V. R. Bhardwaj

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.

Coincidence imaging of polyatomic molecules via laser-induced Coulomb explosion

We extend laser-induced Coulomb explosion imaging to retrieve the structure of the five-atom dichloromethane (CH2Cl2) molecule by developing coincidence imaging and geometry optimization techniques. By detecting all five atoms in coincidence, we show that, from the measured velocity vectors, the geometry of the molecules can be reconstructed.

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.

High-harmonic generation in H2O.

We demonstrate high-harmonic generation in H(2)O using 800 and 1300 nm laser pulses up to a maximum intensity of 5x10(14) W/cm(2). Under optimal phase-matching conditions, photon energies up to approximately 60 and approximately 87 eV are produced by using 800 and 1300 nm light, respectively. The harmonic spectra in H(2)O, when compared with Xe with a similar ionization potential, exhibit significant extension of the cutoff region, indicating suppression of ionization arising from molecular orbital symmetry.

High-order harmonic generation from C{sub 60}-rich plasma

We performed systematic investigation of high-order harmonic generation from fullerene-rich laser-produced plasmas. We studied harmonic generation by varying several experimental parameters, such as the delay between the ablation and driving pulses, and divergence and polarization of the pump laser. Enhancement of harmonic yield is observed near 20 eV, which is attributed to the influence of a broadband plasmon resonance of C{sub 60} on the nonlinear optical response of fullerene-rich plasma. This increase in the harmonic intensity occurs despite the increased absorption by C{sub 60} at these wavelengths. Using simulations based on time-dependent density-functional theory, we confirm that this effect is due to the influence of collective excitations. We compare harmonic generation from fullerenes using lasers with 793 nm and 396 nm wavelengths, which show the influence of plasmon resonance on the conversion efficiency of high-order harmonics for different laser wavelengths.

Signatures of symmetry and electronic structure in high-order harmonic generation in polyatomic molecules

We report detailed measurements of high-order harmonic generation in chloromethane molecules (CCl{sub 4}, CHCl{sub 3}, and CH{sub 2}Cl{sub 2}) to show that fingerprints of symmetry and electronic structure can be decoded from high-order harmonic generation even in complex randomly oriented molecules. In our measurements, orbital symmetries of these molecules are manifested as both extended harmonic cutoffs and a local minimum in the ellipticity dependence of the cut-off harmonics, suggesting the occurrence of quantum interferences during ionization. The harmonic spectra exhibit distinct interference minima at {approx}42 and {approx}60 eV. We attribute the former to the Cooper minimum in the photoionization cross section and the latter to intramolecular interference during the recombination process.

High-order harmonics from C 60 fullerenes -toward attosecond dynamics of complex systems

We demonstrate, for the first time, high-order harmonic generation from C<inf>60</inf> pumped by intense femtosecond Ti: sapphire laser. Laser-produced plasmas from C<inf>60</inf>-rich epoxy and C<inf>60</inf> films were used as the nonlinear media. Harmonics up to the 25^th order were observed. The harmonic yield from fullerene-rich plasma is about 25 times larger compared with those produced from bulk carbon target.

High-order harmonic generation from fullerene plasma

We demonstrate high-order harmonic generation from C60 fullerenes pumped by intense Ti:sapphire laser. We observe a 25 times increase in the harmonic yield compared with bulk carbon, near the 22 eV giant resonance of C60.

Multiphoton Interaction of Light with Dielectrics

When intense femtosecond optical pulses are focused inside fused silica localized material modification leads to self-organized periodic nanoplanes that are few nanometers wide and spaced at lambda/2 in the medium. We provide insight into their origin by invoking the concepts of strong-field physics and nano-plasmonics