arXiv subjects

Martin Wegener

Publications and source records attributed to Martin Wegener.

45 records · Page 3Linked to original sources

arXiv2011

Carrier-envelope phase sensitive inversion in two-level systems

We theoretically study the carrier-envelope phase dependent inversion generated in a two-level system by excitation with a few-cycle pulse. Based on the invariance of the inversion under time reversal of the exciting field, parameters are introduced to characterize the phase sensitivity of the induced inversion. Linear and nonlinear phase effects are numerically studied for rectangular and sinc-shaped pulses. Furthermore, analytical results are obtained in the limits of weak fields as well as strong dephasing, and by nearly degenerate perturbation theory for sinusoidal excitation. The results show that the phase sensitive inversion in the ideal two-level system is a promising route for constructing carrier-envelope phase detectors.

physics.atom-ph↗

arXiv2011

Three-dimensional direct laser writing inspired by stimulated-emission-depletion microscopy

Three-dimensional direct laser writing has become a well established, versatile, widespread, and even readily commercially available "workhorse" of nano- and micro-technology. However, its lateral and axial spatial resolution is inherently governed by Abbe's diffraction limitation - analogous to optical microscopy. In microscopy, stimulated-emission-depletion approaches have lately circumvented Abbe's barrier and lateral resolutions down to 5.6 nm using visible light have been achieved. In this paper, after very briefly reviewing our previous efforts with respect to translating this success in optical microscopy to optical lithography, we present our latest results regarding resolution improvement in the lateral as well as in the much more relevant axial direction. The structures presented in this paper set a new resolution-benchmark for next-generation direct-laser-writing optical lithography. In particular, we break the lateral and the axial Abbe criterion for the first time.

physics.optics↗

arXiv2010

Newtonian photorealistic ray tracing of grating cloaks and correlation-function-based cloaking-quality assessment

Grating cloaks are a variation of dielectric carpet (or ground- plane) cloaks. The latter were introduced by Li and Pendry. In contrast to the numerical work involved in the quasi-conformal carpet cloak, the refractive-index profile of a conformal grating cloak follows a closed and exact analytical form. We have previously mentioned that finite-size conformal grating cloaks may exhibit better cloaking than usual finite-size carpet cloaks. In this letter, we directly visualize their performance using photorealistic ray-tracing simulations. We employ a Newtonian approach that is advantageous compared to conventional ray tracing based on Snell's law. Furthermore, we quantify the achieved cloaking quality by computing the cross-correlations of rendered images. The cross-correlations for the grating cloak are much closer to 100% (i.e., ideal) than those for the Gaussian carpet cloak.

physics.optics↗

arXiv2010

3D Polarization-Independent Visible Invisibility

We fabricate and characterize a three-dimensional polarization-independent invisibility cloak operating at visible wavelengths of light. The required drastic miniaturization has become possible by employing stimulated-emission-depletion inspired direct-laser-writing optical lithography with conceptually diffraction-unlimited axial and lateral resolution.

physics.optics↗

arXiv2010

Conformal carpet and grating cloaks

We introduce a class of conformal versions of the previously introduced quasi-conformal carpet cloak, and show how to construct such conformal cloaks for different cloak shapes. Our method provides exact refractive-index profiles in closed mathematical form for the usual carpet cloak as well as for other shapes. By analyzing their asymptotic behavior, we find that the performance of finite-size cloaks becomes much better for metal shapes with zero average value, e.g., for gratings.

physics.optics↗

arXiv2010

Arrays of Ag split-ring resonators coupled to InGaAs single-quantum-well gain

We study arrays of silver split-ring resonators operating at around 1.5-μm wavelength coupled to an MBE-grown single 12.7-nm thin InGaAs quantum well separated only 4.8 nm from the wafer surface. The samples are held at liquid-helium temperature and are pumped by intense femtosecond optical pulses at 0.81-μm center wavelength in a pump-probe geometry. We observe much larger relative transmittance changes (up to about 8%) on the split-ring-resonator arrays as compared to the bare quantum well (not more than 1-2%). We also observe a much more rapid temporal decay component of the differential transmittance signal of 15 ps for the case of split-ring resonators coupled to the quantum well compared to the case of the bare quantum well, where we find about 0.7 ns. The latter observation is ascribed to the Purcell effect that arises from the evanescent coupling of the split-ring resonators to the quantum-well gain. All experimental results are compared with a recently introduced analytical toy model that accounts for this evanescent coupling, leading to excellent overall qualitative agreement.

physics.optics↗

arXiv2008

Negative-index bi-anisotropic photonic metamaterial fabricated by direct laser writing and silver shadow evaporation

We present the blueprint for a novel negative-index metamaterial. This structure is fabricated via three-dimensional two-photon direct laser writing and silver shadow evaporation. The comparison of measured linear optical spectra with theory shows good agreement and reveals a negative real part of the refractive index at around 3.85 micrometer wavelength - despite the fact that the metamaterial structure is bi-anisotropic due to the lack of inversion symmetry along its surface normal.

physics.optics↗

arXiv2008

Toy model for plasmonic metamaterial resonances coupled to two-level system gain

We propose, solve, and discuss a simple model for a metamaterial incorporating optical gain: A single bosonic resonance is coupled to a fermionic (inverted) two-level-system resonance via local-field interactions. For given steady-state inversion, this model can be solved analytically, revealing a rich variety of (Fano) absorption/gain lineshapes. We also give an analytic expression for the fixed inversion resulting from gain pinning under steady-state conditions. Furthermore, the dynamic response of the 'lasing SPASER', i.e., its relaxation oscillations, can be obtained by simple numerical calculations within the same model. As a result, this toy model can be viewed as the near-field-optical counterpart of the usual LASER rate equations.

physics.optics↗

arXiv2006

Negative-index metamaterial at 780 nm wavelength

We further miniaturize a recently established silver-based negative-index metamaterial design. By comparing transmittance, reflectance and phase-sensitive time-of-flight experiments to theory, we infer a real part of the refractive index of -0.6 at 780 nm wavelength -- which is visible in the laboratory.

physics.optics↗