We have obtained spectra of second-harmonic generation, third harmonic generation, and fourwave mixing from a fishnet metamaterial around its magnetic resonance. The resonant behaviors are distinctly different from those for ordinary materials. They result from the fact that the resonance is plasmonic, and its enhancement appears through the local field in the nanostructure. PACS numbers:Optical metamaterials with nanoscale metal building blocks have been studied extensively in recent years [1]- [8]. With each metal unit much smaller than optical wavelength, they can be viewed as continuous media and the metal units act as 'artificial molecules'. The optical properties of metamaterials can be engineered by proper design of the 'artificial molecules' and they can exhibit unusual behavior nonexistent in nature, such as negative refractive indices. While linear optical properties of metamaterials have been well investigated [2]-[8], nonlinear optical properties began to attract interest only recently [9]-[19]. Such interest stems from possible strong enhancement of nonlinear response from plasmon resonances of the metal nanostructures together with spectral tunability offered by design of 'artificial molecules'. A crucial aspect in understanding nonlinear optical properties of metamaterials is their spectral responses, which have not yet been reported.In this letter, we present the first spectroscopic study of second harmonic generation (SHG), third harmonic generation (THG) and four-wave mixing from a metamaterial comprising a monolayer of "fishnet" structure [20], [21]. It was designed to have negative refractive index in the near-IR range, with a magnetic resonance around 1.55 µm [22]. The spectra of SHG and THG with the fundamental input scanned over the magnetic resonance were obtained with different fundamental and harmonic polarizations. Resonant enhancement were clearly observed. Interestingly, the observed resonances are much sharper than that in linear absorption. This is distinctly different from typical molecular cases, where resonant excitation at the fundamental wavelength yields the same resonance spectrum in linear and nonlinear responses. Such difference originates from the fact that, unlike molecular resonances, the plasmon resonances in metal nanostructures are collective oscillations and their resonance enhancement appears through the local field effect in the nonlinear processes.The measurements were carried out on a "fishnet" metamaterial composed of two silver sheets with hole arrays separated by a SiO 2 layer. It was fabricated us- * Electronic address: evgenia˙kim@berkeley.edu ing combination of nanoimprint lithography (NIL) and electron-beam lithography (EBL) [23]. The SEM image of the structure is shown in Fig. 1a and the structural configuration of the broad wire and its dimensions in Fig. 1b. Linear optical response of this metamaterial exhibits a magnetic resonance at 1.55 µm when the magnetic field component of the input wave threads the loop formed by linking the broad metal wires of...
We present a modular assembly that enables both in situ Raman spectroscopy and laser-based materials processing to be performed in a transmission electron microscope. The system comprises a lensed Raman probe mounted inside the microscope column in the specimen plane and a custom specimen holder with a vacuum feedthrough for a tapered optical fiber. The Raman probe incorporates both excitation and collection optics, and localized laser processing is performed using pulsed laser light delivered to the specimen via the tapered optical fiber. Precise positioning of the fiber is achieved using a nanomanipulation stage in combination with simultaneous electron-beam imaging of the tip-to-sample distance. Materials modification is monitored in real time by transmission electron microscopy. First results obtained using the assembly are presented for in situ pulsed laser ablation of MoS combined with Raman spectroscopy, complimented by electron-beam diffraction and electron energy-loss spectroscopy.
A variety of metamaterials has been demonstrated recently that support backward waves and negative refraction (Negative Index Materials, NIM.) In particular, these materials enable sub-wavelength resolution that makes them even more interesting, especially in optical domain rather than at microwave frequencies where their unusual properties were known for decades. We describe below theoretical and experimental studies of the so-called 'fishnet' metal-spacer holearray metamaterials, which exhibit NIM behavior at optical frequencies, having unit cell size of a few 100s nm. We demonstrate experimentally that their refractive index can be modulated very fast and very strongly (from −2.4 to −1.5) around the communication wavelength of λ=1.55 um, in good agreement with the FDTD results. We also discuss a problem of loss compensation in those materials with hefty Ohmic losses by using gain media and local field enhancement in metallic nanoparticles ensembles that enable SERS.
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