Spatial Modulation Spectroscopy Research Articles

Supercontinuum spatial modulation spectroscopy: Detection and noise limitations

Supercontinuum spatial modulation spectroscopy is a facile tool for conducting single molecule/particle extinction spectroscopy throughout the visible and near infrared (420-1100 nm). The technique's capabilities are benchmarked using individual Au nanoparticles (NPs) as a standard since they are well studied and display a prominent plasmon resonance in the visible. Extinction spectra of individual Au NPs with diameters (d) ranging from d ~ 8 to 40 nm are resolved with extinction cross sections (σ(ext)) of σ(ext) ~ 1 × 10(-13)-1 × 10(-11) cm(2). Corresponding signal-to-noise ratios range from ~30 to ~1400. The technique's limit of detection is determined to be 4.3 × 10(-14) cm(2) (4.3 nm(2)). To showcase supercontinuum spatial modulation spectroscopy's broader applicability, extinction spectra are acquired for other model systems, such as individual single-walled carbon nanotubes (SWCNTs) and CdSe nanowires. We show for the first time extinction spectra of individual (8,3) and (6,5) SWCNTs. For both chiralities, their E11 [(8,3) 1.30 eV (952 nm); (6,5) 1.26 eV (986 nm)] and E22 [(8,3) 1.86 eV (667 nm); (6,5) 2.19 eV (567 nm)] excitonic resonances are seen with corresponding cross sections of σ(ext) ~ 10(-13) cm(2) μm(-1).

Direct measurement of the absolute absorption spectrum of individual semiconducting single-wall carbon nanotubes

The optical properties of single-wall carbon nanotubes are very promising for developing novel opto-electronic components and sensors with applications in many fields. Despite numerous studies performed using photoluminescence or Raman and Rayleigh scattering, knowledge of their optical response is still partial. Here we determine using spatial modulation spectroscopy, over a broad optical spectral range, the spectrum and amplitude of the absorption cross-section of individual semiconducting single-wall carbon nanotubes. These quantitative measurements permit determination of the oscillator strength of the different excitonic resonances and their dependencies on the excitonic transition and type of semiconducting nanotube. A non-resonant background is also identified and its cross-section comparable to the ideal graphene optical absorbance. Furthermore, investigation of the same single-wall nanotube either free standing or lying on a substrate shows large broadening of the excitonic resonances with increase of oscillator strength, as well as stark weakening of polarization-dependent antenna effects, due to nanotube-substrate interaction.

Size-Dependent Surface Plasmon Resonance Broadening in Nonspherical Nanoparticles: Single Gold Nanorods

The dependence of the spectral width of the longitudinal localized surface plasmon resonance (LSPR) of individual gold nanorods protected by a silica shell is investigated as a function of their size. Experiments were performed using the spatial modulation spectroscopy technique that permits determination of both the spectral characteristics of the LSPR of an individual nanoparticle and its morphology. The measured LSPR is shown to broaden with reduction of both the nanorod length and its diameter, which is in contrast with the predictions of existing classical and quantum theoretical models. This behavior can be reproduced assuming the LSPR width linearly depends on the inverse of an effective length proportional to the square root of the particle surface with the same slope as that recently determined for silica-coated silver nanospheres.

Optical Response of Individual Au–Ag@SiO2 Heterodimers

The optical extinction response of individual Au-Ag@SiO2 heterodimers whose individual morphologies are determined by transmission electron microscopy (TEM) is investigated using spatial modulation spectroscopy. The extinction spectra show two resonances spectrally close to the surface plasmon resonances of the constituting Au and Ag@SiO2 core-shell particles. The interparticle electromagnetic coupling is demonstrated to induce a large increase of the optical extinction of the dimer around its Au-like surface plasmon resonance for light polarized along its axis, as compared to that for perpendicular polarization and to that of an isolated Au nanoparticle. For spherical particles, this interaction also leads to comparable shifts with light polarization of the two dimer resonances, an effect masked or even reversed for particles significantly deviating from sphericity. Both amplitude and spectral effects are found to be in excellent quantitative agreement with numerical simulations when using the TEM-measured dimer morphology (i.e., size, shape, and orientation of the individual dimers), stressing the importance of individual morphology characterization for interpreting heterodimer optical response.

A Quantitative Study of the Environmental Effects on the Optical Response of Gold Nanorods

The effects of the dielectric environment on the optical extinction spectra of gold nanorods were quantitatively studied using individual bare and silica-coated nanorods. The dispersion and amplitude of their extinction cross-section, dominated by absorption for the investigated sizes, were measured using spatial modulation spectroscopy (SMS). The experimental results were compared to calculations from a numerical model that included environmental features present in the measurements and the morphology and size of the corresponding nanorods measured by transmission electron microscopy. The combination of these experimental and theoretical tools permits a detailed interpretation of the optical properties of the individual nanorods. The measured optical extinction spectra and the extinction cross-section amplitudes were well reproduced by the numerical model for silica-coated gold nanorods, for which the silica shell provides a controlled environment. In contrast, additional environmental factors had to be assumed in the model for bare nanorods, stressing the importance of controlling and characterizing the experimental conditions when measuring the optical response of bare surface-deposited single metal nanoparticles.

Optical Imaging and Absolute Absorption Cross Section Measurement of Individual Nano-objects on Opaque Substrates: Single-Wall Carbon Nanotubes on Silicon

Optical detection of an individual single nano-object on an opaque substrate and direct determination of its absorption cross section is demonstrated using reflective spatial modulation spectroscopy. This method is applied to optical imaging and investigation of individual single-wall carbon nanotubes in the 1.6 nm diameter range on silicon substrates, which are also individually characterized by atomic force microscopy, scanning electron microscopy, and in situ micro-Raman spectroscopy. Absorption cross sections on the order of 10–17 cm2 per carbon atom are measured for the investigated semiconducting carbon nanotubes, with a light polarization absorption anisotropy of about 2.

Surface Plasmon Resonance Properties of Single Elongated Nano-objects: Gold Nanobipyramids and Nanorods

The spectral characteristics (wavelength and line width) and the optical extinction cross-section of the longitudinal localized surface plasmon resonance (LSPR) of individual gold nanobipyramids have been quantitatively measured using the spatial modulation spectroscopy technique. The morphology of the same individual nanoparticles has been determined by transmission electron microscopy (TEM). The experimental results are thus interpreted with a numerical model using the TEM measured sizes of the particles as an input, and either including the substrate or assuming a mean homogeneous environment. Results are compared to those obtained for individual nanorods and also show the importance of the local environment of the particle on the detailed description of its spectral position and extinction amplitude.

Spectroscopy and electron microscopy imaging of a single metal nano-object

The optical extinction spectra of single metal nano-objects measured by the spatial modulation spectroscopy technique were correlated with their image obtained by scanning electron microscopy. The results show a large influence of both the object shape and the presence of the substrate on the measured optical spectra.

Plasmon Coupling in Silver Nanocube Dimers: Resonance Splitting Induced by Edge Rounding

Absolute extinction cross sections of individual silver nanocube dimers are measured using spatial modulation spectroscopy in correlation with their transmission electron microscopy images. For very small interparticle distances and an incident light polarized along the dimer axis, we give evidence for a clear splitting of the main dipolar surface plasmon resonance which is found to be essentially induced by cube edge rounding effects. Supported by discrete dipole approximation and finite element method calculations, this phenomenon highlights the high sensitivity of the plasmonic coupling to the exact shape of the effective capacitor formed by the facing surfaces of both particles, especially in the regime of very close proximity.

Surface plasmon resonance spectroscopy of single surfactant-stabilized gold nanoparticles

The optical extinction spectra of single gold nanoparticles stabilized by surfactant molecules are investigated using the spatial modulation spectroscopy technique. The experimental results are compared to computed spectra, focussing on the width of the surface plasmon resonance. It is shown to strongly vary from particle to particle, independently of their size and dielectric environment. This demonstrates the key role of the interface conditions on the width of the surface plasmon resonance for surfactant-stabilized metal nanoparticles.

Imaging and Absolute Extinction Cross-Section Measurements of Nanorods and Nanowires through Polarization Modulation Microscopy

This paper describes an experimental technique for measuring the extinction cross-section of anisotropic nanomaterials. The experiments are performed by focusing a laser beam to a diffraction-limited spot under an optical microscope, and using a photoelastic modulator (PEM) to rotate the polarization of the light beam. Monitoring the transmitted beam with a lock-in amplifier referenced to twice the resonant frequency of the PEM yields the difference in extinction for light polarized parallel and perpendicular to the optical axis of the nanostructure. These experiments are the single particle analog of polarization modulation microscopy (PMM). Experimental results for gold nanorods and CdSe nanowires are presented that demonstrate the sensitivity and properties of this technique. In particular, we show that by collecting images at two laser polarizations separated by 45° it is possible to construct an extinction cross-section image. The advantages and disadvantages of the PMM technique compared to existing ways of measuring the extinction of nanoparticles (photothermal heterodyne imaging and spatial modulation spectroscopy) are discussed. Results from experiments where we collected simultaneous absorption and emission images are also presented for different shaped CdSe nanowires. These measurements provide insight into how the structure of the nanowire affects its photophysics.

Absolute optical extinction measurements of single nano-objects by spatial modulation spectroscopy using a white lamp

This article describes a high sensitivity spectrophotometer designed to detect the overall extinction of light by a single nanoparticle (NP) in the 10(-4)-10(-5) relative range, using a transmission measurement configuration. We focus here on the simple and low cost scheme where a white lamp is used as a light source, permitting easy and broadband extinction measurements (300-900 nm). Using a microscope, in a confocal geometry, an increased sensitivity is reached thanks to a modulation of the NP position under the light spot combined with lock-in detection. Moreover, it is shown that this technique gives access to the absolute extinction cross-sections of the single NP provided that the incident electromagnetic field distribution experienced by the NP is accurately characterized. In this respect, an experimental procedure to characterize the light spot profile in the focal plane, using a reference NP as a probe, is also laid out. The validity of this approach is discussed and confirmed by comparing experimental intensity distributions to theoretical calculations taking into account the vector character of the tightly focused beam. The calibration procedure permitting to obtain the absolute extinction cross-section of the probed NP is then fully described. Finally, the force of the present technique is illustrated through selected examples concerning spherical and slightly elongated gold and silver NPs. Absolute extinction measurements are found to be in good consistency with the NP size and shape independently obtained from transmission electron microscopy, showing that spatial modulation spectroscopy is a powerful tool to get an optical fingerprint of the NP.

Quantitative Determination of the Size Dependence of Surface Plasmon Resonance Damping in Single Ag@SiO2 Nanoparticles

The optical extinction spectra of single silver nanoparticles coated with a silica shell were investigated in the size range 10-50 nm. Measurements were performed using the spatial modulation spectroscopy technique which permits independent determination of both the size of the metal nanoparticle under study and the width of its localized surface plasmon resonance (LSPR). These parameters can thus be directly correlated at a single particle level for the first time. The results show a linear increase of the width of the LSPR with the inverse diameter in the small size regime (less than 25 nm). For these nanoparticles of well-controlled environment, this can be ascribed to quantum confinement of electrons or, classically, to increase of the electron surface scattering processes. The impact of this effect was measured quantitatively and compared to the predictions by theoretical models.

Surface Plasmon Resonance of Single Gold Nanodimers near the Conductive Contact Limit

We have studied the spectral response of individual pairs of gold nanoparticles chemically synthesized and stabilized on a transparent thin film. Very thin interspacing distances are explored by taking advantage of a residual layer of surfactant molecules at the metal surface. Absolute extinction cross section measurements by the spatial modulation spectroscopy technique are correlated to the corresponding high magnification images independently obtained from transmission electron microscopy. The signature of a transition from isolated to conductively coupled particles is evidenced in the optical spectra and is discussed in relation with their morphology. The strength of the electromagnetic coupling is controlled not only by their spacing but also by the shape of their opposite surfaces in the gap region.

Optical extinction spectrum of a single metal nanoparticle: Quantitative characterization of a particle and of its local environment

Optical absorption spectroscopy of a single metal nanoparticle is used to characterize its properties and to obtain quantitative information on its local environment. Experiments were performed using the spatial modulation spectroscopy (SMS) technique on 16 nm mean diameter gold nanoparticles embedded in different medium (i.e., deposited on glass or embedded in a polymer layer). Extraction of the nanoparticle characteristics and determination of the dielectric constant of its environment are discussed, focusing on the impact of the particle shape assumption. The refractive index of the local environment deduced from these measurements shows large particle-to-particle variation, yielding information about fluctuations of the dielectric properties of the surrounding medium on a nanometric scale, inaccessible in ensemble measurements. The influence of the environment of a nanoparticle on its optically extracted geometry and its surface plasmon resonance broadening by surface effect are also studied at a single-particle level.

Time-Resolved Investigation of the Acoustic Vibration of a Single Gold Nanoprism Pair

The acoustic vibration of single gold nanoprism pairs on a glass substrate has been investigated in the time-domain combining a spatial modulation spectroscopy microscope with a high-sensitivity femtosecond pump−probe setup. Three modes were observed and ascribed to two in-plane and one out-of-plane vibration of the nanoprisms forming the pair, in agreement with a theoretical analysis. The periods of the two former modes with similar nature show weak (about 10%) and well correlated pair to pair fluctuations that can be unambiguously ascribed to variation of the prism geometry. In contrast, strong fluctuations, by almost a factor of 6, of the mode damping are evidenced with no correlation with their period. This indicates large variations of the prism-substrate coupling, providing a unique way for its local investigation.

Optical response of metal or dielectric nano-objects in strongly convergent light beams

We present a general formalism for describing the optical response of spherical nano-objects located in the focus region of a tightly convergent light beam. The theory can be applied to any polarized light beam emerging from axially symmetric focusing optical devices. The approach is based on the generalized Mie theory equations and the expansion of the angular spectrum plane-wave decomposition of the beam in terms of vector spherical harmonics. The numerical efficiency of the formalism is exemplified in the case of reflecting microscope objectives, in the context of single-particle experiments based on the spatial modulation spectroscopy technique, for spherical gold spheres and pairs of nearly touching gold particles.

Optical response of a single gold nanoparticle

Optical detection and spectroscopy of single gold nanoparticles using the spatial-modulation spectroscopy technique is described, focusing on the connection between the nanoparticle and environment parameters with the measured linear optical extinction spectrum. Characterisation of the size, shape and orientation on the substrate of a nanoparticle through quantitative determination of its optical extinction cross-section spectrum is illustrated in the case of gold nanospheres and nanorods. Extension of this technique to ultrafast nonlinear spectroscopy of a single gold nanoparticle is also discussed.

Quantitative Absorption Spectroscopy of a Single Gold Nanorod

The spectrally- and polarization-resolved absorption cross-sections of a single gold nanorod have been investigated using the spatial modulation spectroscopy technique. The ensemble of its optical features, that is, longitudinal and transverse surface plasmon resonances and interband absorption, has been quantitatively characterized. The results are compared with numerical simulations using the discrete dipole approximation and the finite element method, yielding information on the investigated nanorod size and shape.

Optical response of a single spherical particle in a tightly focused light beam: application to the spatial modulation spectroscopy technique

We develop a new and numerically efficient formalism to describe the general problem of the scattering and absorption of light by a spherical metal or dielectric particle illuminated by a tightly focused beam. The theory is based on (i) the generalized Mie theory equations, (ii) the plane-wave decomposition of the converging light beam, and (iii) the expansion of a plane wave in terms of vector spherical harmonics. The predictions of the model are illustrated in the case of silver nanoparticles. The results are compared with the Mie theory in the local approximation. Finally, some effects related to the convergence of the beam are analyzed in the context of experiments based on the spatial modulation spectroscopy technique.