Nonlinear Plasmonic Sensing with Nanographene

Abstract

Plasmons provide excellent sensitivity to detect analyte molecules through their strong interaction with the dielectric environment. Plasmonic sensors based on noble metals are, however, limited by the spectral broadening of these excitations. Here we identify a new mechanism that reveals the presence of individual molecules through the radical changes that they produce in the plasmons of graphene nanoislands. An elementary charge or a weak permanent dipole carried by the molecule are shown to be sufficient to trigger observable modifications in the linear absorption spectra and the nonlinear response of the nanoislands. In particular, a strong second-harmonic signal, forbidden by symmetry in the unexposed graphene nanostructure, emerges due to a redistribution of conduction electrons produced by interaction with the molecule. These results pave the way toward ultrasensitive nonlinear detection of dipolar molecules and molecular radicals that is made possible by the extraordinary optoelectronic properties of graphene. DOI: 10.1103/PhysRevLett.117.123904 Localized surface plasmons (LSPs) have attracted considerable attention in the nanophotonics community due to their pivotal role in optical sensing applications such as antibody-antigen [1-3], gas [4,5], and pH [6,7] sensors. These excitations also enable the detection and chemical identification of single molecules through their enhancement of molecule-specific Raman scattering intensities [8][9][10][11][12][13]. LSPs are routinely observed in noble metal nanostructures, appearing as pronounced spectral features in their optical absorption and scattering spectra. Plasmonbased sensing heavily relies on the ability of these collective modes to confine and strongly amplify the optical near field. These properties are equally responsible for the large nonlinear optical response observed in metal nanoparticles [14][15][16][17][18][19], which has inspired alternative mechanisms for nonlinear plasmonic sensing. For instance, the aggregation of gold nanoparticles caused by targeted heavy metal ions [20], Escherichia coli bacteria [21], or Alzheimer's disease biomarkers [22] can be detected through an increase in second-harmonic generation (SHG). Additionally, third-harmonic generation has been recently claimed to offer large sensitivity to the dielectric environment compared to the linear response [23].Doped graphene is widely recognized as a promising material platform for plasmonics, capable of supporting electrically tunable plasmons with higher quality factors and spatial confinement than those of metal nanoparticles [24][25][26][27][28][29][30][31][32][33][34][35][36][37]. Moreover, tunable graphene plasmons, so far observed at midinfrared (IR) and THz frequencies, provide the strong near-electric-field confinement needed for sensing [38][39][40][41]. In particular, graphene plasmons have been demonstrated to reveal vibrational fingerprints of biomolecules [38]. Additionally, the anharmonic electron motion associated with the Dirac cones of this material [42,43]...

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