82 research outputs found
Two photon absorption in Mn<SUP>2+</SUP>-doped ZnSe quantum dots
The nonresonant third order nonlinear optical properties of three different sized Mn2+-doped ZnSe quantum dots (QDs) are investigated. The nonlinear absorption is measured at 532 nm using 35 picosecond laser pulses in an open-aperture Z-scan setup. Two photon absorption (2PA) cross-section in ZnSe QDs is found to be three orders of magnitude higher than its bulk value. These nanostructures show size dependent nonlinear absorption coefficients. It is found that the 2PA further enhances with decrease in size of the QD and is twice that of its undoped counterpart due to change in the local electric field
Two-photon absorption in ZnSe and ZnSe/ZnS core/shell quantum structures
The third order nonlinear optical properties of two different sized ZnSe and ZnSe/ZnS quantum dots (QDs) are investigated. The nonlinear absorption is measured at 806 nm using Ti:sapphire 100 fs laser pulses in an open aperture Z-scan setup. Two-photon absorption (2PA) is found to be dominant in core and core shell QDs. 2PA cross section is enhanced by three orders of magnitude compared bulk ZnSe. 2PA cross section is observed to increase with reduction in QD diameter, due to strong confinement effect. ZnSe/ZnS QDs exhibit higher 2PA cross section compared with corresponding ZnSe QDs, indicating better passivation of the QD surface
Contrasting levels of absorption of intense femtosecond laser pulses by solids
The absorption of ultraintense, femtosecond laser pulses by a solid unleashes relativistic electrons, thereby creating a regime of relativistic optics. This has enabled exciting applications of relativistic particle beams and coherent X-ray radiation, and fundamental leaps in high energy density science and laboratory astrophysics. Obviously, central to these possibilities lies the basic problem of understanding and if possible, manipulating laser absorption. Surprisingly, the absorption of intense light largely remains an open question, despite the extensive variations in target and laser pulse structures. Moreover, there are only few experimental measurements of laser absorption carried out under very limited parameter ranges. Here we present an extensive investigation of absorption of intense 30 femtosecond laser pulses by solid metal targets. The study, performed under varying laser intensity and contrast ratio over four orders of magnitude, reveals a significant and non-intuitive dependence on these parameters. For contrast ratio of 10-9 and intensity of 2 × 1019W cm-2, three observations are revealed: preferential acceleration of electrons along the laser axis, a ponderomotive scaling of electron temperature, and red shifting of emitted second-harmonic. These point towards the role of J × B absorption mechanism at relativistic intensity. The experimental results are supported by particle-in-cell simulations
Controlling intense, ultrashort, laser-driven relativistic mega-ampere electron fluxes by a modest, static magnetic field
The guiding and control of ultrahigh flux, femtosecond relativistic electron
pulses through solid density matter is of great importance for many areas of
high energy density science. Efforts so far include the use of magnetic fields
generated by the propagation of the electron pulse itself or the application of
hundreds of Tesla magnitudes, pulsed external magnetic fields driven by either
short pulse lasers or electrical pulses. Here we experimentally demonstrate the
guiding of hundreds of keV mega-ampere electron pulses in a magnetized
neodymium solid that has a very modest, easily available static field of 0.1
tesla. The electron pulses driven by an ultrahigh intensity, 30 femtosecond
laser are shown to propagate beam-like, a distance as large as 5 mm in a high Z
target (neodymium), their collimation improved and flux density enhanced nearly
by a factor of 3. Particle-in-cell simulations in the appropriate parameter
regime match the experimental observations. In addition, the simulations
predict the occurrence of a novel, near-monochromatic feature towards the high
energy end of the electron energy spectrum, which is tunable by the applied
magnetic field strength. These results may prove valuable for fast electron
beam-driven radiation sources, fast ignition of laser fusion, and laboratory
astrophysics.Comment: 10 pages, 5 figure
Magnetic Behavior of Manganese-Doped ZnSe Quantum Dots
Magnetic properties of manganese-doped ZnSe quantum dots with the size of approximately 3.6 nm are investigated. The amount of Mn in the ZnSe quantum dots has been varied from 0.10% to 1.33%. The doping level in the quantum dots is much less than that used in the precursor. The co-ordination of Mn in the ZnSe lattice has been determined by electron paramagnetic resonance (EPR). Two different hyperfine couplings 67.3×10−4 and 60.9×10−4 cm−1 observed in the EPR spectrum imply that Mn atoms occupy two distinct sites; one uncoordinated (near the surface) and other having a cubic symmetric environment (nanocrystal core), respectively. Photoluminescence measurements also confirm the incorporation of Mn in ZnSe quantum dots. From the Curie-Weiss behavior of the susceptibility, the effective Mn-Mn antiferromagnetic exchange constant (J1) has been evaluated. The spin-glass behavior is observed in 1.33% Mn-doped ZnSe quantum dots, at low temperature. Magnetic behavior at a low temperature is discussed
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