Nonresonant Four-wave Mixing Research Articles

Resonant Degenerate Four-Wave Mixing at the Defect Energy Levels of 2D Organic-Inorganic Hybrid Perovskite Crystals.

Two-dimensional organic-inorganic lead halide perovskites are generating great interest due to their optoelectronic characteristics such as high solar energy conversion efficiency and a tunable direct band gap in the visible regime. However, the presence of defect states within the two-dimensional crystal structure can affect these properties, resulting in changes to their band gap emission as well as the emergence of nonlinear optical phenomena. Here, we have investigated the effects of the presence of defect states on the nonlinear optical phenomena of the 2D hybrid perovskite (BA)2(MA)2Pb3Br10. When two pulses, one narrowband pump pulse centered at 800 nm and one supercontinuum pulse with bandwidth from 800-1100 nm, are incident on a perovskite flake, degenerate four-wave mixing (FWM) occurs, with peaks corresponding to the energy levels of the defect states present within the crystal. The longer carrier lifetime of the defect state, in comparison to that of virtual transitions that take place in nonresonant FWM processes, allows for a larger population of electrons to be excited by the second pump photon, resulting in increased FWM signal at the defect energy levels. The quenching of the two-photon luminescence as flake thickness increases is also observed and attributed to the increased presence of defects within the flake at larger thicknesses. This technique shows the potential of detecting defect energy levels in crystals using FWM for a variety of optoelectronic applications.

Highly sensitive Fourier-transform coherent anti-Stokes Raman scattering spectroscopy via genetic algorithm pulse shaping.

We report highly sensitive Fourier-transform coherent anti-Stokes Raman scattering spectroscopy enabled by genetic algorithm (GA) pulse shaping for adaptive dispersion compensation. We show that the non-resonant four-wave mixing signal from water can be used as a fitness indicator for successful GA training. This method allows GA adaptation to sample measurement conditions and offers significantly improved performance compared to training using second-harmonic generation from a nonlinear crystal in place of the sample. Results include a 3× improvement to peak signal-to-noise ratio for 2-propanol measurement, as well as a 10× improvement to peak intensities from the high-throughput measurement of polystyrene microbeads under flow.

Strongly nonresonant four-wave mixing in semiconductors

When semiconductors are optically excited above or slightly below the band gap, the linear and nonlinear responses originate predominantly from the interband polarization. Here, we demonstrate that intraband excitations, i.e., rapidly oscillating currents that originate from the electric-field-induced acceleration of electrons and holes, contribute strongly to transient four-wave mixing when it is performed with center frequencies near half the band gap frequency. Within a two-band model we show that the presence of several pathways arising from different combinations of inter- and intraband excitations and their interference give rise to characteristic signatures in time- and spectrally resolved signals. Our approach is based on the semiconductor Bloch equations and includes the dynamics of off-resonant electron-hole excitations on a microscopic level. The predicted significant broadening and structure appearing in the four-wave-mixing spectra are in good qualitative agreement with experimental results.

High conversion efficiency in resonant four-wave mixing processes.

We propose a new scheme of the resonant four-wave mixing (FWM) for the frequency up or down conversion, which is more efficient than the commonly-used scheme of the non-resonant FWM. In this new scheme, two control fields are spatially varied such that a probe field at the input can be converted to a signal field at the output. The efficiency of probe-to-signal energy conversion can be 90% at medium's optical depth of about 100. Our proposed scheme works for both the continuous-wave and pulse cases, and is flexible in choosing the control field intensity. This work provides a very useful tool in the nonlinear frequency conversion.

Hybrid imaging of fluorescently labeled cancer drugs and label-free four-wave mixing microscopy of cancer cells and tissues.

Fluorescent labels are well suited as tracers for cancer drug monitoring. Identifying cellular target regions of these drugs with a high resolution is important to assess the working principle of a drug. We investigate the applications of label-free nonresonant four-wave mixing (NR-FWM) microscopy in biological imaging in combination with fluorescence imaging of fluorescently labeled cancer drugs. Results from human A431 tumor cells with stained nuclei and incubated with IRdye 800CW labeled cancer drug cetuximab targeting epidermal growth factor receptor at the cell membrane show that NR-FWM is well suited for cellular imaging. A comparison of vibrationally nonresonant FWM imaging with vibrational resonant coherent anti-Stokes Raman scattering signals revealed nearly identical qualitative information in cellular imaging. NR-FWM is also suitable for tumor tissue imaging in combination with fluorescence imaging of IRdye 800CW labeled, human epidermal growth factor 2 targeting cancer drug pertuzumab and provides additional information over transmission microscopy.

The phase-controlled Raman effect.

Unlike spontaneous Raman effect, nonlinear Raman scattering generates fields with a well-defined phase, allowing Raman signals from individual scatterers to add up into a highly directional, high-brightness coherent beam. Here, we show that the phase of coherent Raman scattering can be accurately controlled and finely tuned by using spectrally and temporally tailored optical driver fields. In our experiments, performed with spectrally optimized phase-tunable laser pulses, such a phase control is visualized through the interference of the coherent Raman signal with the field resulting from nonresonant four-wave mixing. This interference gives rise to Fano-type profiles in the overall nonlinear response measured as a function of the delay time between the laser pulses, featuring a well-resolved destructive-interference dip on the dark side of the Raman peak. This phase-control strategy is shown to radically enhance the coherent response from weak Raman modes, thus helping confront long-standing challenges in nonlinear Raman imaging and microspectroscopy.

Signal contrast in coherent Raman scattering: Optical phonons versus biomolecules

We show that the limiting contrast of the coherent anti-Stokes Raman scattering (CARS) signal with respect to the coherent background due to nonresonant four-wave mixing is controlled by the Q factor of the Raman mode and is independent of the parameters of laser pulses. High-Q phonon modes of semiconductor nanoparticles, such as diamond nanoprobes, can therefore substantially enhance the contrast of CARS images, as well as the sensitivity of CARS spectroscopy and microscopy compared to typical Raman-active vibrations of organic molecules in biotissues.

Generation of intense sub-20-fs vacuum ultraviolet pulses compressed by material dispersion

We present our latest results on the generation of ultrashort vacuum UV (VUV) pulses by nonresonant four-wave mixing of chirped broadband pulses generated by filamentation of the fundamental of a Ti:sapphire laser with relatively narrowband pulses at the third harmonic. Positive chirp at the broadband idler yields negatively chirped VUV pulses necessary to compensate for material dispersion of a MgF(2) window in the VUV beam path. Pulse energies exceeding 400 nJ are available for time-resolved experiments. Pulse duration is measured by pump-probe ionization of Xe gas, providing the cross correlation between the fifth harmonic and the fundamental.

Wavelength Conversion Based on Raman- and Non-Resonant Four-Wave Mixing in Silicon Nanowire Rings Without Dispersion Engineering

We propose an efficient wavelength conversion scheme that is based on either Raman-resonant four-wave mixing or non-resonant Kerr-induced four-wave mixing in a silicon nanowire ring, and that does not require dispersion engineering of the nanowire. We rely on the spatial variation of the Raman and Kerr susceptibilities around the ring to quasi-phase match the wavelength conversion processes for TE polarized fields. The flexibility of this quasi-phase-matching scheme can lead to wavelength conversion efficiencies from -26.7 dB to values larger than 0 dB, and in certain circumstances makes it possible to outperform conventional phase-matched conversion in a dispersion-engineered silicon ring by factors exceeding 6 dB.

Picosecond time-resolved pure-rotational coherent anti-Stokes Raman spectroscopy for N_2 thermometry

Time-resolved pure-rotational coherent anti-Stokes Raman spectroscopy using picosecond-duration laser pulses is investigated for gas thermometry. The use of picosecond laser pulses significantly reduces background caused by scattering of the probe beam, and delayed probing of the Raman coherence enables elimination of interference from nonresonant four-wave mixing processes. Temperatures inferred from rotational spectra are sensitive to the probe delay because of the rotational-level dependence of collisional dephasing of Raman coherences. The sensitivity decreases, however, with increasing temperature, and accurate temperature measurements in a flame are demonstrated using a standard frequency-domain analysis of the spectra.

Different contrast information obtained from CARS and nonresonant FWM images

AbstractWe demonstrate that coherent anti‐Stokes Raman scattering (CARS) microscopy can be used not to obtain vibrational contrast related to a particular chemical species only, but furthermore, it directly yields topological contrast. Topological contrast is due to the spatially dependent refractive index of the sample and a number of linear effects as interference, refraction and absorption contribute to the topological contrast inherent to CARS images. In order to distinguish between different contrast mechanisms we propose an image correction method based on the analysis of the nonresonant four‐wave mixing (FWM) signal distribution. Copyright © 2009 John Wiley & Sons, Ltd.

Two-dimensional phase resolution of frequency-resolved optical gating across the midinfrared

Spectrally and temporally heterodyned four-wave mixing (FWM) signals in silver thiogallate are reported over a wavelength range of 3-10μm. We (1) expose the usefulness of the strong nonresonant FWM in optimizing an apparatus for heterodyned 2D infrared experiments and (2) use heterodyning to measure the 2D phase of a frequency-resolved optical gating (FROG) signal. We report the phase of the 2D FROG signal, as opposed to the 1D phase of the excitation pulses routinely determined from conventional FROG analysis. The 2D FROG phase complements the 2D FROG magnitude in providing intuitive visual analysis of the spectral and temporal phases of femtosecond pulses.

Selective excitation of molecular modes in a mixture by optimal control of electronically nonresonant femtosecond four-wave mixing spectroscopy

Femtosecond time-resolved coherent anti-Stokes Raman scattering (fs-CARS) gives access to ultrafast molecular dynamics. Due to the spectrally broad laser pulses, usually poorly resolved spectra result from this spectroscopy. However, it can be demonstrated that by shaping the femtosecond pulses a selective excitation of specific vibrational modes is possible. We demonstrate that using a feedback-controlled optimization technique, molecule-specific CARS spectra can be obtained from a mixture of different substances. A careful analysis of the experimental results points to a nontrivial control of the vibrational mode dynamics in the electronic ground state of the molecules as underlying mechanism.

Observation of all- trans-β-carotene wavepacket motion on the electronic ground and excited dark state using degenerate four-wave mixing (DFWM) and pump–DFWM

Wavepacket dynamics on the ground and on the optically dark, first electronic state of all- trans-β-carotene is presented. Ground state dynamics are studied using resonant and non-resonant degenerate four-wave mixing spectroscopy with 16 fs time resolution. Wavepacket motion on the electronic dark state was monitored using pump-degenerate four-wave mixing spectroscopy.

Multiplex nonresonant pump four-wave mixing

We propose a scheme of multiplex nonresonant pump four-wave mixing (NRP-FWM) process which is highly immune to saturation problems. The process is accomplished with two high-intensity pump beams detuned far from the resonance of the sample under investigation and a resonant probe beam as weak as possible not to give rise to nonlinear absorption. Only a single-broadband probe beam is used for the multiplex experiment to detect the electronic swan band spectrum of C2 molecules in a premixed acetylene–oxygen flame. Comparing the signal of the degenerate four-wave mixing (DFWM) experiment on the same sample, we found that the signal level at which the NRP-FWM starts to show saturation is about 250 times higher than in the case of the DFWM.

Rotational coherence spectroscopy of benzene by femtosecond degenerate four-wave mixing

Femtosecond time-resolved non-resonant degenerate four-wave mixing (fs-DFWM) experiments were applied to study the rotational coherences in benzene molecules in the gas phase at room temperature. The experiments were performed in a cell filled with benzene vapour at reduced pressure (0.01 bar). The technique gives an excellent signal-to-noise ratio for the observed transients. For the first time transients up to a maximum time delay of 1.4 ns, mainly limited by collisional dephasing, have been obtained for a molecule of this size by fs-DFWM. From this data the rotational constant B = 0.18972 ± 0.00010 cm−1 for benzene at room temperature is obtained by a fitted simulation. The application of fs-DFWM to large molecules seems to be possible and might produce some interesting results in the future.

Characterization of Proton-Irradiated InGaAs/GaAs Multiple Quantum Well Structures by Nonresonant Transient Four-Wave Mixing Technique

Effects of proton irradiation and thermal annealing on free carrier lifetime and transport in a InGaAs/GaAs multiple quantum well (MQW) structure and its substrate have been studied by using the nonresonant transient four-wave mixing technique. Under the excitation of the surface region of the structure by 0.53 µm wavelength, we determined the carrier lifetime τ=150–600 ps, bipolar diffusion coefficient D=24–26 cm2 s-1, and mobility-lifetime product µτ≈(2–6)×10-7 cm2 V-1 in 3 MeV proton irradiated and annealed MQW samples. At excitation by 1.06 µm wavelength, we studied the full proton penetration range, including the substrate. We found an unexpected twofold increase in carrier bulk lifetime with increasing irradiation dose. Features of this effect at low photoexcitation and its absence at high photoexcitation are attributed to carrier separation by potential fluctuations in the vicinity of irradiation-created deep defects. These features indicate that proton-irradiated semi-insulating GaAs exhibits properties of mesoscopic pointlike defects, similar to those previously reported in nonstoichiometric crystals of GaAs. This result is important to understand the mechanism of carrier compensation in proton-irradiated semiconductors.

Erratum: Spectral line shape of nonresonant four-wave mixing in Markovian stochastic fields [Phys. Rev. A56, 2267 (1997)

Received 19 February 1999DOI:https://doi.org/10.1103/PhysRevA.59.4850.2©1999 American Physical Society

Femtosecond nonresonant degenerate four-wave mixing at atmospheric pressure and in a free jet

, N2, and CO2. For CO2 additional experiments have been performed at reduced pressure and in a molecular beam. By delaying the probe pulse a periodic recovery of the DFWM signal is observed. The period of these transients can be assigned unambiguously to rotational Raman transitions of the ground state within the laser bandwidth. The decay of the transients yields the collisional dephasing of the Raman-induced polarization. At zero delay also optical-field-induced birefringence of electronic nature contributes to the signal. The different time scales of the Raman and electronic effects allow us to estimate their relative strength.

Nonresonant four-wave mixing in photorefractive CdTe crystals using a picosecond parametric generator

We demonstrate that a parametrically pumped picosecond laser has enough coherence and energy to write transient phase gratings at nonresonant interaction, thus allowing a study of time-resolved carrier transport in CdTe crystals to be made. Autocorrelation trace of light diffraction efficiency on transient grating allowed us to measure a coherence length of the parametric generator. Carrier diffusion, recombination, and drift in light-created internal space-charge (SC) electric fields have been studied in vanadium or germanium doped semi-insulating CdTe crystals by nonresonant four-wave mixing technique at 940 nm wavelength. It was found that modification of the deep level charge state in CdTe:V by As codoping has changed the sign of majority carriers, responsible for the creation of SC field. Dynamics of free carrier grating decay in CdTe:Ge revealed an electron-governed very fast initial grating decay which develops with time into the double-exponential hole-governed grating decay. Time-resolved transient grating technique described in this article provides a powerful tool for investigation of the role of deep traps in photorefractive semiconductors and optimization of their photoelectric properties in a required temporal and spectral range.