Elsevier

Applied Surface Science

Volume 253, Issue 3, 30 November 2006, Pages 1443-1446
Applied Surface Science

Morphology of femtosecond laser-induced structural changes in KTP crystal

https://doi.org/10.1016/j.apsusc.2006.02.020Get rights and content

Abstract

The morphology of structural changes in KTP crystal induced by single femtosecond laser pulse has been investigated by means of CCD camera, scanning electron microscopy (SEM) and atom force microscopy (AFM). The structurally changed region is depressed at energies close to the threshold for producing a structural change and melting ablation morphologies are observed as pulse energy is increased. Furthermore, periodic nanostructures are formed around the edge of the laser-induced spots.

Introduction

In recent years, transparent material modification by femtosecond lasers has attracted attention for the scientific interest and potential applications [1], [2], [3]. For large band-gap materials, based on the mechanisms of local structural change, three-dimensional devices can be fabricated by femtosecond lasers [4], [5], [6], [7]. The growing interest in micromachining of bulk transparent material by femtosecond lasers makes it important to uncover the mechanism responsible for producing permanent structural changes. High laser intensity in the focal volume induces nonlinear absorption of laser energy via multiphoton, tunneling, and avalanche ionization [8], [9]. If enough laser pulse energy is deposited, permanent structural changes are produced in the material at the laser-focused location. Depending on laser energies, focusing conditions, and material properties, different mechanisms will play a role in producing these changes and lead to different morphologies. Unlike previous studies, which paid much attention to glass [10], [11], [12], we firstly demonstrate the morphology of potassium titanyl phosphate (KTP) irradiated by single femtosecond laser pulse. As a typical nonlinear crystal, KTP is widely used in second harmonic generation (SHG), optical parameter oscillation (OPO) and electro-optical modulation for its excellent performances [13]. Therefore, it is very important to study structural changes in KTP crystal induced by femtosecond lasers, and understand the interaction mechanism.

In this paper, the morphology of structural changes produced in the bulk KTP crystal by single femtosecond pulse is investigated. Using CCD camera, scanning electron microscopy (SEM) and atom force microscopy (AFM), the dependence of the morphology of structural changes on the single pulse energy with a fixed focusing condition is presented. Our experimental results show a transition, as the laser pulse energy is increased, from a structural change mechanism dominated by localized melting to one dominated by an explosive expansion.

Section snippets

Experimental details

The experiment was performed in State Key Lab of Laser Technology using a commercially available Ti:sapphire laser system. The laser emits pulses of 50 fs (FWHM) with linearly polarized light at a central wavelength of approximately 800 nm (TEM00 spatial mode, repetition rate of 1 kHz). The laser pulse energy was attenuated by rotating a half waveplate followed by a linear polarizer. A fast acting shutter was used to control the number of laser pulse exposures on sample. The laser beam was focused

Results and discussions

The surface morphologies of KTP crystal produced by single femtosecond pulse with various energies are shown in Fig. 1. We can observe a transition from small structural variations in KTP crystal to severe damage formation as the pulse energy is increased from 6.2 to 80.0 μJ. No trace of surface structural change morphology is observed under CCD as the pulse energy less than 6.2 μJ.

In our experiment, the threshold energy for producing a structural change is measured to be 6.2 μJ (the average

Conclusion

In conclusion, we characterized the morphology of the structural changes produced in KTP crystal by single femtosecond pulse using CCD camera, SEM and AFM. We found that near the threshold energy of structural change, the center of laser-induced spot is depressed and periodic nanostructure is formed at the rim. At higher laser energies, the irradiated spots appear to be melted, and micro hole is observed in the material. Further research is in progress with regard to the related phenomenon.

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