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Register a new userTwo-photon luminescence of single colloidal gold nanorods: revealing the origin of plasmon relaxation in small nanocrystals
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The two-photon luminescence (TPL) of small 10 nm x 40 nm colloidal gold nanorods (GNR) is investigated at the single object level, combining polarization resolved TPL and simultaneously acquired topography. A very high dependence of the TPL signal with both the nanorods longitudinal axis and the incident wavelength is observed confirming the plasmonic origin of the signal and pointing the limit of the analogy between GNRs and molecules. The spectral analysis of the TPL evidences two emission bands peaks: in the visible (in direct connection with the gold band structure), and in the infrared. Both bands are observed to vary quadradically with the incident excitation beam but exhibit different polarization properties. The maximum two-photon brightness of a single GNR is measured to be a few millions higher than the two-photon brightness of fluorescein molecules. We show that the important TPL observed in these small gold nanorods results from resonance effects both at the excitation and emission level : local field enhancement at the longitudinal surface plasmon resonances (LSPR) first results in an increase of the electron-hole generation. Further relaxation of electron-hole pairs then mostly leads to the excitation of the GNR transverse plasmon mode and its subsequent radiative relaxation.
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The two-photon luminescence (TPL) of gold nanoparticles (NP) was shown to result from the excitation of hot carriers, the plasmonic NP resonances playing an important role both for plasmon enhanced absorption and plasmon enhanced emission. However, the exact parameters enabling to control or optimize the NP nonlinear luminescence still need to be understood in detail. In this paper, we report the two-photon excited photoluminescence of single gold nanorods exhibiting identical aspect ratio (close to 4) and thus identical plasmonic resonances, but increasing volumes V (707 <V< 160 103 nm3 i.e. rod diameters varying between 6 and 40 nm). The two-photon luminescence intensity of a high number of colloidal nanorods was investigated at the single object level, combining polarization resolved TPL and simultaneously acquired topography. Non-monotonic TPL variations are evidenced, nanorods with an intermediate size (diameter around 10 nanometers) exhibiting the highest TPL signal intensity. A model is proposed considering both the local field enhancement effects at the NP and the size-dependent electron thermalization processes. BEM (Boundary Elements Method) simulations are used to compute the fields at both the transverse and longitudinal plasmon resonance. A good fitting of the experimental data is obtained considering integration of the fields over the whole the NP volume.
The vibrations of gold nanowires and nanorods are investigated numerically in the framework of continuum elasticity using the Rayleigh-Ritz variational method. Special attention is paid to identify the vibrations relevant in Raman scattering experiments. A comprehensive description of the vibrations of nanorods is proposed by determining their symmetry, comparing with standing waves in the corresponding nanowires and estimating their Raman intensity. The role of experimentally relevant parameters such as the anisotropic cubic lattice structure, the presence of faceted lateral surfaces and the shape of the ends of the nanorods is evaluated. Elastic anisotropy is shown to play a significant role contrarily to the presence of facets. Localized vibrations are found for nanorods with flat ends. Their evolution as the shape of the ends is changed to half-spheres is discussed.
We show that a dimer made of two gold nanospheres exhibits a remarkable efficiency for second-harmonic generation under femtosecond optical excitation. The detectable nonlinear emission for the given particle size and excitation wavelength arises when the two nanoparticles are as close as possible to contact, as in situ controlled and measured using the tip of an atomic force microscope. The excitation wavelength dependence of the second-harmonic signal supports a coupled plasmon resonance origin with radiation from the dimer gap. This nanometer-size light source might be used for high-resolution near-field optical microscopy.
We present a quantitative exploration, combining experiment and simulation, of the mechanical and electronic properties, as well as the modifications induced by an alkylthiolated coating, at the single NP level. We determine the response of the NPs to external pressure in a controlled manner by using an atomic force microscope tip. We find a strong reduction of their Young modulus, as compared to bulk gold, and a significant influence of strain in the electronic properties of the alkylthiolated NPs. Electron transport measurements of tiny molecular junctions (NP/alkylthiol/CAFM tip) show that the effective tunnelling barrier through the adsorbed monolayer strongly decreases with increasing the applied load, which translates in a remarkable and unprecedented increase of the tunnel current. These observations are successfully explained using simulations based on finite element analysis (FEA) and first-principles calculations that permit to consider the coupling between the mechanical response of the system and the electric dipole variations at the interface.
Transition metal dichalcogenides represent an ideal testbed to study excitonic effects, spin-related phenomena and fundamental light-matter coupling in nanoscopic condensed matter systems. In particular, the valley degree of freedom, which is unique to such direct band gap monolayers with broken inversion symmetry, adds fundamental interest in these materials. Here, we implement a Tamm-plasmon structure with an embedded MoSe2 monolayer and study the formation of polaritonic quasi-particles. Strong coupling conditions between the Tamm-mode and the trion resonance of MoSe2 are established, yielding bright luminescence from the polaritonic ground state under non-resonant optical excitation. We demonstrate, that tailoring the electrodynamic environment of the monolayer results in a significantly increased valley polarization. This enhancement can be related to change in recombination dynamics shown in time-resolved photoluminescence measurements. We furthermore observe strong upconversion luminescence from resonantly excited polariton states in the lower polariton branch. This upconverted polariton luminescence is shown to preserve the valley polarization of the trion-polariton, which paves the way towards combining spin-valley physics and exciton scattering experiments.
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