3D spatially-resolved optical energy density enhanced by wavefront shaping

We study the three-dimensional (3D) spatially-resolved distribution of the energy density of light in a 3D scattering medium upon the excitation of open transmission channels. The open transmission channels are excited by spatially shaping the incident optical wavefronts. To probe the local energy density, we excite isolated fluorescent nanospheres distributed inside the medium. From the spatial fluorescent intensity pattern we obtain the position of each nanosphere, while the total fluorescent intensity gauges the energy density. Our 3D spatially-resolved measurements reveal that the local energy density versus depth (z) is enhanced up to 26X at the back surface of the medium, while it strongly depends on the transverse (x; y) position. We successfully interpret our results with a newly developed 3D model that considers the time-reversed diffusion starting from a point source at the back surface. Our results are relevant for white LEDs, random lasers, solar cells, and biomedical optics

Selective coupling of optical energy into the fundamental diffusion mode of a scattering medium

We demonstrate experimentally that optical wavefront shaping selectively couples light into the fundamental diffusion mode of a scattering medium. The total energy density inside a scattering medium of zinc oxide (ZnO) nanoparticles was probed by measuring the emitted fluorescent power of spheres that were randomly positioned inside the medium. The fluorescent power of an optimized incident wave front is observed to be enhanced compared to a non-optimized incident front. The observed enhancement increases with sample thickness. Based on diffusion theory, we derive a model wherein the distribution of energy density of wavefront-shaped light is described by the fundamental diffusion mode. The agreement between our model and the data is striking not in the least since there are no adjustable parameters. Enhanced total energy density is crucial to increase the efficiency of white LEDs, solar cells, and of random lasers, as well as to realize controlled illumination in biomedical optics.Comment: 5 pages, 5 figure

Few-emitter lasing in single ultra-small nanocavities

Funding: We acknowledge support from EPSRC grants EP/G060649/1, EP/L027151/1, EP/G037221/1, EP/T014032/1, EPSRC NanoDTC, and from the European Research Council (ERC) under Horizon 2020 research and innovation programme PICOFORCE (Grant Agreement No. 883703), THOR (Grant Agreement No. 829067) and POSEIDON (Grant Agreement No. 861950). O.S.O acknowledges the support of a Rubicon fellowship from the Netherlands Organisation for Scientific Research.Lasers are ubiquitous for information storage, processing, communications, sensing, biological research, and medical applications. To decrease their energy and materials usage, a key quest is to miniaturize lasers down to nanocavities. Obtaining the smallest mode volumes demands plasmonic nanocavities, but for these, gain comes from only single or few emitters. Until now, lasing in such devices was unobtainable due to low gain and high cavity losses. Here, we demonstrate a form of “few emitter lasing” in a plasmonic nanocavity approaching the single-molecule emitter regime. The few-emitter lasing transition significantly broadens, and depends on the number of molecules and their individual locations. We show this non-standard few-emitter lasing can be understood by developing a theoretical approach extending previous weak-coupling theories. Our work paves the way for developing nanolaser applications as well as fundamental studies at the limit of few emitters.Publisher PDFPeer reviewe

Efficient Generation of Two-Photon Excited Phosphorescence from Molecules in Plasmonic Nanocavities.

Nonlinear molecular interactions with optical fields produce intriguing optical phenomena and applications ranging from color generation to biomedical imaging and sensing. The nonlinear cross-section of dielectric materials is low and therefore for effective utilisation, the optical fields need to be amplified. Here, we demonstrate that two-photon absorption can be enhanced by 108 inside individual plasmonic nanocavities containing emitters sandwiched between a gold nanoparticle and a gold film. This enhancement results from the high field strengths confined in the nanogap, thus enhancing nonlinear interactions with the emitters. We further investigate the parameters that determine the enhancement including the cavity spectral position and excitation wavelength. Moreover, the Purcell effect drastically reduces the emission lifetime from 520 ns to <200 ps, turning inefficient phosphorescent emitters into an ultrafast light source. Our results provide an understanding of enhanced two-photon-excited emission, allowing for optimization of efficient nonlinear light-matter interactions at the nanoscale

Nanoscopy through a plasmonic nanolens.

Plasmonics now delivers sensors capable of detecting single molecules. The emission enhancements and nanometer-scale optical confinement achieved by these metallic nanostructures vastly increase spectroscopic sensitivity, enabling real-time tracking. However, the interaction of light with such nanostructures typically loses all information about the spatial location of molecules within a plasmonic hot spot. Here, we show that ultrathin plasmonic nanogaps support complete mode sets which strongly influence the far-field emission patterns of embedded emitters and allow the reconstruction of dipole positions with 1-nm precision. Emitters in different locations radiate spots, rings, and askew halo images, arising from interference of 2 radiating antenna modes differently coupling light out of the nanogap, highlighting the imaging potential of these plasmonic "crystal balls." Emitters at the center are now found to live indefinitely, because they radiate so rapidly.We acknowledge EPSRC grants EP/N016920/1, EP/L027151/1, and NanoDTC EP/L015978/1. OSO acknowledges support of Rubicon fellowship from the Netherlands Organisation for Scientific Research, and RC thanks support from Trinity College Cambridge

Microcavity-like exciton-polaritons can be the primary photoexcitation in bare organic semiconductors.

Strong-coupling between excitons and confined photonic modes can lead to the formation of new quasi-particles termed exciton-polaritons which can display a range of interesting properties such as super-fluidity, ultrafast transport and Bose-Einstein condensation. Strong-coupling typically occurs when an excitonic material is confided in a dielectric or plasmonic microcavity. Here, we show polaritons can form at room temperature in a range of chemically diverse, organic semiconductor thin films, despite the absence of an external cavity. We find evidence of strong light-matter coupling via angle-dependent peak splittings in the reflectivity spectra of the materials and emission from collective polariton states. We additionally show exciton-polaritons are the primary photoexcitation in these organic materials by directly imaging their ultrafast (5 × 106 m s-1), ultralong (~270 nm) transport. These results open-up new fundamental physics and could enable a new generation of organic optoelectronic and light harvesting devices based on cavity-free exciton-polaritons.EPSRC (EP/R025517/1), EPSRC (EP/M025330/1), ERC Horizon 2020 (grant agreements No 670405 and No 758826), ERC (ERC-2014-STG H2020 639088), Netherlands Organisation for Scientific Research, Swedish Research Council (VR, 2014-06948), Knut and Alice Wallenberg Foundation 3DEM-NATUR (no. 2012.0112), Royal Commission for the Exhibition of 1851, CNRS (France), US Department of Energy, Office of Science, Basic Energy Sciences, CPIMS Program, Early Career Research Program (DE-SC0019188)

Looking inside a 3D scattering medium to observe the 3D spatially-resolved optical energy density that is enhanced by wavefront shaping

It is well known that a thick scattering medium (e.g. a slab of paint) is opaque since incident waves are thoroughly scrambled [1, 2]. In the diffusive transport regime, the scattered light has an (ensemble-averaged) energy density that linearly increases with depth from the front surface to about one mean free path 1, and then decreases linearly with depth to the back surface. Two main questions arise: (A) Can one increase (or decrease) the energy density? (B) What is the new position-dependence? Answers to these questions are crucial for light-matter interactions with applications to white LEDs, random lasers, solar cells, and biomedical optics.Therefore, we report here on a wavefront shaping experiment on ZnO samples ((=580 nm [2]). Dilute single fluorescent nanospheres probe the local energy density. The depth z of each single sphere is obtained by modelling the observed intensity pattern with diffusion theory for a point source. The wavefront is shaped to yield a bright spot at the back surface, which closely corresponds to the excitation of a so-called open transmission channel [5]. The resulting energy density enhancement is obtained from the ratio of the emitted fluorescence power measured with shaped incident wavefronts to that measured with reference wavefronts. Fig. 1 shows that results on a 3D sample with thickness L/l = 28 provide affirmative answers to both questions above: (A) the internal energy density is strongly redistributed by wavefront shaping. (B) The redistribution is strongly depth dependent. The energy density is enhanced compared to the diffusive case, and increases when approaching the back surface, contrary to 1D theory. In contrast, our newly developed 3D theory successfully models the data without adjustable parameters