Erica Bricchi

Form birefringence and negative index change created by femtosecond direct writing in transparent materials

Although femtosecond lasers have proved to be of great utility for micromachining within bulk transparent materials, little is known about the fundamental physics that drive the process. Depending on the laser intensity delivered to the sample, any of three types of feature can be written into the glass. We observed that in the intermediate regime there is a correlation among the negative sign of the effective index change, the presence of anisotropic reflection, and birefringence. We propose a model that can explain all three principal characteristics. Results show that the local index change can be as high as 10(-1).

“Quill” writing with ultrashort light pulses in transparent materials

A remarkable phenomenon in ultrafast laser processing of transparent materials, in particular, silica glass, manifested as a change in material modification by reversing the writing direction is observed. The effect resembles writing with a quill pen and is interpreted in terms of anisotropic trapping of electron plasma by a tilted front of the ultrashort laser pulse along the writing direction.

Birefringent Fresnel zone plates in silica fabricated by femtosecond laser machining

We demonstrate maskless, single-step fabrication of strongly birefringent Fresnel zone plates by focusing of femtosecond laser pulses deep within silica substrates. The process allows us to produce alternate zone rings directly by inducing a local refractive-index modification of the order of n~10(-2) . The embedded zone plates shown in this Letter exhibit efficiencies that vary by as much as a factor of ~6 for orthogonal polarizations. Focal lengths of primary and secondary foci are shown to compare well with theory.

Self-assembled periodic sub-wavelength structures by femtosecond laser direct writing

Self-assembled, sub-wavelength periodic structures are induced in fused silica by a tightly focused, linearly polarized, femtosecond laser beam. Two different types of periodic structures, the main one with period (Lambda(E)) in the direction of the laser beam polarization and the second with period (Lambda(k)) in the direction of the light propagation, are identified from the cross-sectional images of the modified regions using scanning electron microscopy. We demonstrate the spatial coherence of these nanogratings in the plane perpendicular to the beam propagation direction. The range of effective pulse energy which could produce nanogratings narrows when the pulse repetition rate of writing laser increases. The period Lambda(E) is proportional to the wavelength of the writing laser and period Lambda(k) in the head of the modified region remains approximately the wavelength of light in fused silica.

Embedded anisotropic microreflectors by femtosecond-laser nanomachining

Directly written embedded structures created within fused silica by a femtosecond Ti:sapphire laser are observed to strongly reflect blue light. Reflection emerges only in a direction parallel to the polarization axis of the writing laser. This anisotropic-effect is caused by a periodic modulation of refractive index of amplitude Delta.n ~ 10–2 with a characteristic period Lambda ~ 150nm over a spot size ~1.5µm. We show that the origin of the anisotropic reflection is the primary cause of other anisotropic phenomena reported in recent experiments.

Extraordinary stability of anisotropic femtosecond direct-written structures embedded in silica glass

In this letter we report the different response to temperature displayed by isotropic femtosecond written structures (type I_fs), and anisotropic ones (type II_fs), which are characterized by the presence of a self-assembled subwavelength periodic structure within the irradiated volume. We observe that the anisotropic structures display an extraordinary annealing behavior, namely, their photoinduced change in refractive index increases with the annealing temperature. We explain our experimental results with a theoretical model.

SnO2 nanoparticles in silica: Nanosized tools for femtosecond-laser machining of refractive index patterns

We show that SnO2 nanoclusters in silica interact with ultrashort infrared laser pulses focused inside the material generating a hydrostatic compression and photoelastic response of the surrounding glass. This effect, together with the laser-induced nanocluster amorphization, gives rise to positive or negative refractive-index changes, up to 10−2, depending on the beam-power density. This result points out a wide tuning of the refractive index patterns obtainable in silica-based optical technology.

Self-Assembled Nanostructures and Two-Plasmon Decay in Femtosecond Processing of Transparent Materials

Self-assembled nanostructures in transparent materials irradiated by ultrashort light pulses reveal two-dimensional periodicity. The mechanism of the phenomenon based on interference of bulk plasma waves excited via two plasmon parametric decay is proposed.

Robustness of femtosecond direct written structures in silica glass

We demonstrate the extraordinary stability of femtosecond directly written structures to temperature increase. A temperature close to the glass transition is required to degrade birefringence properties and the related nanostructures.

The lightwave road show

In order to change the way that school children view physics, OSAs University of Southampton Student Chapter has developed an optic roadshow. By taking simple optics experiments and demonstrations to primary schools we aim to dispel the negative image of classroom science and help teachers to convey the fascination of physics.