Two-color Pump-probe Technique Research Articles

Magnetic polarons reach a hundred thousand Bohr magnetons.

The ability of light to manipulate fundamental interactions in a medium is central to research in optomagnetism and applications in electronics. A prospective approach is to create composite quasiparticles, magnetic polarons, highly susceptible to external stimuli. To control magnetic and transport properties by weak magnetic and electric fields, it is important to find materials that support photoinduced magnetic polarons with colossal net magnetic moments. Here, we demonstrate that magnetic polarons with a record-high magnetic moment, reaching and exceeding a hundred thousand Bohr magnetons, can be optically generated in EuO, an archetypal ferromagnetic semiconductor. The phenomenon is established employing the photoinduced Faraday effect studied in EuO films by a two-color pump-probe technique. The giant magnetic polarons are generated just above the Curie temperature once EuO is exposed to photons of an energy exceeding the bandgap. Picosecond temporal dynamics of magnetic polarons follows relaxation processes in the spin-split 5d conduction band occupied by the photoexcited electron. The study is expected to provide a platform for implementation of an efficient optical control over the magnetic state in solids.

Picosecond Lifetime Hot Electrons in TiO2 Nanoparticles for High Catalytic Activity

A large number of studies have examined the origins of high-catalytic activities of nanoparticles, but very few have discussed the lifetime of high-energy electrons in nanoparticles. The lifetime is one of the factors determining electron transfer and thus catalytic activity. Much of the lifetime of electrons reported in the literature is too short for a high transfer-efficiency of photo-excited electrons from a catalyst to the attached molecules. We observed TiO2 nanoparticles using the femtosecond laser two-color pump-probe technique with photoemission electron microscopy having a 40 nm spatial resolution. A lifetime longer than 4 ps was observed together with a fast decay component of 100 fs time constant when excited by a 760 nm laser. The slow decay component was observed only when the electrons in an intermediate state pumped by the fundamental laser pulse were excited by the second harmonic pulse. The electronic structure for the asymmetry of the pump-probe signal and the origin of the two decay components are discussed based on the color center model of the oxygen vacancy.

Synthesis, structure and femtosecond nonlinear absorption properties of Ce-ZnO films

In this work, Ce-ZnO (CZO) films with excellent nonlinear absorption (NA) properties were successfully prepared via the magnetron sputtering process, where the conditions depended morphology were characterized. The cluster surface appeared with the increased sputtering time at low power, and disappeared at high power. Energy dispersive spectrometer and X-ray photoelectron spectroscopy analysis confirmed the presence of Ce ions into ZnO lattice. The functional groups, optical bandgap value and NA properties were revealed by Raman spectroscopy studies, UV–Visible absorption spectra and Z-scan technique, respectively. By adjusting the direct current sputtering power and excitation intensities, the NA mechanism of samples was converted from saturable absorption (SA) to reverse saturable absorption (RSA) behavior. Under different laser excitation intensities, the NA mechanism of samples was that TPA induced RSA. Furthermore, the results show that the order of magnitude of the NA coefficient for samples is ~10−8 m/W. In addition, the ultrafast carrier dynamics was studied using two-color pump-probe technique. When excited with 380 and 400 nm pumps and probed over a broad range in the visible region, the biexponential decay of positive signal is same to the mechanism of pump-induced deep-level or excited state absorption. CZO films will be promising candidate for developing optoelectronic devices.

Ultrafast electron hole plasma dynamics in chemically pristine and Ag-doped ZnO nanorods

The ultrafast dynamics of femotosecond laser excited carriers in the assembly of chemically pristine and Ag-doped ZnO nanorods is studied using the two-color pump-probe spectroscopy technique with the central photon energy of the pump pulse equal to the bandgap of the nanorods ensemble (∼3.14 eV) and the central photon energy of the probe pulse kept at 1.57 eV. On photo-excitation at 3.14 eV with an excited carrier density of ∼1020cm−3, about an order higher than the Mott density of bulk ZnO, the time delayed probe transmission displays tri-exponential decay exhibiting three decay mechanisms: (i) ultrafast electron-phonon thermalization of the order of ∼0.40 ps, (ii) trap mediated decay with a time constant of ∼9 ps, and (iii) e-h recombination with a time constant of ∼650 ps. It is observed that as the carrier density increases, the first two decay processes get longer on contrary to the recombination time which gets shorter. A detailed analysis of the faster decay process through the Two Temperature Model of ultrafast carrier relaxation supports our observation. Doping of Ag in these nanorods is found to introduce new defect states which alter the carrier dynamics significantly.

Nonlinear absorption properties and excited-state charge-transfer dynamics of Er doped ZnO films

This work reports on the nonlinear optical properties of Er doped ZnO (EZO) films, measured by the Z-scan technique with different laser parameters (pulse width, wavelengths and energy). The nonlinear absorption mechanism of EZO films is analyzed by a singlet state three and four-level model, respectively. The effect of the direct current sputtering power of EZO films on the nonlinear optical properties was studied. From the results, the samples show the self-focusing effect and two photon absorption (TPA) induced excited state reverse saturable absorption (RSA) behavior. Under the different laser parameter excitation, it has been determined that the TPA induced saturable absorption (SA) or RSA properties for EZO films with 8 W dc sputtering power. Moreover, these findings indicate that free carrier density increases with Er doping and this behavior leads to excited state absorption. In addition, the ultrafast dynamics of EZO films have been investigated using the two-color pump-probe (TCPP) technique with 325, 380 and 400 nm wavelength excitation and probing over a broad range in the visible region. The decay of the positive signal is found to be biexponential, which we have assigned to the pump-induced deep-level (DL) or excited state absorption. We also present experimental results that the few decade picosecond component has been assigned to vibrational relaxation in the excited electronic states, and the slow components represent the decay from singlet excited state to the ground state ( 1.2 ns). Moreover, these findings indicate that the excitation wavelength increases and the detection wave band widens. Our results show that EZO films are a promising candidate in further optoelectronic device applications.

Temporal-spatial measurement of electron relaxation time in femtosecond laser induced plasma using two-color pump-probe imaging technique

The femtosecond (fs) laser is a powerful tool to study ultrafast plasma dynamics, especially electron relaxation in strong ionization of dielectrics. Herein, temporal-spatial evolution of femtosecond laser induced plasma in fused silica was investigated using a two-color pump-probe technique (i.e., 400 nm and 800 nm, respectively). We demonstrated that when ionized electron density is lower than the critical density, free electron relaxation time is inversely proportional to electron density, which can be explained by the electron-ion scattering regime. In addition, electron density evolution within plasma was analyzed in an early stage (first 800 fs) of the laser-material interaction.

The photoluminescence, ultrafast nonlinear optical properties and carrier dynamics of 1D In-doped ZnO nanostructures: Experiment and mechanism

1D In-doped ZnO (IZO) nanostructures with different morphologies, namely nanowires, nanoseries(serial), nanorods and nanoprisms, have been successfully synthesized by chemical vapour deposition method. The structural, morphology and optical properties of IZO nanowires have been characterized. The ultrafast nonlinear optical and carrier dynamics characteristics of IZO were studied using femtosecond Z-scan, time-resolved photoluminescence (TRPL) and two-color pump-probe (TCPP) technique, respectively. The TRPL showed that the luminescence decay lifetimes of about several 10 ps and nanoseconds time response scale can be attributed to the shallow trap (ST) state, and deep trap (DT) state emissions, respectively. The mechanisms of the TRPL behavior in the different emission band were analyzed. The result of TCPP experiment showed that the bleach recovery with lifetimes on the fast (100–200 ps), and slow (>1 ns) time scale, caused by the shallow donor mixed with donor bound recombination, and donor−acceptor pair recombination (DAP), respectively. The optical transition process in the IZO nanostructures has demonstrated that the energy transfer process from the host electron-hole pairs can efficiently excite the metal ions. Comparing with the results obtained from the TRPL experiment, we found the fast decay process was resulted from the exciton migration and the slow decay to the recombination process of the relaxed excitons. Our results show that IZO nanostructures are a promising candidate in further optoelectronic device applications.

Phonon-assisted population inversion of a single InGaAs/GaAs quantum dot by pulsed laser excitation.

We demonstrate a new method to realize the population inversion of a single InGaAs/GaAs quantum dot excited by a laser pulse tuned within the neutral exciton phonon sideband. In contrast to the conventional method of inverting a two-level system by performing coherent Rabi oscillation, the inversion is achieved by rapid thermalization of the optically dressed states via incoherent phonon-assisted relaxation. A maximum exciton population of 0.67±0.06 is measured for a laser tuned 0.83meV to higher energy. Furthermore, the phonon sideband is mapped using a two-color pump-probe technique, with its spectral form and magnitude in very good agreement with the result of path-integral calculations.

Heterodyne pump probe measurements of nonlinear dynamics in an indium phosphide photonic crystal cavity

Using a sensitive two-color heterodyne pump-probe technique, we investigate the carrier dynamics of an InP photonic crystal nanocavity. The heterodyne technique provides unambiguous results for all wavelength configurations, including the degenerate case, which cannot be investigated with the widely used homodyne technique. A model based on coupled mode theory including two carrier distributions is introduced to account for the relaxation dynamics, which is assumed to be governed by both diffusion and recombination.

Coherent acoustic phonon generation and detection by femtosecond laser pulses in ZnTe single crystals

We report experiments of coherent acoustic phonon generations and detections using a two-color femtosecond pump-probe technique in [001] and [110] oriented ZnTe single crystals. The above-bandgap pump excitation initiates strain waves to yield coherent acoustic phonons. The generation and propagation of coherent acoustic waves not only modulate the intensity of the reflected probe light but also change its polarization periodically due to strain-induced birefringence. The nonuniform excitation due to Gaussian spatial distribution of pump pulses takes an important role in the polarimetric detection of coherent acoustic phonon oscillations.

Nonlinear Optical Properties and Ultrafast Dynamics of Undoped and Doped Bulk SiC

Ultrafast third-order nonlinear optical response of bulk 6H-SiC undoped and doped with different nitrogen concentrations are investigated utilizing femtosecond Z-scan and optical Kerr effect (OKE) techniques at the wavelength of 800nm. The Z-scan measurement shows that the third-order nonlinear optical susceptibilities of the doped samples are improved in comparison to the intrinsic sample. The OKE results additionally reveal that the instantaneous nonlinear optical response of the samples can be ascribed to the distortion of the electron cloud. The ultrafast transient spectroscopic measurements with the one-color and two-color pump-probe techniques demonstrate that the ultrafast recovery process in subpicosecond domain is induced by two-photon absorption process, while the slow relaxation component reflects the carrier dynamics of the excited electrons.

Revealing Anharmonic Couplings and Energy Relaxation in DNA Oligomers by Ultrafast Infrared Spectroscopy

The identification and characterization of NH 2 hydrogen-bonded stretching vibrations [nu(NH 2)] in DNA oligomers is usually hampered by the all-dominating absorption of the water stretching band in the spectral range of 3050-3600 cm(-1). Here, we use the two-color IR pump-probe technique to overcome the limitations of linear absorption spectroscopy by exciting adenine-thymine (A-T) oligomer vibrations in the fingerprint region and analyzing induced transient spectral changes in the nu(NH2) spectral region. These transient changes are related to anharmonic couplings to the modes excited in the fingerprint region and to modes populated by intra- and intermolecular energy redistribution and relaxation. The combination of calculated anharmonic coupling parameters and experimental transient IR data allows the assignment of a transition at 3215 cm(-1) to the nu(NH2) vibration of adenine in dA(20)-dT(20) DNA oligomers.

Direct Observation of Unstable Intermediate Species in the Reaction of trans-2-Butene on Ferrierite Zeolite by Picosecond Infrared Laser Spectroscopy

The reaction dynamics of trans-2-butene adsorbed to acidic hydroxyl groups on the surface of ferrierite zeolite is examined by time-resolved spectroscopy using a tunable infrared picosecond pulse laser system. The transient absorption spectra measured by a two-color pump-probe technique at 188-243 K reveal bleaching and hot bands of the OD stretching mode 2 ps after excitation. This vibrationally excited state relaxes within 20 ps at 188 K, while the bleaching band includes a long-lifetime component that lasts for more than 100 ps at 243 K. Thus, the OD (isotope-exchanged hydroxy groups) stretching band does not entirely recover in this period and is mirrored by an analogous weakening of the CH bending band of the adsorbed trans-2-butene. Simultaneously, three new bands in CH stretching region were observed at 3045, 3095, and 3130 cm(-1). This result suggests the presence of a short-lived intermediate formed by reaction between the acidic hydroxyl groups and adsorbed trans-2-butene.

Measurement of self- and cross-gain saturation dynamics using two-color heterodyne pump-probe technique

The gain dynamics of a symmetrical multiple-quantum-well semiconductor optical amplifier (SOA) with wide gain-bandwidth are measured by a two-color heterodyne pump-probe technique. Short optical pulses are generated by using optical filters to select spectral components from compressed supercontinuum optical pulses. To maintain durations of the order of 100 fs over the entire gain bandwidth of the SOA, the pulse broadening caused by frequency shifters is compensated by fibers with lengths optimized for respective wavelengths. We measure the wavelength dependences of the recovery time of carrier heating (CH) and the gain reduction due to CH. The gain reduction caused by spectral hole burning is observed.

Intersubband relaxation dynamics inGaN∕AlNmultiple quantum wells studied by two-color pump-probe experiments

We have investigated intersubband relaxation in $\mathrm{Ga}\mathrm{N}∕\mathrm{Al}\mathrm{N}$ multiple quantum wells using a two-color pump-probe technique in a wide energy range around $800\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ $(1.55\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m})$. We have observed not only absorption bleaching signals but also, for the first time, induced absorption signals with an ultrafast and a slow component. Absorption bleaching signals are attributed to a phase space filling of the upper subband and a carrier cooling process in the lower subband, with relaxation times of $140\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$ and $300--400\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$, respectively. The ultrafast induced absorption corresponds to the absorption of electrons during intra-subband relaxation and the slow component of $1.3\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$ corresponds to the carrier cooling process. We have also found that the intrinsic absorption width of highly doped materials is as large as the inhomogeneous width, because the dispersion difference of subbands is enhanced by exchange interaction.

Lateral coupling of InxGa1−xAs∕GaAs quantum dots investigated using differential transmission spectroscopy

Coupling between quantum dots is investigated using differential transmission spectroscopy. Two-color pump-probe techniques are used to spectrally resolve the carrier dynamics, revealing carrier transfer between quantum dots at room temperature. The time constant for this process is shown to increase from at room temperature to at .

Measurement of Ultrafast Cross-Gain Saturation Dynamics of a Semiconductor Optical Amplifier Using Two-Color Pump-Probe Technique

The gain dynamics of a multiple-quantum-well semiconductor optical amplifier with wide gain-bandwidth are measured by a two-color pump-probe technique using optical pulses with a pulse duration on the order of 100 fs. The short optical pulses are generated by selecting narrow spectral components from compressed supercontinuum optical pulses using optical filters.

Cooperative Phenomena in Two-pulse, Two-color Laser Photocoagulation of Cutaneous Blood Vessels¶

A novel laser system has been developed to study the effects of multiple laser pulses of differing wavelengths on cutaneous blood vessels in vivo, using the hamster dorsal skin flap preparation and in vitro, using cuvettes of whole or diluted blood. The system permits sequenced irradiation with well-defined intrapulse spacing at 532 nm, using a long-pulse frequency-doubled Nd:YAG laser, and at 1064 nm, using a long-pulse Nd:YAG laser. Using this system, we have identified a parameter space where two pulses of different wavelengths act in a synergistic manner to effect permanent vessel damage at radiant exposures where the two pulses individually have little or no effect. Using a two-color pump–probe technique in vitro, we have identified a phenomenon we call green-light–induced infrared absorption, where a pulse of green light causes photochemical and photothermal modifications to the chemical constituents of blood and results in enhanced infrared absorption. We identify a new chemical species, met-hemoglobin, not normally present in healthy human blood but formed during laser photocoagulation which we believe is implicated in the enhanced near-infrared absorption.

Cooperative phenomena in two-pulse, two-color laser photocoagulation of cutaneous blood vessels.

A novel laser system has been developed to study the effects of multiple laser pulses of differing wavelengths on cutaneous blood vessels in vivo, using the hamster dorsal skin flap preparation and in vitro, using cuvettes of whole or diluted blood. The system permits sequenced irradiation with well-defined intrapulse spacing at 532 nm, using a long-pulse frequency-doubled Nd:YAG laser, and at 1064 nm, using a long-pulse Nd:YAG laser. Using this system, we have identified a parameter space where two pulses of different wavelengths act in a synergistic manner to effect permanent vessel damage at radiant exposures where the two pulses individually have little or no effect. Using a two-color pump-probe technique in vitro, we have identified a phenomenon we call greenlight-induced infrared absorption, where a pulse of green light causes photochemical and photothermal modifications to the chemical constituents of blood and results in enhanced infrared absorption. We identify a new chemical species, met-hemoglobin, not normally present in healthy human blood but formed during laser photocoagulation which we believe is implicated in the enhanced near-infrared absorption.

Pump-probe measurement of ultrafast all-optical modulation based on intersubband transition in n-doped quantum wells

Modulation of interband-resonant light (∼800 nm) by intersubband-resonant light (5–7 μm) was investigated in n-doped AlGaAs/GaAs multiple quantum wells by a two-color femtosecond pump-probe technique. Modulation with a recovery time of ∼1 ps is observed in a plainer-type modulation device at room temperature. The modulation of interband absorption coefficient is ∼1000 cm−1 when the energy density of the intersubband light pulse is ∼4 fJ/μm2. The modulation efficiency indicates that 99% modulation can be achieved with a control pulse energy of ∼1 pJ when a conventional waveguide-type device structure is utilized. The mechanism which determines the modulation speed is discussed in terms of carrier relaxation process. It is shown that the modulation speed is mainly determined by the inter- and intrasubband relaxation times, where the latter is influenced by hot phonon effects.