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We demonstrate third harmonic generation in plasmonic antennas consisting of highly doped germanium grown on silicon substrates and designed to be resonant in the mid-infrared frequency range that is inaccessible with conventional nonlinear plasmonic materials. Owing to the near-field enhancement, the result is an ultrafast, subdiffraction, coherent light source with a wavelength tunable between 3 and 5 µm, and ideally overlapping with the fingerprint region of molecular vibrations. To observe the nonlinearity in this challenging spectral window, a high-power femtosecond laser system equipped with parametric frequency conversion in combination with an all-reflective confocal microscope setup is employed. We demonstrate spatially resolved maps of the linear scattering cross section and the nonlinear emission of single isolated antenna structures. A clear third-order power dependence as well as mid-infrared emission spectra prove the nonlinear nature of the light emission. Simulations support the observed resonance length of the double-rod antenna and demonstrate that the field enhancement inside the antenna material is responsible for the nonlinear frequency mixing., Nanoantennas: Concentrated infrared light source for molecular analysis A nanoantenna resonant in the mid-infrared frequency range demonstrates promise for optical devices that analyze molecules. Mid-infrared frequency light can be used to detect molecular vibrations, enabling detailed analyses of individual molecules at the quantum level, which is useful for medical and environmental applications. Previous plasmonic nanoantennas developed to concentrate such light and drive nonlinear processes were not effective at Mid-infrared wavelengths. Now, Daniele Brida and co-workers have built a plasmonic nanoantenna using highly-doped germanium grown on silicon substrates, which acts as an ultrafast, tunable nonlinear light source when it is excited by a laser. Their nanoantenna is capable of third harmonic generation – the creation of a light wave with a tripled frequency (one third of the original wavelength) - allowing for enhanced interaction of light and molecules in the Mid-infrared.

Graphene is an ideal material for integrated nonlinear optics thanks to its strong light-matter interaction and large nonlinear optical susceptibility. Graphene has been used in optical modulators, saturable absorbers, nonlinear frequency converters, and broadband light emitters. For the latter application, a key requirement is the ability to control and engineer the emission wavelength and bandwidth, as well as the electronic temperature of graphene. Here, we demonstrate that the emission wavelength of graphene$'$ s broadband hot carrier photoluminescence can be tuned by integration on photonic cavities, while thermal management can be achieved by out-of-plane heat transfer to hexagonal boron nitride. Our results pave the way to graphene-based ultrafast broadband light emitters with tunable emission., 22 pages, 5 Figures

Imparting orbital angular momentum (OAM) to particle beams opens unique opportunities for applications and exploration of fundamental science. Electron beams carrying OAM are commonly achieved via passive phase masks that imprint a transverse modulation on the particle wave function. Here, we show that a chiral plasmonic near field can be used to generate an OAM-carrying electron beam and dynamically control its vorticity. By monitoring the phase change of the transverse electron wave function in both real space and reciprocal space using ultrafast transmission electron microscopy, we demonstrate that an optically-excited chiral near-field with azimuthal order m = 1 is able to imprint an azimuthally varying 2πℓm phase shift on the electron wave function, where ℓ is the number of exchanged plasmons. In contrast to other passive monolithic methods, our approach enables a dynamic coherent control over the vortex beam properties with attosecond precision. This is achieved by properly modifying the optical field distribution using a sequence of two phase-locked light pulses with independent polarization states and delayed by fractions of their optical cycles. We discuss the potential extension of our method to composite charged particles to yield insights into their inner structure, with particular attention to the proton and the origin of its spin.

Imparting orbital angular momentum (OAM) to particle beams opens unique opportunities for applications and exploration of fundamental science. Electron beams carrying OAM are commonly achieved via passive phase masks that imprint a transverse modulation on the particle wave function. Here, we show that a chiral plasmonic near field can be used to generate an OAM-carrying electron beam and dynamically control its vorticity. By monitoring the phase change of the transverse electron wave function in both real space and reciprocal space using ultrafast transmission electron microscopy, we demonstrate that an optically-excited chiral near-field with azimuthal order m = 1 is able to imprint an azimuthally varying 2πℓm phase shift on the electron wave function, where ℓ is the number of exchanged plasmons. In contrast to other passive monolithic methods, our approach enables a dynamic coherent control over the vortex beam properties with attosecond precision. This is achieved by properly modifying the optical field distribution using a sequence of two phase-locked light pulses with independent polarization states and delayed by fractions of their optical cycles. We discuss the potential extension of our method to composite charged particles to yield insights into their inner structure, with particular attention to the proton and the origin of its spin.

Recent efforts in the field of surface-enhanced spectroscopies have focused on the paradigm of superchirality, entailing the engineering of the local electromagnetic fields to boost the enantiospecific interaction between light and chiral molecules. In this framework, approaches based on both metallic and dielectric nanostructures have been proposed and have also recently been extended to vibrational circular dichroism in the mid-infrared. In this work, we design, fabricate and characterize arrays of chiral plasmonic slits featuring enhanced chiral fields in the mid-infrared. We exploit collective lattice resonances to further enhance the local intensity and to generate sharp features in the circular dichroism spectra of the platform. Such features are ideally suited to test the superchiral coupling with the vibrational resonances of chiral molecules.

We demonstrate third harmonic generation in plasmonic antennas made of highly doped germanium and designed to be resonant in the mid infrared. Owing to the near-field enhancement, the result is an ultrafast, sub-diffraction, coherent light source tunable between 3 and 5 micrometer wavelength on a silicon substrate. To observe nonlinearity in this challenging spectral region, a high-power femtosecond laser system equipped with parametric frequency conversion in combination with an all-reflective confocal microscope setup is employed. We show spatially resolved maps of the linear scattering cross section and the nonlinear emission of single isolated antenna structures. A clear third order power dependence as well as the mid-infrared emission spectra prove the nonlinear nature of the light emission. Simulations support the observed resonance length of the double rod antenna and demonstrate that the field enhancement inside the antenna material is responsible for the nonlinear frequency mixing.

The validity of Kirchhoff’s laws in plasmonic nanocircuitry is investigated by studying a junction of plasmonic two-wire transmission lines. We find that Kirchhoff’s laws are valid for sufficiently small values of a phenomenological parameter κ relating the geometrical parameters of the transmission line with the effective wavelength of the guided mode. Beyond such regime, for large values of the phenomenological parameter, increasing deviations occur and the equivalent impedance description (Kirchhoff’s laws) can only provide rough, but nevertheless useful, guidelines for the design of more complex plasmonic circuitry. As an example we investigate a system composed of a two-wire transmission line and a nanoantenna as the load. By addition of a parallel stub designed according to Kirchhoff’s laws we achieve maximum signal transfer to the nanoantenna.

The demand for single photon sources at $\lambda~=~1.54~\mu$m, which follows from the consistent development of quantum networks based on commercial optical fibers, makes Er:O$_x$ centers in Si still a viable resource thanks to the optical transition of $Er^{3+}~:~^4I_{13/2}~\rightarrow~^4I_{15/2}$. Yet, to date, the implementation of such system remains hindered by its extremely low emission rate. In this Letter, we explore the room-temperature photoluminescence (PL) at the telecomm wavelength of very low implantation doses of $Er:O_x$ in $Si$. The emitted photons, excited by a $\lambda~=~792~nm$ laser in both large areas and confined dots of diameter down to $5~\mu$m, are collected by an inverted confocal microscope. The lower-bound number of detectable emission centers within our diffraction-limited illumination spot is estimated to be down to about 10$^4$, corresponding to an emission rate per individual ion of about $4~\times~10^{3}$ photons/s., Comment: 13 pages, 4 figures

Heavily-doped semiconductor films are very promising for application in mid-infrared plasmonic devices because the real part of their dielectric function is negative and broadly tunable in this wavelength range. In this work we investigate heavily n-type doped germanium epilayers grown on different substrates, in-situ doped in the $10^{17}$ to $10^{19}$ cm$^{-3}$ range, by infrared spectroscopy, first principle calculations, pump-probe spectroscopy and dc transport measurements to determine the relation between plasma edge and carrier density and to quantify mid-infrared plasmon losses. We demonstrate that the unscreened plasma frequency can be tuned in the 400 - 4800 cm$^{-1}$ range and that the average electron scattering rate, dominated by scattering with optical phonons and charged impurities, increases almost linearly with frequency. We also found weak dependence of losses and tunability on the crystal defect density, on the inactivated dopant density and on the temperature down to 10 K. In films where the plasma was optically activated by pumping in the near-infrared, we found weak but significant dependence of relaxation times on the static doping level of the film. Our results suggest that plasmon decay times in the several-picosecond range can be obtained in n-type germanium thin films grown on silicon substrates hence allowing for underdamped mid-infrared plasma oscillations at room temperature., 18 pages, 10 figures

We discuss the possibility of enhancing the chiroptical response from molecules uniformly distributed around nanostructures that sustain localized plasmon resonances. We demonstrate that the average optical chirality in the near field of any plasmonic nanostrucure cannot be significantly higher than that in a plane wave. This conclusion stems from the quasi-static nature of the nanoparticle-enhanced electromagnetic fields and from the fact that, at optical frequencies, the magnetic response of matter is much weaker than the electric one.

In the presence of matter there is no fundamental limit preventing confinement of visible light even down to atomic scales. Achieving such confinement and the corresponding intensity enhancement inevitably requires simultaneous control over atomic-scale details of material structures and over the optical modes that such structures support. By means of self-assembly we have obtained side-by-side aligned gold nanorod dimers with robust atomically-defined gaps reaching below 0.5 nm. The existence of atomically-confined light fields in these gaps is demonstrated by observing extreme Coulomb splitting of corresponding symmetric and anti-symmetric dimer eigenmodes of more than 800 meV in white-light scattering experiments. Our results open new perspectives for atomically-resolved spectroscopic imaging, deeply nonlinear optics, ultra-sensing, cavity optomechanics as well as for the realization of novel quantum-optical devices.

An experimentally realizable prototype nanophotonic circuit consisting of a receiving and an emitting nano antenna connected by a two-wire optical transmission line is studied using finite-difference time- and frequency-domain simulations. To optimize the coupling between nanophotonic circuit elements we apply impedance matching concepts in analogy to radio frequency technology. We show that the degree of impedance matching, and in particular the impedance of the transmitting nano antenna, can be inferred from the experimentally accessible standing wave pattern on the transmission line. We demonstrate the possibility of matching the nano antenna impedance to the transmission line characteristic impedance by variations of the antenna length and width realizable by modern microfabrication techniques. The radiation efficiency of the transmitting antenna also depends on its geometry but is independent of the degree of impedance matching. Our systems approach to nanophotonics provides the basis for realizing general nanophotonic circuits and a large variety of derived novel devices.