Vesselin Z Kolev

Passive Mode Locking of a Nd:YVO 4 Laser with an Extra-long Optical Resonator

We describe a mode-locked, diode-pumped Nd:YVO4 laser with a very long optical cavity operating at 1064 nm. High-modulation, InGaAs quantum-well, semiconductor saturable-absorber mirrors were used for passive mode locking, providing a stable train of 13-ps pulses. A novel zero-q-transformation multipass cell provided a variable-length optical cavity as much as 100 m long. The output beam had M2 < 1.1 at average powers of 4.1, 3.9, and 3.5 W at repetition rates of 4.1, 2.6, and 1.5 MHz, respectively. To the best of our knowledge the last of these is the lowest repetition rate ever generated directly from a mode-locked nonfiber solid-state laser without cavity dumping.

Ion-implanted InGaAs single quantum well semiconductor saturable absorber mirrors for passive mode-locking

We demonstrate that ion implantation can be used for response time tailoring to create high-performance indium gallium arsenide (InGaAs) quantum well semiconductor saturable absorber mirrors (SESAMs). The design and manufacture of the SESAMs are described, and their nonlinear optical and temporal responses, relevant to the mode-locking of picosecond type pulses, are given. The implanted devices shown here have response times as short as 11 ps, compared with several hundred picoseconds without implantation. They have small non-bleachable losses and low saturation fluence (<20 µ Jc m −2 ), allowing for operation far below the damage threshold. An implantation- and annealing-induced quantum well intermixing effect is shown to be present in the SESAMs, resulting in an energy shift in the excitonic feature. This effect can be taken into account in the SESAM design or minimized, if necessary. Using the SESAMs to mode-lock a low-power Nd:YVO4 laser at 1064 nm, we have generated pulsewidths between 9 and 23 ps, depending on the cavity configuration and the SESAM used.

Compact high-power optical source for resonant infrared pulsed laser ablation and deposition of polymer materials.

We propose a novel tuneable table-top optical source as an alternative to the free electron laser currently used for resonant infrared pulsed laser deposition of polymers. It is based on two-stage pulsed optical parametric amplification using MgO doped periodically poled lithium niobate crystals. Gain in excess of 10(6) in the first stage and pump depletion of 58% in the second stage were achieved when the system was pumped by a high-power Nd:YVO(4) picosecond laser source at 1064 nm and seeded by a CW tuneable diode laser at 1530 nm. An average power of 2 W was generated at 3.5 microm corresponding to 1.3 microJ pulse energy.

Passive mode locking of a self-frequency-doubling Yb:YAl 3 (BO 3 ) 4 laser

We report passive mode-locking experiments with a novel self-doubling laser crystal Yb:YAl(3)(BO(3))(4) (Yb:YAB). The diode-pumped laser was mode locked by an ion-implanted semiconductor saturable absorber mirror. Far off phase matching, soliton mode locking produced pulse widths of 198 fs to 1.4 ps, with up to 660-mW output and optical efficiency of 24% at 1040 nm. The shortest pulses had a peak power of 28 kW with 440-mW average power and 16% efficiency. A few degrees off phase matching, a total of 60 mW of green femtosecond pulses was generated simultaneously. Close to phase matching, the laser produced picosecond pulses and, without infrared output, a total of 270 mW of green output, corresponding to 10% conversion efficiency (absorbed pump to green output).

Ablation of metals with picosecond laser pulses: Evidence of long-lived non-equilibrium surface states

Experiments on laser ablation of metals in air, in vacuum, and in similar irradiation conditions, revealed that the ablation thresholds in air are up to three times lower than those measured in vacuum. Our analysis shows that this difference is caused by the existence of a long-lived transient non-equilibrium surface state at the solid-vacuum interface. The energy distribution of atoms at the surface is Maxwellian-like but with its high-energy tail truncated at the binding energy. We find that in vacuum the rate of energy transfer from the bulk to the surface layer to build the high-energy tail, which determines the lifetime of this non-equilibrium state, exceeds other characteristic timescales such as the surface cooling time. This prohibits thermal evaporation in vacuum for which the high-energy tail is essential. In air, however, collisions between the gas atoms and the surface markedly reduce the lifetime of this non-equilibrium surface state allowing thermal evaporation to proceed before the surface cools. It was experimentally observed that the difference between the ablation depth in vacuum and that in air disappears at the laser fluencies 2–3 times in excess of the vacuum threshold value. The material removal at this level of the deposited energy density attains the features of the non-equilibrium ablation similar for both cases. We find, therefore, that the threshold in vacuum corresponds to non-equilibrium ablation during the pulse, while thermal evaporation after the pulse is responsible for the lower ablation threshold observed in air. This paper provides direct experimental evidence of how the transient surface effects may strongly affect the onset and rate of a solid-gas phase transition.

Control over a phase state of the laser plume ablated by femtosecond laser: Spatial pulse shaping

The conditions for the formation of a fully atomized laser-ablated plume using subpicosecond laser pulses have been studied theoretically and implemented experimentally. It is shown that the low-intensity wings, which generally exist in the spatial distribution of laser intensity in the focal plane and can contain a substantial part of the incident laser energy, are responsible for low-threshold phase transformations of the target and this is the major source of particulates in the ablated plume. By truncating these wings with a simple aperture and positioning the target in the image plane of that aperture, particulates can be eliminated from the plume. This is demonstrated in experiments on deposition of Si films using the truncated beam in combination with the proper choice of laser fluence. This results in an almost totally atomized plume and consequently in droplet-free deposition of thin films.

Large phase shifts in As₂S₃ waveguides for all-optical processing devices

A large phase shift of 4.7pi at 1.53 microm has been observed from a low-loss (0.2 dB/cm), small-core As2S3 waveguide fabricated by dry etching. The strength of the nonlinear response was limited by photosensitivity and photocrystallization of the As2S3 films at 1.53 microm, far below the material bandgap.

Corrections to refractive index data of stoichiometric lithium tantalate in the 5-6 μm range

We propose corrections to the coefficients in the published Sellmeier equation for stoichiometric LiTaO3 [Opt. Lett.28, 194 (2003)] that allow the extension of the wavelength range within the region of midinfrared absorption edge up to 6 microm. The required extraordinary refractive index data for this range were obtained using single-pass optical parametric fluorescence measurements with a pump wavelength of 1064.4 nm. We also observed efficient parasitic second-harmonic generation that could affect some quasi-phase-matching interactions. The corrected Sellmeier equation improves the accuracy of poling period calculations where the idler wavelength is within the region.

In Situ Second Harmonic Generation in Disperse Red 1-Doped Polymer and Sol-Gel Films

ABSTRACT We investigated linear optical and second-order nonlinear optical (NLO) properties of films of poly(bisphenol A carbonate) resin, and a newly synthesised titania/silica/hybrid sol-gel polymer resin doped with Disperse Red 1 (DR1) molecular chromophore. DC electric field induced second harmonic generation (SHG) technique has been used to monitor in situ the polar alignment and relaxation of orientation of the guest molecules in the corona-poled films. The temporal behaviour of the second harmonic signal was remarkably different in these two polymers both during poling and when the poling field was turned off. A fast decay of the second harmonic intensity was observed at room temperature in 3 wt% DR1-doped sol-gel film, and a long relaxation time constant was found in 13 wt% DR1-doped polycarbonate film. Resonantly enhanced deff coefficient of about 9 pm/V was found in the sol-gel film, and 25 pm/V in the polycarbonate film at the fundamental wavelength 1053 nm. The d33 values were about 20 pm/V, and 60 pm/V in these films, respectively. The SHG results were used to estimate the average rotation mobility parameter of DR1 molecules in the host polymers.

Ultrafast trapping times in ion implanted InP

As+ and P+ implantation was performed on semi-insulating (SI) and p-type InP samples for the purpose of creating a material suitable for ultrafast optoelectronic applications. SI InP samples were implanted with a dose of 1×1016 cm−2 and p-type InP was implanted with doses between 1×1012 and 1×1016 cm−2. Subsequently, rapid thermal annealing at temperatures between 400 and 700 °C was performed for 30 sec. Hall-effect measurements, double-crystal x-ray diffraction, and time-resolved femtosecond differential reflectivity showed that, for the highest-annealing temperatures, the implanted SI InP samples exhibited high mobility, low resistivity, short response times, and minimal structural damage. Similar measurements on implanted p-type InP showed that the fast response time, high mobility, and good structural recovery could be retained while increasing the resistivity.