Jan Heckmann

Directed emission of CdSe nanoplatelets originating from strongly anisotropic 2D electronic structure

Intrinsically directional light emitters are potentially important for applications in photonics including lasing and energy-efficient display technology. Here, we propose a new route to overcome intrinsic efficiency limitations in light-emitting devices by studying a CdSe nanoplatelets monolayer that exhibits strongly anisotropic, directed photoluminescence. Analysis of the two-dimensional k-space distribution reveals the underlying internal transition dipole distribution. The observed directed emission is related to the anisotropy of the electronic Bloch states governing the exciton transition dipole moment and forming a bright plane. The strongly directed emission perpendicular to the platelet is further enhanced by the optical local density of states and local fields. In contrast to the emission directionality, the off-resonant absorption into the energetically higher 2D-continuum of states is isotropic. These contrasting optical properties make the oriented CdSe nanoplatelets, or superstructures of parallel-oriented platelets, an interesting and potentially useful class of semiconductor-based emitters.

The dual annihilation of a surface plasmon and a photon by virtue of a three-wave mixing interaction

The enhanced nonlinear interactions that are driven by surface-plasmon resonances have readily been exploited for the purpose of optical frequency conversion in metallic structures. As of yet, however, little attention has been payed to the exact particulate nature of the conversion process. We show evidence that a surface plasmon and photon can annihilate simultaneously to generate a photon having the sum frequency. The signature for this nonlinear interaction is revealed by probing the condition for momentum conservation using a two-beam k-space spectroscopic method that is applied to a gold film in the Kretschmann geometry. The inverse of the observed nonlinear interaction-an exotic form of parametric down-conversion-would act as a source of surface plasmons in the near-field that are quantum correlated with photons in the far-field.

Directed Two-Photon Absorption in CdSe Nanoplatelets Revealed by k-Space Spectroscopy

We show that two-photon absorption (TPA) is highly anisotropic in CdSe nanoplatelets, thus promoting them as a new class of directional two-photon absorbers with large cross sections. Comparing two-dimensional k-space spectroscopic measurements of the one-photon and two-photon excitation of an oriented monolayer of platelets, it is revealed that TPA into the continuum is a directional phenomenon. This is in contrast to one-photon absorption. The observed directional TPA is shown to be related to fundamental band anisotropies of zincblende CdSe and the ultrastrong anisotropic confinement. We recover the internal transition dipole distribution and find that this directionality arises from the intrinsic directionality of the underlying Bloch and envelope functions of the states involved. We note that the photoemission from the CdSe platelets is highly anisotropic following either one- or two-photon excitation. Given the directionality and high TPA cross-section of these platelets, they may, for example, find employment as efficient logic AND elements in integrated photonic devices, or directional photon converters.

Surface-wave phenomena and anisotropic photoluminescence in nano-film structures

The investigation of surfaces and thin films is of particular interest in current research as it provides a basis for a multiplicity of applications such as waveguides, sensors, solar cells and optoelectronics. The losses of light emitting structures, here CdSe nano-platelets, can be reduced by harmonizing the orientation of the transition dipoles with the optical mode that the light is coupled to. The electronic structure of the emitting nanoparticle can be optimized via its shape and the density of states strongly depends on the dielectric function of the environment which can be tuned to modify the emission characteristics.

Low-Loss Surface-Wave Nonlinear Optics with Strongly Absorbing Materials

Our experimental investigation of molybdenum in the Otto configuration shows that an absorbing film is essentially transparent to long-range surface exciton-polaritons, yet it allows a nonlinear interaction leading to the enhanced emission of second-harmonic radiation.

A Hidden Surface Plasmon Resonance Generates Second-Harmonic Light from a Metal Film

The surface plasmon resonance in a metal film is apparent as absorption at one angle of incidence. Our Kretschmann study of gold reveals a separate angle for second-harmonic generation. These phenomena relate to mode hybridization.