Rohit P. Prasankumar

An extended cavity femtosecond Cr:LiSAF laser pumped by low cost diode lasers.

We describe an extended cavity femtosecond Cr:LiSAF laser pumped by inexpensive single spatial mode diodes. Using a multi-pass cavity (MPC) to lower the repetition rate and a saturable Bragg reflector (SBR) for mode-locking, pulse energies of 0.75 nJ at a repetition rate of 8.6 MHz are achieved with durations of 39 fs and bandwidths of 20 nm in a prismless configuration. Pulse energies of 0.66 nJ at a repetition rate of 8.4 MHz with durations of 43 fs and bandwidths of 18.5 nm are generated using prisms for dispersion compensation. This laser offers performance approaching that of standard Ti:sapphire lasers at a fraction of the cost.

Self-starting mode locking in a Cr:forsterite laser by use of non-epitaxially-grown semiconductor-doped silica films

We demonstrate RF sputtered, non-epitaxially-grown semiconductor nanocrystallite-doped silica films for mode locking a Cr:forsterite laser. We controlled the size and the optical properties of the nanocrystallites by varying the ratio of InAs to SiO(2) during fabrication. Femtosecond pump-probe measurements were performed to characterize the nonlinear optical properties of these films, revealing their lower saturation fluences. Using the InAs-doped silica films as saturable absorbers permitted self-starting Kerr-lens mode locking (KLM), generating pulses of 25-fs duration with 91-nm spectral bandwidth at 1.3 microm . We also describe saturable-absorber mode-locked operation without KLM and investigate its dependence on intracavity dispersion.

High-speed femtosecond pump-probe spectroscopy with a smart pixel detector array

A new femtosecond pump-probe spectroscopy technique is demonstrated that permits the high-speed, parallel acquisition of pump-probe measurements at multiple wavelengths. This is made possible by use of a novel, two-dimensional smart pixel detector array that performs amplitude demodulation in real time on each pixel. This detector array can not only achieve sensitivities comparable with lock-in amplification but also simultaneously performs demodulation of probe transmission signals at multiple wavelengths, thus permitting rapid time- and wavelength-resolved femtosecond pump-probe spectroscopy. Measurements on a thin sample of bulk GaAs are performed across 58 simultaneous wavelengths. Differential probe transmission changes as small as approximately 2 x 10(-4) can be measured over a 5-ps delay scan in only approximately 3 min. This technology can be applied to a wide range of pump-probe measurements in condensed matter, chemistry, and biology.

Ultra-low-threshold, low cost, Kerr lens modelocked Ti:Al/sub 2/O/sub 3/ laser

We report the development of an ultralow-threshold modelocked Ti:Al/sub 2/O/sub 3/ laser. Since the pulse energy of a laser is given by its average power divided by its repetition rate, increasing the cavity length produces an increase in pulse energy. This increased pulse energy enables high performance KLM operation at low powers while also decreasing the spot size of the laser mode to reduce pump thresholds. We report what is, to our knowledge, the lowest threshold achieved to date for a Kerr lens mode locked Ti:Al/sub 2/O/sub 3/ laser.

Ultrafast dynamics of non-epitaxially grown semiconductor-doped silica film saturable absorbers

Summary form only given. Semiconductor saturable absorbers are a widely used technology for generating femtosecond pulses in solid state lasers, providing advantages such as self-starting operation and the potential for compact laser cavities. The most common semiconductor saturable absorber devices are fabricated by molecular beam epitaxy (MBE) and have been used for both saturable absorber modelocking and initiation of Kerr lens modelocking (KLM) in many solid state laser systems. However, MBE has disadvantages including high complexity and cost as well as lattice matching constraints that limit material choice. Recently, we have developed non-epitaxially grown saturable absorber devices and applied them to self-starting KLM in a Ti:Al/sub 2/O/sub 3/ laser. The devices consist of InAs nanocrystallites doped into SiO/sub 2/ films and deposited on sapphire substrates using a non-magnetron radio frequency (RF) sputtering system. The current work focuses on methods for reducing the saturation fluence in these devices.

Saturable absorber modelocking using non-epitaxially grown semiconductor-doped films

Summary form only given. We demonstrate novel, non-epitaxially grown semiconductor saturable absorber devices for laser modelocking. These devices consist of semiconductor nanocrystallites doped into silica films using RF sputtering. These films can be deposited on virtually any substrate, including oxides such as glass and dielectric coatings as well as metal mirrors. By varying the doping density of the semiconductor quantum dots, one can adjust the absorption coefficient of the device. A wide range of semiconductor materials can be doped into the silica films. Therefore, appropriate choice of the semiconductor and knowledge of quantum confinement effects allow one to control the operating wavelength and absorption edge of the device.

Characterization of composite microstructure and damage using optical coherence tomography

Optical coherence tomography (OCT) is a non-destructive and non-contact technique that images microstructure within scattering media. In this work, the versatility of OCT for non-destructive evaluation is demonstrated through imaging of composite microstructure and damage. Imaging of composite microstructure is demonstrated through tomographic reconstructions of an epoxy/unidirectional E-glass composite and an epoxy/0-90 degree woven E-glass composite. Imaging of damage is shown by tomographic reconstruction of impact damage in a epoxy/unidirectional E-glass composite. The volumetric reconstruction of the composite is re-sliced along the thickness axis to reveal the propagation of cracks through the reinforcement layers. Advantages and limitations of OCT are discussed.