Degenerate Pump-probe Experiments Research Articles

Pump-induced terahertz anisotropy in bilayer graphene

We investigate the intraband nonlinear dynamics in doped bilayer graphene in the presence of strong, linearly polarized, in-plane terahertz fields. We perform degenerate pump-probe experiments with 3.4-THz fields on doped bilayer graphene at low temperature ($12\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) and find that when the pump is copolarized with the probe beam, the differential pump-probe signal is almost double that found in the cross-polarized case. We show that the origin of this pump-induced anisotropy is the difference in the average electron effective mass in the probe direction when carriers are displaced in $k$ space by the pump either parallel or perpendicular to the direction of the probe polarization. We model the system using both a simple semiclassical model and a Boltzmann equation simulation of the electron dynamics with phenomenological scattering and find good qualitative agreement with experimental results.

Disentangling electronic, lattice, and spin dynamics in the chiral helimagnet Cr1/3NbS2

We investigate the static and ultrafast magneto-optical response of the hexagonal chiral helimagnet $Cr_{1/3}NbS_{2}$ above and below the helimagnetic ordering temperature. The presence of a magnetic easy plane contained within the crystallographic ab-plane is confirmed, while degenerate optical pump-probe experiments reveal significant differences in the dynamic between the parent, $NbS_{2}$, and Cr-intercalated compounds. Time resolved magneto-optical Kerr effect measurements show a two-step demagnetization process, where an initial, sub-ps relaxation and subsequent buildup ($\tau > 50$ ps) in the demagnetization dynamic scale similarly with increasing pump fluence. Despite theoretical evidence for partial gapping of the minority spin channel, suggestive of possible half metallicity in $Cr_{1/3}NbS_{2}$, such a long demagnetization dynamic likely results from spin lattice-relaxation as opposed to minority state blocking. However, comparison of the two-step demagnetization process in $Cr_{1/3}NbS_{2}$ with other 3d intercalated transition metal dichalcogenides reveals a behavior that is unexpected from conventional spin-lattice relaxation, and may be attributed to the complicated interaction of local moments with itinerant electrons in this material system.

Bandgap renormalization in single-wall carbon nanotubes

Single-wall carbon nanotubes (SWNTs) have been extensively explored as an ultrafast nonlinear optical material. However, due to the numerous electronic and morphological arrangements, a simple and self-contained physical model that can unambiguously account for the rich photocarrier dynamics in SWNTs is still absent. Here, by performing broadband degenerate and non-degenerate pump-probe experiments on SWNTs of different chiralities and morphologies, we reveal strong evidences for the existence of bandgap renormalization in SWNTs. In particularly, it is found that the broadband transient response of SWNTs can be well explained by the combined effects of Pauli blocking and bandgap renormalization, and the distinct dynamics is further influenced by the different sensitivity of degenerate and non-degenerate measurements to these two concurrent effects. Furthermore, we attribute optical-phonon bath thermalization as an underlying mechanism for the observed bandgap renormalization. Our findings provide new guidelines for interpreting the broadband optical response of carbon nanotubes.

Dispersion studies of optical nonlinearity and excited state dynamics in cyclo[4]naphthobipyrroles

Herein we present results from the dispersion studies of nonlinear optical (NLO) properties of three novel cyclo[4]naphthobipyrroles at wavelengths of 600nm, 640nm, 680nm, and 800nm using picosecond (ps) pulses. NLO coefficients were extracted from Z-scan measurements using ∼1.5ps pulses. Large two-photon absorption cross sections in the range of 0.2–5.2×104 GM and n2 values of ∼10−16cm2/W were recorded for these moieties. The excited state decay dynamics were investigated using degenerate pump-probe experiments with ∼70 femtosecond pulses near 600nm. Double exponential fits of the pump-probe data suggested two decay times for all the molecules investigated.

Effect of two-beam coupling in strong-field optical pump-probe experiments

Nonlinear optics experiments measuring phase shifts induced in a weak probe pulse by a strong pump pulse must account for coherent effects that only occur when the pump and probe pulses are temporally overlapped. It is well known that a weak probe beam experiences a greater phase shift from a strong pump beam than the pump beam induces on itself. The physical mechanism behind the enhanced phase shift is diffraction of pump light into the probe direction by a nonlinear refractive index grating produced by interference between the two beams. For an instantaneous third-order response, the effect of the grating is to simply double the probe phase shift, but when delayed nonlinearities are considered, the effect is more complex. A comprehensive treatment is given for both degenerate and nondegenerate pump-probe experiments in noble and diatomic gases. Results of numerical calculations are compared to a recent transient birefringence measurement [Loriot et al., Opt. Express 17, 13429 (2009)] and a recent spectral interferometry experiment [Wahlstrand et al., Phys. Rev. A 85, 043820 (2012)]. We also present results from two new experiments using spectrally-resolved transient birefringence with 800 nm pulses in Ar and air and degenerate chirped pulse spectral interferometry in Ar. Both experiments support the interpretation of the negative birefringence at high intensity as arising from a plasma grating.

Long intersubband relaxation times in n-type germanium quantum wells

We measured the non-radiative intersubband relaxation time in n-type modulation-doped Ge/SiGe multi-quantum wells of different thickness by means of degenerate pump-probe experiments. The photon energy was tuned to be resonant with the lowest conduction band intersubband transition energy (14-29 meV), as measured by terahertz absorption spectroscopy and in agreement with band structure calculations. Temperature-independent lifetimes in excess of 30 ps were observed.

Ultrafast Switching Time and Third Order Nonlinear Coefficients of Microwave Treated Single Walled Carbon Nanotube Suspensions

Microwave treated water soluble and amide functionalized single walled carbon nanotubes have been investigated using femtosecond degenerate pump-probe and nonlinear transmission experiments. The time resolved differential transmission using 75 femtosecond pulse with the central wavelength of 790 nm shows a bi-exponential ultrafast photo-bleaching with time constants of 160 fs (130 fs) and 920 fs (300 fs) for water soluble (amide functionalized) nanotubes. Open and closed aperture z-scans show saturation absorption and positive (negative) nonlinear refraction for water soluble (amide functionalized) nanotubes. Two photon absorption coefficient, beta0 approximately 250 cm/GW (650 cm/GW) and nonlinear index, gamma approximately 15 cm2/pW (-30 cm2/pW) are obtained from the theoretical fit in the saturation limit to the data for two types of nanotubes.

Ultrafast relaxation and optical saturation of intraband absorption of GaN/AlN quantum dots

We report on the measurements of ultrafast relaxation and absorption saturation of the s-pz intraband transition at λ=1.55 μm in GaN/AlN quantum dot superlattice. The recovery time of the intraband absorption is assessed using degenerate pump-probe experiments at room temperature. Measurements reveal a multiexponential decay of the differential transmission with an ultrafast (∼160 fs) and a slower (∼1.5 ps) time constant attributed to the excited state lifetime and to the thermalization time in the ground state, respectively. The saturation intensity of the intraband absorption (<140 MW/cm2) is one order of magnitude smaller than the value measured in nitride quantum wells.

Ultrafast Exciton Dynamics in a ZnO Thin Film

We report the behavior of utrafast exciton dynamics in a ZnO thin film based on temperature-, pump-energy-, and pump-power-dependent degenerate pump–probe experiments. When the pump energy is higher than the emission peak, either dominated by free exciton or donor-bound exciton, a Stokes shift of about 20 meV can be observed. In this situation, rapid thermalization process (in several ps) followed by an exciton relaxation stage (7 to 104 ps) is recorded. The relaxation into the donor-bound exciton level takes a longer time because a spatial transport process is required. When the pump energy is lower than the emission peak, the optical-field-induced red shift of the absorption spectrum results in an enhanced absorption for the probe can be observed. This effect recovers in several ps.

Ultrafast electron dynamics and cubic optical nonlinearity of freestanding thin film of double walled carbon nanotubes

Ultrafast degenerate pump-probe experiments performed on a freestanding film of double walled carbon nanotubes near the first metallic transition energy of outer tube show ultrafast (97 fs) photobleaching followed by a photoinduced absorption with a slow relaxation of 1.8 ps. Femtosecond closed and open aperture z-scan experiments carried out at the same excitation energy show saturation absorption and negative cubic nonlinearity. From these measurements, real and imaginary part of the third order nonlinear susceptibility are estimated to be Re(χ(3))∼−2.2×10−9 esu and Im(χ(3))∼4.5×10−11 esu.

Femtosecond excitation dynamics in gold nanospheres and nanorods

Femtosecond visible photoluminescence is detected from gold nanoparticles using time-resolved fluorescence upconversion spectroscopy. We directly compared this fast luminescence from gold nanospheres $(\ensuremath{\emptyset}25\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ with that obtained from nanorods ($\ensuremath{\emptyset}15\ifmmode\times\else\texttimes\fi{}40\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, $\ensuremath{\emptyset}15\ifmmode\times\else\texttimes\fi{}27\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$) under vis $(3.02\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ and $\mathrm{UV}(4.65\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ excitation. A fast $(\ensuremath{\sim}50\phantom{\rule{0.3em}{0ex}}\mathrm{fs})$ decay was obtained for the nanoparticles and the emission was depolarized. Degenerate femtosecond pump-probe experiments in the low excitation intensity regime demonstrated much slower electron thermalization and/or equilibration dynamics on the time scale of a few hundred femtoseconds. These features strongly indicate a $d$-hole-conduction electron recombination process as the origin of this photoluminescence. A direct comparison of the fast emission spectra from nanorods and nanospheres is used to discuss the emission enhancement mechanism. These results suggest that the classical local field enhancement theory describes quantitatively well many of the emission features of nanorods with respect to those for nanospheres without invoking more complex models.

Dilute nitride absorbers in passive devices for mode locking of solid-state lasers

We report on the growth, material and nonlinear optical properties of GaInNAs quantum wells (QWs) used in semiconductor saturable absorber mirrors (SESAMs) for passive mode locking of solid-state lasers in the telecommunication wavelength range from 1.3 to 1.55 μm. The antiresonant SESAMs were grown by molecular beam epitaxy with a nitrogen concentration of 1.6% and 2.6%, respectively. They were subject to rapid thermal annealing to fine-tune the absorption wavelength by blueshifting the photoluminescence wavelength. The appearance of QW intermixing upon thermal annealing was studied by X-ray rocking curve measurements. The thermal annealing procedure was proved not to alter the GaInNAs SESAM by QW intermixing for temperatures up to 800 °C. Optical characterization was applied to investigate the nonlinear SESAM properties. Degenerate pump-probe experiments revealed similar recovery times for both SESAMs in the tens of picoseconds range. We demonstrated stable self-starting passive cw mode locking with sub-10 ps pulses at 1314 and 1534 nm.

Ultrafast carrier dynamics in an InGaN thin film

We perform femtosecond degenerate pump-probe experiments on an InGaN thin film of 800 nm in thickness. The observed temperature-, pump-photon-energy-, and pump-intensity-dependent variations of ultrafast carrier dynamics manifest the variation of the space-averaged density of state with energy level in this sample. The carrier dynamics is controlled by the shift of effective band gap and hence the behavior of band filling, which are determined by the combined effect of band-gap renormalization and phonon effect (band-gap shrinkage with increasing temperature). Two-photon absorption and free-carrier absorption can be observed when the corresponding density of state is low and hence the band-filling effect is weak. The variation of the space-averaged density of state with energy level can be due to the existence of indium-composition-fluctuation nanostructures, which is caused by the spinodal decomposition process, in the sample.

Organometallic Complexes for Nonlinear Optics. 33. Electrochemical Switching of the Third-Order Nonlinearity Observed by Simultaneous Femtosecond Degenerate Four-Wave Mixing and Pump−Probe Measurements

The molecular inorganic compound 1,3,5-{trans-[RuCl(dppe)2]C≡C-4-C 6 H 4 C≡C} 3 C 6 H 3 (1) is reversibly oxidized in solution using an optically transparent thin-layer electrochemical (OTTLE) cell, the oxidation to 1 3 + resulting in the appearance of a strong absorption band at 11 200 cm - 1 in contrast to 1, which is optically transparent at frequencies below 20 000 cm - 1 . This linear electrochromic behavior is accompanied by third-order nonlinear optical electrochromic behavior, which has been probed by a combination of femtosecond degenerate four-wave mixing (DFWM) and pump-probe experiments at 12 500 cm - 1 . DFWM studies of 1 and 1 3 + reveal an order of magnitude increase in the hyperpolarizability, ‖γ‖, upon oxidation with ‖γ‖ values of 2000 × 10 - 3 6 esu and 20 000 x 10 - 3 6 esu, respectively; the DFWM signal of the latter is dominated by a delayed response (relaxation time ca. 1 ps). The transient absorption (TA) data for 1 reveal efficient two-photon absorption (σ 2 10 3 GM), whereas the TA data for 1 3 + reveal saturable absorption (decay time ca. 1 ps). The switching of third-order nonlinearity in the 1/1 3 + pair represents the first demonstration of femtosecond time-scale processes being responsible for nonlinear electrochromism.

Photogeneration and Quenching of Reactive Oxygen Species by Urocanic Acid

Urocanic acid, UCA, is characterized by two electronic transitions in the UV-B (280-320 nm) which comprise its broad absorption spectrum and give rise to wavelength-dependent isomerization quantum yields. The absorption spectrum of UCA extends into the UV-A (320-400 nm). Given the UV-A component of sunlight is significantly greater than the UV-B component it is hypothesized even weak UV-A photochemistry of UCA could be important for in vivo responses to UV radiation. Degenerate pump-probe experiments performed on t-UCA at several wavelengths in the UV-A reveal an excited-state absorption that undergoes a rapid, approximately 1 ps decay. Photoacoustic experiments performed on both the cis and trans isomers reveal the formation of a long-lived intermediate following UV-A excitation. The efficiency and action spectra for this latter photoactive process are presented and are similar for both isomers of UCA. Cholesterol hydroperoxide assays designed to investigate the nature of the UV-A photoreactivity of t-UCA confirm the production of reactive oxygen species. The bimolecular rate constant for the quenching of singlet oxygen by t-UCA is determined to be 3.5 x 10(6) M(-1) s(-1). Taking into consideration recent theoretical calculations and jet expansion studies of the electronic structure of gas-phase t-UCA, a model is proposed to explain the isomerization and photoreactivity of t-UCA in solution over the UV-A region.

Intermolecular hydrogen bonding in chlorine dioxide photochemistry: A time-resolved resonance Raman study

The geminate-recombination and vibrational-relaxation dynamics of chlorine dioxide (OClO) dissolved in ethanol and 2,2,2-trifluoroethanol (TFE) are investigated using time-resolved resonance Raman spectroscopy. Stokes spectra are measured as a function of time following photoexcitation using degenerate pump and probe wavelengths of 398 nm. For OClO dissolved in ethanol, subpicosecond geminate recombination occurs resulting in the reformation of ground-state OClO with a quantum yield of 0.5±0.1. Following recombination, intermolecular-vibrational relaxation of OClO occurs with a time constant of 31±10 ps. For OClO dissolved in TFE, recombination occurs with a time constant of 1.8±0.8 ps and a quantum yield of only 0.3±0.1. The intermolecular-vibrational-relaxation time constant of OClO in TFE is 79±27 ps. The reduced geminate-recombination quantum yield, delayed recombination, and slower vibrational relaxation for OClO in TFE is interpreted in terms of greater self-association of the solvent. Degenerate pump–probe experiments are also presented that demonstrate decay of the Cl-solvent charge-transfer complex on the ∼1-ns time scale in ethanol and TFE. This time is significantly longer than the abstraction times observed for other systems demonstrating that Cl hydrogen abstraction from alcohols occurs in the presence of a significant energy barrier.

The Microcavity AC Stark Triplet: Excitation in the Exciton Continuum

The coherent optical response of high quality microcavities containing quantum wells is investigated. The dynamics of the ac Stark effect is investigated by degenerate pump-probe experiments, by exciting the quantum well excitons at various intensities and detunings. The evolution from a simple dynamical excitonic Stark shift to a splitting is dearly observed, even when the excitation is degenerate with the continuum states. The experimental results agree well with the predictions of a theoretical model based on the semiconductor Maxwell-Bloch equations.

Femtosecond degenerate and nondegenerate pump-probe experiments in bulk GaAs below the band gap

We perform degenerate and nondegenerate pump-probe experiments on bulk GaAs at 100 K below the band gap. We mostly observe a negative differential transmission signal both in the degenerate and nondegenerate experiments. We interpret our signal as due to two-photon absorption. This negative signal has a different origin from the normally considered band gap renormalization for resonant excitations.

Pump-probe studies of carrier capture processes in semiconductor multiple-quantum-well waveguides

Carrier capture processes in GaAs-AlGaAs multiple-quantum-well (MQW) waveguides are monitored using the time-resolved pump-probe technique. With appropriate selections of wavelength and pulse recompression, subpicosecond pulses at various wavelengths can be obtained for both nondegenerate and degenerate pump-probe experiments with various polarization combinations. Two samples of different quantum-well structures are compared. It is found that the carrier capture times (defined as the time period to reach probe transmission maximum after the pump) are in the range from 15 to 50 ps in the samples used. In the degenerate pump-probe measurements, a two-component decay is observed for one of the samples in the case of TM pump and TE probe.

Preparation and non-linear optical properties of CdS quantum dots in Na2O-B2O3-SiO2 glasses by the sol-gel technique

Non-linear optical properties of CdS quantum dots embedded in the sol-gel derived Na2O-B2O3-SiO2x3 glass matrix have been studied using nanosecond degenerate four-wave mixing (DFWM), pump-probe experiments, and time-resolved subpicosecond DFWM measurements. The concentration of CdS microcrystallites was varied from 1.4 to 10.2 wt% while the particle size was controlled to be in the range of 3–6 nm in diameter so that the confinement effects can be realized. The third-order susceptibility, χ3, was determined to be of the order of 10−7-10−6 e.s.u. near the resonant wavelength between 450 and 470 nm by the nanosecond DFWM and pump-probe experiments, and of the order of 10−11-10−10 e.s.u. at the off-resonant wavelength, 580 nm, by a time-resolved DFWM measurement with 400 fs laser pulse. The decay time changed from 0.5 to 50 picoseconds as a function of the size, size distribution and number density of CdS microcrystallites in the glasses. Photodarkening reduced the optical non-linearity of the melt-quenched samples by a factor of ≈ 20, while it had no appreciable effect on that of the sol-gel derived samples.[/p]