Periodic dielectric structures are typically integrated with a planar waveguide to create photonic band-edge modes for feedback in one-dimensional distributed feedback lasers and two-dimensional photonic-crystal lasers. Although photonic band-edge lasers are widely used in optics and biological applications, drawbacks include low modulation speeds and diffraction-limited mode confinement. In contrast, plasmonic nanolasers can support ultrafast dynamics and ultrasmall mode volumes. However, because of the large momentum mismatch between their nanolocalized lasing fields and free-space light, they suffer from large radiative losses and lack beam directionality. Here, we report lasing action from band-edge lattice plasmons in arrays of plasmonic nanocavities in a homogeneous dielectric environment. We find that optically pumped, two-dimensional arrays of plasmonic Au or Ag nanoparticles surrounded by an organic gain medium show directional beam emission (divergence angle <1.5° and linewidth <1.3 nm) characteristic of lasing action in the far-field, and behave as arrays of nanoscale light sources in the near-field. Using a semi-quantum electromagnetic approach to simulate the active optical responses, we show that lasing is achieved through stimulated energy transfer from the gain to the band-edge lattice plasmons in the deep subwavelength vicinity of the individual nanoparticles. Using femtosecond-transient absorption spectroscopy, we verified that lattice plasmons in plasmonic nanoparticle arrays could reach a 200-fold enhancement of the spontaneous emission rate of the dye because of their large local density of optical states.

Abstract. Direct electrical recording and stimulation of neural activity using microfabricated silicon and metal micro-wire probes have contributed extensively to basic neuroscience and therapeutic applications; however, the dimensional and mechanical mismatch of these probes with the brain tissue limits their stability in chronic implants and decreases the neuron-device contact. Here, we demonstrate the realization of a 3D macroporous nanoelectronic brain probe that combines ultra-flexibility and subcellular feature sizes to overcome these limitations. Built-in strains controlling the local geometry of the macroporous devices are designed to optimize the neuron/probe interface and to promote integration with the brain tissue while introducing minimal mechanical perturbation. The ultra-flexible probes were implanted frozen into rodent brains and used to record multiplexed local field potentials (LFPs) and single-unit action potentials from the somatosensory cortex. Significantly, histology analysis revealed filling-in of neural tissue through the macroporous network and attractive neuron-probe interactions, consistent with long-term biocompatibility of the device.Currently, there is intense interest in the development of materials and electronic devices that can extend and/or provide new capabilities for probing neural circuitry and afford long-term minimally-invasive brain-electronics interfaces 1,2,3,4 . Conventional brain probes have contributed extensively to basic neuroscience 5,6 and therapeutic applications 7,8,9,10 , although they suffer from chronic stability and poor neuron-device contacts 4,11,12,13 . Recent studies of smaller 14,15 and more flexible 16,17 probes suggest that addressing size and mechanical factors could help overcome current limitations. 3The most common neural electrical probes are fabricated from metal 18 and silicon 19,20 , materials that have very different structural and mechanical properties compared to brain tissue 21 .Evidence suggests that mechanical mismatch is an important reason leading to abrupt and chronically unstable interfaces within the brain 4,22 . For example, motion of skull-affixed rigid probes in chronic experiments can induce shear stresses and lead to tissue scarring 13,23 , and thereby compromise the stability of recorded signals on the time scale of weeks to months 4,24, 25 .More recent work has shown that flexible probes fabricated on polymer substrates 12,17 and smaller-sized probes 11,14 can reduce deleterious tissue response. More generally, there has also been effort developing flexible bioelectronics 26, 27, 28 and nanoscale devices for single cell recording 29, 30 . We have also shown that 3D macroporous electronic device arrays can function as a scaffold for and allow for 3D interpenetration of cultured neuron cell networks without an adverse effect on cell viability 31 , and such networks can be injected by syringe through needles into materials, including brain tissue 32 . In the latter case, it remains challenging to make electrical in...

Plasmonic nanostructures concentrate optical fields into nanoscale volumes, which is useful for plasmonic nanolasers, surface enhanced Raman spectroscopy and white-light generation. However, the short lifetimes of the emissive plasmons correspond to a rapid depletion of the plasmon energy, preventing further enhancement of local optical fields. Dark (subradiant) plasmons have longer lifetimes, but their resonant wavelengths cannot be tuned over a broad wavelength range without changing the overall geometry of the nanostructures. Also, fabrication of the nanostructures cannot be readily scaled because their complex shapes have subwavelength dimensions. Here, we report a new type of subradiant plasmon with a narrow (∼5 nm) resonant linewidth that can be easily tuned by changing the height of large (>100 nm) gold nanoparticles arranged in a two-dimensional array. At resonance, strong coupling between out-of-plane nanoparticle dipolar moments suppresses radiative decay, trapping light in the plane of the array and strongly localizing optical fields on each nanoparticle. This new mechanism can open up applications for subradiant plasmons because height-controlled nanoparticle arrays can be manufactured over wafer-scale areas on a variety of substrates.