C. Kalpouzos

Femtosecond transient reflection from polymer surfaces during femtosecond UV photoablation

A subpicosecond KrF laser system (248 nm, 0.5 ps) was used as a light source for ablation of PMMA Mylar and Kapton. The time-dependent reflectivity of the light-induced plasma mirror as measured by 496 nm, 0.5 ps long probe pulses showed an increase of up to 94% with 0.4–1 ps rise time and 10–15 ps fall time. The highdensity plasma mirror shows perfect optical quality, and seems to be a promising light-controlled ultrafast switch for UV and visible light. The spectrum of the UV light reflected from the ablated spot is blue shifted by 0.5 nm and shows 1 nm broadening.

High current and directional electron beams produced from gold photocathodes by ultrashort excimer laser pulses

The surface photoelectric effect induced by 500 fs excimer laser pulses in a gold metallic target has been studied. Unexpectedly high current densities of 4 kA/cm2, induced by laser intensities in the 3 GW/cm2 range have been measured. Intense photoelectron emission could also be observed at laser intensities up to 10 GW/cm2 without plasma formation. Furthermore, an electron angular distribution of 18°±2° was obtained for laser intensities in the range of 1–3 GW/cm2. These results are compared to those obtained in previous work for single‐photon and multiphoton photoeffects by using longer laser pulses, and their potential exploitation for free‐electron laser and accelerator applications is discussed.

Coherent continuum extreme ultraviolet radiation in the sub-100-nJ range generated by a high-power many-cycle laser field

High-energy coherent continuum radiation is generated by the interaction of rare gases with a high-power many-cycle laser field, utilizing the interferometric polarization gating technique. A narrow temporal gate is formed in the laser pulse within which the extreme ultraviolet (XUV) emission is restricted. An analytical expression for the gate function is derived. A super-XUV continuum down to 15 nm, broad enough to support synthesis of single pulses of 260 as duration and a few tens of nanojoule energy, has been measured. These results directly challenge the perspectives of single-attosecond pulse XUV-pump-XUV-probe applications.

Role of As precipitates on ultrafast electron trapping in low-temperature-grown GaAs and AlGaAs alloys

The role and interplay of basic structure parameters of arsenic precipitates on the ultrafast trapping of conduction band electrons have been studied in a series of low-temperature-grown GaAs and AlGaAs epilayers grown at various temperatures ranging from 170 up to 325 °C and annealed at 600 °C. Cross-sectional electron-transmission microscopy was used to determine the density and size of the precipitated arsenic clusters as a function of the growth temperature. The dependence of the electron trapping times (τ) on the spacing (R) and radius (α) of the arsenic precipitates has been systematically investigated by time-resolved pump–probe transient transmission spectroscopy. The electron trapping times follow a τ∝R3/α law which holds for low-temperature-grown GaAs and is applicable for low-temperature-grown AlGaAs as well. This trend generalizes the electron trapping mechanism for low-temperature-grown and annealed GaAs and related semiconductor alloys.

Ultrafast electron trapping times in low-temperature-grown gallium arsenide: The effect of the arsenic precipitate spacing and size

The role and interplay of basic structure parameters of arsenic precipitates on the ultrafast trapping of conduction band electrons has been studied in a series of low-temperature-grown GaAs epilayers grown at various temperatures ranging from 170 up to 325 °C and annealed at 600 °C. Cross sectional electron-transmission characterization was used to determine the density and size of the precipitated arsenic clusters with growth temperature. The dependence of the electron trapping times (τ) on the spacing (R) and radius (α) of the arsenic precipitates has been systematically studied by time-resolved pump-probe transient transmission spectroscopy. The present work demonstrates that the electron trapping time follows a τ∝R3/α law.

Full temporal reconstruction of a lower order harmonic superposition

We demonstrate full spectral phase/amplitude distribution retrieval of an arbitrary superposition of the third and fifth harmonic fields of 800?nm central wavelength, fs laser radiation, through a cross-correlation approach. Using the retrieved distributions, the temporal profile of the total harmonic field has been reconstructed and found to be in agreement with simulations. The results reveal the suitability of the approach for full temporal characterization of a low-order harmonic superposition and thus for attosecond pulses.

Effect of diffraction on early-arriving photons during femtosecond laser transillumination of highly scattering media of biological significance.

Early-arriving photons of 100-fs laser pulses transmitted through highly scattering media have been detected by a streak camera. Because of their partial spatial coherence, they are affected by diffraction from small hidden discontinuities. The experimental data of the patterns are analyzed with Fresnel diffraction theory and then corrected accordingly. Submillimeter hidden objects were scanned and imaged. Diffraction correction resulted in a significantly improved contrast in the hidden objects image.

Femtosecond And Picosecond Laser Ablation Of Intraocular Lenses: An Advanced Technique For Their Surface Modification

Ophthalmology is entering a very interesting period with new diffractive multifocals, improved refractive multifocals, and accommodative lenses, all coming out at the same time. A new diffractive‐refractive design for providing intermediated vision is apodization. In an apodized pattern, physical diffractive step heights are reduced in height, in an almost continuously varying manner. This study is aimed to investigate the use of ultrashort laser pulses to ablate the surface of intraocular lenses, and thus provide an alternative to conventional techniques. Ablation experiments were performed on hydrophilic and hydrophobic intraocular lenses (IOLs). The samples were irradiated with a Ti:Sapphire laser at λ = 0.785 μm, pulse duration 150 fs, repetition rate 1 kHz and with a Nd:YAG 4ω laser at λ = 0.266 μm, pulse duration 155 ps, repetition rate 10 Hz. We investigated the ablation efficiency and the surface modification with a Scanning Electron Microscope (SEM). The experimental results and the theoretical a...

Generation of intense coherent continuum XUV radiation by manycycle laser fields

During the last decade continuing efforts in ultrashort pulse engineering have led to the generation of attosecond (asec) pulse trains and isolated pulses. Although trains of intense multiple pulses can be generated through the interaction of many-optical-cycle pulses with gases [1] and solids [2] the generation of intense isolated asec pulses, remains a challenge.

Development of table-top ultrashort hybrid laser by amplification of a tuned Ti:Sa laser beam in an excimer double cavity

We present a new scheme of Ti:Sa laser amplification, based on second harmonic generation from a Ti:Sa laser tuned at 496 nm by an OPA system and amplification in an excimer double cavity.