Time domain investigation of the intrachain vibrational dynamics of a prototypical light-emitting conjugated polymer

Coherent nuclear motion of the carbon backbone is monitored in real time in substituted poly(phenylene)-vinylene in both excited and ground electronic state using sub-10-fs light pulses. Characteristic features of the intrachain vibrational dynamics are obtained. We observe vibrational dephasing in the excited state within 1 ps (time constant T(2e) approximately 300 fs). These findings are in agreement with the molecular picture for photoexcitation in conjugated polymers.     

Self-referenced spectral interferometry for ultrashort infrared pulse characterization

We demonstrate for the first time (to our knowledge) characterization of ultrashort IR pulses by self-referenced spectral interferometry. Both sub-55-fs pulses from 1.4 μm to 2 μm and broadband 2.5-cycle pulses at 1.65 μm (13 fs FWHM) are characterized.     

Two-color pump-probe ionization spectroscopy of gaseous water enhanced by preliminary impulsive Raman excitation

We use sub-10-fs pulses at 400 nm and 15-fs pulses at 800 nm to ionize water molecules and their isotopomers HDO and D₂O in a pump–probe scheme. Pulses are generated via spectral broadening of 25-fs pulses of a 1-kHz Ti:sapphire amplifier system by self-phase modulation in a noble-gas-filled hollow waveguide and subsequent compression using chirped mirrors. At this time scale vibronic excitation of the first bending mode of water in the electronic ground state by impulsive Raman scattering is possible (e.g. the fundamental bending mode of H₂O: t_vib=20 fs). The effect of this pre-excitation on the ionization rate is shown. Received: 14 May 2001 / Revised version: 24 August 2001 / Published online: 19 September 2001     

Generation of high-energy broadband femtosecond deep-ultraviolet pulses by highly nondegenerate noncollinear four-wave mixing in a thin transparent solid

We present a simple and efficient technique for the generation of ultrashort deep-ultraviolet pulses based on four-wave mixing of noncollinear laser pulses in a thin solid. Sub-30-fs pulses (Fourier-limit of 13 fs) centered at 270 nm, with energies up to 6 μJ, were obtained by mixing the fundamental and the second harmonic of a Ti:sapphire amplifier in fused silica. Temporal characterization was performed with a dispersionless self-diffraction FROG setup. Spectra as broad as 20 nm were also obtained that can in principle support sub-4-fs deep-ultraviolet pulses.The results are well described by two-dimensional numerical simulations.     

High-Efficiency Generation of 0.12 mJ,8.6 Fs Pulses at 400 nm Based on Spectral Broadening in Solid Thin Plates

We demonstrate efficient generation of continuous spectrum centered at 400 nm from solid thin plates. By frequency doubling of 0.8 mJ, 30 fs Ti:sapphire laser pulses with a BBO crystal, 0.2 mJ, 33 fs laser pulses at400 nm are generated. Focusing the 400-nm pulses into 7 thin fused silica plates, we obtain 0.15 mJ continuous spectrum covering 350-450 nm. After compressing by 3 pairs of chirped mirrors, 0.12 mJ, 8.6 fs pulses are achieved.To the best of our knowledge,this is the first time that sub-10-fs pulses centered at 400 nm are generated by solid thin plates, which shows that spectral broadening in solid-state materials works not only at 800 nm but also at different wavelengths.     

Sub-100 femtosecond pulses from a SESAM modelocked thin disk laser

We present the first passively modelocked thin disk laser (TDL) with sub-100-fs pulse duration using the broadband sesquioxide gain material Yb:LuScO₃ and an optimized SEmiconductor Saturable Absorber Mirror (SESAM). In this proof-of-principle experiment, we obtained 5.1 W of average power at a repetition rate of 77.5 MHz and a pulse duration of 96 fs. We carefully explored and optimized the different parameters on the soliton pulse formation process for the generation of short pulses. In particular, SESAMs combining fast recovery time, high modulation depth and low nonsaturable losses proved crucial to achieve this result even though they are expected to only play a minor role in soliton modelocking. To our knowledge, these are the shortest pulses ever obtained with a modelocked TDL, reaching for the first time the sub-100-fs milestone. This result opens the door to sub-100-fs oscillators with substantially higher power levels in the near future.     

Pulse-front-matched optical parametric amplification for sub-10-fs pulse generation tunable in the visible and near infrared

A noncollinear optical parametric amplifier is presented that generates transform-limited sub-10-fs pulses that are tunable in both the visible and the near infrared (NIR). The pulse-front-matched pump geometry realizes tilt-free signal amplification, and pulses as short as 6.1 fs can be obtained from 550 to 700 nm. The large angular dispersion of the idler specific to the group-velocity-matching interaction is effectively eliminated by a grating-telescope compensator, and 9-fs NIR pulses are also successfully obtained from 900 to 1300 nm. This is believed to be the first tunable sub-10-fs light source.     

High-order harmonics generation by few-cycle and multi-cycle femtosecond laser pulses

This paper investigates experimentally high-order harmonic generation(HHG) of neon gas with 5-fs and 25-fs driving laser pulses.It has been demonstrated that the cutoff energy of the harmonic extreme ultraviolet photons is extended to 131 eV and the HHG spectrum near the cutoff region becomes continuum as the driving laser pulse duration is 5 fs;whereas much lower cutoff photon energy and discrete harmonic spectrum near the cutoff region are presented as the laser pulse duration is 25 fs.The results can be explained by the fact that neutral atoms can be exposed to more intense laser field before they are depleted by ionization because of the extremely short rising time of the few-cycle pulse.The 5-fs driving laser pulse paves the way of generation of coherent x-ray in the water window and single attosecond pulse.     

Spectral analysis of high-order harmonics generated by 30-fs and sub-10-fs laser pulses

The spectral properties of harmonic radiation generated in neon by laser pulses with duration from 30 fs down to 5 fs have been investigated. In the 30-fs temporal regime we have achieved continuously tunable sub-20-nm radiation by changing the gas jet position relative to the laser beam waist. We show that the results of a one-dimensional propagation code partially including three-dimensional focusing effects are in good agreement with the experimental results. Finally, we have performed a spectral characterization of harmonic radiation in the sub-10-fs temporal regime.     

30-fs, cavity-dumped optical parametric oscillator

We demonstrate cavity dumping of a synchronously pumped optical parametric oscillator (OPO). The OPO delivers 30-fs pulses with energies as high as 13 nJ at up to 400-kHz repetition rates. The OPO's tunability ranges from 570 to 600 nm and is limited by the resonator mirror coatings. The time jitter between pulses of a Ti:sapphire laser and the cavity-dumped OPO pulses is smaller than 30 fs, permitting multicolor time-resolved spectroscopic experiments.     

Pulse characteristics of an optical parametric oscillator pumped by sub-30-fs light pulses

We obtained nearly transform-limited light pulses of 34 fs near 1.2 microm by pumping an optical parametric oscillator with a 2-mm-long KTP crystal by 26-fs pulses from a Ti:sapphire laser. The average power of the pulses that were obtained was greater than 50 mW, at an 80-MHz repetition rate. Attempts to downscale the pulse duration by decreasing the pump-pulse duration revealed remarkable limitations of the attainable pulse length for sub-30-fs pump pulses, in accordance with a recent theoretical study [J. Opt. Soc. Am. B 17, 741 (2000)].     

Sub-100 fs SDPF optical soliton compressor for diode laser pulses

We report our studies on a fiber-optic soliton compressor for generation of sub-100 femtosecond (fs) optical pulses out of picosecond (ps) diode laser pulses. The soliton compressor is rather simple and composed of a 15 ～ 20 m-long step-like dispersion profiled fiber (SDPF) in conjunction with a single Er-doped fiber amplifier (EDFA). Careful design of such a SDPF compressor was performed, leading to demonstration of 20-fs class compression performance, and experimental investigation was carried out in detail on the optical pulse propagation in the compression processes. In addition, nonlinear fiber loops were applied to suppression of pulse pedestals, resulting successfully in high quality optical pulses of the 100-fs range.     

Simultaneous femtosecond-pulse compression and second-harmonic generation in aperiodically poled KTiOPO(4)

We report both extracavity and intracavity simultaneous second-harmonic generation and compression of pulses at 1.25 mum from a synchronously pumped RbTiOAsO(4) -based optical parametric oscillator, using an aperiodically poled crystal of KTiOPO(4) . The 290-fs input pulses were temporally compressed to 120 fs, with average output powers as great as 120 mW. The experimental results are compared with a numerical model that uses data obtained by characterization of the input pulses by use of the frequency-resolved optical gating technique.     

Nonlinear dynamics of femtosecond pulse propagation through single mode optical fiber

At the cascade SRS in the region of negative chromatic dispersion of a single mode fiber, 70-to 100-fs light pulses have been obtained. It is shown that a 70-fs multisoliton pulse in a single mode fiber breaks up into “color” solitons. Pulses of 18 fs have been formed by multisoliton compression. The pulse duration corresponds to about three optical cycles at the wavelength of 1.6 μm. From the results of the experiment, the relaxation time of the silica-glass electron nonlinearity is estimated to be 2 to 4 fs.     

Generation of synchronized femtosecond and picosecond pulses in a dual-wavelength femtosecond Ti:sapphire laser

We obtain synchronized 45-fs and 0.848-ps pulses by achieving cross-mode locking in a double-cavity dual-wavelength femtosecond Ti:sapphire laser. Autocorrelation and cross correlation show that the femtosecond and picosecond pulses are well synchronized, with a timing jitter of 41 fs. Cross-phase modulation dominates the processes of cross-mode locking and synchronization.     

Compact sub-100-fs Nd:silicate laser

We present a compact Nd:silicate laser pumped by a single 1-W high-brightness commercial laser diode that generates pulses as short as 88 fs (nearly Fourier limited) when passively mode-locked with a saturable absorber mirror. We also tested a prismless cavity setup employing a single Gires-Tournois mirror, yielding 100-fs pulses. These setups are significantly simpler and more compact than those reported previously for short pulse Nd:silicate lasers.     

Collinear type II second-harmonic-generation frequency-resolved optical gating for the characterization of sub-10-fs optical pulses

For the first time to our knowledge, we demonstrate a collinear frequency-resolved optical gating (FROG) technique that is suitable for the characterization of sub-10-fs pulses. This FROG variant does not suffer from geometrical blurring effects, and a temporal resolution of 1 fs can be achieved without the need for additional aperturing. The apparatus is suitable for subnanojoule pulse energies. We apply this technique for the full characterization of pulses from a Kerr-lens mode-locked Ti:sapphire laser.     

Dual-wavelength chirped-pulse amplification system

We demonstrate a chirped-pulse amplification system that simultaneously amplifies two pulses from a dual-wavelength oscillator in a single regenerative amplifier. The two wavelengths can be tuned from 800 to 890 nm. The total energy of 1.5 mJ can be variably split between the two pulses. The pulses are 150 fs in duration and have 60-fs timing jitter.     

Piezo-driven deformable mirror for femtosecond pulse shaping

We have developed a deformable, gold-coated mirror based on piezoelectric actuators with 15-micros response time. With 20 independent channels we were able to compress 72-fs pulses from a Ti:sapphire oscillator down to 45 fs in a 4f zero-dispersion compressor arrangement. Spectral interference was used to measure the mirror performance, while the spectral phase interferometry for direct electric field reconstruction (SPIDER) technique was used for the laser pulse characterization.     

Tunable visible and near-infrared pulse generator in a 5 fs regime

Transform-limited (TL) visible pulses with as short a duration as 4.7 fs with a 5 7J pulse energy have been generated for the first time from a novel noncollinear optical parametric amplifier (NOPA). Both signal-idler group-velocity matching and pulse-front matching are essential to generate coherent down-converted pulses compressible to the TL pulse with more than a 150-THz bandwidth. Tunable operation with bandwidth-limited sub-10-fs pulses in the visible (550-700 nm) and near-infrared (900-1300 nm) ranges, achieved by increasing the seed chirp, is also a remarkable property. The NOPA is believed to be on useful light source for ultrafast spectroscopy on an extremely short time-scale.