Ghazal Hajisalem

Probing the Quantum Tunneling Limit of Plasmonic Enhancement by Third Harmonic Generation

Metal nanostructures provide extreme focusing of optical energy that is limited fundamentally by quantum tunneling. We directly probe the onset of the quantum tunneling regime observed by a sharp reduction in the local field intensity in subnanometer self-assembled monolayer gaps using third harmonic generation. Unlike past works that have inferred local limits from far-field spectra, this nonlinear measurement is sensitive to the near-field intensity as the third power. We calculate the local field intensity using a quantum corrected model and find good quantitative agreement with the measured third harmonic. The onset of the quantum regime occurs for double the gap size of past studies because of the reduced barrier height of the self-assembled monolayer, which will be critical for many applications of plasmonics, including nonlinear optics and surface enhanced Raman spectroscopy.

Interband transition enhanced third harmonic generation from nanoplasmonic gold

We measure third harmonic generation (THG) as a function of infrared fundamental wavelength for gold nanoparticles over a gold film. An order of magnitude enhancement in THG is found at ~2.5 eV. We ensure that this enhancement is away from plasmonic resonances, so that it may be attributed directly to the ultrafast third-order susceptibility of gold. Using a simple relation between linear and third-order susceptibilities in conjunction with the linear response of gold, we show that the experimental results agree well with the enhancement from interband transitions. This result is interesting for potential applications leveraging telecom-band sources, such as third harmonic deep-tissue imaging with plasmonic nanoparticle markers.

Effect of surface roughness on self-assembled monolayer plasmonic ruler in nonlocal regime

Recently, self-assembled monolayers (SAMs) have been used for plasmonic rulers to measure the nonlocal influence on the Au nanoparticle - metal film resonance wavelength shift and probe the ultimate field enhancement. Here we examine the influence of surface roughness on this plasmonic ruler in the nonlocal regime by comparing plasmonic resonance shifts for as-deposited and for ultra-flat Au films. It is shown that the resonance shift is larger for ultra-flat films, suggesting that there is not the saturation from nonlocal effects previously reported for the spacer range from 0.7 nm to 1.6 nm. We attribute the previously reported saturation to the planarization of the as-deposited films by thinner SAMs, as measured here by atomic-force microscopy. This work is of interest both in probing the ultimate limits of plasmonic enhancement with SAMs for applications in Raman and nonlinear optics, but also in the study of SAMs planarization as a function surface roughness.

One-step integration of metal nanoparticle in photonic crystal nanobeam cavity.

A single step process of integrating a resonantly tuned silver nanoparticle into photonic crystal nanobeam cavities fabricated by focused ion beam milling is presented. Even though the quality factor of the cavities is reduced by a factor of 20, the emission peak at the cavity resonance is enhanced by 5-fold with respect to the cavities without the metal nanoparticle. The fluorescence is also compared before and after etching away the nanoparticle. Experimental quality factors and wavelength shifts are found to agree reasonably well with simulation values. These results are promising for future single photon emission studies involving the incorporation of quantum dot or NV center emitters into hybrid plasmonic/photonic crystal cavities for enhanced emission rates while retaining reasonably high quality factors.

Plasmon hybridization for enhanced nonlinear optical response

We report the plasmon hybridization between silver nanoprisms and a thin gold film as a means to tune the plasmon resonance and to achieve enhanced optical second harmonic generation. The hybridization enhances the second harmonic counts by nearly three orders of magnitude when varying the spacer layer between the nanoprisms and the gold film. Finite-difference time-domain calculations agree within a factor of 2 with the experimental findings in terms of the predicted enhancement factor. This plasmon hybridization approach is promising for future applications, including multi-photon lithography and nonlinear sensing using metal nanoparticles.

Antenna Design for Directivity-Enhanced Raman Spectroscopy

Antenna performance can be described by two fundamental parameters: directivity and radiation efficiency. Here, we demonstrate nanoantenna designs in terms of improved directivity. Performance of the antennas is demonstrated in Raman scattering experiments. The radiated beam is directed out of the plane by using a ground plane reflector for easy integration with commercial microscopes. Parasitic elements and parabolic and waveguide nanoantennas with a ground plane are explored. The nanoantennas were fabricated by a series of electron beam evaporation steps and focused ion beam milling. As we have shown previously, the circular waveguide nanoantenna boosts the measured Raman signal by 5.5x with respect to a dipole antenna over a ground plane; here, we present the design process that led to the development of that circular waveguide nanoantenna. This work also shows that the parabolic nanoantenna produces a further fourfold improvement in the measured Raman signal with respect to a circular waveguide nanoantenna. The present designs are nearly optimal in the sense that almost all the beam power is coupled into the numerical aperture of the microscope. These designs can find applications in microscopy, spectroscopy, light-emitting devices, photovoltaics, single-photon sources, and sensing.

Probing the acoustic vibrations of complex-shaped metal nanoparticles with four-wave mixing

We probe the acoustic vibrations of silver nanoprisms and gold nano-octahedrons in aqueous solution with four-wave mixing. The nonlinear optical response shows two acoustic vibrational modes: an in-plane mode of nanoprisms with vertexial expansion and contraction; an extensional mode of nano-octahedrons with longitudinal expansion and transverse contraction. The particles were also analyzed with electron microscopy and the acoustic resonance frequencies were then calculated by the finite element analysis, showing good agreement with experimental observations. The experimental mode frequencies also fit with theoretical approximations, which show an inverse dependence of the mode frequency on the edge length, for both nanoprisms and nano-octahedrons. This technique is promising for in situ monitoring of colloidal growth.

Nanorod Surface Plasmon Enhancement of Laser-Induced Ultrafast Demagnetization

Ultrafast laser-induced magnetization dynamics in ferromagnetic thin films were measured using a femtosecond Ti:sapphire laser in a pump-probe magneto-optic Kerr effect setup. The effect of plasmon resonance on the transient magnetization was investigated by drop-coating the ferromagnetic films with dimensionally-tuned gold nanorods supporting longitudinal surface plasmon resonance near the central wavelength of the pump laser. With ~4% nanorod areal coverage, we observe a >50% increase in demagnetization signal in nanorod-coated samples at pump fluences on the order of 0.1 mJ/cm2 due to surface plasmon-mediated localized electric-field enhancement, an effect which becomes more significant at higher laser fluences. We were able to qualitatively reproduce the experimental observations using finite-difference time-domain simulations and mean-field theory. This dramatic enhancement of ultrafast laser-induced demagnetization points to possible applications of nanorod-coated thin films in heat-assisted magnetic recording.

Localized surface plasmon resonance enhanced magneto-optical Kerr effect in Ni80Fe20 thin films coated with Au nanorods

The influence of the localized surface plasmon resonance on the magneto-optical Kerr effect (MOKE) was investigated in Ni80Fe20 thin films coated with gold nanorods. The nanorods are dimensionally tuned to support localized surface plasmon resonance near the incident laser frequency. A significant enhancement of magneto-optical response is observed, in which the MOKE signal is increased by over 40% compared to the reference Ni80Fe20 films. The spectral dependence of MOKE signals exhibits the maximum magneto-optical response centered around the longitudinal surface plasmon resonance wavelength of gold nanorods. Finite-difference time-domain modeling confirms that the excitation of the localized surface plasmons leads to the enhancement of magneto-optical responses.The influence of the localized surface plasmon resonance on the magneto-optical Kerr effect (MOKE) was investigated in Ni80Fe20 thin films coated with gold nanorods. The nanorods are dimensionally tuned to support localized surface plasmon resonance near the incident laser frequency. A significant enhancement of magneto-optical response is observed, in which the MOKE signal is increased by over 40% compared to the reference Ni80Fe20 films. The spectral dependence of MOKE signals exhibits the maximum magneto-optical response centered around the longitudinal surface plasmon resonance wavelength of gold nanorods. Finite-difference time-domain modeling confirms that the excitation of the localized surface plasmons leads to the enhancement of magneto-optical responses.

Trace cancer biomarker quantification using polystyrene-functionalized gold nanorods.

We demonstrate the application of polystyrene-functionalized gold nanorods (AuNRs) as a platform for surface enhanced Raman scattering (SERS) quantification of the exogenous cancer biomarker Acetyl Amantadine (AcAm). We utilize the hydrophobicity of the polystyrene attached to the AuNR surface to capture the hydrophobic AcAm from solution, followed by drying and detection using SERS. We achieve a detection limit of 16 ng/mL using this platform. This result shows clinical potential for low-cost early cancer detection.