Controllable optical delay line using a Brillouin optical fiber ring laser

A controllable optical delay line using a Brillouin optical fiber ring laser is demonstrated and a large timedelay is obtained by cascading two optical fiber segments. In experiment, a single-mode Brillouin opticalfiber ring laser is used to provide Stokes wave as probe wave. We achieve a maximum tunable time delayof 61 ns using two cascading optical fiber segments, about 1.5 times of the input probe pulse width of 40ns. In the meantime, a considerable pulse broadening is observed, which agrees well with the theoreticalprediction based on linear theory.     

Controllable cascaded four-wave mixing by two chirped femtosecond laser pulses

We investigated the generation of cascaded four-wave mixing (CFWM) sidebands by using two crossing chirped femtosecond pulses with the same central wavelength in tellurite glass (Te glass). Sidebands of broadband spectra, which contained non-degenerate and degenerate CFWM signals, were obtained at different delay time between two input pulses. The CFWM sidebands observed on different sides of input beams were flexibly controlled by adjusting the delay time.     

Ultrafast coherent population transfer driven by two few-cycle laser pulses

We investigate ultrafast coherent population transfer driven by few-cycle pump and Stokes laser pulses in the Λ-type three-level system with the stimulated Raman adiabatic passage technique beyond the rotating-wave approximation. In contrast to the case with the rotating wave approximation, the most efficient population transfer may be realized without the satisfaction of the one-photon resonances or two-photon resonance and the transfer efficiency depends critically on the Rabi frequencies and initial optical phases of the two laser fields when the peak Rabi frequencies are much larger than the respective transition frequencies. Moreover, complete and robust population transfer can still be obtained with the variations of the Rabi frequencies, pulse durations, and one-photon or two-photon detuning in a moderate range, though a considerable transient population may reside in the excited state. These abnormal behaviors result from the counterrotating terms, which are not taken into account in the traditional rotating wave approximation.     

Symmetries and asymmetries in coherent atomic excitation by chirped laser pulses

We analyze the effects of laser-induced Stark shift and irreversible population loss on the technique of chirped-frequency adiabatic passage, and the ensuing symmetries and asymmetries in the ionization and fluorescence signals. We find that the properties of the detection signal depend critically on the fashion in which it is collected: for example, the post-pulse populations of the ground and excited states, and the ionization signal collected during the excitation, possess different symmetry properties with respect to the frequency chirp rate and the static frequency detuning. We illustrate these features with two exactly soluble analytic models, which describe simultaneous excitation and ionization of a two-state quantum system, as it typically occurs in atomic excitation with femtosecond laser pulses. We find that the ionization signal may exhibit unexpected oscillations and derive the conditions for maximizing their contrast.     

Controllable generation of partially coherent light pulses with direct space-to-time pulse shaper

We demonstrate the possibility of creating user-defined partially coherent light pulses by means of a slight modification of the direct space-to-time pulse shaper. Specifically, we generate a mutual coherence function that corresponds to the independent-elementary-pulse representation model. The theoretical limits in the parameter of global coherence and the efficiency of the system are studied. Our result opens the door to a new way of quantum control in laser-assisted chemical reactions, namely, control by partial coherence.     

Temporal compression of short-wavelength laser pulses by coherent control in rare gases

We present a theoretical study of temporal compression of a short-wavelength laser pulse predicted in a real, Doppler-broadened, atomic system. The compression is the result of the coherent control peculiarities of electromagnetically induced transparency-propagation dynamics. Numerical results are reported and discussed, showing a temporal compression of 2 orders of magnitude (from 10 ns to 100 ps) of a 106.7-nm laser pulse in argon atoms at room temperature.     

Temporal compression of nanosecond laser pulses using coherent control

We present a study of the temporal compression of nanosecond laser pulses resulting from the coherent control peculiarities of the propagation dynamics in a regime of electromagnetically induced transparency. We describe the general theoretical framework and discuss the crucial conditions required in order to experimentally realize such a temporal compression scheme. A proof-of-concept experimental realization of a scheme of this type in a sample of hot sodium vapors is currently being implemented: we describe in detail the experimental setup designed for this purpose.     

Solid state coherent transient measurements using hard optical pulses

An isolated and spectrally narrow absorptive feature is prepared via a novel spectral hole burning process in an inhomogeneously broadened optical transition in Eu(3+):Y(2)SiO5. With the narrow feature it is shown that it is feasible to apply complex optical pulse sequences analogous to rf pulse sequences used extensively in NMR.     

Kinetics of optical Kerr effect induced by picosecond laser pulses

By use of a double beam technique, we were able to observe, in the OKE dynamics of many non saturated organic compounds, a very fast decay response superimposed upon the much slower decline of the usual effect of molecular origin.     

Fully coherent x-ray pulses from a regenerative-amplifier free-electron laser

We propose and analyze a regenerative-amplifier free-electron laser (FEL) to produce fully coherent, hard x-ray pulses. The method makes use of narrow-bandwidth Bragg crystals to form an x-ray feedback loop around a relatively short undulator. Self-amplified spontaneous emission (SASE) from the leading electron bunch in a bunch train is spectrally filtered by the Bragg reflectors and is brought back to the beginning of the undulator to interact repeatedly with subsequent bunches in the bunch train. The FEL interaction with these short bunches regeneratively amplifies the radiation intensity and broadens its spectrum, allowing for effective transmission of the x rays outside the crystal bandwidth. The spectral brightness of these x-ray pulses is about 2 to 3 orders of magnitude higher than that from a single-pass SASE FEL.     

Filamentational instability of partially coherent femtosecond optical pulses in air

The filamentational instability of spatially broadband femtosecond optical pulses in air is investigated by means of a kinetic wave equation for spatially incoherent photons. An explicit expression for the spatial amplification rate is derived and analyzed. It is found that the spatial spectral broadening of the pulse can lead to stabilization of the filamentation instability. Thus optical smoothing techniques could optimize current applications of ultrashort laser pulses, such as atmospheric remote sensing.     

Controlled dumping of pulses of laser radiation ranging from MIR to the MM by means of optical semiconductor switching

A simple method to produce sub-ns pulses of laser radiation with wavelength ranging from middle infrared (MIR) to millimeter (MM) is experimentally demonstrated. The system operates with a single OSS (optical semiconductor switching) driven by a ns Nd laser. Pulse control is produced by varying the distance of a mirror from an OSS used as a cavity dumping. The effectiveness of this sub-ns MM pulse generation is larger than the one observed in other schemes. Pulses with 500-ps FWHM and 15-W power were produced at 2.65 mm with a time compression compared to the 2-ns FWHM of the 1.06-μm driving laser. Received: 30 October 2000 / Revised version: 26 January 2001 / Published online: 9 May 2001     

Filtration of partially coherent optical radiation by means of multiphonon Bragg diffraction

The filtering properties of three-phonon acousto-optical Bragg diffraction for increasing the degree of coherence of a partially coherent optical field are investigated by the example of multiphonon acousto-optical interaction in a TeO₂ single crystal. Two possible mechanisms of coherent scattering upon the formation of the highest orders are taken into account. It is shown that the three-phonon interaction is more efficient by at least a factor of two than any mode of one-phonon diffraction realized at the same frequency and the same acousto-optical interaction length. The experimental investigation, using partially coherent radiation of a He-Ne laser (λ = 0.63 μm) diffracted from a transverse acoustic wave propagating in a TeO₂ single crystal, on the whole confirmed the basic theoretical foundations.     

Stability theory of coherent laser pulses in an inverted medium

Analytic steady-state solutions describing pulse propagation in a homogenously broadened laser amplifier are known. In the present paper we show the stability of these solutions.     

Nanostructuring of thin Au films by means of short UV laser pulses

The particle size distribution, morphology and optical properties of the Au nanoparticle (NP) structures for surface enhanced Raman signal (SERS) application are investigated in dependence on their preparation conditions. The structures are produced from relatively thin Au films (10–20 nm) sputtered on fused silica glass substrate and irradiated with several pulses (6 ns) of laser radiation at 266 nm and at fluencies in the range of 160–412 mJ/cm². The SEM inspection reveals nearly homogeneously distributed, spherical gold particles. Their initial size distribution of the range of 20–60 nm broadens towards larger particle diameters with prolonged irradiation. This is accompanied by an increase in the uncovered surface of the glass substrate and no particle removal is observed. In the absorption profiles of the nanostructures, the broad peak centred at 546 nm is ascribed to resonant absorption of surface plasmons (SPR). The peak position, halfwidth and intensity depend on the shape, size and size distribution of the nanostructured particles in agreement with literature. From peak intensities of the Raman spectra recorded for Rhodamine 6G in the range of 300–1800 cm⁻¹, the relative signal enhancement by factor between 20 and 603 for individual peaks is estimated. The results confirm that the obtained structures can be applied for SERS measurements and sensing.     

Production of nanoparticles of different materials by means of ultrashort laser pulses

Ultrashort pulsed laser ablation in vacuum of different targets was performed in order to investigate the possibility of producing nanoparticles with controlled size and shape. A systematic morphology characterization of deposited products was performed for nickel and silicon as a function of laser pulse intensity and wavelength, at a fixed pulse repetition rate. The nanoparticles were investigated by atomic force microscopy, and clear trends for their size and shape anisotropy were evidenced. The best conditions to obtain nanosized particles of oblate ellipsoidal shape, with the minor axis below 10 nm, were determined in the case of nickel targets. Our results show that ultrashort pulse laser deposition can be considered as an interesting technique for the tailoring of nanogranular films with the desired particles dimension and shape, according to the peculiar properties required in specific applications. Moreover, the preliminary features are very promising from the point of view of the production of magnetoresistive films with specific anisotropy.     

The pumping of an X-ray laser by means of an optical laser

A new mechanism for pumping of an X-ray laser by an optical laser is suggested. The inverse population between the inner levels of atoms is attained by means of the use of fast laser plasma electrons.     

Generation of few-cycle laser pulses by coherent synthesis based on a femtosecond Yb-doped fiber laser amplification system

We demonstrate a coherent synthesis system based on femtosecond Yb-doped fiber laser technology. The output pulse of the amplification system is divided into two replicas and seeded into photonic crystal fibers of two parallel branches for nonlinear pulse compression. Because of the different nonlinear dynamics in the photonic crystal fibers, the compressed pulses show different spectra, which can be spliced to form a broad coherent spectrum. The integrated timing jitter between the pulses of two branches is less than one tenth of an optical cycle.By coherently synthesizing pulses from these two branches, 8 fs few-cycle pulses are produced.