Fumiya Kusa

Spectral response of localized surface plasmon in resonance with mid-infrared light

We study spectral responses of localized surface plasmons (LSPs) in gold nanorods, which resonate at mid-infrared frequencies, by transmission spectroscopy and electromagnetic field analyses. The resonance linewidth is found to be linearly proportional to the resonance frequency, indicating that the dephasing due to Drude relaxation is suppressed and that the overall dephasing is dominated by radiative damping. Owing to the reduced radiative/non-radiative damping and large geometrical length of the nanorod, near-field intensity enhancement exceeds several hundred times. Nonetheless the resonance linewidth is comparable with or larger than the bandwidth of a 100-fs pulse, and therefore the enhanced near-field as short as 100-fs can be created upon pulsed excitation. The large enhancements with appropriate bandwidths make LSPs promising for enhanced nonlinear spectroscopies, coherent controls, and strong-field light-matter interactions in the mid-infrared range.

Optical field emission from resonant gold nanorods driven by femtosecond mid-infrared pulses

We demonstrate strong-field photoelectron emission from gold nanorods driven by femtosecond mid-infrared optical pulses. The maximum photoelectron yield is reached at the localized surface plasmon resonance, indicating that the photoemission is governed by the resonantly-enhanced optical near-field. The wavelength- and field-dependent photoemission yield allows for a noninvasive determination of local field enhancements, and we obtain intensity enhancement factors close to 1300, in good agreement with finite-difference time domain computations.

Propagation length of mid-infrared surface plasmon polaritons on gold: Impact of morphology change by thermal annealing

We studied the propagation length of surface plasmon polaritons (SPPs) at the gold/air interface in the mid-infrared range. We showed that SPPs propagate for a distance of about or above 10 mm at a wavelength of 10.6 μm, in good agreement with the value predicted from the dielectric constant of polycrystalline gold. We also demonstrated that a simple treatment of thermal annealing led to noticeable elongation of SPP propagation length, accompanied by increased grain size and decreased surface roughness. Quantitative evaluation of SPP propagation length, in correlation with materials morphology, is important in designing plasmonic devices and beneficial for understanding the mechanisms of SPPs losses that underlie electric-field enhancement.

Enhanced ultrafast infrared spectroscopy using coupled nanoantenna arrays

Surface-enhanced nonlinear vibrational spectroscopy using periodic gold nanoantenna arrays is demonstrated. The dipolar coupling among arrayed nanoantennas is shown to have striking impact on near-field enhancements of femtosecond pulsed-fields and on nonlinear signal enhancements. The condition near the collective-resonance achieves averaged signal enhancement of 850 times and local signal enhancement of 1.8 × 106 times, substantially reducing the required pump energy from micro-joule to nano-joule level. The scheme is useful for characterizing structure and dynamics of minute-volume molecular samples, monolayers, and interfaces, as well as paves the way to nonlinear vibrational spectroscopy with compact light sources of oscillator-level.

Antenna-enhanced nonlinear infrared spectroscopy under internal reflection geometry

Nonlinear infrared (IR) spectroscopy, including pump-probe IR spectroscopy and two-dimensional IR spectroscopy, is a powerful tool to study detailed molecular structures and dynamics [1]. In particular, these techniques have great potential to characterize protein conformation and its dynamics [2]. However, small oscillator strengths of molecular vibrations and strong IR absorption bands of liquid water limit the application of nonlinear IR spectroscopy to biomolecules in aqueous environments. One promising approach is to amplify the interaction of molecular vibrations with IR light by using IR-resonant metal nanoantennas and detect the vibrational signals in reflection from nanoantennas [3]. Here, we apply this approach to ‘nonlinear’ IR spectroscopy for the first time. We obtain a dramatic signal amplification of ∼107 and show that the enhanced nonlinear signals reflect the intrinsic vibrational dynamics of sample molecules.

Enhancement of linear/nonlinear optical responses of molecular vibrations using metal nanoantennas

Plasmonic enhancements of optical near-fields with metal nanostructures offer extensive potential for amplifying lightmatter interactions. We analytically formulate the enhancement of linear and nonlinear optical responses of molecular vibrations through resonant nanoantennas, based on a coupled-dipole model. We apply the formulae to evaluation of signal enhancement factors in the antenna-enhanced vibrational spectroscopy.

Electron Tunneling and Acceleration at Gold Nanostructures Driven by Ultrashort Mid-Infrared Pulses

Strong-field photoemission from resonant and non-resonant gold nanostructures is studied using ultrashort mid-infrared pulses. The photoelectron yield and kinetic energy spectra are governed by both antenna resonances and optical near-field distributions.

Electric-field enhancement of mid-infrared light by using Au nano-rod structures

We study resonant enhancement of mid-infrared fields by use of gold nano-rod structures. Such enhancement should be useful for nonlinear vibrational spectroscopy, molecular coherent controls, and other nonlinear optical phenomena.