Complex energies of the coherent longitudinal optical phonon-plasmon coupled mode according to dynamic mode decomposition analysis.

In a dissipative quantum system, we report the dynamic mode decomposition (DMD) analysis of damped oscillation signals. We used a reflection-type pump-probe method to observe time-domain signals, including the coupled modes of long-lived longitudinal optical phonons and quickly damped plasmons (LOPC) at various pump powers. The Fourier transformed spectra of the observed damped oscillation signals show broad and asymmetric modes, making it difficult to evaluate their frequencies and damping rates. We then used DMD to analyze the damped oscillation signals by precisely determining their frequencies and damping rates. We successfully identified the LOPC modes. The obtained frequencies and damping rates were shown to depend on the pump power, which implies photoexcited carrier density. We compared the pump-power dependence of the frequencies and damping rates of the LOPC modes with the carrier density dependence of the complex eigen-energies of the coupled modes by using the non-Hermitian phenomenological effective Hamiltonian. Good agreement was obtained between the observed and calculated dependences, demonstrating that DMD is an effective alternative to Fourier analysis which often fails to estimate effective damping rates.

Normal mode analysis of a relaxation process with Bayesian inference.

Measurements of relaxation processes are essential in many fields, including nonlinear optics. Relaxation processes provide many insights into atomic/molecular structures and the kinetics and mechanisms of chemical reactions. For the analysis of these processes, the extraction of modes that are specific to the phenomenon of interest (normal modes) is unavoidable. In this study we propose a framework to systematically extract normal modes from the viewpoint of model selection with Bayesian inference. Our approach consists of a well-known method called sparsity-promoting dynamic mode decomposition, which decomposes a mixture of damped oscillations, and the Bayesian model selection framework. We numerically verify the performance of our proposed method by using coherent phonon signals of a bismuth polycrystal and virtual data as typical examples of relaxation processes. Our method succeeds in extracting the normal modes even from experimental data with strong backgrounds. Moreover, the selected set of modes is robust to observation noise, and our method can estimate the level of observation noise. From these observations, our method is applicable to normal mode analysis, especially for data with strong backgrounds.

Optical control of spin-polarized photocurrent in topological insulator thin films.

Dirac electrons in topological insulators (TIs) provide one possible avenue to achieve control of photocurrents and spin currents without the need to apply external fields by utilizing characteristic spin-momentum locking. However, for TI crystals with electrodes it is actually difficult to characterize the net flow of spin-polarized photocurrents because of the coexistence of surface carriers and bulk carriers generated by optical excitations. We demonstrate here that the net flow directions of spin-polarized photocurrents in TI polycrystalline thin films without electrodes can be precisely and intentionally controlled by the polarization of the excitation pulse alone, which is characterized by performing time-domain terahertz (THz) wave measurements and time-resolved magneto-optical Kerr rotation measurements that are non-contact methods. We show that the amplitudes of s-polarized THz waves radiated from photocurrents under right- and left-circularly polarized excitations are inverted relative to one another. Moreover, we observe the inversion of time-resolved magneto-optical Kerr rotation signals between the two excitations. Our results will open the way as innovative methods to control spin-polarized electrons in optoelectronic and spintronic TI devices without the need to apply external fields.

Quantitative characterization of highly efficient correlated photon-pair source using biexciton resonance.

A high efficiency method for the generation of correlated photon pairs accompanied by reliable means to characterize the efficiency of that process is needed in the study of entangled states, which have important potential applications in quantum information and quantum communication. In this study, we report the first characterization of the efficiency of generation of correlated photon pairs emitted from a CuCl single crystal using the biexciton-resonance hyper-parametric scattering (RHPS) method which is the highly efficient method of generation of correlated photon pairs. In order to characterize the generation efficiency and signal-to-noise ratio of correlated photon pairs using this method, we investigated the pump power dependence on the photon counting rate and coincidence counting rate under resonant excitation. The pump power dependence shows that the power characteristic of the photon counting rates changes from linear to quadratic dependence of the pump power. This behavior represents a superposition of contributions from correlated photon pairs and non-correlated photons. The analysis of the pump power dependence shows that one photon-pair is produced by a pump pulse with 2 x 106 photons. Moreover, the generation efficiency of this method obtained by calculating the number of generated photon pairs per pump power is comparable to that of several methods based on the χ(3) parametric process.

Dynamics of coherent anharmonic phonons in bismuth using high density photoexcitation.

We have investigated the dynamical properties of the coherent anharmonic phonons generated in Bi under high density excitation. The time-resolved reflectivity in the intensely photoexcited Bi film is modulated by the coherent A(1g) phonon oscillation with a time-dependent oscillation period. As the pump power density is increased, the line shape of the A(1g) mode in the Fourier transformed spectra becomes asymmetric, and the redshift of the phonon frequency is observed. Analysis of the transient redshift with a wavelet transform reveals that the frequency of the A(1g) mode depends on the squared amplitude of the oscillation, which is attributed to an anharmonicity of the lattice potential.