a large localized electric field is built near the Ag particle surface which enhances the absorption of excitation light. Furthermore, excellent overlap of the plasmonic resonance of Ag NPs with the absorption and photoluminescence spectrum of the dye molecules promotes more dye molecules excited simultaneously to the higher energy levels and leads to more effective random lasing performance.

Diffusive to quasi-ballistic random laser: incoherent and coherent models

We study the crossover between the diffusive and quasi-ballistic regimes of random lasers. In particular, we compare incoherent models based on the diffusion equation and the radiative transfer equation (RTE), which neglect all wave effects, with a coherent wave model for the random laser threshold. We show that both the incoherent and the coherent models predict qualitatively similar thresholds, with a smooth transition from a diffuse to a quasi-ballistic regime. The shape of the intensity distribution in the sample as predicted by the RTE model at threshold is also in good agreement with the coherent model. The approximate incoherent models thus provide useful analytical predictions for the threshold of random lasers as well as the shape of the random laser modes at threshold.

Dopamine sensing and measurement using threshold and spectral measurements in random lasers.

We developed a novel dopamine sensing and measurement technique based on aggregation of gold nanoparticles in random lasers. Dopamine combined with copper ions triggers the aggregation of gold nanoparticles and thus affects the performance of random lasers. Dopamine sensing can be achieved using four parameters which are sensitive to the presence of dopamine, that is emission peak shift, emission linewidth, signal-to-noise ratio (peak emission intensity / noise) and random lasing threshold. The dopamine is most sensitively detected by a change in the emission linewidth with a limit of detection of 1 × 10(-7) M, as well as by an increase in the lasing threshold. The dopamine concentration from 1 × 10(-7) M to 1 × 10(-2) M can be determined by calibrating with the laser threshold.