FWM Signal Research Articles

Multi-dressing suppression and enhancement and all-optical switching in parametrically amplified four-wave mixing

We study different dressing effects in parametrically amplified four-wave mixing (PA-FWM) processes. By seeding a weak probe laser into the Stokes or anti-Stokes channel of the FWM, the gain process is generated in the so-called bright twin beams which are the probe and conjugate beams. The dressing types dramatically affect the gain factors in both the probe and conjugate channels. The gain factor of the FWM signal decreases under the cascade-type dressing and the signal's shape splits into two dips under this dressing type. However, the intensity of the FWM signal changes from suppression to enhancement under the parallel-type dressing. We will apply this switching process to all-optical switching.

In Situ Accurate Analysis of Colloidal Nanoparticles via Four Wave Mixing

ABSTRACTFour-wave mixing (FWM) is used to measure the vibrational modes of nanoparticles in solution. The vibrations give information about the particle size, material properties and shape. This method has been used for in-situ monitoring of the growth of nanoparticles with high accuracy, as confirmed by electron microscopy analysis. We observe a threshold in the FWM signal which we believe is from a cavity forming around the nanoparticles that reduces viscous damping. We have observed this effect in molecular dynamics simulations as well.

Vacuum-induced suppression and enhancement of four-wave mixing in an optical cavity

We report on an experimental study of vacuum- induced suppression and enhancement of four-wave mix- ing (FWM) signal in a composite atom-cavity system. By scanning the additional dressing field, the suppression ratio of the FWM signal can reach 90 % compared with 40 % without cavity. We attribute the enhanced suppression and enhancement to the atom-cavity coupling arising from a vacuum-induced Raman process, which amplifies the dressing effect from the additional field. Also, the dress - ing asymmetry of the atom-cavity coupling is discussed and used to estimate the nonlinearity of atomic medium in the cavity. The suppression and enhancement can be inter- preted by a dressed-state picture and agree with theoreti- cal calculations. The investigation may find applications in optical switch and quantum memory controlled by cavity.

Investigate the Effect of Line Width at Various Laser Sources on FWM Power in WDM System

AbstractIn this paper, the dominating effect of four wave mixing is analysed with line width of laser source in wave length division multiplexed system. Power of FWM signal is minimum, Q factor maximum and bit error rate is minimized at optimized value of line width at all laser source power.

Observations of Autler-Townes spatial splitting of four-wave mixing image

We report the self- and external-dressed Autler-Townes (A-T) splittings of the images of the generated four-wave mixing signal (FWM) and electromagnetically induced transparency (EIT) of probe images in cascade three-level atomic system. Such spatial properties of probe and FWM signals are induced by the enhanced cross-Kerr nonlinearity. We demonstrate the controlled electromagnetically induced spatial dispersion (EISD), splitting and focusing of probe and FWM signals images by adjusting self- and external-dressing fields. Studies on such controllable A-T spatial splitting and spatial EIT effect can be very useful in applications of spatial signal processing and optical communication.

Repeatedly dressed four-wave mixing in N5B five-level atomic system

We present a theoretical treatment for the repeatedly dressed FWM processes in N5B five-level atomic system. Based on our previous work, the evolution of Autler-Townes splitting in a multi-dressed FWM spectrum is expounded using the characteristic method of Independence effect. Under condition that the coupling and probe fields which induced the FWM are strong, we further demonstrate the suppression and enhancement of FWM signal intensity. The results indicate that one cpupling field can dresse the atom twice and produce the donbly dressed state and triply dressed states. Many applications of multi-dressed FWM spectrum can benefit from the investigation on the atomic energy level and the nonlinear processes of mutiwave mixing(MWM).

Coherent control of polariton modes in pulse‐transmission and four‐wave‐mixing experiments

AbstractWe report coherent optical control of polariton modes in pulse‐transmission and four‐wave‐mixing experiments realized on a ZnSe/ZnSSe heterostructure containing a 25 nm thin active ZnSe layer. It is shown in linear pulse‐transmission at low excitation density that the modes can be separately controlled. The coherent control of nonlinear polaritonic FWM polarization is different for both directions of first‐order diffraction. The FWM signal observed in direction 2k1‐k2 shows that the polariton modes can be separately manipulated. Contrasting to that, in the direction 2k2‐k1 all polariton modes are simultaneously enhanced or diminished. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Biexcitons and their dephasing processes in ZnO

We have investigated free excitons and biexcitons of bulk ZnO by time-integrated and spectrally-resolved four-wave mixing. The polarization- and wave-vector-dependent FWM signals for A and B excitons demonstrate clearly that the valence-band ordering is A–Γ9, B–Γ7, and C–Γ7. Next, we gave qualitative explanation of the FWM signals induced by excitons and biexcitons in ZnO. For negative delay times, bound and unbound AB-biexcitons (A exciton-B exciton correlation), which have been considered to play a minor role in GaAs heterostructures, was found to play an important role and give a significant qualitative impact to the optical response of ZnO. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Measurements of exciton–polariton dynamics in ZnO by using nonlinear spectroscopic techniques

We have studied the exciton–polariton dynamics in a c-face bulk ZnO crystal grown by the seeded vapor transport method. We have performed two-pulse spectrally resolved four-wave mixing (SR-FWM) and three-pulse SR-FWM at T = 10 K. In two-pulse FWM experiment, obtained FWM signals showed the fast dephasing which has been attributed to the strong polariton dispersion of the upper polariton of B exciton. On the other hand, in three-pulse FWM experiment, the signal decay shows the free exciton population relaxation. Observed FWM signal has decayed with time constant ∼ 12 ps which may represent exciton–polariton lifetime.

Light-polarization and intensity dependence of higher-order nonlinearities in excitonic FWM signals

A theory-experiment comparison for the intensity and polarization dependence of spectrally resolved transient four-wave-mixing signals is presented for a ZnSe single quantum well. Results for high intensities deviate from a simple model based on the optical Bloch equations for a five-level system whereas a microscopic theory for coherent excitonic and biexcitonic nonlinearities shows excellent agreement.

Biexciton formation and exciton–exciton correlation effects in bulk ZnO

We demonstrate that exciton–exciton correlations strongly affect FWM signals. In particular, A exciton–B exciton correlation (bound and unbound AB-biexcitons), which can be generated even in co-circular polarizations, has a significant qualitative impact on the optical response of bulk ZnO.

Exciton–exciton correlation effects on FWM in GaN

AbstractBound and unbound biexcitons in a free‐standing bulk GaN are investigated by time‐integrated and spectrally‐resolved four‐wave mixing measurements, where the formation of hetero‐biexcitons that consist of A and B excitons (XXAB) as well as A biexcitons and their unbound biexciton are clearly observed. Through the FWM spectra and delay‐time dependence, it is found that the interaction between A and B excitons gives rise to the phase shifts of the quantum beating and the energy shifts of the spectra, which is considered as the effect of the unbound state of XXAB (i.e. XX*AB). This interaction is found to play an important role in the FWM signals for all polarizations. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Polarization dependence of excitonic and biexcitonic contributions to FWM signals

The time-averaged polarization angles of excitonic and biexcitonic four-wave-mixing signals are investigated in spectrally resolved measurements for different angles between the linear excitation polarizations, delay times and pulse energies. For an angle close to 90° between the two incident polarizations the signal emitted at the spectral position of the excitonic resonance clearly shows different polarization angles as the delay is changed from negative to positive. Additionally, the polarization angles of the signals exhibit oscillations which are differently pronounced for negative and positive delay times. The experimental results are compared with numerical calculations based on a phenomenologically extended five-level model of the exciton-biexciton system.

Biexcitonic effects in the time integrated four-wave mixing with picosecond pulses

We propose a simple method to estimate the biexcitonic contribution to the excitonic non-linearity. The method is based on the time integrated four-wave mixing (TI FWM) with picosecond pulses. The TI FWM signal, which is measured as a function of the delay between pump and test pulses, shows shift towards positive and negative time delay when the laser is tuned at the exciton and biexciton resonance, respectively. We show theoretically that the magnitude of the shift allows us to estimate the biexcitonic contribution to the third-order non-linearity at the fundamental band edge.

Exciton-exciton interaction and heterobiexcitons in GaN

The formation of not only A biexcitons ${(XX}_{\mathrm{AA}})$ but also heterobiexcitons that consist of A and B excitons ${(XX}_{\mathrm{AB}})$ in a free-standing bulk GaN is identified by polarization-sensitive spectrally resolved FWM measurements. The FWM spectra and delay-time dependence show that the interaction between A and B exciton gives rise to the energy shifts of the spectra and the phase shifts of the quantum beating, which is considered as the effect of the unbound state of ${\mathrm{XX}}_{\mathrm{AB}}$ (i.e., ${\mathrm{XX}}_{\mathrm{AB}}^{*})$ and ${\mathrm{XX}}_{\mathrm{AB}}^{*}$ is found to play an important role in the FWM signals for all polarizations. The unbound A biexciton ${(XX}_{\mathrm{AA}}^{*})$ is also observed clearly in spectral and temporal domains.

Study and Characterization of Resonances in the Four-Wave Mixing Signal of a Two-Level System with Intramolecular Coupling

The Liouville formalism in frequency space is used in the present study to investigate the effect of the intramolecular coupling on the four wave mixing signal (FWM) of a two-level system. The internal rovibrational structure is incorporated by including the spin–orbit residual interaction in the total Hamiltonian of the system. The behavior of the FWM signal as a function of the coupling parameters (v and V0) has been studied. The main properties affected were the permanent and transition dipole moments of the solute molecules when the rotating wave approximation is neglected. Reduced polarization expressions are obtained from the resonant terms and from the larger coherence and population density contributions to the macroscopic polarization, which in turn allows us to characterize individual resonances in the resulting frequency spectrum.

Coherent control of the molecular four-wave-mixing response by phase and amplitude shaped pulses

Femtosecond pulse shaping using a spatial light modulator and degenerate four-wave-mixing (DFWM) spectroscopy have been combined to control the vibrational wave packet motion in the electronic ground and excited states of the prototype molecule K 2 and, thus, study the influence of phase and amplitude modulated pulses on the molecular FWM response. It is shown that the vibrational motion of the potassium dimer is sensitive to the temporal interpulse separation and the adjusted phase differences. A simple theoretical model for such arbitrary shaped pulse excitation in a FWM process has been developed, which proves to be in excellent agreement with the observed FWM signal. In addition, the spectrally resolved DFWM signal works as a molecular FROG providing information about the activated molecular vibrations, the DFWM signal itself and the shapes of the pulses used in the excitation sequence. Furthermore, the experiment was ideal to test the robustness of the evolutionary algorithm previously used to find new control processes.

Four-wave mixing of picosecond pulses in hollow fibers: expanding the possibilities of gas-phase analysis

Summary form only given. Our experiments have shown that the use of short light pulses allows the sensitivity of gas phase analysis based on (i) third-harmonic generation (ii) two-color difference frequency generation (DFG) /spl omega//sub DF/ = 2/spl omega//sub 1/ - /spl omega//sub 2/, and (iii) two-color sum-frequency generation /spl omega//sub SF/ = 2/spl omega//sub 1/ + /spl omega//sub 2/ in hollow fibers to be improved as compared with the nanosecond regime. The use of picosecond pulses allowed us to couple pump beams with a power density up to 10/sup 12/ W/cm/sup 2/ into a hollow fiber, thus considerably increasing the intensity of FWM signals generated through third harmonic, sum-frequency and difference-frequency generation processes in hollow fibers as compared with the nanosecond regime. It was shown that hollow fibers permit third-harmonic and sum-frequency generation, which give zero signals in the regime of tight focusing in a medium with normal dispersion, to be employed for gas-phase monitoring and analysis.

Four-wave mixing in CdMnTeSe: In crystals

It is shown that the four-wave mixing technique can be used as a spectroscopic tool for studying the properties of bistable centers in semiconductors. Two metastable centers with different lattice relaxation energy have been identified in the Cd 1− x Mn x Te 1− y Se x : In crystal. The power dependence of the FWM signal provides additional support for the “negative-U” model of metastable centers in this material.

Four-wave mixing of incoherent light in a dispersion-shifted fiber using a spectrum-sliced fiber amplifier light source

We investigate the FWM efficiency of incoherent light in a dispersion-shifted fiber (DSF) using a spectrum-sliced fiber amplifier light source. A theoretical model is provided to describe the FWM mechanism of incoherent light. The FWM efficiencies of coherent and incoherent light are compared theoretically and experimentally. Unlike the FWM of coherent light, the FWM signals of incoherent light are mostly generated by nondegenerate FWM regardless of the number of input signals. Thus, when two input signals are mixed, incoherent light has about 6 dB higher mixing efficiency than coherent light due to the difference in their degeneracy factor.