Nonlinear Optical Properties of Core-Shell Nanocavities for Enhanced Second-Harmonic Generation

Abstract

A nonlinear optical plasmonic core-shell nanocavity is demonstrated as an efficient, subwavelength coherent light source through second-harmonic generation. The nonlinear optical plasmonic nanocavity incorporates a noncentrosymmetric medium, which utilizes the entire mode volume for even-order nonlinear optical processes. In previous plasmonic nanocavities, enhancement of such processes was only possible at the interface but symmetry prohibited in the body. We measured an enhancement of over 500 times in the second-harmonic radiation power. Calculations show that an enhancement of over 3500 times is achievable. DOI: 10.1103/PhysRevLett.104.207402 PACS numbers: 78.67.Bf, 42.65.Ky, 73.20.Mf, 81.16.Dn Nonlinear optical (NLO) harmonic generation [1] is important in a broad range of technologies and has become of increasing interest in photonics [2][3][4], chemistry [5,6], material science [7], and biosensing [8,9]. The interest was recently renewed [10][11][12][13][14][15][16][17] by the increasing demands for subwavelength coherent light sources, a critical component in the sciences of physics, information, and biology involving nanometer length scale. As one of the most important means for this purpose, second-harmonic generation (SHG) is prohibited in bulk centrosymmetric materials, though nanoscale SHG was often achieved by breaking the symmetry through interfaces [18], imperfect spheres [19], and asymmetric shapes [20]. Nanoscale SHG is also achieved though the used of noncentrosymmetric nanocrystals [11,[13][14][15][16]21], asymmetric geometric arrangements [22], and coating of NLO material over nanospheres [23]. Practically, however, NLO harmonic generation is generally inefficient at such a small scale. Plasmonic nanocavities are thus intriguing for the construction of more efficient coherent NLO light sources because of their compact mode volume [24] resulting in a highly concentrated local field at resonance [25]. We have synthesized plasmonic core-shell nanocavities using a noncentrosymmetric NLO cavity medium, which enables the entire mode volume of the cavity to harness the enhanced local field. This results in an intrinsically NLO plasmonic nanocavity that is fundamentally different from previous dielectric core plasmonic nanoshells [26], where intracavity SHG is symmetry prohibited, and the plasmonic enhancement in NLO harmonic generation is so far limited to the interface [23,27]. Our calculations show that a geometry-tunable enhancement factor of up to several thousands in the SHG efficiency is theoretically achievable in these NLO plasmonic nanocavities. We experimentally demonstrate enhancements of 500 and 70 times under resonant and off-resonance conditions, respectively, using 100 nm BaTiO 3 =Au core-shell structures. The results agree well with the theoretical prediction.The principle of intracavity SHG enhancement in a plasmonic nanocavity is illustrated in Fig. 1(a). The structure consists of a noncentrosymmetric crystalline core (tetragonal BaTiO 3 as an example) of radius r 1 enclosed i...

Enhanced second-harmonic generation from metal-integrated semiconductor nanowires via highly confined whispering gallery modes

“…For example, a gallium arsenide (GaAs)-WGM microresonator (GaAs-WGM) can produce very strong SHG signal (5 Â 10 À 2 W À 1 ) owing to the large size (dB5.2 mm) and high nonlinear coefficient 14 but has a large footprint. Small nanoparticles (o100 nm), for example, BaTiO 3 /Au nanocavity 24 , may find applications in bio-imaging but may not produce strong enough SHG signal power (7.3 Â 10 5 GM) to be coupled into other devices such as waveguiding and optical manipulation for on-chip applications. Here, we have designed Ag-coated CdS NWs (B200 nm) for efficient, tunable and directional SHG (3.0 Â 10 À 6 W À 1 or 3.1 Â 10 8 GM).…”

“…Here P o (P 2o ) represents the peak power of FW (SHG). The SHG scattering cross-section, s 2o ¼ P 2o =I 2 o (I o , peak intensity), is another commonly used metric to evaluate the conversion efficiency 24 . As to these reported values (Figs 1c and 4c and Supplementary Fig.…”

“…Hybrid systems, for example, nanoscale nonlinear dielectrics coupled to metallic structures, utilize the local-field enhancement of SPRs to enhance SHG, but suffer from highOhmic losses at the metal-dielectric interface, and poor spatial overlap with the nonlinear material 23 , and may not be used for waveguiding applications. For example, sub-100 nm dielectric nanoparticles coupled with plasmonic cavities have demonstrated large enhancements of the SHG signal based on SPRs 24 , in which the enhanced field on the surface can significantly overlap with the nonlinear dielectrics. However, for nanostructures such as NW with dimensions smaller than the wavelength of the incident light, WGMs are loosely confined and exhibit high loss 25 , and hence not very useful for nonlinear optical applications such as SHG.…”

Stimulated Raman adiabatic passage as a route to achieving optical control in plasmonics

“…Recently, it has been proposed to combine plasmonic systems with highly nonlinear media [10][11][12]. Utilizing strongly inhomogeneous electromagnetic (EM) fields associated with the surface plasmonpolariton (SPP) resonance, one can achieve a significant spatial dependence of density of the conductive electrons in metals resulting in nonlinear phenomena such as second harmonic generation [13,14].…”

Bioinspired optical antennas: gold plant viruses

“…In current nanotechnology, the development of nanoscale optical antennas that are capable of receiving and transmitting light in unique light-electron interactions called 'surface plasmons' 1 is of considerable interest because of the potential applications of such antennas in molecular detection, 2-4 multiple harmonic generation, 5,6 plasmonic photovoltaics, 7 optoelectronics, 8 negative-index materials 9 and more. In particular, the use of an optical antenna as a molecular sensor permits the dynamic detection of chemical signatures in a non-invasive and label-free manner.…”