Transient Reflectivity Measurements Research Articles

Electron–phonon coupling and defect scatterings in Ar<sup>+</sup>-ion implanted graphite

Dynamics of coherent G-mode and D-mode phonons in Ar + -ion implanted graphite has been investigated using transient reflectivity measurement. With an increase in the Ar + -ion dose, coherent D-mode phonon appears at which the dephasing rate of the coherent G-mode phonon begins to increase. The increase in the amplitude of the coherent D-mode phonon and those in dephasing rates of the coherent D-mode and G-mode phonons indicate effective carrier-defect scatterings in the defective graphite. The dephasing rate of the D-mode is larger than that of the G-mode, indicating an existence of an additional dephasing process for the D-mode. We attribute it to the broad spectral distribution of the D-mode phonon, which is understood as a coherent excitation of large wavevector phonons with different wavevectors through inter-Dirac-cone scattering. The frequency chirping of the D-mode and its defect density dependence are also discussed based on this scattering mechanism.

Quasiparticle Dynamics in FeSe Superconductors Studied by Femtosecond Spectroscopy

The ultrafast quasiparticle dynamics in FeSe single crystals were measured by using dual-color transient reflectivity measurements (ΔR/R) from 4.4 to 290 K. In general, the typical ΔR/R of FeSe includes two significant components. One is the relaxation of photoinduced quasiparticles, which has been used to estimate the electron–phonon coupling strength (λ=0.16). The other is the oscillation component due to the acoustic phonon. Moreover, the acoustic phonon’s energy estimated from the period of oscillation in ΔR/R markedly shrinks around 90 K, which is the so-called phonon softening.

Picosecond Kinetics of Strongly Coupled Excitons and Surface Plasmon Polaritons

Coupling between excitons of CdSe nanocrystal quantum dots (NQDs) and surface plasmon polaritons (SPPs) of an Ag film attached to a prism have been studied by steady-state and transient reflectivity measurements in the Kretschmann geometry. In these experiments, the angle of incidence of the probe beam selects hybrid exciton/SPP states with different wavevectors and exciton/SPP compositions. The dynamics measured in the transient reflectivity experiments are sensitive to the composition of the hybrid states. Specifically, fast dynamics are observed at probe wavevectors where the lower hybrid state has predominant SPP character. In contrast, at probe wavevectors where the lower hybrid state is predominantly excitonic, the dynamics are similar to that measured for CdSe NQDs on glass.

Ultrafast dynamics and phonon softening in Fe1+ySe1−xTex single crystals

The ultrafast quasiparticle dynamics of Fe1+ySe1−xTex single crystals were investigated by dual-color transient reflectivity measurements (ΔR/R) from 4.3 to 290 K. The electron–phonon coupling strength λ (=0.16–0.01) and the temperature-dependent energy of longitudinal-acoustic phonons were, respectively, obtained from the relaxation time of a fast component and the period of an oscillation component in ΔR/R. Such a small λ demonstrates the unconventional origin of superconductivity in FeSe. Moreover, the temperature-dependent ΔR/R exhibits anomalous changes at Tc, 90 K and 230 K, unambiguously revealing the phase transition as well as the phonon softening via the magnetoelastic effect.

Ultrafast photoinduced mechanical strain in epitaxial BiFeO3 thin films

We studied ultrafast dynamics and photoinduced mechanical strain of BiFeO3 thin films by dual-color transient reflectivity measurements (ΔR/R). Anisotropic photostriction in BiFeO3 is found to be mainly driven by the optical rectification effect. Results of the photostriction at various thicknesses show that the estimated sound velocity along [110] direction of BiFeO3 is 4.76 km/s.

Femtosecond excitation of radial breathing mode in 2-D arrayed GaN nanorods

Radial breathing oscillation of 2-D arrayed GaN nanorods was successfully excited in rods with different diameters by using femtosecond transient reflectivity measurement. Through analyzing thus measured diameter dependent oscillation frequency, we discovered that modification of the mechanical property appeared in the 2-D arrayed piezoelectric GaN nanorods, fabricated on top of a bulk substrate, when the rod diameter was on the order of or less than 50 nm. Our measurement observed a much reduced elastic stiffness constant (C11) of 193 ± 24 GPa in 35nm diameter nanorods, compared with the 365 ± 2 GPa in bulk GaN. This size-reduction induced mechanical modification would be a critical factor to be considered for future sensing and energy applications. Our study also provides a new spectroscopic method to explore the size-reduction-induced softening effect through the measurement of the radial breathing oscillations.

Quasiparticle Dynamics and Phonon Softening in FeSe Superconductors

Quasiparticle dynamics of FeSe single crystals revealed by dual-color transient reflectivity measurements (ΔR/R) provides unprecedented information on Fe-based superconductors. The amplitude of the fast component in ΔR/R clearly gives a competing scenario between spin fluctuations and superconductivity. Together with the transport measurements, the relaxation time analysis further exhibits anomalous changes at 90 and 230 K. The former manifests a structure phase transition as well as the associated phonon softening. The latter suggests a previously overlooked phase transition or crossover in FeSe. The electron-phonon coupling constant λ is found to be 0.16, identical to the value of theoretical calculations. Such a small λ demonstrates an unconventional origin of superconductivity in FeSe.

Laser-Induced Backside Wet Etching Employing Green DPSS Laser and Liquid Metallic Absorber

An indirect laser processing, laser-induced backside wet etching (LIBWE) is one of effective techniques for flexible laser-direct-write (LDW) type micromachining of transparent materials. Micromachining by LIBWE employing a diode-pumped solid state (DPSS) green laser and a galvanometer-based point scanning system was examined with employing organic dye solution and liquid metal as laser-absorbing media. LIBWE using liquid metal absorber showed smaller threshold pulse energies for etching (9.1 μJ/pulse) than that using organic dye solution (33 μJ/pulse). Different morphologies of etched surfaces were observed for different laser-absorbing media, indicating different mechanisms involved in the etching processes. The laser-induced dynamic phenomena were investigated by means of transient reflection measurements. The different contribution of direct and indirect actions of laser pulses was indicated for LIBWE employing different laser-absorbing media.

Fabrication of high resistivity cold-implanted InGaAsP photoconductors for efficient pulsed terahertz devices

A multiple-energy, high fluence, MeV Fe ion implantation process was applied at 83 K to heavily damage a low band gap (0.79 eV) epitaxial InGaAsP layer. Optimal rapid thermal annealing conditions were found and produced a fast photoconductor with high resistivity (up to 2500 Ωcm) and Hall mobility around 400 cm2V−1s−1. Short photocarrier trapping times (0.3 ps – 3 ps) were observed via transient differential reflectivity measurements. Furthermore, photoconductive terahertz devices with coplanar electrodes were fabricated and validated. Under pulsed excitation with a 1550 nm femtosecond fiber laser source, antennas based on Fe-implanted InGaAsP are able to emit broadband radiation exceeding 2 THz. Given such specifications, this new material qualifies as a worthy candidate for an integration into optical terahertz spectrometer designs.

Comparison of optical and electrical transient response during nanosecond laser pulse-induced phase transition of Ge 2Sb 2Te 5 thin films

The phase transition dynamics of Ge 2Sb 2Te 5 thin films induced by single nanosecond laser pulses were studied by transient optical reflectivity and electrical resistance measurements with nanosecond resolution. It was found that the real-time responses of optical and electrical signals were different under the same pumping condition. It needs much little time to reach equilibrium state for electrical resistance than optical reflectivity. The dependence of crystallization time determined respectively by optical and electrical evolution curve on laser pulse fluence was compared and analyzed. A 2-dimensional percolation model was used to explain the differences between electrical and optical transient responses.

Femtosecond carrier dynamics in bulk graphite and graphene paper

Femtosecond white-light continuum probe transient absorption and reflectivity measurements of bulk graphite and graphene paper are reported. In graphite, the relaxation of photoinduced electron–hole pairs happens through in-plane electron–electron and electron–phonon scattering in ≃200fs, while the ps dynamics is due to the modulation of the electronic structure by out-of-plane structural motions. The ps dynamics of the optical signal is strongly reduced in graphene paper, where the out-of-plane bond is disrupted, while the short component of the dynamics is identical in both materials. These results show that in 2D-graphene, the carrier relaxation occurs in ≃200fs.

Direct Detection of the Ultrafast Response of Charges and Molecules in the Photoinduced Neutral-to-Ionic Transition of the Organic Tetrathiafulvalene-p-Chloranil Solid

Ultrafast dynamics of charge and molecular degrees of freedom in a photoinduced neutral-ionic transition of an organic solid, tetrathiafulvalene-p-chloranil, were directly detected for the first time by transient reflectivity measurements using 15 fs laser pulses. The results reveal that ionic domains are photogenerated in the neutral lattice via collective charge-transfer processes within 20 fs, and subsequently molecular deformations and bendings occur, giving rise to large changes in the molecular ionicity and charge redistributions in molecules, respectively. We show that such couplings between charge and molecular degrees of freedom play important roles in photoinduced transitions of organic molecular compounds.

Femtosecond carrier dynamics in native and high resistivity iron-doped GaxIn1−xAs

We report femtosecond transient reflectivity measurements of as-grown and iron-doped GaxIn1−xAs. A hybrid vertical Bridgman and gradient freezing directional solidification process was employed for the growth of high quality Ga0.69In0.31As:Fe crystals with the uniform impurity doping concentration necessary for high resistivity (1.6×107 Ω cm) and high mobility [(2–3)×103 cm2/V s] material. Carrier lifetimes range from ∼62 fs for as-grown Ga0.09In0.91As to ∼306 fs for Ga0.69In0.31As:Fe. The high carrier mobility along with high resistivity and subpicosecond carrier lifetimes make Ga0.69In0.31As:Fe an excellent candidate for photoconducting antenna based terahertz emitters.

Time-resolved optical studies of spin and quasiparticle dynamics in colossal magnetoresistance materials:La0.67Ca0.33MnO3,La0.67Sr0.33MnO3, andSr2FeMoO6

We report on transient reflectivity and magneto-optical Kerr measurements from the colossal magnetoresistance compounds ${\text{La}}_{0.67}{\text{Ca}}_{0.33}{\text{MnO}}_{3}$ (LCMO), ${\text{La}}_{0.67}{\text{Sr}}_{0.33}{\text{MnO}}_{3}$ (LSMO), and ${\text{Sr}}_{2}{\text{FeMoO}}_{6}$ (SFMO) as a function of temperature and magnetic field. In LCMO and LSMO, an unusually slow $(\ensuremath{\sim}1\text{ }\ensuremath{\mu}\text{s})$ carrier relaxation component is revealed in the transient reflectivity traces. The component disappears as the transition temperature is approached from below. This slow decay process is attributed to spin-lattice relaxation of carriers in localized states and shows a close relationship with the spectral weight near the Fermi surface. The attribution is further supported by our pump-probe magneto-optical Kerr measurements. In addition to the clear observation of magnetic precessions, a long-lived exponentially decaying background reflects the spin-related relaxation of photoexcited electrons. In contrast to manganites, the temperature dependence of transient reflectivity is negligible, although there is a significant change in transient Kerr rotation in SFMO. Our results show that the dynamics of charge, spin, and lattice are strongly correlated with each other in the manganites, but the spin degree of freedom is thermally insulated from the electron and lattice systems.

Effects of Si film thickness and substrate temperature on melt duration observed in excimer laser-induced crystallization of amorphous Si thin films using in-situ transient reflectivity measurements

The influences of Si film thickness and substrate temperature on the melt duration of molten Si during excimer laser crystallization (ELC) have been theoretically and experimentally investigated. The melt duration of molten Si significantly changed with the increase in Si film thickness, especially at higher substrate temperature. In-situ time-resolved (∼ 1 ns) optical reflectivity measurements revealed that the longest melt duration of 400-nm-thick amorphous silicon (a-Si) film at substrate temperature of 500 °C was 1714 ns which is 4.54 times longer than that of 90-nm-thick a-Si film. The resulting polycrystalline silicon (poly-Si) thin films were characterized using Raman spectroscopy, field-emission scanning electron microscope and atomic force microscopy. It is found that the longest melt duration of molten Si is proportional to the Si film thickness and substrate temperature for Si thin film smaller than 200 nm. On the other hand, in the thicker Si thin film (> 200 nm), the grain size of poly-Si is not proportionally affected by variation in Si film thickness and substrate temperature, which is probably related to the different recrystallization mechanisms of Si thin films of different thickness. Micro-Raman scattering measurements revealed that the crystallinity of poly-Si in the ELC region improved to reach an excellent level, very near that of the single-crystal Si by preheating the substrate during ELC.

Azimuth Dependence of Light-Induced Ultrafast Insulator-to-Metal Phase Transition in VO2 Thin Film

Using femtosecond laser excitation the light-induced ultrafast phase transition (PT) was observed in VO2 thin films deposited on different optically transparent substrates. The PT was characterized by transient absorption, reflection and nonlinear optical (NLO) response measurements. Degenerate-four-wave- mixing (DFWM) technique was also applied for time-resolved measurement of light-induced PT. While strong NLO response can always be observed in films on fused quartz substrates the response in films on C-cut sapphire substrates was diminished. However, it is much enhanced in films on R-cut sapphire. Further orientation dependent measurements show that the light- induced PT from insulator to metallic state is connected to the following lattice relaxation. In the interface layers a lattice distortion along c axis of the rutile structure or off-axis displacement may be engaged in the resultant PT. While the substrate lattice symmetry is in favour of this type of distortion the light-induced PT becomes enhanced.

Femtosecond carrier dynamics in implanted and highly annealed polycrystalline silicon

We have studied the ultrafast optical response of highly implanted and highly annealed polycrystalline silicon films using 400 nm ultrashort amplified pulses with fluence ranging between 8 mJ cm−2 to 56 mJ cm−2. Transient reflection measurements reveal differences both in the short and long temporal behaviour between the implanted non-annealed and annealed samples. Important contributing factors to the dynamics of the non-annealed sample are the carrier recombination centres and traps induced by ion implantation. In contrast to the non-annealed sample, the Auger recombination process is a key factor in the dynamics in the first few picoseconds for the sample annealed at 1100 °C. A model based on two coupled differential equations has been employed to investigate in detail the carrier dynamics in these systems. Parameters including carrier trapping times, diffusion coefficients and the Auger coefficient have been extracted.

Transient pump–probe measurements for polarized excitons in strained GaN epitaxial layers

We have studied exciton spin relaxation in strained GaN epilayers on various sapphire substrates using spin-dependent transient reflectivity and three-pulse four-wave mixing measurements. Strikingly fast exciton spin relaxation (<1 ps) at 10 K was observed in the lowest exciton level (A-exciton) both in bulk and GaN layer on c-sapphire. In contrast, the transient signals in the GaN layer on a-sapphire show the break down of the spin polarization caused by the exchange interaction.

Ultrafast carrier dynamics in InxGa1−xN (0001) epilayers: Effects of high fluence excitation

Ultrafast carrier dynamics in InxGa1−xN (0001) epilayers were investigated, using femtosecond transient differential transmission and reflection measurements for x=0.07, 0.15, and 0.33, over a fluence range of 1–12mJ∕cm2. Stimulated emission as well as band gap renormalization play a crucial role in the dynamics of the photogenerated carriers. Threshold fluence leading to saturation of the differential reflectivity and transmission signals related to the In mole fraction has been observed, which is attributed to band gap renormalization, Auger process, and carrier recombination through In-rich nanoclusters. Furthermore, coherent acoustic phonon oscillations have also been observed in the In0.15Ga0.85N at high fluence excitation.

Carrier dynamics in α-Fe2O3 (0001) thin films and single crystals probed by femtosecond transient absorption and reflectivity

Femtosecond transient reflectivity and absorption are used to measure the carrier lifetimes in α-Fe2O3 thin films and single crystals. The results from the thin films show that initially excited hot electrons relax to the band edge within 300fs and then recombine with holes or trap within 5ps. The trapped electrons have a lifetime of hundreds of picoseconds. Transient reflectivity measurements from hematite (α-Fe2O3) single crystals show similar but slightly faster dynamics leading to the conclusion that the short carrier lifetimes in these materials are due primarily to trapping to Fe d-d states in the band gap. In the hematite single crystal, the transient reflectivity displays oscillations due to the formation of longitudinal acoustic phonons generated following absorption of the ultrashort excitation pulse.