Olavi Ollikainen

Femtosecond stimulated Raman scattering in pressurized gases in the ultraviolet and visible spectral ranges

We investigate femtosecond stimulated Raman scattering (SRS) in H2,D2, HD, and CH4 excited with 300-fs-duration pulses at 390-nm wavelength and with as much as 0.1 mJ of energy. We show that the SRS-generation threshold and conversion efficiency are due to the transient nature of SRS in femtosecond regime. We determine optimal conditions for efficient generation in the broad spectral range 289–797 nm and show how self-phase modulation and white-light generation limit the ultimate conversion efficiency.

Stimulated Raman scattering in hydrogen by frequency-doubled amplified femtosecond Ti:sapphire laser pulses

We present results on stimulated Raman scattering in hydrogen excited by 200-fs-duration 1-kHz repetitionrate and 0.6-W average-power Ti:sapphire (780-nm) and 0.2-W average-power second-harmonic (390-nm) laser pulses with linear and circular polarizations. Spectral and temporal measurements of the vibrational and rotational components at Stokes and anti-Stokes frequencies in the wavelength range of 289-1154 nm are reported. We observe threefold shortening of the first vibrational Stokes component pulses, excited by 400-fsduration second-harmonic pulses.

Stimulated Raman amplification of femtosecond pulses in hydrogen gas.

We report efficient amplification of weak femtosecond supercontinuum pulses by a stimulated Raman scattering process in pressurized H(2) gas excited with 350-fs-duration frequency-doubled pulses from a regenerativeamplified Ti:sapphire laser. An amplification factor of 10(9) is obtained at the wavelength of 465 nm for seed pulses produced by supercontinuum generation in glass.

Efficient noncollinear parametric amplification of weak femtosecond pulses in the visible and near-infrared spectral range.

We report measurement of efficient amplification of weak femtosecond supercontinuum seed pulses by use of a noncollinear optical parametric process in BBO crystal pumped with 150-fs pulses from a frequency-doubled regenerative-amplified Ti:sapphire laser at 390nm . The highest amplification factor, 10(8) , was achieved for 3x10(-16)J energy seed pulses at wavelength of 560nm.

Terahertz bit-rate parallel multiplication by photon echo in low-temperature dye-doped polymer film

We demonstrate multiplication of two 8 bit binary words in less than 10 ps by generating two-pulse photon echo in a cryogenically-cooled polymer film doped with organic dye molecules. Input bits are coded in the intensity spectrum of 100 fs duration excitation pulses and the outcome of the multiplication is obtained by measuring the intensity spectrum of the photon echo signal. We study the dependence of the echo spectrum on the energy of the excitation pulses and investigate excitation conditions when the photon echo has maximum efficiency and minimum distortions.

Femtosecond time-and-space domain holograms: diffraction on the edge of time

Abstract We study the diffraction properties of time-and-space domain holograms stored with femtosecond light pulses with a coherence length much smaller than the lateral geometrical dimension of the hole-burning sample. We show that one can observe diffraction of read out beam on the virtual ‘time edge’ separating the spatial regions where the delay between the reference and the object pulses is either positive or negative. We also demonstrate that by placing a Fourier-transforming lens in the beam diffracted from the ‘time edge’ a spatial intensity pattern proportional to the homogeneous hole-burning spectrum is observed.

High-efficiency parametric amplification of weak femtosecond pulses in the visible spectral range

Gaussian pulse shapes. The energy dependence in Fig. 3 follows the GVM behavior and the energy increases with the interaction length (dashed lines with circles in Fig. 3) when lengthening the pump (and consequently the idler) pulses up to 0.5-1 ps. Near 4 p m the output idler energy always decreases for longer pump pulses. Saturation stabilizes the energy output and in the saturated regime the KNb0,-OPA exhibits an excellent pulse to pulse reproducibility ofbetter than 5%. Extremely low seed powers (20 mW of average power or 10 W of peak power) are sufficient to boost the idler pulse energy above 1 pJ. Continuous reduction of the temporal and spectral widths could be observed at lower output energies leading to bandwidth-limited pulses as short as 100 fs. At the given pump pulse duration the chosen crystal length is rather optimal since the idler pulses do not exceed in duration the pump pulses. That this was possible in the whole tuning range is attributed to the increased bandwidth ofthe parametric amplification as a result of the applied pump intensity levels, which counteracts the pulse broadening effect resulting from the temporal walk-off.

New coherent transient technique for femtosecond single-shot homogeneous spectrum measurement

Summary form only given. Coherent optical transients such as photon echo are a versatile method far measurement of the homogeneous absorption spectrum of molecular systems in the presence of strong inhomogeneous broadening. Such experiments require measurement of the decay of the optical coherence, typically accomplished by gradually varying the time delay between the excitation pulses. Time-domain information is then Fourier transformed to frequency domain by a computer. In this paper we demonstrate a new method that yields the homogeneous spectrum in a single shot of femtosecond laser pulse illumination by performing Fourier transformation of coherent optical transient signal with a lens. We apply this method to measure the homogeneous spectrum of dye molecules in a polymer film in a broad range of temperatures.

Terahertz bit-rate optical processing by femtosecond two-pulse photon echo

We apply low-temperature polymer film doped with a near IR-absorbing naphthalocyanine-type dye molecules with 10 THz-broad inhomogeneous absorption band to demonstrate multiplication of two 8-bit binary vectors in less than 10 ps by two-pulse photon echo.

Amplification of femtosecond signals by stimulated Raman scattering in hydrogen gas

We report efficient amplification of weak femtosecond signals by a stimulated Raman scattering process on vibrational and rotational components of pressurized H2 gas excited with 200-fs-duration frequency-doubled pulses from a regenerative-amplified Ti:sapphire laser. The amplification factor of 108 is obtained at the wavelength of 465 nm for weak seed pulses produced by white light continuum generation in glass.