Kyriaki Kosma

Excited-state dynamics of cytosine tautomers.

We report the relaxation dynamics of keto and enol or keto-imino cytosine, photoexcited in the wavelength range of 260-290 nm. Three transients with femtosecond to hundreds of picoseconds lifetimes are observed for the biologically relevant keto tautomer and are assigned to internal conversion and excited-state tautomerization. Only two transients with femtosecond and picosecond lifetimes are identified for the enol or keto-imino tautomer and are assigned to internal conversion processes. The results are discussed in the context of published ab initio theory.

Vacuum ultraviolet pulses of 11 fs from fifth-harmonic generation of a Ti:sapphire laser

We demonstrate that in a short Ar cell, generation of the fifth harmonic from 12 fs pulses at 810 nm directly results in ultrashort vacuum UV pulses at 162 nm. They have a spectral width of approximately 5 nm and a duration of 11+/-1 fs (1.4 times the transform limit), as measured by cross correlation with the fundamental pulses. Their energy (estimated to 4 nJ) turned out to be sufficient for use as a pump in time-resolved experiments.

Sub-10-fs supercontinuum radiation generated by filamentation of few-cycle 800 nm pulses in argon

Focusing 12 fs pulses of 800 nm with moderate energy (0.35 mJ) into atmospheric-pressure argon (Ar) gives rise to filamentation (self-focusing) and a supercontinuum with a very broad pedestal, extending to 250 nm. According to the present understanding, the short wavelengths are produced by self-phase modulation in the self-steepened trailing edge of the pulse. Pulses in this spectral range might thus be intrinsically short. Indeed we demonstrate this by extracting the light near the end of the filament, terminating self-focusing by a pressure gradient at a pinhole, beyond which the Ar is pumped away. We obtain pulses of 9.7 fs in the region of 290 nm without the necessity of compression.

Whispering gallery mode microsphere resonator integrated inside a microstructured optical fiber

Here we report on a novel in-fiber microresonator coupler, which combines a microstructured optical fiber (MOF) and a polystyrene microsphere (PM), embedded into one of its empty capillaries. The MOF consists of a germanium-doped silica core (~2.5 m), suspended within three hollow channels with a maximum dimension of ~11 m. The microsphere has a diameter of 10.4 m and a refractive index n=1.59 at 589 nm. After selection of a single PM from a glass micropipette and its controllable infiltration inside the fiber capillary with the help of a liquid solvent, the microsphere is placed inside the MOF, in contact with the core, ~3 cm from the fiber endface. The operation of the coupler when radiation is guided along the fiber has been investigated under two different launch/collection schemes, i.e. in the core-input/sphere-output (C/S) and the sphere-input/core-output (C/S) mode.

Extending the supercontinuum spectrum down to 200 nm with few-cycle pulses

By focusing 805 nm pulses of low energy (0.2-1 mJ) into atmospheric-pressure argon, a supercontinuum is generated with a short- wavelength cutoff of 640, 250 and 210 nm for initial pulse durations of 45, 10 and 6 fs, respectively. It is shown numerically that the large shift of the UV cutoff and many features of the spectrum are caused by terms beyond the slowly-varying- envelope approximation (SVEA). Their effect on pulse compression and filament length is also discussed.

CRASY: mass- or electron-correlated rotational alignment spectroscopy.

A technique merging rotational spectroscopy with mass spectrometry facilitates analysis of a complex isotopic mixture. Rotational spectra have traditionally been measured without a concurrent means of differentiating the molecular constituents of the sample. Here, we present an all-optical multipulse experiment that allows the correlated measurement of rotational and mass or photoelectron spectra by combining Fourier transform rotational coherence spectroscopy with resonance-enhanced multiphoton ionization. We demonstrate the power of this method with the determination of ground-state rotational constants and fragmentation channels for 10 different isotopes in a natural carbon disulfide sample. Three of the reported rotational constants were previously inaccessible by conventional spectroscopic techniques.

Characterization of the supercontinuum radiation generated by self-focusing of few-cycle 800 nm pulses in argon

Self-focusing of few-cycle pulses in atmospheric-pressure argon results in a supercontinuum which differs remarkably from the case of longer pulses: under single-filament conditions it extends to 200 nm and 250 nm with 6 fs and 10 fs pulses, respectively; the radiation, including the shortest wavelengths, is collimated and shows no conical emission. The short-wavelength part is intrinsically at least as short as the incoming fundamental pulse. These features make the few-cycle supercontinuum attractive as a source of widely tunable 10 fs pump pulses for spectroscopic applications. We present extensive experimental results including the dependence of the spectrum on pulse energy, duration and chirp, filament length, gas pressure and a comparison with nitrogen and air. We discuss them and other features including the role of the third harmonic and identify the conditions required to get a single highly stable filament. We also present a model, based on self-guiding, which predicts useful scaling rules.

Multiple Light Coupling and Routing via a Microspherical Resonator Integrated in a T-Shaped Optical Fiber Configuration System

We demonstrate a three-port, light guiding and routing T-shaped configuration based on the combination of whispering gallery modes (WGMs) and micro-structured optical fibers (MOFs). This system includes a single mode optical fiber taper (SOFT), a slightly tapered MOF and a BaTiO3 microsphere for efficient light coupling and routing between these two optical fibers. The BaTiO3 glass microsphere is semi-immersed into one of the hollow capillaries of the MOF taper, while the single mode optical fiber taper is placed perpendicularly to the latter and in contact with the equatorial region of the microsphere. Experimental results are presented for different excitation and reading conditions through the WGM microspherical resonator, namely, through single mode optical fiber taper or the MOF. The experimental results indicate that light coupling between the MOF and the single mode optical fiber taper is facilitated at specific wavelengths, supported by the light localization characteristics of the BaTiO3 glass microsphere, with spectral Q-factors varying between 4.5 × 103 and 6.1 × 103, depending on the port and parity excitation.

Light coupling and routing using a microsphere attached on the endface of a microstructured optical fiber

Herein, we demonstrated a T-shaped whispering gallery modes (WGMs) excitation system including a tapered single mode fiber (SMF), a tapered microstructured optical fiber (MOF) and a BaTiO3 microsphere for efficient light coupling and routing between the two fibers. The BaTiO3 microsphere is semi-immersed into the capillary of a tapered MOF, while the tapered SMF is placed perpendicularly to MOF in a contact with equatorial region of the microsphere. Based on that, three channels joined by the microsphere are formed, and excitation and measurement of WGMs is possible either using the SMF or the MOF taper. The measured WGMs spectra reveal light routing along Q-factors between 4500 and 6100, along with scattering signal with all three fiber ports and parities.

A microspherical resonator embedded inside a microstructured optical fiber taper

The integration of a microspherical whispering gallery mode (WGM) resonator inside a microstructured optical fiber taper is demonstrated. Preliminary WGM spectra of this in-fiber resonator in transmission mode are presented and discussed.