Effects of laser polarization and interface orientation in harmonic generation microscopy

Uwe Petzold,Thomas Halfmann,Andreas Büchel

doi:10.1364/oe.20.003654

Abstract

We present systematic experimental investigations on the effects of laser polarization and interface orientation in second and third harmonic generation microscopy. We find that the laser polarization has no measurable effect on signal strength and resolution in third harmonic microscopy, while the second harmonic strongly depends upon the polarization direction of the driving laser. Moreover, we observe a strong effect of the interface orientation with respect to the laser beam direction-both in second and third harmonic generation. This affects the signal strength, as well as the obtained transversal and longitudinal resolution in microscopic imaging. As an (on the first glance) surprising feature, also surfaces parallel to the optical axis of the laser beam yield strong harmonic signal. This enables applications of harmonic microscopy in specific geometries. As an example we monitor the flow of immiscible microfluids in lateral cut by third harmonic microscopy.

Highlights

In the past fifteen years coherent nonlinear microscopy (CNM) advanced towards a powerful tool for three-dimensional visualization of transparent samples
We present systematic experimental investigations with regard to the effects of laser polarization and interface orientation in second harmonic generation (SHG) and third harmonic generation (THG) microscopy
Barille et al did not perform any systematic measurements on the dependence of the THG signal with regard to polarization directions and interface orientations—which is the aim of the following work

Summary

Introduction

In the past fifteen years coherent nonlinear microscopy (CNM) advanced towards a powerful tool for three-dimensional visualization of transparent samples (see e.g [1,2,3]. and refs. therein). By scanning the laser focus across the sample, we obtain a three-dimensional image We note, that such nonlinear processes result in effective excitation volumes smaller than the diffraction-limited volumes of the fundamental field—which enables larger resolution. Numerical simulations for THG at surfaces oriented parallel to the laser beam direction predicted a considerable dependence of THG vs polarization direction [18, 19]— though different authors gave different values and signs of the predicted changes. There are only numerical simulations on the effect of surface orientation in THG microscopy [19] This dependence is very important for applications—as specific surface orientations may not yield harmonic signal, i.e. may not permit imaging of complex structures or specific geometries. We apply our findings to image a microfluidic system in lateral cut by THG microscopy

Experimental implementation

Experimental results

Dependence of harmonic generation upon laser polarization

Dependence of harmonic generation upon interface orientation

Conclusion