Widely Tuning Surface Plasmon Polaritons with Laser‐Induced Bubbles

Abstract

so the SPPs could be actively tuned by varying the refractive index of the adjacent medium. To achieve the tunable plasmonic devices, [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] various tuning methods were proposed to vary the refractive index in the experiment, including the electro-optical, thermo-optical, magneto-optical, and all-optical tuning schemes. By utilizing the electro-optical tuning method, the double-slit array, [11] the nanohole array, [12] and the metamaterial [13] were fabricated to tune the SPPs. The modulation depth was about 6 dB for an equivalent index variation of about Δn = 0.05 in the nanohole array covered with the liquid crystal, and the corresponding wavelength shift was about Δλ = 28 nm. [12] By utilizing the thermo-optical tuning scheme, the metallic photonic crystals, [14] the plasmonic waveguide, [15] and the hole array [16] were designed to realize the modulators. The modulation depth could reach up to about 13 dB for the index variation of about Δn = −0.06 in the hole array, and the corresponding wavelength shift was about Δλ = −33 nm. [16] Utilizing the magneto-optical tuning scheme, the plasmonic modulators were achieved in the metal/ferromagnetic/metal structures [17][18][19] and magneto-plasmonic crystal. [20] The modulation depth was up to about 17 dB, and the corresponding wavelength shift was about Δλ = 24 nm for an equivalent index variation of about Δn = 0.06 in the plasmonic crystal. [20] Herein, the values of Δn, Δλ, and dB are calculated based on the shift of the experimental spectra in these papers, and the largest values are chosen for each tuning method. The all-optical tuning scheme, where the signal beam is controlled by the pump beam, is preferred by the alloptical active devices and all-optical plasmonic circuits. However, because of the weak light-matter interactions in natural materials, the index variation induced by the pump beams is very limited. As a result, various nanostructures, including the prism structure, [21] nanohole array, [22] asymmetric nano-slit, [9] grating, [23] T-shape slit, [24] and the micro-ribbon structure, [10] were designed to enhance the light-matter interactions. [4] For example, the asymmetric slit was designed to enhance the light-matter interaction based on the cavity effect. [9] Consequently, the modulation depth was as high as 20 dB under an index variation of about Δn = −0.05, and the corresponding wavelength shift was about Δλ = −30 nm. [9] Besides the variations of the real part of the refractive index, the tunable The wide-range tunable all-optical plasmonic devices are of importance for the multifunctional photonic circuits of high densities. Here, the laserinduced bubbles are utilized to widely tune the surface plasmon polaritons (SPPs) in the metallic hole array. Because of the bubbles generation under a pump beam, the equivalent refractive index of the dielectric medium adjacent to the metal surface decreases. This decrease blue-shifts the transmission spectrum of the hole array. Experimental...

Plasmonic Sensing and Modulation Based on Fano Resonances

“…Hence, by using the Fano resonances, a small variation of the structural parameters or the local refractive indices in the metallic nanostructures can bring a remarkable and observable wavelength shift or intensity change . For the optical modulators, a key figure of merit to assess their performance is the modulation depth (MD), which is defined as…”

“…To obtain the same modulation depth, the required wavelength shift in the sharp asymmetric Fano profiles should be much smaller than that of the traditional symmetric Lorentzian‐like profiles. Hence, by using the sharp asymmetric Fano profiles, only a small variation of the structural parameters or the local refractive indices is required to realize the large modulation depth . Moreover, based on Equation , the smaller the initial intensity ( I 0 → 0 indicating a high spectral contrast), the larger the modulation depth .…”

“…Therefore, the sharp Fano resonances provide a platform for achieving the high‐performance and ultrasmall optical modulators and switches. In the past decades, various tuning methods, such as the mechanical, thermal, magnetic, electrical, and optical tuning schemes, were utilized to improve the performance of the Fano modulators. All of these modulation approaches have applications in active plasmonic devices .…”

“…Compared with the above tuning methods, the optical tuning method, where the signal beam is modulated by a pump beam, is much preferred by the all‐optical active devices and all‐optical plasmonic circuits . However, because of the weak nonlinear light–matter interactions in natural materials and the short propagation distances of nanoscale‐confined SPP modes, the achievement of the all‐optical modulation with a large modulation depth in a small metallic nanostructure is an enormous challenge.…”

“…However, because of the weak nonlinear light–matter interactions in natural materials and the short propagation distances of nanoscale‐confined SPP modes, the achievement of the all‐optical modulation with a large modulation depth in a small metallic nanostructure is an enormous challenge. The Fano resonances with sharp asymmetric profiles and strong field enhancements are extremely sensitive to the variations of the refractive index, and thus they provide an effective solution to this problem . By placing a photorefractive polymer on the metallic hole arrays, the Fano resonance (linewidth of Δλ ≈ 80 nm and spectral contrast of C ≈ 0.93) around λ = 750 nm was tuned based on the photorefractive effect under the pump beam of λ = 387 nm, as shown in Figure a .…”