Eliminating thermal effects in z-scan measurements of thin PTCDA films.

N Wickremasinghe,H P Wagner,H Schmitzer,X Wang

doi:10.1364/oe.22.023955

Abstract

We investigate the two-photon absorption (TPA) and nonlinear refraction of a micrometer thick 3,4,9,10-perylentetracarboxyl-dianhydride (PTCDA) film using z-scans with tightly focused 100 fs laser pulses. The PTCDA film was grown by organic molecular beam deposition on a Pyrex substrate. To study the influence of sample heating, the pulse repetition rate was varied between 4 MHz and 50 kHz with an acousto-optic pulse selector. We find that thermal effects diminish for pulse repetition times longer than 5 and 0.75 µs when using a 10x or 20x microscope lens, respectively, resulting in a TPA coefficient of 6 cm/GW and a nonlinear refractive index of 1.2 x 10⁻¹³ cm²/W at a wavelength of 820 nm.

Highlights

The z-scan technique is a highly sensitive optical method that allows measuring both the two photon absorption (TPA) coefficient and the nonlinear refractive index [1,2]
As an alternative approach to eliminate heat accumulation in thin, dielectric films we propose the application of low fluence 100 fs laser pulses at reduced repetition rates using an acousto-optic modulator (AOM) as pulse selector
We performed z-scan measurements on an organic molecular beam deposition (OMBD) grown PTCDA thin film with tightly focused 100 fs laser pulses at a wavelength of 820 nm

Summary

Introduction

The z-scan technique is a highly sensitive optical method that allows measuring both the two photon absorption (TPA) coefficient and the nonlinear refractive index [1,2]. The pulses are tightly focused to reduce the thermal diffusion time from the area of the laser excitation into the unexcited film area Using this modified z-scan technique we were able to determine the nonlinear optical constants at 820 nm of a 3,4,9,10-perylentetracarboxylic dianhydride (PTCDA) film which has been grown by organic molecular beam deposition (OMBD) on a Pyrex substrate. Of a vacuum deposited PTCDA film using degenerate four wave mixing experiments at 602 nm [35] have been previously reported These values serve as reference to test our modified tightly focused z-scan technique using 100 fs pulses at reduced pulse repetition rate

Experimental details

Open aperture z-scan measurements

Closed aperture z-scan measurements

Summary