Degenerate Four-wave Mixing Signal Research Articles

Single-scan polarization-resolved degenerate four-wave mixing spectroscopy using a vector optical field

We report on a new method to achieve the single-scan polarization-resolved degenerate four-wave mixing (DFWM) spectroscopy in a Rb atomic medium using a vector optical field, in which two pump beams are kept linearly polarized and a vector beam is employed as the probe beam. As the polarization and intensity of the DFWM signal are closely dependent on the polarization state of the probe beam, a vector probe beam with space-variant states of polarization is able to generate a DFWM signal with space-variant states of polarization and intensity across the DFWM image. Accordingly, the polarization-resolved spectra can be retrieved from a single DFWM image. To the best of our knowledge, this is the first time that the single-scan polarization-resolved spectrum detection has been realized experimentally with a vector beam. This work provides a simple but efficient single-scan polarization-resolved spectroscopic method, which would be of great utility for the samples of poor light stability and fast optical processes.

Multi-wave mixing using a single vector optical field

The traditional multiwave mixing setups are always complicated because multiple laser beams are involved, and they are required to be aligned into certain spatial configurations to get the phase-matching condition satisfied. Here, we report on a multiwave mixing method using a single vector optical field. The spatially separated phase-matching beams are obtained after the vector light passes through a polarizer. Filtering out part(s) of them and focusing the remaining ones into the Rb vapor cell, we detected degenerate four-wave mixing, degenerate six-wave mixing, and coexisting degenerate four-wave mixing signals under different circumstances. As the beams participating in the multiwave mixing processes originate from the same phase plane of the vector laser beam, their relative positions are fixed for the setup so that this method has the potential to resist for turbulence compared with the traditional methods. Besides, the entire system is simple and easy to align because the complicated optical setup is avoided. This work provides a convenient tool for multiwave mixing generation and has great potential in spectroscopy, light squeezing, nonlinear optics, and so on.

Enhancement of phase conjugation degenerate four-wave mixing using a Bessel beam

We report on the enhancement of phase conjugation degenerate four-wave mixing (DFWM) in hot atomic Rb vapor by using a Bessel beam as the probe beam. The Bessel beam was generated using cross-phase modulation based on the thermal nonlinear optical effect. Our results demonstrated that the DFWM signal generated by the Bessel beam is about twice as large as that generated by the Gaussian beam, which can be attributed to the extended depth and tight focusing features of the Bessel beam. We also found that a DFWM signal with reasonable intensity can be detected even when the Bessel beam encounters an obstruction on its way, thanks to the self-healing property of the Bessel beam. This work not only indicates that DFWM using a Bessel beam would be of great potential in the fields of high-fidelity communication, adaptive optics, and so on, but also suggests that a Bessel beam would be of significance to enhance the nonlinear process, especially in thick and scattering media.

Snap-shot optical polarization spectroscopy using radially polarized pulses

By using radially polarized pulses for excitation in degenerate four-wave mixing (DFWM) spectroscopy, we realize highly sensitive snap-shot measurement of optical anisotropies in semiconductor films. Because the radially polarized pulses exhibit spatially varying linear polarizations, the optical anisotropies in samples can be evaluated from the spatial distributions of DFWM signals without rotating the polarization of the excitation pulses. We measure the excitons in GaN layers and evaluate the energies of uniaxial strain and the spin-exchange interaction constant. Our results prove both the accuracy and sensitivity of the snap-shot measurement.

Low-noise mid-IR upconversion detector for improved IR-degenerate four-wave mixing gas sensing.

We compare a nonlinear upconversion detector with a conventional cryogenic InSb detector for the detection of coherent infrared light showing near-shot-noise-limited performance in the upconversion system. The InSb detector is limited by dark noise, which results in a 500 times lower signal-to-noise ratio. The two detectors are compared for the detection of a coherent degenerate four-wave mixing (DFWM) signal in the mid-infrared, and applied to measure trace-level acetylene in a gas flow at atmospheric pressure, probing its fundamental rovibrational transitions. In addition to lower noise, the upconversion system provides image information of the signal, thus adding new functionality compared to standard point detection methods. We further show that the upconversion detector system can be implemented as a simple replacement of the cryogenic detector.

Ultra-trace Rb detection with degenerate four-wave mixing in a graphite furnace

Degenerate four-wave mixing (DFWM) is employed to detect rubidium (Rb) in a graphite furnace atomizer as a versatile spectroscopic method for trace element analysis at high spectral resolution. In order to further reduce the detection limit for ultra-trace Rb detection, HNO3, NaNO3, KNO3 and CsNO3 are used as chemical modifiers and the corresponding effects on the Rb DFWM signal are investigated. The experimental results show that NO3− has an enhancement, while the effects of the metals mainly depend on their ionization potentials. Among these chemical modifiers, CsNO3 can significantly enhance the Rb DFWM signal intensity. Using CsNO3, the detection limit was reduced from 7.02 ng mL−1 to 0.90 ng mL−1. This investigation indicates that nitrates, especially those with metallic ions of lower ionization potential can effectively enhance the DFWM signal intensity and reduce the detection limit, which is of special significance in ultra-trace element detection.

Effects of acid anions, major metallic elements, and chlorides on Li detection by DFWM in graphite furnace

In trace Li analysis with degenerate four-wave mixing (DFWM) method, acid anions and major metallic elements are dominant interferences in Li-containing samples. To better use DFWM technique to analyze trace Li in actual samples, we study their effects on Li DFWM signal intensity. It is found that K, Cs, and Ni can enhance the Li DFWM signal, SO2-4..., PO3-4..., Cl-, and Ca can cause significant suppression, and NO3-, Mg, Ba, Sr, and Na almost have no effects. Finally, we use H3BO3 to eliminate the depressive effects of chlorides on Li DFWM signal. The result is also of reference in other trace elements analysis with DFWM.

Anisotropy in Third-Order Nonlinear Optical Susceptibility of the Stretch-Oriented Chrysophenine-Doped Poly(vinyl alcohol) Films

Stretch-oriented chrysophenine-doped poly(vinyl alcohol) (PVA) films were prepared at temperatures above the glass transition temperature of PVA. The optical absorption spectra of the stretch-oriented films exhibited a very high anisotropy (anisotropy ratio = 13). The third-order nonlinear optical properties of chrysophenine in stretch-oriented PVA films were measured by the resonant femtosecond degenerate four-wave mixing (DFWM) technique. The anisotropy ratio of the DFWM signal intensity was greater than 1000. The value of third-order nonlinear optical susceptibility of a uniaxially stretch-oriented chrysophenine-doped PVA film was 6.9×10-13 esu.

Large resonant third-order optical nonlinearity of thin film containing J-like aggregates of a bis[4-(N-dibutylamino)phenyl]squarylium dye

The third-order optical nonlinearity and response of thin film containing J-like aggregates of a bis[4-(N-dibutylamino)phenyl]squarylium dye were measured by degenerate four-wave mixing (DFWM) technique under resonant conditions. The temporal profile of DFWM signal was obtained with a time resolution of 0·3 ps (FWHM), and was found to consist of at least two components, i.e. the coherent instantaneous nonlinear response (electronic response) and the slow response due to the excited state population grating. The effective χ (3) value of thin squarylium dye film was evaluated to be as high as 1·1 × 10 − 7 esu, and the figure of merit of third-order nonlinearity F (F = χ (3)/α), was calculated to be about 2·1 × 10 − 13 esu cm.

Imagine Techniques for Aligning in the Degenerate Four-Wave Mixing Experiment

Phase-match in degenerate four-wave mixing (DFWM) is not automatically achieved in the forward folded boxcars geometry. In this paper, we first design a simple image aligning system composed of an optical system, CCD camera and the related software. We can feasibly obtain well overlapped spot in the sample cell without moving the sample cell under this system. Also, we have proven the three pumping light beams are well overlapped in the sample cell when the three pumping light beams are on the same spot on CCD camera. This method minimizes the deviation by our eyes aligning. We can easily and timely know the effect of the pointing stability on the optical path by monitoring facula changing with the laser beam pointing and disturbs of the environment. So, this is a good choice for fast adjusting the light path and finding the overlapped spot. Finally, Steady DFWM signals have been obtained in the iodine vapor. This system makes it feasible that the potential application of FG-DFWM is used as a diagnostic tool in combustion research and environment monitoring.

Room temperature degenerate four‐wave mixing in CuCl thin films

AbstractWe report observation of anomalous temperature dependences of degenerate four‐wave mixing (DFWM) spectra in CuCl thin films with high crystalline quality and thicknesses in the order of 10–100 nm. The behavior of temperature dependence can be explained by the change in the relative intensities among component spectra. The excitonic state with large radiative width can be observed at high temperatures due to the superradiance faster than the nonradiative decay process. We succeeded in observing the DFWM signal up to room temperature based on the extremely large radiative width peculiar to the size region beyond the long‐wavelength approximation regime.

Strong field effects in rotational femtosecond degenerate four-wave mixing

We study theoretically strong field effects in rotational femtosecond degenerate four-wave-mixing (DFWM). First, we developed a perturbative approach and calculated the leading correction to the standard (weak field) formula for femtosecond DFWM signal. Second, we constructed a nonperturbative approach for computing femtosecond DFWM signals excited by (short) pulses of arbitrary intensity. Third, we worked out the theory to describe femtosecond DFWM with an extra aligning pulse. We show that the strong-pulse-induced nonadiabatic alignment does explain many experimentally detected features that develop in femtosecond DFWM signals with increasing laser intensity beyond the standard weak field regime. However, we also show that this nonadiabatic alignment cannot solely be responsible for the onset of the heterodyne detection and pronounced constant background in DFWM signals excited by high intensity laser pulses.

Probing the vibrational dynamics of high-lying electronic states using pump-degenerate four-wave mixing

To investigate the vibrational dynamics in high-lying electronic states of molecules, a degenerate four-wave mixing (DFWM) process in combination with an initial pump pulse was employed. Applying this technique, the vibrational dynamics occurring in the high-lying E(0(2)g) ion-pair state of molecular bromine were investigated. The initial pump pulse is used to excite the B(3)Pi(0(+)u) state, from which the E(0(2)g) ion-pair state can be accessed in a subsequent DFWM process. By introducing an internal time delay in the DFWM process, the vibrational dynamics of the E(0(2)g) and B(3)Pi(0(+)u) states can be probed and analyzed. In most cases, the signals of the E(0(2)g) and B(3)(0(+)u) state dynamics are overlapping, which makes it difficult to identify the contributions of the respective states. To this end we show that it is possible to extract the contributions of the E(0(2)g) state by a spectral decomposition of the DFWM signal. These findings are circumstantiated by quantum dynamical calculations.

Detection of C2H2 and HCl using mid-infrared degenerate four-wave mixing with stable beam alignment: towards practical in situ sensing of trace molecular species

A stable and convenient optical system to realize the forward phase-matching geometry for degenerate four-wave mixing (DFWM) is demonstrated in the mid-infrared spectral region by measuring DFWM signals generated in acetylene (C2H2) and hydrogen chloride (HCl) molecules by probing the fundamental ro-vibrational transitions. IR laser pulses tunable from 2900 cm−1 to 3350 cm−1 with a 0.025 cm−1 linewidth were obtained using a laser system composed of an injection seeded Nd:YAG laser, a dye laser, and a frequency mixing unit. At room temperature and atmospheric pressure, a detection limit of 35 ppm (∼ 9.5×1014 molecules/cm3) for C2H2 was achieved in a gas flow of a C2H2/N2 mixture by scanning the P(11) line of the (010(11)0)–(0000000) band. The detection limit of the HCl molecule was measured to be 25 ppm (∼6.8×1014 molecules/cm3) in the same environment by probing the R(4) line. The dependences of signal intensities on molecular concentrations and laser pulse energies were demonstrated using C2H2 as the target species. The variations of the signal line shapes with changes in the buffer gas pressures and laser intensities were recorded and analyzed. The experimental setup demonstrated in this work facilitates the practical implementation of in situ, sensitive molecular species sensing with species-specific, spatial and temporal resolution in the spectral region of 2.7–3.3 μm (3000–3700 in cm−1), where various molecular species important in combustion have absorption bands.

Multiple beats of weakly confined excitons with inverted selection rule

The phenomenon of multiple beats (MBs) arising from nondipole-type excitons weakly confined in a thin film is theoretically elucidated using a nonlocal transient-response theory. Kojima et al. previously demonstrated for a GaAs thin film that the degenerate four-wave mixing signals from the quantized levels of the center-of-mass motion of excitons exhibit complex interference between beats under femtosecond-order pulse incidence [Kojima et al., J. Phys. Soc. Jpn. 77, 044701 (2008)]. This leads to an ultrafast optical response on the order of femtoseconds. This effect occurs in a size region beyond the long-wavelength approximation regime due to the resonant enhancement of the internal field, wherein the usual dipole selection rule is violated. Our analysis of MBs employs a model of the nonlocal multilevel system that considers the spatial interplay between excitonic waves and the radiation field to elucidate the mechanism behind the observed ultrafast response.

Ultrafast coherent vibronic oscillations in regioregular poly(3‐alkylthiophene)

AbstractUltrafast degenerate four‐wave mixing (DFWM) signals of the regioregular (RR) poly(3‐hexylthiophene) (P3HT) film have been investigated by the experiments using sub‐20 fs pulses generated from a noncollinear optical parametric amplifier (NOPA) system. Strong DFWM signals were observed owing to a large third‐order nonlinear susceptibility χ(3) of the RR‐P3HT film. The time profile of the DFWM signals exhibits clear coherent oscillation on a decaying signal. The oscillation turn out to be caused by the C=C stretching mode that gives strong Raman signal. The time profile of the DFWM signal is simulated by a numerical calculation. The result of simulation reveals that decay constants of electronic population and of vibronic oscillation are 200 fs and 210 fs, respectively. This is the first determination of the decay constants for the RR‐P3HT film. The obtained result demonstrates that the χ(3) signal of the RR‐P3HT film decays very rapidly. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Cover Picture: Phys. Status Solidi C 6/S1

AbstractThe article by Katsuichi Kanemoto et al. demonstrates that regioregular poly(3‐hexylthiophene) (RR‐P3HT) is promis‐ing as nonlinear optical material. The cover picture shows strong nonlinear optical signals obtained from degenerate four‐wave mixing (DFWM) experiments using sub‐20 fs pulses for the RR‐P3HT film. The picture also shows ultrafast coherent oscillations of the DFWM signals with a period of about 25 fs. The authors reveal by a nu‐merical simulation that the oscillations arise from molecular vibrations of the C=C stretching mode that gives strong Raman signals. The simulation also determines for the first time the decay constants of the electronic popula‐tion and of the vibronic oscillation in RR‐P3HT as being 200 fs and 210 fs, respectively (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Phase‐relaxation processes of excitons in semiconducting single‐walled carbon nanotubes

AbstractWe have investigated the phase‐relaxation process of excitons in semiconducting single‐walled carbon nanotubes (SWNTs) by a time‐resolved degenerate four‐wave mixing (DFWM) method. The phase‐relaxation time (T2) estimated from the time evolution of the diffracted DFWM signals strongly depends on the temperature and excitation intensity, shorter T2 with higher temperature and excitation intensity. The experimental trend can be explained by the enhancement of phase‐relaxation rate due to frequent exciton–phonon scattering at high temperature and exciton–exciton scattering under high‐density excitation conditions. The T2 value at the weak excitation limit measured for semiconducting SWNT with a tube diameter of 1.1 nm at 2 K is estimated to be 350 fs. This T2 value is in good agreement with the homogeneous width estimated from microluminescence measurements. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Quantitative estimation of collisional dephasing rate in the X1A1–A1A2 (410) band of formaldehyde with degenerate four wave mixing spectroscopy

AbstractThe application of degenerate four wave mixing (DFWM) spectroscopy to the study of collisional dephasing in the 1A2(410) excited band of formaldehyde has been carried out. The method relies on the measurement of DFWM signal dependence on the partial pressures of different buffer gases and in the saturated regime. Experiments were conducted with CO2, N2 and He as colliding partners to H2CO, and collisional dephasing rates γpd of the order of 1010 s−1 atm−1 were estimated. The experimental findings clearly indicate that coherence decay is the dominant mechanism, with respect to rotational energy transfer (RET), in determining the DFWM signal decreasing with pressure. The comparison between the different values of γpd measured and the literature data for γRET has been used to shed light on key factors mostly ruling the competition between decoherence and population relaxation effects. Copyright © 2007 John Wiley & Sons, Ltd.

Enhancement of nonlinear optical response of weakly confined excitons in GaAs thin films by spectrally rectangle-shape-pulse-excitation

We report the enhancement of the nonlinear optical response of the weakly confined excitons with use of spectrally rectangular pulse. The nonlinear optical response was investigated as a function of excitation energy by a degenerate four-wave-mixing (DFWM) technique. In the case that the laser pulse with the controlled spectral shape excites the plural exciton states simultaneously, the DFWM signal intensity is enhanced by a factor of two in comparison with the intensity under the excitation of a single exciton state. This enhancement is caused by the superposition of the nonlinear optical responses from the plural exciton states.