skip to main content

Title: Inverse design of nanoparticles for enhanced Raman scattering

We show that topology optimization (TO) of metallic resonators can lead to ∼102 × improvement in surface-enhanced Raman scattering (SERS) efficiency compared to traditional resonant structures such as bowtie antennas. TO inverse design leads to surprising structures very different from conventional designs, which simultaneously optimize focusing of the incident wave and emission from the Raman dipole. We consider isolated metallic particles as well as more complicated configurations such as periodic surfaces or resonators coupled to dielectric waveguides, and the benefits of TO are even greater in the latter case. Our results are motivated by recent rigorous upper bounds to Raman scattering enhancement, and shed light on the extent to which these bounds are achievable.

Authors:
; ; ; ;
Publication Date:
NSF-PAR ID:
10133034
Journal Name:
Optics Express
Volume:
28
Issue:
4
Page Range or eLocation-ID:
Article No. 4444
ISSN:
1094-4087; OPEXFF
Publisher:
Optical Society of America
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    2D freestanding nanocrystal superlattices represent a new class of advanced metamaterials in that they can integrate mechanical flexibility with novel optical, electrical, plasmonic, and magnetic properties into one multifunctional system. The freestanding 2D superlattices reported to date are typically constructed from symmetrical constituent building blocks, which have identical structural and functional properties on both sides. Here, a general ligand symmetry‐breaking strategy is reported to grow 2D Janus gold nanocrystal superlattice sheets with nanocube morphology on one side yet with nanostar on the opposite side. Such asymmetric metallic structures lead to distinct wetting and optical properties as well as surface‐enhanced Raman scattering (SERS) effects. In particular, the SERS enhancement of the nanocube side is about 20‐fold of that of the nanostar side, likely due to the combined “hot spot + lightening‐rod” effects. This is nearly 700‐fold of SERS enhancement as compared with the symmetric nanocube superlattices without Janus structures.

  2. Abstract

    Plasmonic nanostructures exhibit intriguing optical properties due to spectrally selective plasmon resonance and thus have broad applications, including biochemical sensing and photoelectric detections. However, excited plasmons are often strongly influenced by the substrates supporting the metallic nanostructures, which not only weakens the intrinsic plasmon coupling effect, but also results in a great reduction of optical near‐field enhancement. Here, a plasmonic nanostructure combining collapsible Au‐nanofingers with selective‐etching that enables Au to be suspended is demonstrated, thus avoiding the undesirable influence of the substrates on the local near‐field distribution and forming symmetric electromagnetic‐field enhancements at both the top and bottom surfaces. The polymer support of the Au‐nanofingers is selectively etched by oxygen plasma, while the Au‐cap retains its original size. After an ultrathin dielectric coating is applied on the Au‐nanofingers, suspended Au‐caps with extremely small dielectric gaps are formed via the collapse of neighboring Au‐nanofingers by exposing them to ethanol. These nanostructures can provide a surface‐enhanced Raman scattering (SERS) enhancement of up to ≈109, which is nearly twice that in the nonsuspended system. As a highly active SERS substrate, the label‐free detection of low‐concentration harmful plastic phthalates in a child's urine without any pretreatment is successfully demonstrated, which suggests that thismore »method is suitable for medical prediagnosis.

    « less
  3. Abstract

    Raman scattering is a powerful probe oflocal structure (LS)of glasses. In Sodium Phosphate Glasses (SPGs), we show that bothLScomposed of Qnspecies andExtended Range Structures (ERS)composed of Long Chains (LCs), Large Rings (LRs), and Small Rings (SRs) can be decoded by Raman scattering. The trimodal distribution of P‐Oterminalstretch modes of Q2species and P‐Obridgingatx < 50% are manifestations of theseERS. These two pairs of triads of modes are uniquely identified with Q2units present in either LCs, or LRs, or SRs. The existence three phases of c‐NaPO3composed of 3‐membered rings, 6‐membered rings, and infinitely long chains has facilitated the identification. The Intermediate Phase (IP) in SPGs extends in the 37.5 < x < 46.0% range, the Stressed‐rigid Phase in the 46.0% < x < 50%, and the Flexible Phase in the 18% < x < 37.5% range of soda. We show the IP consists predominantly of LCs (82%), with a minority of LRs (15%) and SRs (3%). The LR‐ and SR‐fractions increase measurably in the non‐IP phases. The structural finding is in harmony with the high configurational entropy of the IP glasses that leads aging to be qualitatively suppressed.

  4. Abstract

    Whereas electron-phonon scattering relaxes the electron’s momentum in metals, a perpetual exchange of momentum between phonons and electrons may conserve total momentum and lead to a coupled electron-phonon liquid. Such a phase of matter could be a platform for observing electron hydrodynamics. Here we present evidence of an electron-phonon liquid in the transition metal ditetrelide, NbGe2, from three different experiments. First, quantum oscillations reveal an enhanced quasiparticle mass, which is unexpected in NbGe2with weak electron-electron correlations, hence pointing at electron-phonon interactions. Second, resistivity measurements exhibit a discrepancy between the experimental data and standard Fermi liquid calculations. Third, Raman scattering shows anomalous temperature dependences of the phonon linewidths that fit an empirical model based on phonon-electron coupling. We discuss structural factors, such as chiral symmetry, short metallic bonds, and a low-symmetry coordination environment as potential design principles for materials with coupled electron-phonon liquid.

  5. Abstract

    The gold–sulfur (Au–S) and silver–sulfur (Ag–S) bonds are integral to the surface modification of metal films with alkanethiol monolayers. Although the metal–sulfur bond can be characterized with surface‐enhanced Raman spectroscopy (SERS) at roughened metal films, some applications require or perform better when using a smooth metal surface, which is not suitable for SERS signal enhancement. Directional‐surface‐plasmon‐coupled Raman scattering (directional Raman scattering) is an approach to measure metal–sulfur bonds on smooth metal films with sub‐monolayer sensitivity. The metal–sulfur bonds formed from a benzenethiol monolayer on smooth planar gold or silver films are observed in the directional Raman scattering spectra between 240 and 270 cm−1; the signal‐to‐noise ratio of the Au–S Raman peak is 60. Importantly, the directional Raman scattering signal measured with smooth metal surfaces can be simply modeled and easily compared across many samples. Directional Raman scattering can also be measured at roughened metal films, which makes it applicable for many analyses.