Bringing tunability to ultrafast nanoplasmonics

The nanosciences are stimulating the possibility of engineering novel nanophotonics with tunable, tailored properties. In particular, nanocomposites with metallic nanoparticles embedded in an insulating host matrix (as shown in Figure 1), represent a distinctive class of nanoplasmonics: they have specific nonlinear characteristics due to enhancement of the local field.^1,2 The cost effectiveness of these materials could open up photonic applications in wave mixing, heterodyning, and modulation, and could expand their role in laser applications.

In addition, nanoplasmonics could be an integrated component of nonlinear optical (NLO) devices: the global market for NLO materials was estimated at $856.1 million in 2005 and is expected to grow to 1.656 billion by 2009³. Since nanoplasmonics are macroscopically isotropic, their nonlinear response is of the third order. The χ³_composite(ω) enhancement -- which takes place in the neighborhood of the plasmon frequency ω_p -- has been confirmed experimentally.⁴ In addition, they are fast: degenerate four-wave mixing and Z-scan in Au, Ag, and Cu-based nanoplasmonics have given values of about 10^-7 to 10^-9esu with response times on the picosecond scale.⁴ Further, because the notions of centro-symmetry and phase matching are not major concerns with these new materials, they should open up new possibilities for NLO nanophotonics.

One of the technological challenges has been to fabricate these nanoplasmonics with the additional feature of reversible χ³_composite(ω) frequency tunability: tuning ω_p via an external stimulus. As ω_p is proportional to nm_eff/2ε_host(ω), one could anticipate its tunability via the carrier density (n), the effective electronic mass (m_eff) of the metallic nanoparticles, or through the dielectric function of the dielectric host matrix (omega_host). While the first two of these parameters can be varied by changing the nature of the nanoparticles, the third option requires a radical change in the host matrix. The latter is more effective, as has been illustrated experimentally.⁵ Results with gold nanoparticles embedded in different host matrices -- including Fe₂O₃, SiO₂, TiO₂, and Nb₂O₅ -- have exhibited plasmon tunability via the variation of the refractive index of these different host matrices. The variation of ω_p is substantial: from 2.4eV to 1.7eV. Such a variation entails an explicit host-matrix change. This is not a practical way to achieve plasmon-frequency tunability and a possible externally-driven χ³_composite(ω) modulation within the same nanodevice.

We have developed an elegant method for the preparation of tunable and reversible nanoplasmonics where both the nature of the metallic nanoparticles and the host matrix remain fixed. This allows us to thermally and reversibly tune ω_p and χ³_effective(ω) in a controllable manner for both linear and nonlinear applications. The new idea is based on using vanadium dioxide VO₂) as an intelligent host matrix because it exhibits a reversible, thermally-induced metal-semiconductor phase transition^1,2 when heated above its critical value of 68°C. This concurs with an optical transition, and hence a variation in the material's optical constants. This reversible change in ε(T) is pronounced in the infrared. The limit dielectric constants ε(0) and ε(∞) are 43 and 10.0 in the dielectric regime, respectively, and 18.3 and 9.0 in the metallic regime.^1,2In the near infrared, ε(ω) has values of ≈imately 8.64 and 5.7 in the dielectric and metallic states respectively.

To demonstrate the utility of this approach, we laser-deposited Au-VO₂ nanoplasmonics onto Corning glass. The embedded Au nanocrystallites were ≈13nm in diameter.^1,2 Figure 2 depicts the ultraviolet--visible--near-infrared transmission profiles at different temperatures. We limited our interest to the 425--750nm spectral range in order to focus on the shift of the nanostructured gold omega_p. Initially, the Au-VO₂ wavelength was about 648nm with the temperature below T_c ≈ 68°C (i.e. with the host matrix in its dielectric state). However, as the temperature increased past the transition point, changing the material into its metallic state, the wavelength shifted towards the blue (to ≈603nm). This sizeable variation of ≈45nm confirms the tunability of ω_p(T) through temperature change.

In Figure 3, we can see the evolution of ω_p(T) with the thermal cycling over the 25--120°C range: this result confirms its thermal reversibility with a jump-like change. Such a sharp transition with a hysteresis of ≈16°C is inherent to the first order transition of the VO₂, which lasts ≈500fs.⁶ Such an ultrafast temperature change in the refractive index should enable ultrafast nanoplasmonics. As shown in Figure 2, the ultraviolet-visible transmission is low: about 22.5 and 19.7% at ≈525nm (below and above T_c, respectively). As a result, such Au-VO₂ nanoplasmonics would be impractical in their current form for tunable third-order frequency χ³_composite(ω) generation. What is required is that the optical ultraviolet--visible transmission be enhanced using an optimal optical design: for example, by sandwiching Au-VO₂ nanoplasmonics between thin layers of TiO₂.⁷