Godofredo Bautista

Second-Harmonic Generation Imaging of Metal Nano-Objects with Cylindrical Vector Beams

We introduce an imaging technique based on second-harmonic generation with cylindrical vector beams that is extremely sensitive to three-dimensional orientation and nanoscale morphology of metal nano-objects. Our experiments and second-harmonic field calculations based on frequency-domain boundary element method are in very good agreement. The technique provides contrast for structural features that cannot be resolved by linear techniques or conventional states of polarization and shows great potential for simple and cost-effective far-field optical imaging in plasmonics.

Nonlinear chiral imaging of subwavelength-sized twisted-cross gold nanodimers

This work was funded by the ANIMOS Consortium project under the Research Program on Photonics and Modern Imaging Techniques of the Academy of Finland (project 134973). MJH acknowledges support from the Graduate School of Modern Optics and Photonics in Finland. The Karlsruhe team acknowledges support by the Deutsche Forschungsgemeinschaft (DFG), the State of Baden-Wurttemberg, and the Karlsruhe Institute of Technology (KIT) through the DFG Center for Functional Nanostructures (CFN) within subproject A1.5. The project PHOME acknowledges the financial support of the Future and Emerging Technologies (FET) programme within the Seventh Framework Programme for Research of the European Commission, under FET-Open grant number 213390. The project METAMAT is supported by the Bundesministerium fur Bildung und Forschung (BMBF).

Second-harmonic generation imaging of semiconductor nanowires with focused vector beams.

We use second-harmonic generation (SHG) with focused vector beams to investigate individual vertically aligned GaAs nanowires. Our results provide direct evidence that SHG from oriented nanowires is mainly driven by the longitudinal field along the nanowire growth axis. Consequently, focused radial polarization provides a superior tool to characterize such nanowires compared to linear polarization, also allowing this possibility in the native growth environment. We model our experiments by describing the SHG process for zinc-blende structure and dipolar bulk nonlinearity.

Third- and second-harmonic generation microscopy of individual metal nanocones using cylindrical vector beams

We demonstrate third- (THG) and second-harmonic generation (SHG) microscopy of individual silver nanocones using tightly focused cylindrical vector beams (CVBs). Although THG is expected to be a weaker process than SHG, the yield for THG with radial polarization was higher than for SHG. We also found an excellent correlation between the imaging properties of THG and SHG, suggesting that both are governed by the same overall features of the individual nanocone. We also found that the transverse spatial resolution of THG with CVBs, particularly RP, exceeds that of SHG. Our work establishes the potential of THG microscopy with CVBs for structure-sensitive imaging of three-dimensional (3D) metal nano-objects.

Direct laser writing of photostable fluorescent silver nanoclusters in polymer films.

Metal nanoclusters consist of a few to a few hundred atoms and exhibit attractive molecular properties such as ultrasmall size, discrete energy levels, and strong fluorescence. Although patterning of these clusters down to the micro- or nanoscale could lead to applications such as high-density data storage, it has been reported only for inorganic matrices. Here we present submicron-scale mask-free patterning of fluorescent silver nanoclusters in an organic matrix. The nanoclusters were produced by direct laser writing in poly(methacrylic acid) thin films and exhibit a broadband emission at visible wavelengths with photostability that is superior to that of Rhodamine 6G dye. This fabrication method could open new opportunities for applications in nanophotonics like imaging, labeling, and metal ion sensing. We foresee that this method can be further applied to prepare other metal nanoclusters embedded in compositionally different polymer matrices.

Multipolar second-harmonic emission with focused Gaussian beams

We show that electric-dipole-allowed surface second-harmonic (SH) generation with focused Gaussian beams can be described in terms of Mie- type multipolar contributions to the SH signal. In contrast to the traditional case, where Mie multipoles arise from field retardation across nanoparticles, the multipoles here arise from the confined source volume and the tensorial properties of the SH response. We demonstrate this by measuring strongly asymmetric SH emission into reflected and transmitted directions from a nonlinear thin film with isotropic surface symmetry, where symmetric emission is expected using traditional formalisms based on plane-wave excitation. The proposed multipole approach provides a convenient way to explain the measured asymmetric emission. Our results suggest that the separation of surface and bulk responses, which have dipolar and higher-multipolar character, respectively, may be even more difficult than thought. On the other hand, the multipolar approach may allow tailoring of focal conditions in order to design confined and thin nonlinear sources with desired radiation patterns.

Polarized THG Microscopy Identifies Compositionally Different Lipid Droplets in Mammalian Cells

Cells store excess lipids as two major compounds, triacylglycerols (TAGs) and cholesteryl esters (CEs), inside lipid droplets (LDs). The degree of lipid ordering is considered to play a major role in the mobility and enzymatic processing of lipids in LDs. Here, we provide evidence that polarized third-harmonic generation (THG) microscopy distinguishes between native TAG- and CE-enriched LDs in cells due to the different ordering of the two lipid species. We first demonstrate that the responses from synthetic TAG- and CE-enriched LDs using THG microscopy with linear and circular polarizations differ according to their different intrinsic ordering. We then employ simulations to dissect how polarization effects influence the THG from an isotropic LD. Finally, we induce TAG- and CE-enriched LDs in murine macrophages and demonstrate that polarized THG responses increase in a nonlinear fashion with increasing CE/TAG ratio. This suggests that with an increasing CE content, there is a rather sharp transition toward increased LD ordering. Our results demonstrate that polarized THG microscopy enables label-free quantitative analysis of LD ordering and discriminates between compositionally different LDs in intact mammalian cells.

Measurement of optical second-harmonic generation from an individual single-walled carbon nanotube

We show that optical second-harmonic generation (SHG) can be observed from individual single-walled carbon nanotubes (SWCNTs) and, furthermore, allows imaging of individual tubes. Detailed analysis of our results suggests that the structural non-centrosymmetry, as required for SHG, arises from the non-zero chiral angle of the SWCNT. SHG thus has potential as a fast, non-destructive and simple method for imaging of individual nanomolecules and for probing their chiral properties.

Sub-micron scale patterning of fluorescent silver nanoclusters using low-power laser

Noble metal nanoclusters are ultrasmall nanomaterials with tunable properties and huge application potential; however, retaining their enhanced functionality is difficult as they readily lose their properties without stabilization. Here, we demonstrate a facile synthesis of highly photostable silver nanoclusters in a polymer thin film using visible light photoreduction. Furthermore, the different stages of the nanocluster formation are investigated in detail using absorption and fluorescence spectroscopy, fluorescence microscopy, and atomic force microscopy. A cost-effective fabrication of photostable micron-sized fluorescent silver nanocluster barcode is demonstrated in silver-impregnated polymer films using a low-power continuous-wave laser diode. It is shown that a laser power of as low as 0.75 mW is enough to write fluorescent structures, corresponding to the specifications of a commercially available laser pointer. The as-formed nanocluster-containing microstructures can be useful in direct labeling applications such as authenticity marking and fluorescent labeling.

Nonlinear optical activity effects in complex anisotropic three-dimensional media

We perform numerical modelling of nonlinear optical (NLO) microscopy of complex anisotropic three-dimensional (3D) media using the uncoupled dipole approximation. The modelling is applied to 3D biological microstructures resembling collagen fibers and multilamellar vesicles. The results elucidate how nonlinear optical activity effects, such as second-harmonic generation circular dichroism, can arise from 3D morphological chirality, in addition to molecular level chirality. We also show how third-harmonic generation circular dichroism could act as a contrast mechanism for visualizing local structural ordering in 3D anisotropic materials.