Carrier-LO phonon interactions in Si(001)

We present a study of the interaction between high density photoexcited carriers and coherent LO phonons in Si(001). Through high-density (10¹⁹–10²⁰ carriers/cm³) photoexcitation of Si(001) near its E₁ critical point with 10 fs duration, 400 nm laser pulses, the zone-center coherent LO (q ≈ 0) phonon is excited. The coherent LO phonon gains a complex self-energy (i.e., frequency shift and contribution to decay rate) by interacting with the photogenerated nonequilibrium electron-hole plasma through the deformation potential interaction mechanism. We measure the time dependent renormalization of the LO phonon frequency and dephasing time by analyzing the anisotropic transient reflectivity of variously doped Si samples over a delay time of 6 ps between pump and probe pulses. We study how both these quantities depend on the initial photoexcited carrier density, and the level and type of doping. We further study the LO phonon excitation and detection mechanism monitoring the LO phonon amplitude change with the pump and probe polarizations orientations with respect to Si sample crystalline axes. The phonon amplitude exhibits a sin(2Ѳ)dependence on either the pump or the probe polarization angular rotation, where θ is the angles between the pulse electric field polarization and the [110] axis of the sample in both cases. This result is consistent both with the Г25’ symmetry and the deformation potential mechanism excitation of the LO phonon of Si. We also measure the dependence of the coherent LO phonon on the excitation light wavelength. We find that the phase does not fit into the theoretical picture developed to date for the coherent LO phonon generation. Moreover, the Fourier Transforms of the time-domain signals show Fano-like profiles for the LO phonon peak, with the asymmetry strongly dependent on the excitation wavelength. The wavelength dependence of both the phase and the LO phonon lineshape suggest that the mechanism for the LO coherent phonon generation should be described by a model where the generation pathways via the Raman process and the carrier excitation are coupled through the deformation potential interaction

Coherent phonon induced optical modulation in semiconductors at terahertz frequencies

The coherent modulation of electronic and vibrational nonlinearities in atoms and molecular gases by intense few-cycle pulses has been used for high-harmonic generation in the soft X-ray and attosecond regime, as well as for Raman frequency combs that span multiple octaves from the Terahertz to Petahertz frequency regions. In principle, similar high-order nonlinear processes can be excited efficiently in solids and liquids on account of their high nonlinear polarizability densities. In this paper, we demonstrate the phononic modulation of the optical index of Si and GaAs for excitation and probing near their direct band gaps, respectively at ~3.4 eV and ~3.0 eV. The large amplitude coherent longitudinal optical polarization due to the excitation of longitudinal optical (LO) phonon of Si (001) and LO phonon-plasmon coupled modes in GaAs (001) excited by 10-fs laser pulses induces effective amplitude and phase modulation of the reflected probe light. The combined action of the amplitude and phase modulation in Si and GaAs generates phonon frequency combs with more than 100 and 60 THz bandwidth, respectively.Comment: 15 pages, 11 figure

Coherent phonon frequency comb generated by few-cycle femtosecond pulses in Si

We explore the coherent phonon induced refractive index modulation of a Si(001) surface upon the excitation in near-resonance with the direct band gap of Si. Through the anisotropic e–h pair generation and coherent Raman scattering, ∼ 10-fs laser pulses exert a sudden electrostrictive force on Si lattice launching coherent LO phonon oscillations at 15.6 THz frequency. The concomitant oscillatory change in the optical constants modulates the reflected probe light at the fundamental LO phonon frequency, generating a broad comb of frequencies at exact integer multiples of the fundamental frequency extending to beyond 100 THz. On the basis of an analytical model, we show that the simultaneous amplitude and phase modulation of the reflected light by the coherent lattice polarization at 15.6 THz generates the frequency comb

Optimizing vortex pinning in YBa2Cu3O7-x superconducting films up to high magnetic fields

The magnetic flux pinning capabilities of YBa2Cu3O7-x (YBCO) coated conductors vary strongly across different regions of the magnetic field–temperature phase diagram and with the orientation of the magnetic field θ. Here, we determine the optimal pinning landscape for a given region of the phase diagram by investigating the critical current density Jc(H,θ,T) in the 5–77 K temperature range, from self-field to high magnetic fields of 35 T. Our systematic analysis reveals promising routes for artificially engineering YBCO coated conductors in any region of interest of the phase diagram. In solution-derived nanocomposites, we identify the relevance of coexisting high amounts of short stacking faults, Cu-O vacancy clusters, and segmentation of twin boundaries, in combination with nanoparticles, for enhanced pinning performance at high magnetic fields and low temperatures. Moreover, we demonstrate that twin boundaries preserve a high pinning energy in thick YBCO films, which is beneficial for the pinning performance at high magnetic fields and high temperatures.The authors acknowledge financial support from Spanish Ministry of Economy and Competitiveness through the “Severo Ochoa” Programme for Centres of Excellence in R&D (Grant No. SEV-2015-0496), ULTRASUPERTAPE (ERC-2014-ADG-669504), EUROTAPES project (FP7-NMP-Large-2011-280432), the CONSOLIDER Excellence Network (Grant No. MAT2015-68994-REDC), COACHSUPENERGY project (Grant No. MAT2014-56063-C2-1- R and SuMaTe RTI2018-095853-B-C21, co-financed by the European Regional Development Fund), and from the Catalan Government with Grant No. 2014-SGR-753 and 2017-SGR-1519. Authors also thank the network collaboration of EU COST action NANOCOHYBRI CA16218. We also acknowledge the Scientific Services at ICMAB and Z. Li and P. Cayado for the growth of the studied samples. A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by the National Science Foundation Cooperative Agreement No. DMR-1644779 and the State of Florida.With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).Peer reviewe