P. P. Rajeev

Pulse duration dependence of femtosecond-laser-fabricated nanogratings in fused silica

Femtosecond laser radiation tightly focused in bulk fused silica is used to generate self-ordered nanogratings when the sample is translated under the lens at constant speed. The nanogratings are preserved over a length scale of millimeters. We demonstrate that nanogratings are formed for all pulse durations tested, ranging from 40to500fs, and that the pulse energy threshold for this phenomenon increases with decreasing pulse duration. We use high spatial resolution diagnostics based upon selective chemical etching followed by atomic force microscopy and scanning electron microscopy to reveal the morphology of the nanogratings.

Femtosecond laser erasing and rewriting of self-organized planar nanocracks in fused silica glass

Tightly focused, linearly polarized, femtosecond laser radiation can produce highly birefringent nanograting structures inside fused silica glass. Here we report that when the polarization direction of the femtosecond light is changed, old nanogratings are erased and simultaneously replaced with new ones whose orientation is solely determined by the polarization of the rewrite beam. We also show that these volume nanogratings can be rewritten 1000 times with little degradation in their quality.

Transient nanoplasmonics inside dielectrics

Intense ultrashort light pulses interacting inside dielectrics can create nanoplasmas due to localized inhomogeneous nonlinear ionization. These nanoplasmas are bound inside the dielectric and are transient as their density changes during the light pulse—from underdense to quasi-metallic plasma densities. Interaction of light at the transient plasma–dielectric interface can lead to local field enhancements, similar to that observed in the metal-dielectric interface, which control the growth of nanoplasmas. We discuss the differences in the interaction of light at these two interfaces and demonstrate that transient nanoplasmonics can imprint periodic nanostructures inside the dielectric.

Femtosecond laser writing of porous capillaries inside fused silica glass.

We demonstrate that within a restricted optical pulse duration-pulse energy parameter space tightly focused femtosecond laser radiation can be used to fabricate porous capillaries in bulk fused silica glass by simply moving the laser focus through the material. We show that the rate of penetration of liquids into the porous capillaries can be controlled by the laser polarization, which determines their morphology. The fluid propagation is measured using the form birefringence of nanocrack/nanovoid structures produced inside the capillaries. We also demonstrate the nanofiltration capabilities of the capillaries by separating the relatively small molecules of Rhodamine 6G dye from their solvent.

Strong field ionization inside transparent solids

Femtosecond light pulses focused inside transparent materials permanently modify their dielectric properties locally within the focal volume due to multiphoton ionization. Under certain conditions, the localization of dielectric modification can be severe and confined to nanometre dimensions, much less than the diffraction limit of light. To better understand the complexity of the light-solid interaction, we review multiphoton processes in isolated systems such as atoms, molecules and clusters, and discuss how collective and local field effects influence the interaction of light with transparent materials.

Polarization Dependence of Nanostructure Formation in Transparent Solids

We demonstrate how the polarization of light controls the geometry of nanostructures formed by ultrashort laser pulses in fused quartz.

Intense Field Science in Dielectrics

We discuss fundamental aspects of the interaction intense field with dielectrics that underpin femtosecond laser dielectric modification. We establish that sub-cycle dynamics can be observed in dielectrics, introducing possibilities for attosecond science in solids.

Cold avalanche ionisation

When intense laser pulses interact with matter, ionisation is initiated by multiphoton effects. In some cases the electrons, gaining energy from the laser field, can collide with other atoms and ionise them, triggering an avalanche ionisation. It is generally believed that the laser pulse has to be sufficiently long (≫ 100 fs) for the electrons to gain energy required for an avalanche. However, this is still a well-debated issue as some experimental observations suggest that avalanche effects can prevail even for shorter pulses [1]. Recent theoretical works suggest that in the presence of intense fields, even electrons with lower-than-the-bandgap energy can cause ionisation [2]. This field-assisted collisional ionisation or cold avalanche could be presen even for few cycle pulses. In solids for example, several collisions are possible during the presence of intense fields due to the high atomic density. However, it is difficult to distinguish this from the normal avalanche process and the presence of field assisted collisional ionisation has not yet been experimentally verified.

Memory in nonlinear ionization of transparent dielectrics

We show a reduction in the ionization threshold at previously ionized regions inside transparent solids. This forms a shot-to-shot memory that can lead to several unique nonlinear phenomena including the formation of nanostructures.

Measuring attosecond ionization dynamics inside dielectrics

We resolve attosecond dynamics of multiphoton ionization in solids. We subdivide the laser cycle using differential absorption between the major and the minor axes of elliptically polarized beam.