180 research outputs found
Effect of picosecond strain pulses on thin layers of the ferromagnetic semiconductor (Ga,Mn)(As,P)
The effect of picosecond acoustic strain pulses (ps-ASP) on a thin layer of
(Ga,Mn)As co-doped with phosphorus was probed using magneto-optical Kerr effect
(MOKE). A transient MOKE signal followed by low amplitude oscillations was
evidenced, with a strong dependence on applied magnetic field, temperature and
ps-ASP amplitude. Careful interferometric measurement of the layer's thickness
variation induced by the ps-ASP allowed us to model very accurately the
resulting signal, and interpret it as the strain modulated reflectivity
(differing for probe polarizations), independently from dynamic
magnetization effects.Comment: 6 pages, 5 figure
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Coherent Coupling of Multiple Transverse Modes in Quantum Cascade Lasers
Quantum cascade lasers are a unique laboratory for studying nonlinear laser dynamics because of their high intracavity intensity, strong intersubband optical nonlinearity, and an unusual combination of relaxation time scales. Here we investigate the nonlinear coupling between the transverse modes of quantum cascade lasers. We present evidence for stable phase coherence of multiple transverse modes over a large range of injection currents. We explain the phase coherence by a four-wave mixing interaction originating from the strong optical nonlinearity of the gain transition. The phase-locking conditions predicted by theory are supported by spectral data and both near- and far-field mode measurements.Engineering and Applied Science
Green's function for metamaterial superlens: Evanescent wave in the image
We develop a new method to calculate the evanescent wave, the subdivided
evanescent waves (SEWs), and the radiative wave, which can be obtained by
separating the global field of the image of metamaterial superlens. The method
is based on Green's function, and it can be applied in other linear systems.
This study could help us to investigate the effect of evanescent wave on
metamaterial superlens directly, and give us a new way to design new devices.Comment: 15 pages, 3 figure
Near-field magneto-caloritronic nanoscopy on ferromagnetic nanostructures
Near-field optical microscopy by means of infrared photocurrent mapping has rapidly developed in recent years. In this letter we introduce a near-field
induced contrast mechanism arising when a conducting surface, exhibiting a
magnetic moment, is exposed to a nanoscale heat source. The
magneto-caloritronic response of the sample to near-field excitation of a
localized thermal gradient leads to a contrast determined by the local state of magnetization. By comparing the measured electric response of a magnetic reference sample with numerical simulations we derive an estimate of the field enhancement and the corresponding temperature profile induced on the sample surface.This work was supported by the Deutsche Forschungsgemeinschaft through grant HE 2063/5-1 to JH. The work also received funding from the ERC synergy grant No. 61011
Emergent Phenomena Induced by Spin-Orbit Coupling at Surfaces and Interfaces
Spin-orbit coupling (SOC) describes the relativistic interaction between the
spin and momentum degrees of freedom of electrons, and is central to the rich
phenomena observed in condensed matter systems. In recent years, new phases of
matter have emerged from the interplay between SOC and low dimensionality, such
as chiral spin textures and spin-polarized surface and interface states. These
low-dimensional SOC-based realizations are typically robust and can be
exploited at room temperature. Here we discuss SOC as a means of producing such
fundamentally new physical phenomena in thin films and heterostructures. We put
into context the technological promise of these material classes for developing
spin-based device applications at room temperature
Absorption Enhancement in Lossy Transition Metal Elements of Plasmonic Nanosandwiches
Combination of catalytically active transition metals and surface plasmons offers a promising way to drive chemical reactions by converting incident visible light into energetic electron-hole pairs acting as a mediator. In such a reaction enhancement scheme, the conversion efficiency is dependent on light absorption in the metal. Hence, increasing absorption in the plasmonic structure is expected to increase generation of electron-hole pairs and, consequently, the reaction rate. Furthermore, the abundance of energetic electrons might facilitate new reaction pathways. In this work we discuss optical properties of homo- and heterometallic plasmonic nanosandwiches consisting of two parallel disks made of gold and palladium. We show how near-field coupling between the sandwich elements can be used to enhance absorption in one of them. The limits of this enhancement are investigated using finite-difference time-domain simulations. Physical insight is gained through a simple coupled dipole analysis of the nanostructure. For small palladium disks (compared to the gold disk), total absorption enhancement integrated over the near visible solar AM 1.5 spectrum is 8-fold, while for large palladium disks, similar in size to the gold one, it exceeds three
Electric-field-coupled resonators for negative permittivity metamateri-als
Abstract: Complex and interesting electromagnetic behavior can be found in spaces with non-flat topology. When considering the properties of an electromagnetic medium under an arbitrary coordinate transformation an alternative interpretation presents itself. The transformed material property tensors may be interpreted as a different set of material properties in a flat, Cartesian space. We describe the calculation of these material properties for coordinate transformations that describe spaces with spherical or cylindrical holes in them. The resulting material properties can then implement invisibility cloaks in flat space. We also describe a method for performing geometric ray tracing in these materials which are both inhomogeneous and anisotropic in their electric permittivity and magnetic permeability
Planar integrated metasurfaces for highly-collimated terahertz quantum cascade lasers
We report planar integration of tapered terahertz (THz) frequency quantum cascade lasers (QCLs) with metasurface waveguides that are designed to be spoof surface plasmon (SSP) out-couplers by introducing periodically arranged SSP scatterers. The resulting surface-emitting THz beam profile is highly collimated with a divergence as narrow as ~4° × 10°, which indicates a good waveguiding property of the metasurface waveguide. In addition, the low background THz power implies a high coupling efficiency for the THz radiation from the laser cavity to the metasurface structure. Furthermore, since all the structures are in-plane, this scheme provides a promising platform where well-established surface plasmon/metasurface techniques can be employed to engineer the emitted beam of THz QCLs controllably and flexibly. More importantly, an integrated active THz photonic circuit for sensing and communication applications could be constructed by incorporating other optoelectronic devices such as Schottky diode THz mixers, and graphene modulators and photodetectors
Interactions of nanorod particles in the strong coupling regime
The plasmon coupling in a nanorod dimer obeys the exponential size dependence
according to the Universal Plasmon Ruler Equation. However, it was shown
recently that such a model does not hold at short nanorod distance (Nano Lett.
2009, 9, 1651). Here we study the nanorod coupling in various cases, including
nanorod dimer with the asymmetrical lengths and symmetrical dimer with the
varying gap width. The asymmetrical nanorod dimer causes two plasmon modes: one
is the attractive lower- energy mode and the other the repulsive high-energy
mode. Using a simple coupled LC-resonator model, the position of dimer
resonance has been determined analytically. Moreover, we found that the plasmon
coupling of symmetrical cylindrical (or rectangular) nanorod dimer is governed
uniquely by gap width scaled for the (effective) rod radius rather than for the
rod length. A new Plasmon Ruler Equation without using the fitting parameters
has been proposed, which agrees well with the FDTD calculations. The method has
also been extended to study the plasmonic wave-guiding in a linear chain of
gold nanorod particles. A field decay length up to 2700nm with the lateral mode
size about 50nm (~wavelength/28) has been suggested.Comment: 31 pages, 6 figures, 58 reference
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