Strong coupling between vibrational modes and cavity optical modes leads to the formation of vibration-cavity polaritons, separated by the vacuum Rabi splitting. The splitting depends on the square root of the concentration of absorbers confined in the cavity, which has important implications on the response of the coupled system after ultrafast infrared excitation. In this work, we report on solutions of W(CO) in hexane with a concentration chosen to access a regime that borders on weak coupling. Under these conditions, large fractions of the W(CO) oscillators can be excited, and the anharmonicity of the molecules leads to a commensurate reduction in the Rabi splitting. We report excitation fractions > 0.4, depending on excitation pulse intensity, and show drastic increases in transmission that can be modulated on the picosecond time scale. In comparison to previous experiments, the transient spectra that we observe are much simpler because excited-state transitions lie outside of the transmission spectrum of the cavity, thereby contributing only weakly to the spectra. We find that the Rabi splitting recovers with the characteristic vibrational relaxation lifetime and anisotropy decay of uncoupled W(CO), implying that polaritons are not directly involved in the relaxation we observe after the first few ps. The results help corroborate the model that we proposed to describe the results at higher concentrations and show that the ground-state bleach of cavity-coupled molecules has a broad, multisigned spectral response.

Nanoscale control over the second-order photon correlation function g (2) (τ ) is critical to emerging research in nonlinear nanophotonics and integrated quantum information science. Here we report on quasiparticle control of photon bunching with g (2) (0) > 45 in the cathodoluminescence of nanodiamond nitrogen vacancy (NV 0 ) centers excited by a converged electron beam in an aberrationcorrected scanning transmission electron microscope. Plasmon-mediated NV 0 cathodoluminescence exhibits a 16-fold increase in luminescence intensity correlated with a three fold reduction in photon bunching compared with that of uncoupled NV 0 centers. This effect is ascribed to the excitation of single temporally uncorrelated NV 0 centers by single surface plasmon polaritons. Spectrally resolved Hanbury Brown-Twiss interferometry is employed to demonstrate that the bunching is mediated by the NV 0 phonon sidebands, while no observable bunching is detected at the zero-phonon line. The data are consistent with fast phonon-mediated recombination dynamics, a conclusion substantiated by agreement between Bayesian regression and Monte Carlo models of superthermal NV 0 luminescence.PACS numbers: 42.50. Ar, 78.60.Hk, 73.20.Mf The efficiency of second-order nonlinearities scales proportionally with g (2) (0), the second-order photon correlation function at zero delay of the driving optical field [1,2]. Nanoscale superthermal light sources exhibiting photon bunching with g (2) (0) > 2 thus provide a path toward high-efficiency nonlinear nanophotonics. Moreover, control of g (2) (τ ) is increasingly critical for quantum nanophotonics applications [3,4]. However, despite increasing evidence of coherent quantum behavior in nanoplasmonic systems [5,6], experimental plasmonic control of g (2) (τ ) has been realized only in Purcell enhancement of the anti-bunching dynamics of plasmoncoupled emitters [7].Compared with photoluminescence (PL) spectroscopy, cathodoluminescence (CL) yields vastly improved spatial resolution in measurements of g (2) (τ ). This fact was leveraged in the first explorations of CL photon statistics, in which photon antibunching was observed from individual NV 0 centers in nanodiamonds and from point defects in hexagonal boron nitride excited by an 80-keV electron beam [8-10]. More critically, photon bunching has been observed in the CL of ensembles of quantum emitters * Matthew.Feldman@vanderbilt.edu † lawriebj@ornl.gov whose PL exhibits g (2) (τ ) ≈ 1 because of the absence of temporal correlations between optically excited emitters. In contrast to PL, the scanning transmission electron microscope (STEM) primarily excites higher-energy modes, such as the 30-eV bulk plasmon in diamond [11]. The subsequent cascading excitation of multiple excitons and color centers for each plasmon, within an ∼ 10 fs excitation window, explains recent observations of photon bunching of g (2) (0) − 1 > 4 in CL spectroscopy of ensembles of NV 0 centers in nanodiamond [12,13]. However, understanding the classical and quantum optical proper...

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Saturable absorption, in which optical absorption decreases as the incident intensity increases, is commonly utilized in the visible and near-infrared for laser applications. In the mid-infrared, most vibrational transitions are too weak and optical fluences too low to achieve saturable absorption. In this work, we demonstrate saturable absorption in a narrow band centered at 1983 cm–1 in solution-phase W­(CO)6 with a readily accessible saturation fluence. Furthermore, in a system where coupling between the strongly absorbing vibrational mode of W­(CO)6 and an optical cavity gives rise to two polariton modes (i.e., the strong coupling regime), we demonstrate that saturation of the splitting between polariton peaks leads to a fluence-dependent cavity transmission with a saturation fluence that scales counterintuitively with the cavity length and molecular concentration.

Understanding the near-field electromagnetic interactions that produce optical orbital angular momentum (OAM) is crucial for integrating twisted light into nanotechnology. Here, we examine the cathodoluminescence (CL) of plasmonic vortices carrying OAM generated in spiral nanostructures. The nanospiral geometry defines a photonic local density of states that is sampled by the electron probe in a scanning transmission electron microscope (STEM), thus accessing the optical response of the plasmonic vortex with high spatial and spectral resolution. We map the full spectral dispersion of the plasmonic vortex in spiral structures designed to yield increasing topological charge. Additionally, we fabricate nested nanospirals and demonstrate that OAM from one nanospiral can be coupled to the nested nanospiral, resulting in enhanced luminescence in concentric spirals of like handedness with respect to concentric spirals of opposite handedness. The results illustrate the potential for generating and coupling plasmonic vortices in chiral nanostructures for sensitive detection and manipulation of optical OAM.

Abstract:The enhanced electric field at plasmonic resonances in nanoscale antennas can lead to efficient harmonic generation, especially when the plasmonic geometry is asymmetric on either inter-particle or intra-particle levels. The planar Archimedean nanospiral offers a unique geometrical asymmetry for second-harmonic generation (SHG) because the SHG results neither from arranging centrosymmetric nanoparticles in asymmetric groupings, nor from non-centrosymmetric nanoparticles that retain a local axis of symmetry. Here, we report forward SHG from planar arrays of Archimedean nanospirals using 15 fs pulses from a Ti:sapphire oscillator tuned to 800 nm wavelength. The measured harmonic-generation efficiencies are 2.6·10 −9 , 8·10 −9 and 1.3·10 −8 for left-handed circular, linear, and right-handed circular polarizations, respectively. The uncoated nanospirals are stable under average power loading of as much as 300 µW per nanoparticle. The nanospirals also exhibit selective conversion between polarization states. These experiments show that the intrinsic asymmetry of the nanospirals results in a highly efficient, two-dimensional harmonic generator that can be incorporated into metasurface optics.Keywords: nonlinear plasmonics, asymmetric nanoparticles, polarization conversion, metasurfaces, near-field enhancement, Archimedean nanospirals The second-order susceptibility governs a host of important nonlinear optical phenomena, including frequency mixing, sum-frequency and harmonic generation, and optical rectification. In crystalline and molecular materials, second-order nonlinearities are nonvanishing only at surfaces or in materials with a noncentrosymmetric crystal structure; moreover, the efficient generation of a second-order nonlinear effect requires that the fundamental incident and the nonlinear outgoing waves be phase matched through a macroscopic volume of material, typically on the order of cubic millimeters [1].Plasmonic nanostructures and nanostructure arrays also exhibit second-order nonlinearities, and can generate forward second harmonics if their geometries are not centrosymmetric. Such structures are inherently planar, and therefore compatible with thin-film optical and optoelectronic technologies and with metasurface optics. With advances in nanofabrication, the symmetry of the structures can be exquisitely controlled at the level of a few nanometers [2]. Combining these effects with ultrafast, high-intensity laser pulses yields massive electricfield enhancements and correspondingly greater secondharmonic yield. The localized surface plasmon resonance enhances efficiency and can be designed by selecting the nanoparticle shape for a given wavelength; selective polarization response can also be designed into the nanoparticle. A number of asymmetric plasmonic geometries have been used for harmonic generation, including L-and V-shaped nanoparticles, nanocups, and asymmetric trimers [3][4][5][6][7]. Larger plasmonic structures-such as the ratchet wheel-have also been shown to affect the polarization o...

Efficient frequency conversion techniques are crucial to the development of plasmonic metasurfaces for information processing and signal modulation. In principle, nanoscale electricfield confinement in nonlinear materials enables higher harmonic conversion efficiencies per unit volume than those attainable in bulk materials. Here we demonstrate efficient secondharmonic generation (SHG) in a serrated nanogap plasmonic geometry that generates steep electric field gradients on a dielectric metasurface. An ultrafast pump is used to control plasmon-induced electric fields in a thin-film material with inversion symmetry that, without plasmonic enhancement, does not exhibit an an even-order nonlinear optical response. The

Hexagonal boron nitride (hBN) is a wide, indirect bandgap semiconductor that holds great promise for optoelectronic devices in the ultraviolet and midinfrared spectral regimes. The efficiency of optoelectronic devices is dominated by the dynamic behavior of photogenerated carriers. Here we report on the dynamics of photoexcited free carriers in exfoliated 10 B-enriched (99%) hBN at room temperature. Through implementation of ultrafast ultraviolet-pump−infrared-probe transient transmission spectroscopy, we identify two characteristic recombination rates. Initially, at high free carrier density, the pump fluence dependence is bimolecular with a characteristic rate constant of ∼2.0 × 10 −7 cm 3 /s. This is followed by an exponential recombination of the free carriers at a rate of ∼2.3 × 10 9 s −1 , which we assign to the influence of the impurities and defects in the lattice. These initial results offer insight into the radiative recombination processes for deep ultraviolet optoelectronic devices and toward realizing active control of mid-IR nanophotonic responses.