Andrei Naumov

Ultrashort pulse non-linear optical absorption in transparent media

A focused ultrashort pulse can reach high enough intensity that non-linear ionization dominates its interaction with transparent media while still having relatively low fluence. In this case, the energy extracted from the beam can counter self-focusing by energy depletion and plasma formation, providing controlled energy deposition that can modify the material in a highly local manner. We demonstrate that non-linear absorption limits the intensity that can be reached and that the energy is deposited prior to the focus. We model the energy distribution, and predict and measure the energy transmitted through the focus. We establish the threshold intensity for non-linear ionization in dielectrics at ~ 10(1)(3) W cm-(2). We use the refractive index modification that the non-linear ionization causes in glass to image the spatial distribution of the energy deposition.

Tailored semiconductors for high-harmonic optoelectronics

Hitting the highs in solid state The ability to generate high harmonics of optical frequencies through the nonlinear interaction between intense light pulses and gas atoms has opened up the area of ultrafast optics and spectroscopy. Sivis et al. now show that high harmonics can also be generated with a solid-state sample. They used nanofabricated structured targets of ZnO and varied the chemical composition of the sample to demonstrate that (modest) high harmonics can be generated as the light interacts with the target materials. The results present the possibility of developing solid-state ultrafast optical devices. Science, this issue p. 303 Nanofabricated structures and chemical composition can tune the generation of high harmonics from solid-state targets. The advent of high-harmonic generation in gases 30 years ago set the foundation for attosecond science and facilitated ultrafast spectroscopy in atoms, molecules, and solids. We explore high-harmonic generation in the solid state by means of nanostructured and ion-implanted semiconductors. We use wavelength-selective microscopic imaging to map enhanced harmonic emission and show that the generation medium and the driving field can be locally tailored in solids by modifying the chemical composition and morphology. This enables the control of high-harmonic technology within precisely engineered solid targets. We demonstrate customized high-harmonic wave fields with wavelengths down to 225 nanometers (ninth-harmonic order of 2-micrometer laser pulses) and present an integrated Fresnel zone plate target in silicon, which leads to diffraction-limited self-focusing of the generated harmonics down to 1-micrometer spot sizes.

Microstructuring with femtosecond laser inside silica glasses

Summary form only given. Modification of the refractive index of transparent materials by UV laser radiation is widely used for writing waveguides and Bragg gratings in different type of glasses. Extending controlled writing to the inside of transparent materials is important since it opens a wide range of applications for integrated optical elements. In this work we report on direct writing of diffraction gratings and other optical elements inside different silica glasses with a train of femtosecond laser pulses at high repetition rate and study the influence of laser repetition rate and the number of pulses per focused area.

Photosensitivity in glasses: comparing ultrafast lasers with vacuum-ultraviolet lasers

Summary form only given. Laser microfabrication technology is a promising photonics processing approach with parallels to the current use of lasers in semiconductor lithography, trimming, repair, and inspection. To this end, our groups are exploring two extreme forefronts of laser technology - ultrafast (UF) and deep-ultraviolet (UV) lasers - to drive strong interactions in transparent materials for shaping photonic structures. We recently provided head-to-head comparisons of F/sub 2/-laser and 1-ps UF-laser approaches in smooth surface microsculpting of optical glasses, and introduced a new UF-laser processing mode called burst machining that offers crack-free ablation. In this paper, we present an extension to more subtle laser-glass interactions that drive internal refractive-index changes. Photosensitivity processing rates, spatial resolution, and processing windows for both laser types are discussed together with the prospects for printing and trimming of optical waveguides and circuits.

Plane-by-plane inscription of grating structures in optical fibers

Plane-by-plane fabrication of fiber Bragg gratings in optical fibers using short-pulse femtosecond IR laser is proposed and demonstrated. By incorporating a cylindrical lens in the fabrication setup, a plane of index modification can be directly inscribed in fiber core by a single laser pulse through the proposed method. This plane-by-plane method simplifies the grating inscription process and allows for the fabrication of complicated grating structures.

Tailored high-harmonic generation in nanostructured semiconductors

High harmonic generation (HHG) [1, 2] in gas-phase atomic or molecular targets has been extensively studied and developed over the past few decades, enabling attosecond spectroscopy [3] and tomographic imaging of molecular orbitals [4]. Recently, HHG could also been demonstrated in solid-state systems [5-8], which allows for applying attosecond spectroscopy techniques to condensed matter in an all-optical fashion as well as utilizing a solids electrons as a probe to investigate structural anisotropies [9]. Here, we extend these novel approaches by nanoscale engineering of the solid targets and demonstrate that HHG in semiconductors can be tailored and controlled by modification of the local chemical composition or the microstructure.

An All-optical Characterization of the Attosecond Pulse in Space and Time

We demonstrate an all-optical spatio-temporal characterization method for the attosecond pulses produced through high harmonic generation. A spatio-temporal profile is retrieved from the spatial modulation of the harmonic spectra.