We report the first experimental demonstration of curvature-tunable random lasers to control the transport mean free path of emitted photons.

Enamel is the outermost layer of the tooth that protects it from invasion. In general, an acidic environment accelerates tooth demineralization, leading to the formation of cavities. Scanning electron microscopy (SEM) is conventionally used as an in vitro tool for the observation of tooth morphology changes with acid attacks. Yet, SEM has intrinsic limitations for the potential application of in vivo detection in the early demineralization process. In this study, a high-resolution optical coherence tomography (OCT) system with the axial and transverse resolutions of 2.0 and 2.7 μm in teeth has been utilized for characterizing the effect of the acidic environment (simulated by phosphoric acid) on the enamel topology. The scattering coefficient and the surface roughness of enamel can be directly derived from the OCT results, enabling a quantitative evaluation of the topology changes with demineralization. The dynamic process induced by the acid application is also recorded and analyzed with OCT, depicting the evolution of the demineralization process on enamel. Notably, the estimated enamel scattering coefficient and surface roughness significantly increase with the application time of acid and the results illustrate that the values of both parameters after demineralization are significantly larger than those obtained before the demineralization, illustrating both parameters could be effective to differentiate the healthy and demineralized teeth and determine the severity. The obtained results unambiguously illustrate that demineralization of the tooth surface can be successfully detected by OCT and further used as an indicator of early-stage cavity formation.

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.

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This work provides experimental evidence for photon quenching in InGaN and discusses its relevance to efficiency droop problem in InGaN-based light emitters. An equilibrium rate equation model demonstrates that radiative efficiency for this loss mechanism not only has a similar dependence on carrier density as Auger recombination process, but it also possesses the right magnitude making it difficult to distinguish between the two and possibly leading to errors in interpretation. The impact of photon quenching processes on device performance is emphasized by demonstrating loss of efficiency for spectral regions where there is experimental evidence for photon quenching.

We describe a random laser that uses the ZnO nanorods randomly orientated on a flexible polyimide (PI) substrate as disorderedly optical scatterers to stimulate coherent random lasing actions. Repeatable and reversible tuning of spectral emission is demonstrated by exerting a bending strain on the PI substrate, which enables us to activate the random laser on either below or above the lasing threshold. Furthermore, our random laser functions as a stable and durable optical strain gauge with a gauge factor of ≈37.7 ± 5.4 under a bending strain of 40%, which is comparable to that of traditional electrical strain gauges. The study validates that the reported strain-gauge random laser is able to be used in certain fields where the electrical gauge is restricted and the optical gauge is considered to preferable as an alternative solution.

Transfer-free graphene has been directly grown on ZnO nanorods (NRs)/p-GaN LEDs by a feasible approach involving rapid thermal annealing of amorphous carbon and nickel layers. Compared with conventional ZnO NRs/p-GaN LEDs without a current spreading layer, the current–voltage characteristic and electroluminescence performance are enhanced, mainly attributed to more efficient current spreading by the transfer-free graphene that conformally covers the entire surface of ZnO NRs, leading to better carrier injection. It is hence expected that our scheme will be of great importance for nanostructured LED arrays with large-scale fabrication.

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