Letter pubs.acs.org/NanoLett
Observation of Plasmon Wave Packet Motions via Femtosecond Time-Resolved Near-Field Imaging Techniques Yoshio Nishiyama,† Kohei Imura,§ and Hiromi Okamoto*,†,‡ †
Institute for Molecular Science, Myodaiji, 38 Nishigonaka, Okazaki, Aichi 444-8585, Japan The Graduate University for Advanced Studies, Myodaiji, 38 Nishigonaka, Okazaki, Aichi 444-8585, Japan § School of Advanced Science and Engineering, Waseda University, Okubo, Shinjuku, Tokyo 169-8555, Japan ‡
S Supporting Information *
ABSTRACT: The generation and dynamics of plasmon wave packets in single gold nanorods were observed at a spatiotemporal scale of 100 nm and 10 fs via time-resolved near-field optical microscopy. Following simultaneous excitation of two plasmon modes of a nanorod with an ultrashort near-field pulse, a decay and revival feature of the time-resolved signal was obtained, which reflected the reciprocating motion of the wave packet. The timeresolved near-field images were also indicative of the wave packet motion. At some period of time after the excitation, the spatial features of the two modes appeared alternately, showing motion of plasmonic wave crests along the rod. The wave packet propagation was clearly demonstrated from this observation with the aid of a simulation model. The present experimental scheme opens the door to coherent control of plasmon-induced optical fields in a nanometer spatial scale and femtosecond temporal scale. KEYWORDS: Plasmon wave packet, near-field optical microscopy, ultrafast dynamics, nanoscale
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short pulses and achieved site-selective excitation of a silver island structure that exhibited several hot spots by optimizing adaptively the polarization characteristics of the ultrashort pulses.14,15 To further improve the controllability of the spatial and temporal characteristics of the nano-optical fields, coherent control of plasmon wave packet dynamics is promising. For atomic and molecular systems, the coherent control technique of quantum wave packets was established,16−18 and manipulation of the chemical reaction pathways was demonstrated.19,20 In a similar manner to quantum wave packets, it was theoretically proposed that propagation/localization of the nano-optical fields associated with the plasmon wave packet was controllable in a deterministic fashion.21 In connection with plasmon wave packet control, it was recently demonstrated experimentally in an air-gap plasmon waveguide that the waveform at a targeted port was deterministically controllable by introducing the pulse shaping technique.22 Such schemes have potential utility in ultrafast information processing, for example, and experimental achievement of coherent excitation and control of plasmon wave packet is of great significance in these regards. In this article, we report observation of plasmon wave packet dynamics via femtosecond time-resolved near-field optical microscopy. In the conventional far-field optical methods with propagating light, dipole-forbidden plasmon modes
ontrol of the spatial and temporal characteristics of optical fields provides a valuable tool for fundamental research studies in physics and chemistry at the microscopic level, and it is also an essential technique to promote the progress of optical technologies, including optical communication and information processing.1 In conventional macroscopic far-field optical systems, spatial manipulation of the optical field distribution is restricted by the diffraction limit of light (of the order of submicrometers for the visible wavelength region). Experimental techniques based on plasmonic excitation of metal nanostructures can overcome this limit. Plasmonic excitation allows for confinement of light into nanospaces and provides locally enhanced optical fields in the vicinity of the nanostructures.2 The properties of the confined optical fields reflect the characteristics of plasmon eigenmodes that are determined by the geometries of the metal nanostructures.3 Optical excitation of plasmons by ultrashort pulses not only provides an experimental means for investigation of the dynamic response of the plasmons4,5 but also yields controlled optical fields on the nanometer and femtosecond spatiotemporal scale.6−10 In particular, a sequence of ultrashort pulses or shaped ultrashort pulses enables the nano-optical field to be actively controllable.11,12 Kubo et al. demonstrated this scheme of optical control by irradiation of a pair of 10 fs pulses, followed by nanoscale imaging via photoelectron emission microscopy.6,13 They manipulated the location of the optical field enhancement on a silver grating with protruding island structures by adjusting the interpulse delay. Aeschlimann et al. demonstrated another scheme using polarization-shaped ultra© XXXX American Chemical Society
Received: September 8, 2015 Revised: October 12, 2015
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DOI: 10.1021/acs.nanolett.5b03610 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
pump and probe pulses. The resulting time-resolved signals exhibit the interference pattern similar to that found in the fringe-resolved SHG autocorrelation of the pulses. The time profiles of the signals depend on the population and/or coherence dynamics of the intermediate states in the twophoton excitation process. Because the plasmon resonance of the nanorod was an intermediate state involved in the TPI-PL process of gold,25 we can observe plasmon dynamics from the time-resolved TPI-PL signals. The time-resolved measurements were conducted at every position of the scanning area of the sample. We then constructed transient near-field images by collecting TPI-PL signals at a fixed time delay between the pump and probe pulses. In the time-resolved measurements, the irradiation power was kept as low as possible (0.6 mW before the fiber probe, which is sufficiently below the damage threshold of the gold nanostructures and the probes according to our past experiences) to avoid the damage of nanorods and near-field probe in use. To clarify the static nature of the plasmon modes, that is, resonant wavelengths and spatial structures of the modes, we performed static near-field transmission spectroscopy and twophoton excitation imaging measurements. In the static transmission measurement, we used a Xe discharge lamp as a light source and the transmitted light was analyzed by a spectrometer. Static two-photon excitation images were obtained by detecting TPI-PL under irradiation of narrow spectral-band pulses from a wavelength-tunable femtosecond Ti:sapphire laser (wavelength range 680−1080 nm, spectral width
