Nonlinear Spectroscopy on the Plasmonic Analog of Electromagnetically Induced Absorption: Revealing Minute Structural Asymmetries

Abstract

Tailoring not only the linear but also the nonlinear properties of plasmonic structures has been a longstanding idea. The plasmonic dolmen structure with its many degrees of freedom in design has been of particular interest. We are investigating this system in detail in the retarded weak-coupling regime, the so-called plasmonic analog of electromagnetically induced absorption. While it is generally expected that the enhanced absorbance leads to an increased nonlinear generation, we find that the details are more complex. A thorough wavelength and polarization resolved study reveals two distinct nonlinear contributions. Our nonlinear spectroscopy method exhibits a surprisingly high sensitivity to minute structural asymmetries. Our experimental results are corroborated by finite-element simulation. We envision that our findings will stimulate further research into phase tuning, structural symmetries, and manipulation in nonlinear plasmonic systems in order to fully exploit the ability to tailor the linear and specifically the nonlinear optical properties of the nanostructured matter.

Nonlinear Light Amplification Governed by Structural Asymmetry

“…Therefore, recorded x ‐polarized SHG emissions can be described by in which A (α) and B (α) refer to these two contributions that are proportional to cosα and sinα, respectively. When assuming a relative phase (φ) term between these two contributions, [ 12 ] we obtain in which A and B are the amplitude of nonlinear harmonic fields, respectively, excited by x ‐ and y ‐polarized FF beams, and are proportional to and . Experimental data can thus be fitted with Equation () (Figure 3a, highlighted in red lines) to extract a limitation of and φ = 85° for AlNanocap(60°).…”

“…[ 11,18 ] Additionally, the decreased asymmetry orientation in AlNanocaps results in the anticlockwise rotation of emission coils for E x (2ω) (Figure 3a–c, upper panel), whereas emission coils for E y (2ω) remain unchanged (Figure 3a–c, bottom panel, highlighted in red double‐headed arrows). According to fitting results (highlighted in red lines), the rotation and the opening of emission coils can be quantitatively attributed to the evolution of both relative amplitude and phase terms between these two contributions, [ 12 ] i.e., φ = 85° for AlNanocap(60°), φ = 75° for AlNanocap(45°), and φ = 65° for AlNanocap(30°), while keeping the limitation of unchanged.…”

“…in which A(α) and B(α) refer to these two contributions that are proportional to cosα and sinα, respectively. When assuming a relative phase (ϕ) term between these two contributions, [12] we obtain…”

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Among these studies, maneuvering the structural asymmetry offers a complementary solution of manipulating optical excitation, subsequent energy coupling, and reemission of resonant modes. [8][9][10][11][12][13][14][15] For instance, this treatment in plasmonic and dielectric nanostructures frequently triggers new optical resonant modes, such as magnetic dipole resonances, [8,9] high-Q Fano resonances, [16,17] and bound states in the continuum, [18,19] resulting in huge nearfield enhancements over a wider operating bandwidth with respect to symmetrical counterparts. As an essential part of nonlinear frequency conversions, second-harmonic generation (SHG) generally calls for media featuring the reduced symmetry under the dipole approximation, [1,6,8,20,21] and has thus been vigorously investigated in these asymmetrical nanodevices.

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Mark Stockman: Evangelist for Plasmonics

“…The ultrafast nonlinearity could also be utilized in nonlinear sensing schemes . Detailed investigations of material and polarization dependence, more complex hybrid geometries, band-structure dependencies of the used metals, utilizing double resonant conditions, nonlinear Fano schemes, and chiral nonlinearities, as well as creating ultranarrow resonances using dielectric fiber cavities, were carried out in our group, − all resulting from the initial stimulus and inspiration of Mark Stockman.…”