Time-resolved Degenerate Four-wave Mixing Research Articles

Line shape analysis of higher order nonlinear processes in time-resolved degenerate four-wave mixing in GaAs

We theoretically calculate the line shapes of the third order, the cascaded third order, and the fifth order nonlinear processes in a femtosecond time-resolved four-wave mixing experiment. On the basis of the calculations, experimentally we distinguish the wavelength dependent contributions to the direct fifth order process and the cascaded third order process in bulk GaAs. Our results show that the cascaded process dominates at wavelengths near band gap resonance; however the direct process dominates away from resonance. The nonlinear susceptibilities χ(3) and χ(5) are also obtained.

Vibrational frequencies and dephasing times in excited electronic states by femtosecond time-resolved four-wave mixing

Time-resolved degenerate four-wave mixing (TRDFWM) for an electronically resonant system in a phase-matching configuration that measures population decay is reported. Because the spectral width of input light exceeds the vibrational Bohr frequency of a strong Raman active mode, the vibrational coherence produces strong oscillations in the TRDFWM signal together with the usual population decay from the excited electronic state. The data are analyzed in terms of a four-level system: ground and excited electronic states each split by a vibrational quantum of a Raman active mode. Absolute frequencies and their dephasing times of the vibrational modes at ≈590 cm −1 are obtained for the excited as well as the ground electronic state. The vibrational dephasing rate in the excited electronic state is about an order of magnitude faster than that in the ground state, the origin of which is speculated upon.

Third-order nonlinear optical susceptibility of acetylenic analogues of tetrathiafulvalene

An analysis of picosecond time-resolved degenerate four-wave mixing at 532 nm in soluble acetylenic analogues of tetrathiafulvalene is presented. Different nonlinear mechanisms are isolated and discussed. The molecular hyperpolarizabilities can reach 2 × 10 −44 SI units (10 5 times greater than that of CS 2).

Resonant subpicosecond degenerate four-wave mixing for a mainchain polymer with a definite short π conjugation

The subpicosecond time-resolved degenerate four-wave mixing (DFWM) technique is applied to evaluate the third-order optical nonlinearity, χ (3) (ω; ω, −ω, ω), and its decay dynamics for a symmetrical short π-conjugated mainchain polymer, pSBAC for short. The χ (3)/α value of 6 × 10 −14 esu cm for pSBAC is the same as that reported for many other π-conjugated polymers. We propose that the concept of real-space filling accounts for the observed χ (3)/ga value in pSBAC. The picosecond dynamics of the transient grating response for pSBAC is also discussed in comparison with that of the π-conjugated polymers.

Resonant nonlinear optical response of the fullerenes C60 and C70.

Time-resolved degenerate four-wave mixing experiments were conducted on films of pure ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$ using a picosecond tunable dye laser. The fullerenes exhibit a large third-order optical response, and their dynamics show wavelength and fluence dependence. The temporal response is characterized by a long-lived component attributed to the triplet excited state and an early decay associated with population of the singlet excited state. This fast response is considerably shortened at higher laser fluences and an excitonic singlet-singlet annihilation mechanism is proposed.

THIRD-ORDER NONLINEAR OPTICAL PROPERTIES OF METALLO-PHTHALOCYANINES

This paper discusses the third-order nonlinear optical (NLO) properties of metal-free, lead and transition metal phthalocyanines, as well as scandium, yttrium and lanthanide bisphthalocyanines measured by time-resolved degenerate four-wave mixing at 1064 nm. The study focuses on the structure-property relationships and explores the possible mechanisms leading to the enhanced third-order optical nonlinearity measured for this interesting class of NLO materials.

Dissipative dynamics of an electronic wavepacket in a semiconductor double well potential

The authors present a detailed study of coherent tunneling oscillations of an optically excited electronic wavepacket in a semiconductor double quantum-well structure. The oscillation of the wavepacket between the two wells is traced by time-resolved pump-probe spectroscopy as well as time-resolved degenerate four-wave mixing. The experimental observations are compared with a simple theoretical model. The motion of the wave packet is studied as a function of both an external electric field and the lattice temperature. While the oscillation frequency shows the expected dependence on electric field, it increases with temperature at fixed electric field when perfectly delocalized electronic states exist at low temperature. >

Time-resolved degenerate four-wave mixing in thin films of C 60 and C 70 using femtosecond optical pulses

Thin films of pure C 60 and C 70 have been studied by time-resolved degenerate four-wave mixing using femtosecond optical pulses with a wavelength of 633 nm. Large optical nonlinearities were measured for both materials, with χ (3) values ≈ 10 −10 esu. The degenerate four-wave mixing signal was monitored as a function of the relative arrival times for the excitation pulses. Along with a long-lived (≈ 100 ps) component to the four-wave mixing decay, response dynamics occurring on both femtosecond and picosecond time scales were observed for both materials.

Third-Order Nonlinear Optical Properties of Phthalocyanines

ABSTRACTThis paper discusses the third-order nonlinear optical (NLO) properties of substituted phthalocyanines measured by time-resolved degenerate four-wave mixing at 1064 nm. The study explores optical pumping as a mechanism contributing to a large third-order optical nonlinearity in one member of this interesting class of NLO materials, PbPc(CP)4.

Off-resonant nonlinear optical properties of C 60 studied by degenerate four-wave mixing

Pure C 60 was deposited on optical substrates in vacuo and spectroscopically characterized. A small linear absorption coefficient α=6 cm −1 was measured at 1.064 μm. Time-resolved degenerate four-wave mixing experiments were conducted on films of pure C 60 using a 35 ps Nd: YAG laser at 1.064 μm. The third-order optical susceptibility x xxxx (3) of C 60, derived from its phase conjugate signal, is 7×10 −12 esu. The nonlinear optical temporal response was largely laser-pulse limited. Evidence for a fifth-order contribution to the nonlinear optical response was observed at high laser intensities.

Exciton-hole collision in staggered type-II Al0.34Ga0.66As/AlAs multiple quantum wells.

We report the phase relaxation of \ensuremath{\Gamma}-\ensuremath{\Gamma} direct excitons in staggered type-II ${\mathrm{Al}}_{0.34}$${\mathrm{Ga}}_{0.66}$As/AlAs multiple quantum wells by means of the time-resolved degenerate four-wave mixing. The dephasing rate increases with the increase in the excitation density. Its excitation density dependence sharply changes at a critical density of 0.2 \ensuremath{\mu}J/${\mathrm{cm}}^{2}$. The dependence agrees with the excitation density dependence of the long-lived \ensuremath{\Gamma}-hole density studied by means of the pump-and-probe experiment. The long-lived hole density saturates at the same excitation density of 0.2 \ensuremath{\mu}J/${\mathrm{cm}}^{2}$. These facts definitely indicate that the phase relaxation rate of \ensuremath{\Gamma}-\ensuremath{\Gamma} excitons is dominated by the collision between the excitons and long-lived holes.

Excitonic diffusion and recombination processes in CuCl

Time-resolved degenerate four-wave mixing experiments are performed on CuCl samples in order to study the recombination and the diffusion dynamics of excitons and biexcitons. The results can be consistently explained by the time evolution of the populations of these quasiparticle states.

Effects of coherent polarization interactions on time-resolved degenerate four-wave mixing

We report the first observation of coherent polarization interactions in dense media in time-resolved degenerate four-wave mixing in semiconductors. For near-resonant ultrashort-optical-pulse excitation, these interactions lead to a diffracted signal at negative delay times, with a slope half that for positive delay times. At higher intensities, two temporal maxima appear in the diffracted signal, in agreement with many-body theory.

Large third-order optical nonlinearities in organic polymer superlattices

Large second hyperpolarizabilities, 〈γxxxx〉, have been measured in solutions of two recently prepared organic polymer superlattices. These compounds have period aromatic and quinoidal structures and differ from each other by a side group substituent. The values observed are 1.6×10−29 esu for the parent copolymer and 3.7×10−30 esu for its acetoxy derivative. The corresponding values of χ(3)xxxx for the solid phase are estimated to be 2.7×10 −7 and 4.5×10−8 esu. Time-resolved degenerate four-wave mixing measurements, made at 532 nm, showed that the dynamics of the parent copolymer were faster than the 25 ps resolution of the instrument while the derivative exhibited an additional slower response component.

Collision broadening of two-dimensional excitons in a GaAs single quantum well.

The phase relaxation of two-dimensional (2D) heavy-hole excitons in a 12-nm GaAs single quantum well subjected to collisions with either free carriers or incoherent heavy-hole excitons is investigated by time-resolved degenerate four-wave mixing. The homogeneous linewidth corresponding to the phase coherence time reveals a collisional broadening due to exciton-exciton scattering and an 8 times stronger exciton-free carrier scattering. Both scattering processes are enhanced for 2D excitons as compared to 3D excitons.

Reflected degenerate four-wave mixing on GaAs single quantum wells

Time-resolved degenerate four-wave mixing (DFWM) on two-dimensional (2D) excitons in GaAs single quantum wells is studied. We observe backward coherent emission (DFWM signal) from the 2D excitons comparable in efficiency to that in forward direction. This reflection or backward DFWM configuration is extremely advantageous and useful in exploring optically thin semiconductor layers.

Nonlinear optical processes in a polydiacetylene measured with femtosecond duration laser pulses

The temporal response of the third-order nonlinear optical susceptibility in poly bis( p-toluene sulfonate) of 2,4-hexadiyne-1,6 diol was determined by time-resolved degenerate four-wave mixing using tunable dye laser pulses with 300 fs duration. In the material’s absorption region these experiments were combined with saturated absorption techniques yielding a measurement for the excited state lifetime of 1.8 ps. Off resonance the nonlinear response was determined to be optical pulse width limited.

Observation of collisional narrowing by time-resolved degenerate four-wave mixing in sodium

The first observation of collisional narrowing by time-resolved degenerate four-wave mixing has been reported. The velocity-changing collision cross section of Na in Ar we obtained is 5.6×10-15cm2.

Phase coherence and line broadening of free excitons in GaAs quantum wells

The optical dephasing of 2D excitons in GaAs single quantum wells is studied by means of time-resolved Degenerate Four-Wave Mixing and transmission spectroscopy. The measured phase coherence time directly corresponds to the absorption line-width confirming the homogeneous nature of the line broadening. The observed linear temperature dependence of the homogeneous linewidth below 80K reveals acoustic-phonon scattering as the dominant optical dephasing process.

QUANTUM BEAT IN TIME-RESOLVED DEGENERATE FOUR-WAVE MIXING

The time-resolved degenerate four-wave mixing in a nearly degenerate quasi-two-level system is treated rigorously in this paper. It is shown that quantum beat effect exists in the time-resolved degenerate four-wave mixing. The behavior of the quantum beat are analysed and its possible applications are discussed.