Optical Pump-probe Technique Research Articles

Investigation of the Carrier Dynamic in GaN-Based Cascade Green Light-Emitting Diodes Using the Very Fast Electrical–Optical Pump–Probe Technique

For the first time, the internal carrier dynamic inside GaN-based green light-emitting diodes (LEDs) during operation has been directly observed using the demonstrated electrical-optical pump-probe technique. Short electrical pulses (~100 ps) were pumped into high-speed cascade green LEDs, and the output optical pulses were probed using high-speed photoreceiver circuits. Using such a method, the recombination time constant of the carriers can be directly measured without any assumption about the recombination process. A high-speed cascade LED structure was adopted in the experiments to eliminate the influence of the RC delay time on the measured responses. Our measurement results indicate that both single- and three-LED cascade structures have the same internal response time due to current continuity. Furthermore, based on responses measured under different temperatures (from 25°C to 200°C), the origin of the efficiency droop in GaN-based green LEDs under a high bias current density may be attributed to the strong nonradiative Auger effect rather than device heating or carrier overflow. The demonstrated measurement scheme and high-speed cascade device structure offer a novel and simple way to straightforwardly investigate the internal carrier dynamic inside the active layers of the LED during forward-bias operation.

Optically Induced Sub-Wavelength Transient Apertures in Sb-Te Based Films

ABSTRACTThe origin of sub-diffraction-limit apertures in Sb-based thin films is discussed. Electromagnetic energy can be channeled by these apertures thus allowing near-field focussing- the Super-RENS effect. The aperture formation within Sb, Sb2Te3, Sb2Te, SbTe and Ge2Sb2Te5 is investigated by time resolved optical pump-probe techniques and found to occur without melting. Density functional calculations have shown that these materials exhibit a thresholdlike change in their optical properties below their melting temperatures. The threshold is shown to be a consequence of thermally induced misalignment of p-orbital bonds. It is the non-linearity of this process that leads to the formation of the sub-diffraction-limit apertures.

Size-dependent phase transition of V O2 nanostructures induced by light excitation

In nanocrystalline V O2 films the light-induced insulator-to-metal phase transition (I-M PT) dynamics was investigated by femtosecond optical pump-probe technique. The size of V O2 nanoparticles and the film morphology were found to be critical for the ultrafast light-induced PT dynamics as well as for thermal hysteresis. The I-M PT within subnanosecond scale demonstrates a nearly linear dependence of characteristic transition time on V O2 particles size. Such a behavior can indicate the importance of the phonon interactions with optically-excited electronic states.

Photoacoustic measurements on thermal properties of hydrogenated amorphous carbon films: the effect of hydrogen dilution

Thermal properties of hydrogenated amorphous carbon (a-C:H) thin-films are measured using an ultrafast optical pump-probe technique. The a-C:H samples were grown in a home-built direct-current (DC) plasma enhanced chemical vapor deposition (PECVD) system with varying hydrogen (H2) diluents to methane (CH4) flow-rate ratios. Thermal diffusivities of samples are extracted by comparing thermoreflectance measurements with numerical calculations so that thermal conductivities (k) can be determined. Although the dependence of thermal property on H2 dilution was not significant, our films show lower k (0.10–0.15 W/mK ± 20%) compared to the results of previous studies.

Thickness effect on ultrafast thermalization of carriers in above-band-gap states in ZnO epitaxial films

Energy-dependent free-carrier dynamics was investigated in 70 nm (thin) and 1 µm (thick) ZnO epifilms using the optical pump–probe technique. The far-above-band-gap dynamics in the thin epifilm reveals the prolonged relaxation and the slow recovery of renormalized band gap. The band-gap renormalization (BGR) effect is affected by the inefficient carrier–phonon scattering. In addition, the loss of excited carrier density via surface trapping results in an energy-dependent BGR buildup time. However, the far-above-band-gap dynamics in the thick epifilm reveals fast relaxation followed by BGR recovery, which is independent of the photon energy. The near-band-gap dynamics shows an ultrafast carrier thermalization both in the thin and the thick epifilms.

Non-melting super-resolution near-field apertures in Sb–Te alloys

The super-resolution near-field structure permits the formation of short-lived subdiffraction limit apertures that channel electromagnetic energy at the aperture boundary. This effect is commonly observed when a laser is focused onto a thin film of Sb based material. The aperture formation within Sb, Sb2Te3, Sb2Te, and SbTe is investigated by time resolved optical pump-probe techniques and found to occur without melting. Ab initio modeling has shown a threshold-like change in the optical properties below the melting temperature which leads to the formation of a near-field aperture. This threshold is shown to be a consequence of thermally induced misalignment of p-type bonding.

Photoassisted amorphization of the phase-change memory alloyGe2Sb2Te5

Subnanosecond time-resolved x-ray absorption measurements have been used to probe dynamical changes in the local structure about Ge atoms in the phase-change alloy ${\text{Ge}}_{2}{\text{Sb}}_{2}{\text{Te}}_{5}$ during the optical recording (amorphization) process using an optical pump and x-ray probe technique to examine the reversible phase transition from the metastable crystalline phase to the amorphous phase. We provide unambiguous evidence that the amorphization process does not proceed via the molten state but is a photoassisted process. We argue that the transition to the amorphous phase is a consequence of photoassisted destabilization of the resonant bonding present in the crystalline phase. This observation challenges the currently existing paradigm of the phase-change process which implicitly assumes the existence of the molten phase as a prerequisite for the creation of the amorphous phase. Implications from this finding are discussed, including the possibility to use the polarization of light as an extra coordinate for data recording.

Ultrafast optical imaging of the spin Hall effect of light in semiconductors

We experimentally demonstrate a general optical pump-probe technique to observe the spin Hall effect of light (SHEL) in an absorbing medium. In essence, a locally confined pump-induced modification of a material's absorptivity can effectively be used as an induced aperture allowing one to detect the transversely displaced circular polarization components of an incident beam through differential transmission techniques. We consider linear absorption mechanisms such as free-carrier absorption and Pauli blocking as well as nonlinear absorption processes such as two-photon absorption. For absorption mechanisms that do not depend on light polarization, the SHEL of the probe beam is obtained directly, while polarization-dependent properties give an effective SHEL displacement that depends on the action of the SHEL on both pump and probe beams. Using 150 fs pump, 820 nm pump and probe pulses we observe SHEL effects in silicon via free-carrier absorption. SHEL effects are also observed via Pauli blocking at 820 nm and two-photon absorption at 1550 nm in GaAs using $\ensuremath{\sim}150\text{ }\text{fs}$ pump and probe pulses.

Ultrafast third-order nonlinear optical properties of ZnPc(OBu)_6(NCS)/DMSO solution

The ultrafast third-order nonlinear optical properties of ZnPc(OBu)(6)(NCS)/dimethyl sulfoxide solution are investigated by optical Kerr effect (OKE) and pump-probe techniques with femtosecond pulses. Using CS(2) as the reference, the third-order susceptibility of the sample solution is obtained as about 1.40x10(-14) esu at a concentration of 3.00x10(-5) M. The second hyperpolarizability for the sample solution is deduced to be as large as 2.35x10(-31) esu. The dynamic characteristics of the nonlinear optical response and the origins of the OKE signal can be attributed to excited singlet state populations.

Carrier mobility in a polar semiconductor measured by an optical pump-probe technique

Ultrafast dephasing of the plasmonlike longitudinal optical phonon-plasmon coupled (LOPC) mode in highly doped n-GaAs has been investigated by using a femtosecond optical pump-probe technique with 40 THz bandwidth as a function of photodoping levels. The direct measurement of plasmon damping with the help of a wavelet analysis enables us to extract carrier (electron) mobility, which decreases with increasing the photodoping levels. It is found that the mobility is suppressed at high photodoping levels due to electron-hole scattering, while it is enhanced near a critical density, being plausibly attributed to the strong coherent coupling of the LO phonon with the plasmon.

Annealing effect on the ultrafast dynamics of Ag nanoparticles embedded in soda-lime silicate glasses

Ag nanoparticles embedded in soda-lime silicate glasses were fabricated by the ion-exchange technique. Effects of thermal treatment on the optical nonlinearity and ultrafast dynamics of Ag nanoparticles were investigated by applying time-resolved optical Kerr effect and pump-probe techniques. The results indicate that thermal treatment is an efficient method to improve the nonlinear optical performance of this kind of material.

Probing rotational relaxation in HBr (v=1) using double resonance spectroscopy

Rotational energy transfer in HBr(v=1)+HBr collisions has been investigated using an optical pump-probe double resonance technique at ambient temperature. Rotationally state selective excitation of v=1 for rotational levels in the range J=0-9 was achieved by stimulated Raman pumping, and the evolution of population was monitored using (2+1) resonantly enhanced multiphoton ionization spectroscopy of the g (3) summation (-)-X (1) summation (+)(0-1) band. Collision-induced population transfer events with DeltaJ<or=8 were observed at a pressure of 0.32 Torr. State-to-field rate constants for loss of population from the initially prepared states ranged from 8.3x10(-10) to 4.9x10(-10) cm(3) s(-1). The full state-to-state rate constant set was analyzed by fitting to numerical solutions of the coupled differential equations describing the relaxation processes. Rate constant matrices were generated using fitting and scaling functions. The rate coefficients were best represented by the statistical power exponential gap law.

Coherent terahertz acoustic vibrations in polar and semipolar gallium nitride-based superlattices

The femtosecond optical pump-probe technique is used to generate and detect the coherent acoustic vibrations with a frequency 0.83×1012Hz in polar (0001) and semipolar (112¯2) InGaN∕GaN superlattices. The measured amplitude of the vibrations in the semipolar sample is about five times smaller that in the polar one. Analysis of the experimental data and theoretical estimates suggest that, in addition to piezoelectric, deformation potential provides essential contribution to both generation and detection of vibrations, especially in the semipolar superlattice.

Dynamics of the coercivity in ultrafast pump–probe experiments

In this paper we devote ourselves to the coercivity as a measured variable in femtosecond magnetism. We stress the fact that an all optical pump–probe technique is in general not suitable to gain access to the time-dependent behaviour of the coercivity, since the switching in a fixed external field is an irreversible process. We comment on the possible mechanisms leading to the observed reduction in the coercivity with increasing pump power and propose a potential solution to clarify the origin of such behaviour.

Towards ultrafast spin-state switching in the solid state

Abstract We report here on the photo-switching dynamics of a Fe(III) molecular material undergoing a spin-crossover. The thermal spin conversion is observed by magnetic and spectroscopic measurements, as well as imaging. The photoinduced switching dynamics between the low-spin and the high-spin states is studied with a femtosecond optical pump–probe technique. The results demonstrate that the switching occurs within a time-scale shorter than one picosecond.

An optical pump-probe technique for measuring the thermal conductivity of liquids

We present a pump-probe optical technique for measuring the thermal conductivity of liquids. The technique uses a reflective geometry which does not depend on the optical properties of the liquid and requires as little as a single droplet to produce a result. An analytical solution is given for bidirectional heat flow in layered media, including the effects of radial heat flow from coaxial Gaussian laser spots, thermal interface resistances, and the accumulation of multiple laser pulses. In addition, several experimental improvements over previous pump-probe configurations are described, resulting in an improved signal to noise ratio and smaller errors at long stage delay times. The technique is applied to a range of liquids and solids. Results are in good agreement with literature values.

Light-Induced Functional VO&lt;sub&gt;2&lt;/sub&gt; Films

Ultrafast optical pump-probe technique employed in a degenerate-four-wave-mixing configuration was used to study the light-induced optical property change during extremely short time period of femtoseconds (fs) to picoseconds (ps) for VO2 thin films. Dramatic change in optical properties serves as basis of light-induced functional materials in ultrafast applications. Upon light illumination the structural change was identified as a phase transition (PT) from insulator to metallic phases. After the light-induced PT a fast recovery process occurred, which was strongly dependent on optical pump energy. It could be governed by pure electronic relaxation excluding thermal contribution at sufficiently low excitation. The insulator and metallic phases are coexistent exhibiting fluctuations of dielectric constants. Different desorption activity was monitored for insulator and metallic VO2 thin solid films under femtosecond optical excitation.

Femtosecond carrier dynamics in electron-beam-irradiated C60 film

Time-resolved temperature-dependent transmission changes were measured for both pristine C60 and electron-beam-irradiated C60 polymer films using an optical pump-probe technique. Only the signals obtained for the C60 polymer show a temperature-dependent slow decay, which appears in the low temperature region below ∼60K. This slow decay component exhibits a monotonic increase in both relaxation time and amplitude with decreasing temperature, providing evidence of gap formation associated with ordering fluctuations.

Femtosecond time-resolved dynamics in electron-doped high-Tc superconductor La2-xCexCuO4

The quasiparticle dynamics in the electron-doped high-Tc superconductor La2-xCexCuO4 (LCCO) was investigated by ultrafast time-resolved optical pump-probe technique. We present results in three temperature regions: namely the low temperature (TTc) region, that around Tc(0.7Tc≤T≤Tc) and the high temperature (T&gt;Tc) region. In general, there was a slowdown of the relaxation as the temperature decreases. However, the quasiparticle lifetime was found to exhibit a quasi-divergence as the temperature approached Tc from below. The relaxation time in normal state was much longer than what was expected for metallic relaxation, indicating the presence of a T-independent pseudogap. We have also observed picosecond rise component of photoinduced change in reflectivity ΔR/R in the electron-doped LCCO, which is attributed to the Cooper pair-breaking dynamics. The rise time became shorter as the temperature increased. The experimental results are analyzed by Rothwarf-Taylor model.

Excited state dynamics in silver nanoparticles embedded in phosphate glass

Excited state dynamics in silver nanoparticles embedded in aluminophosphate glass was studied by ultrafast optical pump–probe technique. The absorption process of pump radiation and the electron–phonon relaxation on the 10 −13–10 −11 s scale were analyzed in the framework of two-temperature model. The time evolution of the light-induced transient diffraction grating shows an uncommon relaxation on the nanosecond time scale. This relaxation is assigned to phonon–phonon scattering process as well as to the energy transfer from photoexcited electronic states in glass matrix to silver nanoparticles.