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The properties of structures written inside dielectrics with high repetitionrate femtosecond lasers are known to depend strongly on the complex interplayof a large number of writing parameters. Recently, ion migration within thelaser-excited volume has been identified as a powerful mechanism for changingthe local element distribution and producing efficient optical waveguides. Inthis work it is shown that the transient plasma distribution induced duringlaser irradiation is a reliable monitor for predicting the final refractiveindex distribution of the waveguide caused by ion migration. By performingin-situ plasma emission microscopy during the writing process inside aLa-phosphate glass it is found that the long axis of the plasma distributiondetermines the axis of ion migration, being responsible for the localrefractive index increase. This observation is also valid when strong positiveor negative spherical aberration is induced, greatly deforming the focal volumeand inverting the index profile. Even subtle changes in the writing conditions,such as an inversion of the writing direction (quill writing effect), show upin the form of a modified plasma distribution, which manifests as a modifiedindex distribution. Finally, it is shown that the superior control over thewaveguide properties employing the slit shaping technique is caused by the moreconfined plasma distribution produced. The underlying reasons for thisunexpected result are discussed in terms of non-linear propagation and heataccumulation.

Controlling plasma distributions as driving forces for ion migration during fs laser writing