Recent advances indicate that enhanced light-matter interaction in plasmonic nanocavities can create hybrid properties in integrated plasmonic metal nanostructures and soft materials. Here, by integrating polyelectrolytes and surface ligands in gold nanorod-on-mirror nanocavities and detecting the nanocavity resonance and vibrational Raman scattering simultaneously, we found that the plasmon-vibration interaction modifies both the nanocavity and molecular responses. Large enhancement of Raman scattering accompanied by the plasmon resonance linewidth broadening are observed as the laser-plasmon detuning approaches the CH vibrational frequency of the molecular systems in the nanocavities. The experimental observations are consistent with the molecular optomechanics theory that predicts dynamical backaction amplification of the vibrational modes and high sensitivity of Raman scattering when the plasmon resonance overlaps with the Raman emission frequency. The results presented here suggests that molecular optomechanics coupling may be manipulated for creating hybrid properties based on quantum mechanical interaction of molecular oscillators and nanocavity electromagnetic optical modes.
In-situ observation of light-induced physico-chemical changes is a long sought experimental approach to gain mechanistic understanding of the underlying fundamental processes. Here, we present experimental results that demonstrate the possibility of using plasmonic nanocavity for inducing and observing physical and chemical changes. This is demonstrated by monitoring photons scattered by a plasmonic nanocavity, in which materials of interest (organic and inorganic systems) are embedded.
In this presentation, the molecular sensitivity of scattering-type near-field scanning optical microscopy (s-SNOM) will be demonstrated by imaging an organic thin film with thickness gradient that continuously vary from zero to over 200 nm on different substrates. We will then present recent s-SNOM experimental results that show phase separation and nanoscale pattern formation in thin films of blended polymers. The evolution of nanoscale domains and hierarchical patterns as a function of composition will be discussed. The results may help to understand the sensitivity of s-SNOM chemical imaging at the molecular “finger print” region of electromagnetic radiation and to realize the capability of the technique to resolve nanoscale domains and phase separation in multicomponent organic thin films.
We will present the manifestations of plasmon-molecule energy and electron transfer processes as observed surface enhanced Raman spectroscopy (SERS). We observe that in presence of intermediary surface ligands, the electron transfer channel is turned off. On the other hand, the presence of surface ligands facilitates plasmon to molecule energy transfer (plasmon pumped adsorbate intramolecular electronic excitation) by orienting the analyte molecules along the surface field vector as well as by prolonging the excited state lifetime. The adsorbate excitation appears to be followed by singlet to triplet intersystem crossing that leads to singlet oxygen generation that initiates N-demethylation reaction as demonstrated using the reactivity of methylene.
Quantum well intersubband polaritons are traditionally studied in large scale systems, over many wavelengths in size. In this presentation, we demonstrate that it is possible to detect and investigate intersubband polaritons in a single subwavelength nanoantenna in the IR frequency range. We observe polariton formation using a scattering-type near-field microscope and nano-FTIR spectroscopy. We will discuss near-field spectroscopic signatures of plasmonic antennae with and without coupling to the intersubband transition in quantum wells located underneath the antenna. Evanescent field amplitude spectra recorded on the antenna surface show a mode anti-crossing behavior in the strong coupling case. We also observe a corresponding strong-coupling signature in the phase of the detected field. We anticipate that this near-field approach will enable explorations of strong and ultrastrong light-matter coupling in the single nanoantenna regime, including investigations of the elusive effect of ISB polariton condensation.
Localized surface plasmon resonances can increase the quantum efficiency of photon emitters through both absorption and spontaneous emission enhancement effects. Despite extensive studies, experimental results that clearly distinguish the two plasmonic enhancement effects are rarely available. Here, we present clear spectral signatures of the plasmonic enhancement effects on the absorption (excitation) and spontaneous emission (Purcell factor) by analyzing the temperature dependent photoluminescence (PL) properties of InGaAs/GaAs single quantum well (QW) coupled to colloidal gold nanorods (AuNRs) at different GaAs capping layer thickness (d). We find that when the emitting InGaAs layer is close to the AuNRs (d = 5 nm), the plasmonic enhancement effect on the QW PL is dominated by the Purcell factor that significantly increases the external quantum efficiency of the QW that otherwise barely emits. When d is increased to 10 nm, the temperature dependence of the PL enhancement factor (F) reflects absorption enhancement in the capping layer followed by carrier diffusion and capture by the well. First F increases with temperature and then decreases following the temperature dependence of the carrier diffusion coefficient in GaAs. By factoring out the contribution of the captured carriers to F, it is shown that carrier transfer to the well reaches saturation with increasing incident laser power. In addition to providing insight into the plasmonic enhancement mechanism, the results presented in this work suggest that colloidal plasmonic nanoparticles can be used as simple probes for understanding carrier transport phenomena in arbitrary semiconductor heterostructures.
Compared to the highly sensitive silicon based affordable visible light detectors, infrared photodetectors require significant improvement. Localized surface plasmon resonances of metal nanoparticles can be utilized for increasing the absorption efficiency of semiconductors suited for detection of infrared radiation. In this work, plasmonic gold nanorods (AuNRs) are used to enhance generation of charge carriers and photon emission by InAs/InGaAs/GaAs quantum dots-in-a-well semiconductor heterostructures. Comparison of measured and calculated scattering spectra reveals that the AuNRs on GaAs exhibit red to green colors depending on their proximity to the GaAs surface. On the other hand, theoretical and experimental near-field optical characterization show that the electric field is tightly localized at the AuNR-GaAs interfacial regions, creating a convenient platform for investigating localized carrier generation and diffusion by monitoring the emission of InAs QDs. The carrier generation and photon emission enhancement is studied as a function of the GaAs thickness (distance) and temperature. Analysis of the QD emission enhancement as a function of distance reveals a Förster radius of 3.85 ± 0.15 nm, a near-field decay length of 4.8 ± 0.1 nm and an effective carrier diffusion length of 64.0 ± 3.0 nm. These distance parameters indicate two emission enhancement mechanisms: plasmon enhanced carrier generation inside the GaAs layer and diffusion to the InAs QDs, and direct near-field excitation of the InAs/InGaAs quantum well. The emission enhancement increases with temperature, confirming the importance of charge carrier diffusion from the GaAs to the InAs QDs, where recombination and photon emission takes place.
We propose a facile approach to fabricate graphene nano-objects (GNOs) using interference lithography (IL) and direct
self-assembly of nanoparticles. Uniformly spaced parallel photoresist (PR) lines and periodic hole arrays are proposed as
an etch mask for producing graphene nanoribbons (GNRs), and graphene nanomesh (GNM), respectively. In a different
experiment, the PR line arrays are transferred to uniform oxide channels, and silica nanoparticle dispersions with an
average size of 10 nm are spun on the patterned surface, leaving a monolayer uniform nanoparticle assembly on the
graphene. Following the particle deposition, the graphene is removed in the narrow spacing between the particles, using
the O2 plasma etch, leaving ordered graphene quantum dot (GQD) arrays. The IL technique and etch process enables
tuning the GNOs dimensions.
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