9 Aperiodic Order in Nanoplasmonics
363
The spatial modifications of the structural color fingerprints induced by small
refractive index variations can be readily quantified using image autocorrelation
analysis performed on the scattered radiation [129], using polychromatic or single
wavelength modalities, as shown in Fig. 9.18 for silk protein monolayers deposited
on DANS surfaces. To construct the image autocorrelation function (ACF), the value
of the field intensity at point (x, y) in the array plane is compared with that at another
point (x ∗ , y ∗ ) and mapped as a function of the distance between the two points. 1D
profiles through the 2D autocorrelation function of the intensity images in Fig. 9.18d.
The initial decay in the ACF reflects local short-range correlations in the aperiodic
structure, and any long-range periodicities in the intensity pattern give rise to periodic
oscillations in the ACF. To quantify the overall change in the colorimetric pattern
induced by the presence of thin molecule layers, we calculate the variance of the
fluctuations in the intensity distribution function, which can be found as the value
of the properly normalized discrete ACF in the limit of zero lateral displacements.
The proposed approach recently led to the demonstration of femto-molar detection
of silk proteins adsorbed on DANS [86].
The refractive index modifications induced by the analytes (silk protein) can be
detected by frequency shifts and a global structural color modification. Combining
Electron Beam Lithography (EBL), dark-field scattering micro-spectroscopy, autocorrelation analysis and rigorous multiple scattering calculations based on the Generalized Mie Theory (GMT), Lee and collaborators [86] have engineered aperiodic
arrays of metal nano-particles on quartz substrates, and showed that the information
encoded in both the spectral and spatial distribution of structural resonances can be
simultaneously utilized for sensitive bio-detection on the nanoscale. The potential of
the proposed approach for rapid, label-free detection and recognition of biomolecular
analytes in the visible spectral range was experimentally demonstrated by the distinct
variation in the spectral and spatial colorimetric fingerprints in response to monolayer increments of protein layers sequentially deposited on the surface of aperiodic
arrays of nanoparticles.
Finally, we remark that this DANS-enabled detection technique can be conveniently integrated into microfludics channels for optical liquid sensing applications.
Our group is currently working to demonstrate full optofludics integration of spatialspectral colorimetric detectors for liquid sensing based on sensitive multiple light
scattering in DANS.
9.3.4 Applications to Light Emission Enhancement
Studies on the far-field and near-field optical behavior of two-dimensional Fibonaccibased lattices fabricated by EBL have demonstrated the presence of strongly localized
plasmon modes whose exact location can be accurately predicted from purely structural considerations. In particular, Dallapiccola at al. [130], by performing near-field
optical measurements in collection mode and three dimensional Finite Difference
Time Domain (FDTD) simulations, showed that dimer coupling in a Fibonacci lat-
363
The spatial modifications of the structural color fingerprints induced by small
refractive index variations can be readily quantified using image autocorrelation
analysis performed on the scattered radiation [129], using polychromatic or single
wavelength modalities, as shown in Fig. 9.18 for silk protein monolayers deposited
on DANS surfaces. To construct the image autocorrelation function (ACF), the value
of the field intensity at point (x, y) in the array plane is compared with that at another
point (x ∗ , y ∗ ) and mapped as a function of the distance between the two points. 1D
profiles through the 2D autocorrelation function of the intensity images in Fig. 9.18d.
The initial decay in the ACF reflects local short-range correlations in the aperiodic
structure, and any long-range periodicities in the intensity pattern give rise to periodic
oscillations in the ACF. To quantify the overall change in the colorimetric pattern
induced by the presence of thin molecule layers, we calculate the variance of the
fluctuations in the intensity distribution function, which can be found as the value
of the properly normalized discrete ACF in the limit of zero lateral displacements.
The proposed approach recently led to the demonstration of femto-molar detection
of silk proteins adsorbed on DANS [86].
The refractive index modifications induced by the analytes (silk protein) can be
detected by frequency shifts and a global structural color modification. Combining
Electron Beam Lithography (EBL), dark-field scattering micro-spectroscopy, autocorrelation analysis and rigorous multiple scattering calculations based on the Generalized Mie Theory (GMT), Lee and collaborators [86] have engineered aperiodic
arrays of metal nano-particles on quartz substrates, and showed that the information
encoded in both the spectral and spatial distribution of structural resonances can be
simultaneously utilized for sensitive bio-detection on the nanoscale. The potential of
the proposed approach for rapid, label-free detection and recognition of biomolecular
analytes in the visible spectral range was experimentally demonstrated by the distinct
variation in the spectral and spatial colorimetric fingerprints in response to monolayer increments of protein layers sequentially deposited on the surface of aperiodic
arrays of nanoparticles.
Finally, we remark that this DANS-enabled detection technique can be conveniently integrated into microfludics channels for optical liquid sensing applications.
Our group is currently working to demonstrate full optofludics integration of spatialspectral colorimetric detectors for liquid sensing based on sensitive multiple light
scattering in DANS.
9.3.4 Applications to Light Emission Enhancement
Studies on the far-field and near-field optical behavior of two-dimensional Fibonaccibased lattices fabricated by EBL have demonstrated the presence of strongly localized
plasmon modes whose exact location can be accurately predicted from purely structural considerations. In particular, Dallapiccola at al. [130], by performing near-field
optical measurements in collection mode and three dimensional Finite Difference
Time Domain (FDTD) simulations, showed that dimer coupling in a Fibonacci lat-