Scanning Second Harmonic Generation Research Articles

Mapping of gold nanostructure-enhanced near fields via laser scanning second-harmonic generation and ablation

The optical near-field of metal films can be modified in a straightforward manner by incorporating nanostructures on the surface. The corresponding field enhancement, which may be due to the lightning rod effect as well as the excitation of plasmon modes, results in a local change of the optical surface response. A transparent thin film on top of the nanostructures can be partially ablated via illumination with near-infrared light. Local variations of the ablation rate due to field enhancement are readily mapped with subdiffractional resolution, as confirmed by a direct comparison to theoretical calculations. Variation of the thickness of the transparent film enables discrimination between localized enhancements at the sharp corners of the structures and collective enhancements at locations between the structures due to surface plasmon polariton modes. In addition, applying the same method to study the effect of nanostructure morphology on localized second-harmonic generation using arrays of rectangular as well as triangular structures, we observed a second-harmonic (SH) signal from both centrosymmetric and noncentrosymmetric nanostructure arrays, indicating that the SH excitation is not due to a collective phenomenon but originates locally from the individual structures.

SHG microscopic observation of Pb(Zn1/3Nb2/3)O3–9%PbTiO3 single crystal

Non-destructive and contactless observations of the domain structures in a single crystal of solid solution Pb(Zn1/3Nb2/3)O3–9%PbTiO3 (PZN–9PT) was performed using the scanning second-harmonic generation (SHG) microscopy with the spatial resolution of optical wavelength scale. A novel method of the 2D mapping of SHG polarization dependence (Pol-D) is proposed. The result indicates that the Pol-Ds correspond well to the observed SHG images of domain structures. The directions of ferroelectric spontaneous polarizations are quantitatively analysed by comparing SHG images and the Pol-Ds. It is confirmed that PZN-9PT has the monoclinic symmetry at room temperature.

Domain structure analysis ofPb(Zn1/3Nb2/3)O3−9%PbTiO3single crystals using optical second harmonic generation microscopy

The domain structures of relaxor-ferroelectric $\text{Pb}({\text{Zn}}_{1/3}{\text{Nb}}_{2/3}){\text{O}}_{3}\text{\ensuremath{-}}9\mathrm{%}{\text{PbTiO}}_{3}$ (PZN-9PT) single crystals with a morphotropic phase boundary (MPB) composition are observed with a scanning second harmonic generation (SHG) microscope. Three-dimensional domain structures are obtained from sectional SH images along the axial direction. The domain structures are explained well by the strain compatibility theory that is based on the ferroelectric/ferroelastic phase transition of $m\overline{3}mFm(p)$ with the monoclinic space group $Pm$. The SHG images are divided into several parts, and the light-polarization dependence (PolD) at each site is calculated by minimizing the least-squares errors of the nonlinear susceptibilities. The PolDs are fitted well by theoretical formulas of the SH intensities for the $Pm$ symmetry, and the two-dimensional map of the PolD coincides well with the corresponding SHG image of the domain structures. The monoclinic $Pm$ symmetry at the MPB of PZN-9PT is determined at the optical diffraction limit of 460 nm.

Three-Dimensional Observations of LiNbO3 and LiTaO3 Quasi-Phase Matching Devices Using Transmission-Type Scanning Second-Harmonic Generation Interference Microscope

Periodically poled domain structures in stoichiometric LiNbO3 (PPSLN) and LiTaO3 (PPSLT) are observed using an optical second harmonic generation (SHG) interference microscope. Three-dimensional section images can be obtained in PPSLN using d31 SHG tensor component. In PPSLT, the images of top and bottom surfaces are obtained using d22 component. The difference in SHG images with and without interference is discussed.

New Dyes With Fast Voltage-Dependent Changes In Membrane SHG

Laser scanning second harmonic generation (SHG) microscopy has shown significant promise for membrane potential imaging with voltage sensitive dyes (VSDs), possessing significant advantages over fluorescence-based imaging modalities. Through simultaneous patch-clamping and non-linear imaging of cells, SHG has been found to exhibit sensitivities to trans-membrane potential that are up to four times better than those obtained under optimal conditions using one-photon fluorescence imaging (Millard et al., 2004). For styryl dyes, while electrochromism is the dominating photophysical mechanism of fluorescence, some ANEP-based dyes display slow SHG voltage responses, suggesting that chromophore membrane reorientation or redistribution may be involved. The mechanism of the SHG response is not entirely understood and necessitates additional study in order to fully optimize this imaging modality. We report on our further investigation of the time dependence of the voltage sensitivity of SHG and simultaneous two-photon fluorescence imaging, using “fast” voltage-switching experiments. The response kinetics of resonance enhanced SHG from several styryl dyes developed in our laboratory, including di-3-ANEPPDHQ, as well as di-4-ANEPPTEA and a new fluorinated derivative, have been determined. Voltage-clamped neuroblastoma cells stained with these dyes were imaged with 1064 nm excitation from a mode locked fiber-based laser source. For SHG, these VSDs were found to exhibit moderately large voltage sensitivities in addition to fast kinetic responses. Our results suggest that voltage sensitive dyes can be developed which have both large SHG signal changes and the requisite speed for use as a practical tool for measuring electrical activity in neuronal systems.(Supported by NIH grant EB001963).

Time-gated imaging for multifocus second-harmonic generation microscopy

We developed a scanning second-harmonic generation (SHG) microscope with a time-gated image intensifier. The combination of a microlens array scanner and the gated image intensifier allows us to perform simultaneous time-resolved detection of SHG at multiple positions in a sample. By tuning the time delay of the gated detection from pulsed laser illumination by a mode-locked Ti:sapphire laser, background noise originating from the image intensifier dark current, fluorescence from the sample, and stray light are effectively reduced.

Local orientational distribution of molecular monolayers probed by nonlinear microscopy

A scanning second harmonic generation (SHG) microscopy technique in conjunction with a polarization resolved detection scheme is applied to probe the local molecular organization in organic monolayers. A full SHG polarization analysis is developed to investigate local effects such as orientational heterogeneities and random orientations in molecular aggregates. The SHG signal measured in hemicyanine-based monolayers deposited on glass substrates evidences spatial micrometric-size domains where the non-centrosymmetric molecular alignment is modeled with a high accuracy based on polarization information. In addition, an incoherent and spatially uniform SHG emission is measured in monolayers displaying molecular aggregation.

The Structure of a Tetraazapentacene Molecular Monolayer

Langmuir molecular monolayers of a unique tetraazapentacene zwitterionic amphiphile were found to form stripe-like domains that responded strongly to an applied magnetic field. These stripe domains were transferred as Langmuir−Blodgett (LB) films to glass substrates, where they were structurally characterized using five complementary imaging techniques, including scanning second-harmonic generation (SHG) microscopy and capacitance force microscopy. The molecular orientations of adjacent single striped domains of a tetraazapentacene monolayer were determined by SHG polarization measurements. Those structures were shown to be consistent across the various imaging techniques.

Mapping piezoelectric-field distribution in gallium nitride with scanning second-harmonic generation microscopy.

Taking advantage of the electric field-enhanced second-harmonic generation effect in bulk gallium nitride (GaN) and indium gallium nitride (InGaN) quantum wells, we demonstrated the piezoelectric field distribution mapping in bulk GaN and InGaN multiple-quantum-well (MQW) samples using scanning second-harmonic generation (SHG) microscopy. Scanning SHG microscopy and the accompanying third-harmonic generation (THG) microscopy of the bulk GaN sample were demonstrated using a femtosecond Cr:forsterite laser at a wavelength of 1230 nm. Taking advantage of the off-resonant electric field-enhanced SHG effect and the bandtail state-resonance THG effect, the second- and third-harmonic generation microscopic images obtained revealed the piezoelectric field and bandtail state distributions in a GaN sample. Combined with 720 nm wavelength excited two-photon fluorescence microscopy in the same sample, the increased defect density around the defect area was found to suppress bandedge photoluminescence, to increase yellow luminescence, to increase bandtail state density, and to decrease residue piezoelectric field intensity. Scanning SHG microscopy of the InGaN MQW sample was resonant excited with 800 nm femtosecond pulses from a Ti:sapphire laser in order to suppress SHG contribution from the bulk GaN substrate. Taking advantage of the strong piezoelectric field inside the InGaN quantum well, the wavelength resonant effect, and the electric field-enhanced SHG effect of InGaN quantum wells, resonant scanning SHG microscopy revealed the piezoelectric field distribution inside the wells. Combined with accompanying three-photon fluorescence microscopy from the bulk GaN substrate underneath the quantum wells, the direct correspondence between the piezoelectric field strength inside the quantum well and the substrate quality can be obtained. According to our study, the GaN substrate area with bright bandedge luminescence corresponds to the area with strong SHG signals indicating a higher stained-induced piezoelectric field. These scanning harmonic generation microscopies exhibit superior images of the piezoelectric field and defect state distributions in GaN and InGaN MQWs not available before. Combining with scanning multiphoton fluorescence microscopy, these techniques open new ways for the physical property study of this important material system and can provide interesting details that are not readily available by other microscopic techniques.

Scanning second-harmonic/third-harmonic generation microscopy of gallium nitride

Scanning second-harmonic generation and third-harmonic generation microscopy of a gallium nitride (GaN) sample was demonstrated using a femtosecond Cr:forsterite laser. Taking advantage of the electric-field enhanced second-harmonic generation effect and bandtail state resonance effect, the obtained second-harmonic and third-harmonic generation microscopic images revealed the piezoelectric field and bandtail state distributions in a GaN sample.