Abstract
We report on plasmonic resonances on VO2 nanodot arrays and associated optical dynamics. The plasmon excitations based on electric field interactions lead to red shifts of the plasmon resonances to lower photon energy with increasing nanodot size. The spectral linewidths of plasmon peaks gradually become narrow with increasing nanodot size. This is related to a reduction in plasmon damping with respect to the electronic band structure of VO2. This specific band structure of VO2 affects the optical dynamics of plasmon resonances at the sub-picosecond scale. The optical excitations of VO2 comprise intraband and interband transitions. The existence of plasmon bands induces long-lived lifetimes on decay processes. Intraband transitions in the conduction band (C.B.) play an important role in producing long lifetimes, attributing to free carriers in the C.B. By contrast, interband transitions related to bound electrons contribute to plasmon damping. The dynamic optical responses are closely related to the electronic band structures of VO2.
Keywords
- VO2
- surface plasmon
- infrared
- dynamics
- Mott insulator
1. Introduction
Recently, plasmonic materials based on oxide and compound semiconductors (e.g., ZnO, CuSe, and InN) have received much attention given that plasmonic responses can be tuned by external fields [1, 2, 3, 4, 5]. Investigation of these materials has led to the identification of a new family of plasmonic materials in the infrared (IR) range, which differ from noble metals (e.g., Ag and Au) that have fixed free electron densities. Oxide and compound semiconductors show ideal Drude terms in the IR range due to the absence of interband transitions in the band gap [6]. Plasmonic tuning can be effected by carrier injections due to control of the Fermi level in the electronic bands [7, 8]. The optical features arising from these emerging plasmonic semiconductors show promise for use in optical applications in the IR range.
Control of free carriers has been reported on oxide materials with strong electron-electron correlations. Of these, materials comprising vanadium dioxide (VO2) show a sharp insulator-metal transition (IMT) based on Mott-related and Peierls-related processes [9], which can be controlled by external fields such as thermal, electrical, and optical inputs. In particular, dramatic change of the specific band structure of VO2 with external fields provides more than a three orders of magnitude change in electrical conductance. The IMT character of VO2 resulting from the spin-orbital interactions makes this material a candidate for use in optoelectronic applications [10, 11, 12, 13]. The widespread interest in VO2 has further focused on elucidating the complicated domain and grain structures in the vicinity of IMT. This determines the spatial and hysteretic nature of the phase transition of VO2. Recent studies have reported optical phenomena from plasmonic structures of VO2 with nanostructures such as nanodots and nanoparticles in an effort to understand the correlation between the optical response and phase transition [14, 15, 16, 17]. These investigations have fueled further interest in nanostructures of VO2 and are a very exciting and promising area of research with respect to active plasmonic materials.
The ultrafast dynamics of free carriers in metals have received much attention. Above all, photo-induced alternation of the plasmonic response is a powerful tool in the fabrication of optically controlled nanophotonic devices. The combination of ultrafast optical manipulations with large reflectance and transmittance is expected to yield promising results in applications concerning active plasmonics and optical switching [18, 19]. Thus far, plasmon modulations at the sub-picosecond scale have been reported on noble metals such as gold and silver, being the most commonly adopted plasmonic materials [20]. In general, effort aimed at exploiting ultrafast dynamics requires the production of intense concentrations of energy (hot electrons and hot holes) in the hosts. The dipolar excitations of all free carriers in plasmonic resonances have large extinction cross-sections, which can easily concentrate light into nanoscopic volumes. These phenomena have been utilized in furthering the development of certain areas of photo-chemistry such as photo-catalytic reactions and surface-enhanced Raman scattering (SERS).
Recently, investigations of the optical dynamics of oxide semiconductors have focused on Sn-doped In2O3 and Ga-doped ZnO [21, 22]. The noble metals have large amounts of free electrons in the host. Their plasmonic characters are hardly tunable. By contrast, oxide materials can be easily controlled by optical excitations such as intraband and interband transitions. For example, an interband transition is based on carrier excitation between the conduction band (C.B.) and valence band (V.B.), which can markedly change the plasma frequency that is dependent on carrier concentration, leading to an optical tuning of a plasmon response. However, an optical dynamics on an interband transition are known to show long plasmon lifetimes of several picoseconds. On the other hand, a carrier excitation (electron or hole) in the C.B. or V.B. is based on an intraband transition, resulting in significantly faster plasmon lifetimes at the sub-picosecond scale [23]. Therefore, oxide semiconductors are expected to show ultrafast optical modulations. Unfortunately, the plasmon dynamics of VO2 have hitherto not been investigated. The electronic band structure of VO2 differs largely from that of oxide semiconductors, which show different optical dynamics due to the strong electron-electron correlations of 3
In this chapter, we highlight the surface plasmons and associated optical dynamics of VO2 with two-dimensional (2
2. Surface plasmons of VO2 nanodot arrays
2.1 Fabrication of nanodot arrays
VO2 nanodot arrays were fabricated by top-down processes as follows [24]. VO2 films were epitaxially grown on Al2O3 (0001) substrates at a substrate temperature of 420°C using pulsed laser deposition. ArF laser pulses were focused on VO2 targets in an oxygen atmosphere of 1.0 Pa. After growing VO2 films, a UV nanoimprint resist with a thickness of 200 nm was spin-coated onto VO2 film surfaces. The lithography process was performed using an UV lithography system to fabricate the nanodot array structure. The coated resists were pushed by quartz molds with UV transparency under a pressure of 3 MPa for 5 min. The residual resist on the VO2 film surfaces after processing the UV lithography was removed by O2 plasma irradiation. Additionally, reactive ion etching (RIE) using SF6 plasma was used to process VO2 films to obtain nanodot arrays. SF6 RIE was conducted under an input of 60 W and a gas pressure of 1 Pa for 30 s. The resist layers that remained on VO2 nanodot arrays were removed in toluene for completion of the VO2 nanodots.
Figure 1 shows X-ray 2
Temperature-dependent Raman measurements were investigated in an effort to identify structural changes in the VO2 nanodot arrays (Figure 2a). The Raman peaks at 195, 224, and 614 cm−1 were mainly attributed to V-V (
2.2 Plasmon resonances and field distributions
Figure 3 shows the experimental and simulated extinction (
We could observe two kinds of resonance peaks (Peak-I and Peak-II) from the nanodot arrays with
2.3 Damping processes
Figure 5a shows the relationship between nanodot size and resonant peak energy. The experimental and simulated data showed that a dominant peak energy (
Besides,
Figure 6a shows the spectral linewidth (
The relationship between spectral linewidth and resonant energy is related to the dielectric functions of VO2. Figure 7a shows complex dielectric functions of metallic VO2 epitaxial films prior to nanofabrication. We discuss the plasmonic response of metallic VO2 nanodots on the basis of bulk dielectric functions. The energy-dependent scattering rate (
where
3. Optical dynamics
Near-infrared laser pulses ranging from 1.9 to 2.5 μm were realized from a single supercontinuum pulse by intrapulse difference-frequency mixing with a spectral focusing scheme. For our pump and probe system, the probe wavelength was the same as that of the pump.
Details regarding the generation of femtosecond laser pulses (pulse width ∼ 150 fs) are described in [35].
We initially investigated differential transmittance (Δ
On the other hand, no carrier dephasing was observed. The relaxation time of carrier dephasing is characterized by the time constant,
Figure 9a shows differential transmittance signals as a function of laser wavelength for VO2 nanodot arrays with
The extinction spectra of VO2 nanodot arrays are characterized by the presence of plasmon bands in the IR range. The surface plasmon is a quantum of
In general, it is known that plasmon lifetimes differ largely between interband and intraband excitations. For an Au metal, the
4. Summary
We investigated infrared plasmonic responses of VO2 nanodot arrays and their optical modulations. Comparison of the experimental plasmon resonances with electromagnetic simulations enabled us to perform spectral assignments and field distributions. In particular, plasmon coupling between metallic VO2 nanodots contributed to the collective excitation mode. Plasmon damping of VO2 was closely related to the specific band structure, which affected the optical dynamics. The plasmonic excitations excited by the fs pulse lasers showed ultrafast optical responses at the sub-picosecond scale, which were dependent on laser wavelength. The optical excitations of VO2 comprised intraband and interband transitions. The long-lived lifetimes were observed at the resonant peaks in terms of free carrier excitations of VO2. This result was attributed to the band structure, which affected the plasmon lifetimes.
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