Ravi Bhardwaj

Fabrication of microlens arrays in polycarbonate with nanojoule energy femtosecond laser pulses.

Microlens arrays were fabricated by focusing a high repetition rate femtosecond laser with low energy pulses inside polycarbonate samples. The microlens was imaged in a scanning electron microscope and was optically characterized using a HeNe laser. It was found that the microlens has a diameter of 80 μm, a focal length of 100 μm, a numerical apperture of 0.63 and a spot diameter of 1.2 μm. Additionally, we found that a void is created under the microlens.

Femtosecond laser induced surface swelling in poly-methyl methacrylate

We show that surface swelling is the first step in the interaction of a single femtosecond laser pulse with PMMA. This is followed by perforation of the swollen structure and material ejection. The size of the swelling and the perforated hole increases with pulse energy. After certain energy the swelling disappears and the interaction is dominated by the ablated hole. This behaviour is independent of laser polarization. The threshold energy at which the hole size coincides with size of swelling is 1.5 times that of the threshold for surface swelling. 2D molecular dynamics simulations show surface swelling at low pulse energies along with void formation below the surface within the interaction region. Simulations show that at higher energies, the voids coalesce and grow, and the interaction is dominated by material ejection.

Ultra-high density optical data storage in common transparent plastics

The ever-increasing demand for high data storage capacity has spurred research on development of innovative technologies and new storage materials. Conventional GByte optical discs (DVDs and Bluray) can be transformed into ultrahigh capacity storage media by encoding multi-level and multiplexed information within the three dimensional volume of a recording medium. However, in most cases the recording medium had to be photosensitive requiring doping with photochromic molecules or nanoparticles in a multilayer stack or in the bulk material. Here, we show high-density data storage in commonly available plastics without any special material preparation. A pulsed laser was used to record data in micron-sized modified regions. Upon excitation by the read laser, each modified region emits fluorescence whose intensity represents 32 grey levels corresponding to 5 bits. We demonstrate up to 20 layers of embedded data. Adjusting the read laser power and detector sensitivity storage capacities up to 0.2 TBytes can be achieved in a standard 120 mm disc.

Mechanism for spherical dome and microvoid formation in polycarbonate using nanojoule femtosecond laser pulses

Spherical domes are created on the surface of polycarbonate samples, and microvoids are formed within the bulk using only a femtosecond oscillator with pulse energy of just 0.47 nJ. Size of spherical domes and shape of microvoids are controlled by changing the laser focus inside the material. Their formation is explained by a combination of heat accumulation and dome formation dynamics, where the dome acts as a microlens shifting the laser focus within the sample. The technique described here provides a simple avenue for fabricating smooth microlens arrays of various sizes and opens the possibility for direct fabrication of complex three-dimensional microfluidic channels in transparent materials.

Rewritable nanogratings in fused silica using a focused femtosecond laser beam

Long-range periodic nanoplane gratings written inside fused silica using a focused femtosecond laser beam can be erased and replaced with new gratings whose orientation is determined by the polarization of the femtosecond laser overwrite beam.

Long-range periodic planar nanostructures produced in glass by femtosecond laser dielectric modification

Polarization dependent periodic nanostructures consisting of long-range arrayed planes of modification, < 20 nm thick and separated by /spl sim/240 /spl plusmn/ 25 nm, are created in fused silica through femtosecond laser irradiation. The periodicity is revealed by ultrahigh spatial resolution chemical etching combined with atomic force microscopy.

Writing buried optical waveguides: Contrasts in ultrafast and ultraviolet lasers

Ultrafast lasers and deep-ultraviolet F2 lasers present stark contrasts in driving multi-photon and single-photon interactions within transparent glasses. Low-loss single mode waveguides and simple photonic components were formed with a 50-fs Ti:Sapphire laser and a 157-nm 15-ns F2 laser inside the bulk of fused silica. Laser interactions were confined by large NA optics within a small ∼8-μm diameter focal volume and scanned with precision motion stages to shape 3-dimensional photonic structures. Refractive index changes of up to 0.01 were induced with the ultrafast laser at a high scanning speed of several hundred microns per second and 100-kHz repetition rate. Weaker refractive index changes of 0.0005 were generated by the F2 laser, owing to a lower 100-Hz repetition rate and weaker absorption in the focal volume. The laser-generated waveguides offer high transmission of ∼1 dB/cm, are transparent to light in the visible and infrared spectrum, and can support multi-mode guiding with low insertion loss into optical fibers. The paper will present examples of fabricating simple three-dimensional photonic components.Ultrafast lasers and deep-ultraviolet F2 lasers present stark contrasts in driving multi-photon and single-photon interactions within transparent glasses. Low-loss single mode waveguides and simple photonic components were formed with a 50-fs Ti:Sapphire laser and a 157-nm 15-ns F2 laser inside the bulk of fused silica. Laser interactions were confined by large NA optics within a small ∼8-μm diameter focal volume and scanned with precision motion stages to shape 3-dimensional photonic structures. Refractive index changes of up to 0.01 were induced with the ultrafast laser at a high scanning speed of several hundred microns per second and 100-kHz repetition rate. Weaker refractive index changes of 0.0005 were generated by the F2 laser, owing to a lower 100-Hz repetition rate and weaker absorption in the focal volume. The laser-generated waveguides offer high transmission of ∼1 dB/cm, are transparent to light in the visible and infrared spectrum, and can support multi-mode guiding with low insertion loss in...

Femtosecond Laser Mass Spectrometry and High Harmonic Spectroscopy of Xylene Isomers

Structural isomers, molecules having the same chemical formula but with atoms bonded in different order, are hard to identify using conventional spectroscopy and mass spectrometry. They exhibit virtually indistinguishable mass spectra when ionized by electrons. Laser mass spectrometry based on photoionization of the isomers has emerged as a promising alternative but requires shaped ultrafast laser pulses. Here we use transform limited femtosecond pulses to distinguish the isomers using two methods. First, we probe doubly charged parent ions with circularly polarized light. We show that the yield of doubly charged ortho-xylene decreases while para-xylene increases over a range of laser intensities when the laser polarization is changed from linear to circular. Second, we probe high harmonic generation from randomly oriented isomer molecules subjected to an intense laser field. We show that the yield of high-order harmonics varies with the positioning of the methyl group in xylene isomers (ortho-, para- and meta-) and is due to differences in the strength of tunnel ionization and the overlap between the angular peaks of ionization and photo-recombination.

Author Correction: Femtosecond Laser Mass Spectrometry and High Harmonic Spectroscopy of Xylene Isomers

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has been fixed in the paper.

Controlled cell adhesion on microstrucured Polydimethylsiloxane (PDMS) surface using femtosecond laser

We demonstrate controlled cell adhesion on Polydimethylsiloxane (PDMS) substrate microstrucured by a femtosecond laser. The effect of micro structures on cell attachment, alignment, patterned growth and cells proliferation were investigated over 24hrs, 48hrs and 72hrs.