Degenerate Pump-probe Measurements Research Articles

Modulating the nonlinear absorption response of SnOx thin films via phase engineering.

Phase (composition) is known to play a key role in determining the electronic and optical properties of amorphous oxide semiconductors. In this work, modulating the ultrafast nonlinear optical (NLO) response of SnO2 and SnO thin films by tuning oxygen partial pressure during film sputtering is explored. Femtosecond Z-scan results demonstrate that intermediate phases have no profound impact on the two-photon absorption (TPA) response of SnO2 and SnO films. Interestingly, the magnitude of the effective nonlinear absorption coefficient (βeff) of both intermediate SnO2-x and SnOx are enhanced after the change of Sn2+/Sn4+ composition ratio, as measured by picosecond Z-scan technique. Femtosecond degenerate pump-probe measurements show that intermediate phases accelerate the carrier trapping and improve the defect-related carrier absorption in SnOx (SnO-rich) film, while intermediate phase suppress the TPA response of SnO2-x (SnO2-rich) films, therefore carrier-induced absorption dominates the NLO behavior of SnO2-x film on picosecond regime. Our results indicate a simple and effective way to modulate the NLO response of transparent conductive oxide SnO2 and SnO.

Measurement of ultrafast carrier dynamics in multilayer MPCVD graphene

Graphene presents unique opto-electronic properties which makes it useful for a wide range of applications and devices, such as high-speed photodetectors, that rely on the relaxation dynamics of photoexcited charge carriers. These demand reliable and reproducible methods for synthesis of high quality graphene. Here we present ultrafast degenerate pump-probe measurements of multilayer graphene coatings grown by microwave plasma chemical vapour deposition (MPCVD) and analyse the impact of the synthesis growth time on the material’s nonequilibrium optical response.

Free carrier-induced optical nonlinearities in β-Ga2O3 single crystals at 355 nm

Gallium oxide (Ga2O3) is a promising candidate in future photonic integrated circuits devices due to its ultra-wide band gap and high stability. However, the nonlinear optical properties of Ga2O3 still remain elusive. In this work, we investigate the free-carrier absorption and refraction in pristine β-Ga2O3 single crystal via the degenerate pump-probe measurement with phase object technique at 355 nm wavelength. The free-carrier absorption cross-section and free-carrier refraction index of β-Ga2O3 on the sub-nanosecond time scale are determined to be ∼0.6 × 10−22 m2, and ∼−0.62 × 10−28 m3, respectively. And the two-photon absorption coefficient of β-Ga2O3 is characterized as ∼0.53 × 10−11 m W−1. It is found that the free-carrier absorption and free-carrier refraction effect in β-Ga2O3 are significantly smaller than that in other wide bandgap semiconductors such as GaN and ZnO. Our results can serve as guidelines for designing β-Ga2O3 based ultra-low loss waveguides and integrated photonic applications in UV spectral range.

Fluence and wavelength dependent ultrafast differential transmission dynamics in graphene

We performed degenerate pump-probe transmission measurements of graphene supported on glass for a range of pump fluences that enable us to observe both positive and negative deferential transmission dynamics. Our results show that at an intermediate pump fluence, where a transition from negative to positive response occurs, the differential transmission dynamics is an order of magnitude faster than at higher and lower pump fluences. This effect can be explained by equal contributions of inter- and intraband transitions with opposite signs to the transient optical conductivity of graphene at an intermediate pump fluence. Moreover, the intermediate threshold pump fluence is shown to increase with decreasing probe energy, which is in agreement with the theoretical model. Furthermore, we show that the relaxation time of the electronic temperature increases monotonically over the range of fluences studied. In perspective, this work is of importance to graphene-based opto-electronic applications such as light modulators.

Hot carrier dynamics in a dispersionless plasmonic system

Hot carrier dynamics in a broadband, dispersionless, plasmonic structure are studied using pump-probe measurements with 45 fs time resolution. The results obtained by simultaneous measurement of differential transmittance and reflectance show the influence of pump generated carriers on the probe generated plasmons. The dependence of decay time on the pump fluence is linear for the interband and intraband carrier excitation near the L-symmetry point in gold. However, 1.908 eV pump excitation, which allows both interband transition at the X-symmetry point and intraband excitation at the L-symmetry point, shows quadratic dependence of decay time on the pump fluence. Degenerate pump-probe measurements at 1.55 eV, which allow L-valley intraband excitations, highlight the difference in surface (reflection) and the bulk (transmission) mechanisms. Hot carrier relaxation time is in the 1–3 ps range for different excitation energies. Rise time, governed by the plasmon to hot carrier conversion and electron-electron scattering processes, is about 200 ± 35 fs for the hot carrier and hot plasmon excitation cases that increased to about 485 ± 35 fs when the pump is resonant with the interband transition at the X-symmetry point. Results show that the hot carrier dynamics are governed by the bulk metal band structure. The dipole matrix element for each of the transitions is estimated by density matrix calculations.

Ultrafast laser pulse chirp effects on laser-generated nanoacoustic strains in Silicon

Nanoacoustic strains are generated in Silicon by chirped femtosecond laser pulses using thin Titanium films as transducers. We investigate the effect that the generating laser pulse chirp has on the amplitude of the induced strains, manifested as Brillouin oscillations observed in degenerate femtosecond pump-probe transient reflectivity measurements. The strain amplitude is larger when negatively chirped pulses are used, which is attributed to the more efficient conversion of laser pulse light into acoustic strain in the Titanium transducer. Our present studies clearly show that the dependence of the Brillouin amplitude and the lattice strain is a non-monotonous function of the laser chirp parameter. An optimum negative laser pulse chirp is found for which the strain amplitude is maximized. A detailed thermomechanical model satisfactorily supports the experimental findings. In such a way, it is possible to suppress or enhance the induced nanoacoustic strain amplitude, thus all-optically controlling it by at least a factor of two.

Impact of stoichiometry on the linear and nonlinear optical response of SnOx thin films

SnO is a promising p-type oxide semiconductor materials for applications such as transparent electronics and solar cells. However, further improvement of its performance is hindered by its diverse stoichiometry. We investigated the nonlinear and saturable absorption characteristics of pristine SnO and O-rich SnOx films by femtosecond degenerate pump-probe measurements at 515 nm. UV-Vis absorption data indicate bandgap blueshift with increasing oxygen concentration. Pristine SnO film exhibit saturable absorption while nonlinear absorption is observed in O-rich SnOx films. Our results shed light on the utilization of SnO in future device applications.

Controlling nanoscale acoustic strains in silicon using chirped femtosecond laser pulses

The influence of femtosecond laser pulse chirp on laser-generated longitudinal acoustic strains in Si (100) monocrystal substrates is studied. Degenerate femtosecond pump-probe transient reflectivity measurements are performed using a layered structure of thin Ti transducer film on an Si substrate. Experimental results show that acoustic strains, manifested as strong Brillouin oscillations, are more effectively induced when negatively chirped femtosecond laser pulses pump the transducer. These results are theoretically supported by a modified thermo-mechanical model based on the combination of a revised two-temperature model and elasticity theory that takes into account the instantaneous frequency of the chirped femtosecond laser pump pulses.

Nonlinear absorption and excited state dynamics of porphyrin and phthalocyanine in the presence of explosive molecules

Nonlinear absorption (NLA) properties and excited state dynamics of a porphyrin and phthalocyanine in the presence of explosive molecules were investigated using the nanosecond Z-scan and femtosecond pump-probe techniques, respectively. The NLA coefficients Is and β increased in the presence of explosive molecules. The change in first excited state decay time was evaluated through degenerate pump-probe measurements near 700nm and it was observed that decay time decreased in the presence of explosive molecules. The reduction in decay time and increase in NLA coefficients varied with different molecules according to their absorption, emission properties and affinity towards the explosive molecule.

High acoustic strains in Si through ultrafast laser excitation of Ti thin-film transducers.

The role of thin-film metal transducers in ultrafast laser-generated longitudinal acoustic phonons in Si (100) monocrystal substrates is investigated. For this purpose degenerate femtosecond pump-probe transient reflectivity measurements are performed probing the Brillouin scattering of laser photons from phonons. The influence of the metallic electron-phonon coupling factor, acoustical impedance and film thickness is examined. An optical transfer matrix method for thin films is applied to extract the net acoustic strain relative strength for the various transducer cases, taking into account the experimental probing efficiency. In addition, a theoretical thermo-mechanical approach based on the combination of a revised two-temperature model and elasticity theory is applied and supports the experimental findings. The results show highly efficient generation of acoustic phonons in Si when Ti transducers are used. This demonstrates the crucial role of the transducer's high electron-phonon coupling constant and high compressive yield strength, as well as strong acoustical impedance matching with the semiconductor substrate.

GaAs在光通讯波段的超快非线性光学研究

We report experimental investigations of nonlinear dynamics and optical nonlinearities of GaA s single crystal in the telecommunication windows. Femtosecond time-resolved degenerate pump-probe measurements indicate that the observed third-order nonlinear process originates from the instantaneous effect whereas the fifth-order effect arises from two-photon absorption( 2PA) induced free-carrier nonlinearities. By performing the Z-scan experiments with femtosecond laser pulses at the wavelengths of 1 300 nm and 1 500 nm,we determine all nonlinear parameters of GaA s crystal,including 2PA coefficient,third-order nonlinear refraction index,2PA-induced free-carrier absorption cross section,and 2PA-induced free-carrier refraction cross section. These results suggest that GaA s crystal is a promising candidate for applications on optical limiting and photodetector at the telecommunication wavelengths.

Ultrafast dynamics of free carriers induced by two-photon excitation in bulk ZnSe crystal

Semiconductor materials exhibiting large optical nonlinearities and ultrafast nonlinear response have received extensive attention because of their potential applications in optical limiting, all-optical devices, optical telecommunication, and so on. As a direct-gap II-VI bulk semiconductor, ZnSe crystal has been exploited as the nonlinear optical devices in the regimes of nanoseconds and picoseconds during the past years. Owing to today's fast advance of laser sources with ultrashort femtosecond pulse duration, it is possible to investigate the ultrafast optical nonlinearities in the bulk ZnSe crystal. In this paper, we experimentally investigate the ultrafast dynamics of free-carriers induced by twophoton excitation in the bulk ZnSe crystal. By performing open-aperture Z-scan experiments with 41 fs laser pulses at the wavelength of 532 nm under the condition of low excitation intensity, the two-photon absorption coefficient is measured. As the excitation intensity exceeds a critical value, the interplay between third- and fifth-order nonlinear absorption processes is observed. To evaluate the ultrafast dynamics of free carriers, we have carried out femtosecond time-resolved degenerate pump-probe measurements with the same laser system used for Z-scan experiments in different levels of pump intensities. It is shown that the transient absorption signals peaked at the zero delay is a linearly increasing function of pump intensity, indicating that the observed instantaneous nonlinear absorption is dominated by the interband two-photon absorption process. At moderate irradiance, the transient absorption signals obviously indicate two components, arising from the two-photon absorption-induced free-carrier absorption, which is equivalent to the fifth-order nonlinear absorption process. Under the excitation of relatively high pump intensity, the magnitude of the reduction of free-carrier absorption signal becomes faster, suggesting that the ZnSe crystal exhibits a new effect and causes a transmittance change of the probe light. The presumable reasons are as follows: intense irradiances will result in the increase of carrier concentration and the rise of the lattice temperature as well as the narrowing of the band gap in the ZnSe crystal, which accelerates the electron-hole interband recombination process. Accordingly, the electron-hole recombination time decreases. Furthermore, when the carrier concentration is larger than 1018 cm-3, the occurrence of the electron-hole plasma is significant. At the same time, the enhancement of the scattering among the carriers results in the reduction of the free carrier absorption cross section. In summary, it is found that the free-carrier absorption cross section decreases whereas the electron-hole recombination time becomes shorter in ZnSe crystal as the excitation intensity increases, owing to both the narrowing of band gap and the occurrence of electron-hole plasma.

Microscopic description of intraband absorption in graphene: the occurrence of transient negative differential transmission.

We present a microscopic explanation of the controversially discussed transient negative differential transmission observed in degenerate optical pump-probe measurements in graphene. Our approach is based on the density matrix formalism allowing a time- and momentum-resolved study of carrier-light, carrier-carrier, and carrier-phonon interaction on microscopic footing. We show that phonon-assisted optical intraband transitions give rise to transient absorption in the optically excited hot carrier system counteracting pure absorption bleaching of interband transitions. While interband transition bleaching is relevant in the first hundreds of fs after the excitation, intraband absorption sets in at later times. In particular, in the low excitation regime, these intraband absorption processes prevail over the absorption bleaching resulting in a zero crossing of the differential transmission. Our findings are in good qualitative agreement with recent experimental pump-probe studies.

Double walled carbon nanotubes as ultrafast optical switches

Pristine and molybdenum filled double walled carbon nanotubes (DWNTs) suspended in D2O show excellent ultrafast optical switching properties investigated through femtosecond Z-scan and degenerate pump-probe method using 50 fs pulses with central photon energy of 1.57 eV. For pristine-DWNT, the two photon absorption coefficient, β and nonlinear refraction coefficient, n2 are 4.9×10−8 cm/W, and 9.5×10−11 cm2/W, respectively, which yield one photon figure of merit, W=133 and two photon figure of merit, T=0.4. The degenerate pump-probe measurements show strong photoinduced bleaching with biexponential decay with time constants ∼150 and 600 fs.

Nonlinear optical properties of 2,4,5-Trimethoxy-4ʹ-nitrochalcone: observation of two-photon-induced excited-state nonlinearities

We report experimental investigations of optical nonlinearities and nonlinear dynamics in acetone solution of 2,4,5-Trimethoxy-4- nitrochalcone. By performing Z-scans with femtosecond laser pulses at low excitation intensity, two-photon absorption (2PA) and third-order nonlinear refraction are measured. As laser excitation intensity exceeds a critical value, however, the interplay between third- and fifth-order nonlinearities is observed. It is also confirmed that fifth-order processes mainly originate from 2PA-induced excited-state nonlinearities by conducting femtosecond time-resolved degenerate pump-probe measurements. All the nonlinear parameters are determined unambiguously in the near infrared region of the 2PA cross-section, second-order hyperpolarizability, excited-state absorption cross-section, excited-state refraction cross-section, lifetime of excited states induced by 2PA, and critical population of the excited states in 2,4,5-Trimethoxy-4-nitrochalcone molecule.

Auger recombination and state filling of resonantly excited ground state in CdSe quantum dots

Dynamics and state filling of the resonantly excited ground state (1se-1S3∕2h) in a strongly confined CdSe quantum dot were investigated by degenerate pump-probe measurements. With increasing the electron-hole (e-h) pairs per dot the state filling and bleaching were observed with ∼2.7 e-h pairs. While radiative recombination was dominant with a small number of the e-h pairs (⪡1), the Auger-type recombination became significant near the bleaching number of e-h pairs (∼2.7). This result suggests that, for resonant excitation, either cold electron or hole of the ground state is scattered into the higher-energy states of a ZnS shell via Auger recombination process (∼28ps).

Gain dynamics and excitonic transition in CdSe colloidal quantum dots

Dynamics of the photo-excited excitons in CdSe colloidal quantum dots was investigated by degenerate pump-probe measurement. Under resonant excitation, the lowest energy state of an electron-hole pair dominates two-step decay. The fast initial decay and long-term decay were attributed to the intrinsic Auger recombination and radiative recombination, respectively. As increasing the excited exciton density, a significant change of the later decay was seen before the absorption bleaching. As our nanocrystal is the case of strong confinement, in which an excitonic Bohr radius is smaller than the dot size, the biexciton-involved recombination is suggested to dominate at high carrier densities. Optical gain was measured by the variable stripe length method and broad gain spectrum was obtained due to the recombination of many excited levels between biexciton and exciton states. This result also supports biexcitons play an important role in the strong confinement regime at high carrier densities.

Dynamics and Bleaching of Ground State in CdSe/ZnS Quantum Dots

For resonant excitation of the ground state $1s^e-1S^h_{3/2}$ , dynamics of 'the electron-hole pair in a CdSe quantum dot was investigated by degenerate pump-probe measurement. At low e-h pair densities, the decay of $1s^e-1S^h_{3/2}$ state is dominated by radiative recombination. As the number of the electron-hole pairs increases, new decay features become significant. Theoretical comparison suggests this is attributed to the bi-molecular and Auger-type scattering.

Comment on “Large Optical Nonlinearity of Semiconducting Single-Walled Carbon Nanotubes under Resonant Excitations”

We measured third-order nonlinear susceptibility (chi(3)) spectra in semiconducting single-walled carbon nanotubes (SWNTs) by the Z-scan method. |Imchi(3)| is remarkably enhanced under resonant excitation to the lowest interband transition, reaching 4.2 x 10(-6) esu and 1.5 x 10(-7) esu in SWNTs grown by the laser ablation and HiPco methods, respectively. A comparison of the transient absorption changes evaluated by degenerate and nondegenerate pump-probe measurements suggests that the resonant enhancement of |Imchi(3)| is dominated by a coherent process rather than by saturation of absorption.

Picosecond carrier dynamics in AlInGaN multiple quantum wells

We report on measurements of stimulated emission in AlInGaN multiquantum wells using ultrafast pump-probe spectroscopy. Low power photoluminescence (PL) of Al0.04In0.015Ga0.945N∕Al0.12In0.01Ga0.87N multiquantum wells (MQW) yields a single MQW PL peak at 347.2 nm at 10 K. Wavelength degenerate pump-probe measurements show a single exponential decay at excitation densities below the stimulated emission threshold having a recombination decay constant of 160 ps at room temperature. At excitation densities greater than 50μJ∕cm2 a biexponential decay is observed, with a fast component (3.3 ps at Iexc=690μJ∕cm2) attributed to accelerated relaxation due to stimulated emission. The fractional strength of this fast component indicates that up to 85% of the excited carriers relax via stimulated emission.