Electronic and phononic Raman scattering in detwinned YBa2_2Cu3_3O6.95_{6.95} and Y0.85_{0.85}Ca0.15_{0.15}Ba2_2Cu3_3O6.95_{6.95}: s-wave admixture to the dx2−y2d_{x^2-y^2}-wave order parameter

Inelastic light (Raman) scattering has been used to study electronic excitations and phonon anomalies in detwinned, slightly overdoped YBa$_2$Cu$_3$O$_{6.95}$ and moderately overdoped Y$_{0.85}$Ca$_{0.15}$Ba$_2$Cu$_3$O$_{6.95}$ single crystals. In both samples modifications of the electronic pair-breaking peaks when interchanging the a- and b-axis were observed. The lineshapes of several phonon modes involving plane and apical oxygen vibrations exhibit pronounced anisotropies with respect to the incident and scattered light field configurations. Based on a theoretical model that takes both electronic and phononic contributions to the Raman spectra into account, we attribute the anisotropy of the superconductivity-induced changes in the phonon lineshapes to a small s-wave admixture to the $d_{x^2-y^2}$ pair wave-function. Our theory allows us to disentangle the electronic Raman signal from the phononic part and to identify corresponding interference terms. We argue that the Raman spectra are consistent with an s-wave admixture with an upper limit of 20 percent.Comment: accepted in Phys. Rev. B, 11 page

Characterization of 1.55-μm pulses from a self-seeded gain-switched Fabry-Pérot laser diode using frequency-resolved optical gating

The intensity and frequency chirp of picosecond pulses from a self-seeded gain-switched Fabry-Perot laser diode have been directly measured using the technique of frequency-resolved optical gating. Measurements over an output sidemode suppression ratio (SMSR) range of 15-35 dB show that higher SMSR's are associated with an increasingly linear frequency chirp across the output pulses. This complete pulse characterization allows the conditions for optimum pulse compression to be determined accurately, and indicates that transform-limited, pedestal free pulses can be obtained at an SMSR of 35 dB

Extreme non-linear response of ultra-narrow optical transitions in cavity QED for laser stabilization

We explore the potential of direct spectroscopy of ultra-narrow optical transitions of atoms localized in an optical cavity. In contrast to stabilization against a reference cavity, which is the approach currently used for the most highly stabilized lasers, stabilization against an atomic transition does not suffer from Brownian thermal noise. Spectroscopy of ultra-narrow optical transitions in a cavity operates in a very highly saturated regime in which non-linear effects such as bistability play an important role. From the universal behavior of the Jaynes-Cummings model with dissipation, we derive the fundamental limits for laser stabilization using direct spectroscopy of ultra-narrow atomic lines. We find that with current lattice clock experiments, laser linewidths of about 1 mHz can be achieved in principle, and the ultimate limitations of this technique are at the 1 $\mu$ Hz level.Comment: 5 pages, 4 figure

Ab initio calculations of edge-functionalized armchair graphene nanoribbons: Structural, electronic, and vibrational effects

We present a theoretical study on narrow armchair graphene nanoribbons (AGNRs) with hydroxyl functionalized edges. Although this kind of passivation strongly affects the structure of the ribbon, a high degree of edge functionalization proves to be particularly stable. An important consequence of the geometric deviations is a severe reduction of the band-gap of the investigated 7-AGNR. This shift follows a linear dependence on the number of added hydroxyl groups per unit cell and thus offers the prospect of a tunable band-gap by edge functionalization. We furthermore cover the behavior of characteristic phonons for the ribbon itself as well as fingerprint modes of the hydroxyl groups. A large down-shift of prominent Raman active modes allows the experimental determination of the degree of edge functionalization.Comment: 6 pages, 9 figure

Characterization of 40-Gbit/s pulses generated using a lithium niobate modulator at 1550 nm using frequency resolved optical gating

The characteristics of 40-Gbit/s pulses generated by exploiting the nonlinear characteristics of a Mach-Zender Lithium Niobate modulator are presented. A high spectral resolution frequency resolved optical gating apparatus has been developed to allow for the complete characterization of the intensity and phase of these pulses. The use of these measurements to simplify the design and optimization of an 80-Gbit/s pulse source, based on this 40-Gbit/s source followed by a nonlinear fiber compressor and multiplexer, is also demonstrated

Complete characterization of ultrashort pulse sources at 1550 nm

This paper reviews the use of frequency-resolved optical gating (FROG) to characterize mode-locked lasers producing ultrashort pulses suitable for high-capacity optical communications systems at wavelengths around 1550 nm, Second harmonic generation (SHG) FROG is used to characterize pulses from a passively mode-locked erbium-doped fiber laser, and both single-mode and dual-mode gain-switched semiconductor lasers. The compression of gain-switched pulses in dispersion compensating fiber is also studied using SHG-FROG, allowing optimal compression conditions to be determined without a priori assumptions about pulse characteristics. We also describe a fiber-based FROG geometry exploiting cross-phase modulation and show that it is ideally suited to pulse characterization at optical communications wavelengths. This technique has been used to characterize picosecond pulses with energy as low as 24 pJ, giving results in excellent agreement with SHG-FROG characterization, and without any temporal ambiguity in the retrieved puls

Horizontal Stratification in Access to Danish University Programmes

In this paper, we use register data to examine horizontal stratification within university institutions and university fields of study in Denmark, a country that has experienced a reduction of the social class gap in access to higher education. First, we argue that it is important to use a relatively detailed classification of parents’ occupations to determine how students are endowed with different forms of capital, even when their parents would typically be characterised as belonging to the same social group. Second, we distinguish among disciplines and among university institutions to explain the dynamics of horizontal stratification in the Danish university system. Using unique and exhaustive register data, including all higher education institutions and the entire 1984 cohort as of the age of 24, we uncover distinct differences in the magnitude and type of horizontal stratification in different fields of study and university institutions. Most importantly, we find distinct patterns of horizontal stratification by field of study and parental occupation that would have remained hidden had we used more aggregated classifications for field of study and social origin.</jats:p