All-optical Pump-probe Technique Research Articles

Sub-gigahertz bandwidth all-optical tunable differentiator with high-energy efficiency based on a micro-ring resonator.

We propose and demonstrate a tunable fractional-order photonic differentiator (DIFF) that can process input pulses with a sub-gigahertz bandwidth. Our scheme utilizes the self-induced optical modulation effect observed in a silicon-on-insulator micro-ring resonator. Gaussian-like pulses with varying pulse widths between 7.5 and 20 ns are employed for differentiation, achieving an energy efficiency over 45%, to the best of our knowledge, which surpasses all previously reported schemes for input pulses with a sub-gigahertz bandwidth. We simulate the temporal dynamics of pulses to gain insight into the physical mechanisms underlying the differentiated outputs and provide a method for differentiation order adjustment, which is experimentally realized using an all-optical pump-probe technique.

Investigation of ultrafast demagnetization and Gilbert damping and their correlation in different ferromagnetic thin films grown under identical conditions

Following the demonstration of laser-induced ultrafast demagnetization in ferromagnetic nickel, several theoretical and phenomenological propositions have sought to uncover its underlying physics. In this work we revisit the three temperature model (3TM) and the microscopic three temperature model (M3TM) to perform a comparative analysis of ultrafast demagnetization in 20 nm thick cobalt, nickel and permalloy thin films measured using an all-optical pump-probe technique. In addition to the ultrafast dynamics at the femtosecond timescales, the nanosecond magnetization precession and damping are recorded at various pump excitation fluences revealing a fluence-dependent enhancement in both the demagnetization times and the damping factors. We confirm that the Curie temperature to magnetic moment ratio of a given system acts as a figure of merit for the demagnetization time, while the demagnetization times and damping factors show an apparent sensitivity to the density of states at the Fermi level for a given system. Further, from numerical simulations of the ultrafast demagnetization based on both the 3TM and the M3TM, we extract the reservoir coupling parameters that best reproduce the experimental data and estimate the value of the spin flip scattering probability for each system. We discuss how the fluence-dependence of inter-reservoir coupling parameters so extracted may reflect a role played by nonthermal electrons in the magnetization dynamics at low laser fluences.

All-optical detection and evaluation of magnetic damping in synthetic antiferromagnet

Synthetic antiferromagnets (SyAFs), which consist of a thin nonmagnetic spacer sandwiched by two nanolayer ferromagnets with antiferromagnetic coupling, are promising artificial magnets for spintronic memory and have attracted attention for use in future ultrafast spintronics devices. Here, we report an observation of the magnetization dynamics in a SyAF with nearly antiparallel magnetizations using an all-optical pump-probe technique. High- and low-frequency precessional dynamics of the SyAF were clearly observed. The damping of both modes was explained theoretically in terms of the dynamic exchange coupling induced by the spin current.

Probing nonlinear magnetization dynamics in Fe/MgO(001) film by all optical pump-probe technique

An all-optical pump-probe technique has been employed to investigate the nonlinear magnetization dynamics of a 10 nm Fe/MgO(001) thin film in time domain. The magnetization precession was excited by pump-laser pulses and modulated by laser fluence variations. With increasing the laser fluence up to 7.1 mJ/cm2, in addition to the uniform precession mode, a second harmonic signal was detected. The time evolution of the second harmonic signal was obtained in time-frequency domain. Based on the Landau-Lifshitz-Gilbert equation, the numerical simulation was performed to reproduce the observed the frequency doubling behaviors in Fe/MgO(001) film.

Precessional switching by ultrashort pulse laser: Beyond room temperature ferromagnetic resonance limit

Conventional pure magnetic recording schemes have a serious and unavoidable problem known as the ferromagnetic resonance (FMR) limit. Here we demonstrate a fast precessional switching via interplay of ultrafast heating by ultrashort pulse laser and large temperature dependency of magnetic resonance. In ferrimagnetic GdFeCo, when the temperature approaches the angular momentum compensation point TA, both frequency of FMR mode and the Gilbert damping parameter increase significantly. High-speed and strongly damped precessional switching was triggered with ultrafast heating of a Gd24.5Fe66.1Co9.4 across its magnetization compensation point TM up to TA, under a static applied magnetic field. To initiate and investigate the magnetization reversal, we have used an all-optical pump-probe technique employing an amplified Ti:sapphire laser system with 90 fs pulses. In particular, following the laser excitation with low pump fluence (0.9 mJ/cm2) the magnetic system relaxes back toward initial state while the rather high pump fluence (3.3 mJ/cm2) excitation induces a metastable opposite magnetic state. After the sudden heating which causes just 30% reduction of magnetization at first breakdown, consequently magnetization was started to rotate across Mz = 0 within 6 ps during first precession and finished the high damped precessional motion within few cycles into opposite direction.

Gilbert damping in perpendicularly magnetized Pt/Co/Pt films investigated by all-optical pump-probe technique

To investigate the correlation between perpendicular magnetic anisotropy and intrinsic Gilbert damping, time-resolved magneto-optical Kerr effect was measured in Pt/Co(dCo)/Pt films. These films showed perpendicular magnetization at dCo=1.0 nm and a perpendicular magnetic anisotropy energy Kueff that was inversely proportional to dCo. With an analysis based on the Landau–Lifshitz–Gilbert equation, the intrinsic Gilbert damping constant α was evaluated by parameter-fitting of frequency and lifetime expressions to experimental data of angular variations in spin precession frequency and life-times. The α values increased significantly with decreasing dCo but not inversely proportional to dCo.

Ultrafast demagnetization for Ni80Fe20 and half-metallic Co2MnSi heusler alloy films

We investigated ultrafast demagnetization for NM/Ni80Fe20 (Py)/NM (NM=Ta,Pt) and epitaxial half-metallic Co2MnSi (CMS) films using an all-optical pump-probe technique to clarify the correlation between demagnetization time τM and magnetic damping constant α or spin polarization. The signal from the all-optical time-resolved magneto-optical Kerr effect exhibited rapid decrease in the sub-ps time regime and damped oscillations for these films. Values of τM and α were evaluated using the three- temperature model and the Landau-Lifshitz-Gilbert equation. The α values for the NM/Py/NM films depended on both the Py thickness and NM materials while τM was almost constant. The τM values for the epitaxial CMS films were almost independent of L21-ordering and a little shorter than those for NM/Py/NM films.

Ultrafast spin demagnetization by nonthermal electrons of TbFe alloy film

An ultrafast spin demagnetization process of an amorphous Tb35Fe65 alloy film has been investigated by means of an all-optical pump-probe technique. Interestingly, steplike demagnetization on a subpicosecond time scale is observed before a much slower change on a time scale of tens of picoseconds. The steplike demagnetization at the subpicosecond scale is explained by the extended three-temperature model considering the interaction between a nonthermal electron and a spin system. The characteristic of subpicosecond demagnetization of TbFe alloy film is expected to be very useful in the manipulation of the spin state in ultrafast regime.

Laser generation and detection of longitudinal and shear acoustic waves in a diamond anvil cell

Laser ultrasonics in a point-source-point-receiver configuration is applied for the evaluation of elastic properties of nontransparent materials in a diamond anvil cell at high pressures. Measurement of both longitudinal and shear acoustic wave velocities in an iron foil at pressures up to 23 GPa does not require any information in addition to the one obtained by all-optical pump-probe technique.

Interface and bulk magnetization dynamics in biaxial Fe/Cr structures induced by ultrashort optical pulses

The interface and bulk magnetization dynamics of single-crystalline, wedge-shaped Fe(001) thin films with Cr cap layers have been studied by time-resolved magneto-optical Kerr effect (MOKE) and time-resolved magnetization-induced second harmonic generation (MSHG) using an all-optical pump-probe technique. We observed long-lived (≈1 ns) MOKE and MSHG oscillations excited by ultrashort (≈150 fs) optical pulses. They exhibit the same main resonance frequency f and damping constant. However, a 90° phase shift was observed between linear and nonlinear responses proving that MOKE and MSHG oscillations are related to the temporal variations of different magnetization components Mz and My. Additionally, we found weak oscillations at the double frequency 2f. Comparing the results of static and dynamic MSHG measurements we evaluate the in-plane amplitude of the optically excited interfacial magnetization oscillations.

Femtosecond optical generation and detection of coherent acoustic phonons in GaN single crystals

We report our experimental and theoretical studies on the time-resolved generation and detection of coherent acoustic phonons (CAPs) in very high quality bulk GaN single crystals, performed using a femtosecond, two-color, all-optical pump-probe technique. A train of ultraviolet laser pulses with energy above the GaN energy gap induced the transient electronic stress at the GaN surface, responsible for the CAP generation. Subsequent CAP oscillations, propagating without any measurable intrinsic attenuation, were observed by scanning the transient differential reflectivity signal $(\ensuremath{\Delta}R∕R)$ of the near-infrared, far-below-band-gap probe beam. The $\ensuremath{\Delta}R∕R$ CAP oscillation amplitude was of the order of ${10}^{\ensuremath{-}5}--{10}^{\ensuremath{-}6}$ and was dependent only on the pump-photon absorption coefficient spectral characteristics. The CAP oscillation frequency was dispersionless (proportional to the probe-beam wave vector) with the slope corresponding to $8002\ifmmode\pm\else\textpm\fi{}22\phantom{\rule{0.3em}{0ex}}\mathrm{m}∕\mathrm{s}$---the speed of sound in GaN---while the CAP signal phase was constant within the entire range of our experiments. The above experimental results are in excellent agreement with our theoretical modeling and the published literature data.

Modified Gilbert damping due to exchange bias in NiFe∕FeMn bilayers

A picosecond all-optical pump-probe technique is used to investigate magnetization dynamics and magnetic damping behavior in the Ni81Fe19∕Fe50Mn50 exchange bias system. Within the laser excitation the spin temperature is elevated resulting in a collapse of the exchange bias field. Thus, a fast anisotropy field pulse is launched which triggers the magnetization precession. The extracted Gilbert damping increases linearly with the exchange bias field magnitude which can be understood by taking the local fluctuations of the interfacial exchange coupling as an additional dissipation mechanism into account.

Irradiance and temperature dependence of the charge carrier lifetimes in bulk Hg0.6Cd0.4Te

We have measured the temperature and irradiance-dependent charge carrier lifetime in bulk Hg0.6Cd0.4Te using an all-optical pump-probe technique. The carrier lifetime is shown to increase by 50% when the ambient sample temperature is increased from 80 to 300 K. At irradiance levels below 1 MW/cm2, the lifetime decreases sharply with increasing incident irradiance. However, for further increase in irradiance level, the lifetime values reach a saturation. A theoretical model is used to explain both temperature and irradiance dependence of the lifetime.

Optical measurement of effective recombination lifetime in silicon epitaxial layers

In this letter we present a novel all-optical pump-probe technique for the measurement of effective recombination lifetime in silicon epitaxial layers. The procedure is based on the measurement of the variation of the attenuation coefficient due to optically injected free carriers of a planar dielectric waveguide defined by the epitaxial layer cladded between the substrate and the air. Perturbation theory is used to take into account nonuniform refractive index distribution due to nonzero surface recombination. The measurement technique is validated by both numerical simulation and experimental results.