High-Sensitivity Detection of Chiro-Optical Effects in Single Nanoparticles by Four-Wave Mixing Interferometry

Abstract

The field of chiral nanoparticles is rapidly expanding, yet measuring the chirality of single nano-objects remains a challenging endeavor. Here, we report a technique to detect chiro-optical effects in single plasmonic nanoparticles by means of phase-sensitive polarization-resolved four-wave mixing interferometric microscopy. Beyond conventional circular dichroism, the method is sensitive to the particle polarizability, in amplitude and phase. First, we demonstrate its application on single chiral nanohelices fabricated by focused ion beam induced deposition. We examined the combination of detected fields, which measures the particle polarizability, and showed that this is a sensitive reporter of chirality, providing dissymmetry factors (g _α) impressively approaching unity. We then applied the method to a set of individual small gold nanoparticles near the dipole limit (60 nm nominal size), having correspondingly small chiral effects from the intrinsic lattice defects and nonperfectly spherical shape. We find that g _α is randomly distributed in the population, consistent with its nondeterministic origin, but again exhibits remarkably high values, an order of magnitude higher than those obtained using conventional light absorption. Considering the importance of chiral plasmonic nanoparticles in fields ranging from catalysis to metamaterials, this technique offers a powerful way to quantify chiro-optical effects at the single particle level with unprecedented sensitivity.

Figure 1

(a) Sketch of the experimental setup with interferometric detection of the FWM field and polarization-resolved configuration. Pump and probe pulses are coupled into an inverted microscope equipped with a high NA microscope objective (MO). The probe optical frequency ν₀ is slightly upshifted by a radio frequency amount ν₂. Incident beams are adjusted to be circularly polarized at the sample by λ/4 and λ/2 wave plates. The reflected circular polarizations are transformed into horizontal (H) and vertical (V) linear polarization by the same wave plates, and both components are simultaneously detected through their interference with a frequency unshifted reference linearly polarized at 45 deg (see text). BP: balanced photodiodes. WP: Wollaston prism. (b) The polarization of the incident probe field E is adjusted to be left-circular (LCP) or right-circular (RCP) at the sample. The sketch highlights that the collection is in reflection geometry, and that the light helicity is inverted upon reflection. (c) Examples of investigated nanostructures; SEM on a single nanohelix (carbon material) and TEM of a nominally spherical gold nanoparticle.

Figure 2

In-plane (xy) images of the reflected probe and FWM field patterns (amplitude A and phase Φ) at the optimum axial focus on a single C and Pt helix, detected as co (+) and cross-circularly polarized component (−) relative to the input circularly polarized probe, comparing the cases of an RCP and LCP helicity of the incident circularly polarized probe, as indicated. The linear gray scale is from 0 to M for field amplitudes (A), where M = 1 corresponds to a detected signal of 11.4 mV. Phases (Φ) are shown from −π to π in a blue-black-red scale, as indicated. Measurements were performed using a 1.27 NA water immersion objective, with a pump power at the sample of 210 μW and a probe power of 22 μW (54 μW) for the Pt (C) helix. The in-plane step size was 42 nm, and the integration time per pixel was 1 ms. Scale bar: 1 μm.

Figure 3

Overview of chiral quantities in single helices. (a,b) Dependence on the axial focus position z for the same Pt helix as in Figure 2. The star symbols with the right axis in a) show the z-dependence of the copolarized detected FWM field amplitude in the RCP incident probe configuration. The phase in b) refers to the polarizability ratio calculated using ; absolute phase drifts between the measurements are compensated in the ratio. Error bars are calculated as one standard deviation, from the photon shot-noise and the laser fluctuations in the experiment (see text). When not shown, errors are smaller than the size of the symbols. (c) Dissymmetry factors g_σ and g_α for all available helices (3 Pt and 3 C) measured at optimum focus versus cross to copolarized FWM amplitude ratio (average of L and R). Symbols relate the corresponding quantities used to calculate the dissymmetry factor, as per legend in (a) and (b). The C and Pt helices shown in Figure 2 correspond to C2 and Pt3. Representative error bars are shown for Pt3.

Figure 4

In-plane (xy) images of the reflected probe and FWM field patterns (amplitude A and phase Φ) on individual AuNPs nominally spherical with 60 nm diameter, detected as co (+) and cross-circularly polarized component (−) relative to the input circularly polarized probe, comparing the cases of an RCP and LCP helicity of the incident circularly polarized probe, as indicated. The linear gray scale is from 0 to M for field amplitudes (A). Here, M = 1 corresponds to a detected signal of 45 mV. Phases (Φ) are shown from −π to π in a blue–black–red scale, as indicated. The bottom row shows the ratio of the cross- relative to the copolarized detected components, on a logarithmic gray scale from m to M as indicated. Measurements were performed using a 1.45 NA oil immersion objective, with a pump (probe) power at the sample of 40 μW (20 μW). The in-plane step size was 20 nm, and the integration time per pixel was 0.5 ms. Scale bar: 1 μm.

Figure 5

Overview of chiral quantities in single nominally spherical AuNPs of 60 nm diameter. Left: dissymmetry factors g_σ and g_α for all available AuNPs, measured at the spatial location corresponding to the center of the objective point-spread function for copolarized FWM, versus cross to copolarized FWM amplitude ratio (average of L and R). Error bars are calculated as one standard deviation, from the photon shot-noise and the relative laser fluctuations in the experiment (see text). The red stars indicate the 3 AuNPs imaged in Figure 4. Right: corresponding histograms of g_σ and g_α, with indicated mean and standard deviation (SD).

Figure 6

(a) Phase of the polarizability ratio versus phase of the cross to copolarized FWM field ratio , in single nominally spherical AuNPs of 60 nm diameter (same as in Figure 5). The red line shows the expected dependence assuming a nonchiral quasi-spherical particle of ellipsoidal shape, with in-plane orientation γ as sketched in the inset. (b) Dissymmetry factor g_α versus the difference between the experimental phase of and the phase expected from the ellipsoid model. Blue symbols are the results for the nanohelices (using the phase measured at the optimum focus z = 0, see Figure 3). The center cross represents the ellipsoid model case.