Metamaterial emitter for thermophotovoltaics stable up to 1400 °C.

High temperature stable selective emitters can significantly increase efficiency and radiative power in thermophotovoltaic (TPV) systems. However, optical properties of structured emitters reported so far degrade at temperatures approaching 1200 °C due to various degradation mechanisms. We have realized a 1D structured emitter based on a sputtered W-HfO2 layered metamaterial and demonstrated desired band edge spectral properties at 1400 °C. To the best of our knowledge the temperature of 1400 °C is the highest reported for a structured emitter, so far. The spatial confinement and absence of edges stabilizes the W-HfO2 multilayer system to temperatures unprecedented for other nanoscaled W-structures. Only when this confinement is broken W starts to show the well-known self-diffusion behavior transforming to spherical shaped W-islands. We further show that the oxidation of W by atmospheric oxygen could be prevented by reducing the vacuum pressure below 10-5 mbar. When oxidation is mitigated we observe that the 20 nm spatially confined W films survive temperatures up to 1400 °C. The demonstrated thermal stability is limited by grain growth in HfO2, which leads to a rupture of the W-layers, thus, to a degradation of the multilayer system at 1450 °C.

Author Correction: Reflection from a free carrier front via an intraband indirect photonic transition.

The original version of this article contained an error in first sentence of the Acknowledgements, which incorrectly read 'M.A.G, D.J., A.Y.P. and M.E. acknowledge the support of the German Research Foundation under grant no. EI 391/13-2, and appreciate the support of CST, Darmstadt, Germany, with their Microwave Studio Software.' The correct version states '261759120' in place of 'EI 391/13-2'. This has been corrected in both the PDF and HTML versions of the article.

Photonic glass for high contrast structural color.

Non-iridescent structural colors based on disordered arrangement of monodisperse spherical particles, also called photonic glass, show low color saturation due to gradual transition in the reflectivity spectrum. No significant improvement is usually expected from particles optimization, as Mie resonances are broad for small dielectric particles with moderate refractive index. Moreover, the short range order of a photonic glass alone is also insufficient to cause sharp spectral features. We show here, that the combination of a well-chosen particle geometry with the short range order of a photonic glass has strong synergetic effects. Using a first-order approximation and an Ewald sphere construction the reflectivity of such structures can be related to the Fourier transform of the permittivity distribution. The Fourier transform required for a highly saturated color can be achieved by tailoring the substructure of the motif. We show that this can be obtained by choosing core-shell particles with a non-monotonous refractive index distribution from the center of the particle through the shell and into the background material. The first-order theoretical predictions are confirmed by numerical simulations.

Ewald sphere construction for structural colors.

Disordered structures producing a non-iridescent color impression have been shown to feature a spherically shaped Fourier transform of their refractive-index distribution. We determine the direction and efficiency of scattering from thin films made from such structures with the help of the Ewald sphere construction which follows from first-order scattering approximation. This way we present a simple geometrical argument why these structures are well suited for creating short wavelength colors like blue but are hindered from producing long wavelength colors like red. We also numerically synthesize a model structure dedicated to produce a sharp spherical shell in reciprocal space. The reflectivity of this structure as predicted by the first-order approximation is compared to direct electromagnetic simulations. The results indicate the Ewald sphere construction to constitute a simple geometrical tool that can be used to describe and to explain important spectral and directional features of the reflectivity. It is shown that total internal reflection in the film in combination with directed scattering can be used to obtain long wavelength structural colors.

Reflection from a free carrier front via an intraband indirect photonic transition.

The reflection of light from moving boundaries is of interest both fundamentally and for applications in frequency conversion, but typically requires high pump power. By using a dispersion-engineered silicon photonic crystal waveguide, we are able to achieve a propagating free carrier front with only a moderate on-chip peak power of 6 W in a 6 ps-long pump pulse. We employ an intraband indirect photonic transition of a co-propagating probe, whereby the probe practically escapes from the front in the forward direction. This forward reflection has up to 35% efficiency and it is accompanied by a strong frequency upshift, which significantly exceeds that expected from the refractive index change and which is a function of group velocity, waveguide dispersion and pump power. Pump, probe and shifted probe all are around 1.5 µm wavelength which opens new possibilities for "on-chip" frequency manipulation and all-optical switching in optical telecommunications.

Single mode thermal emission.

We report on the properties of a thermal emitter which radiates into a single mode waveguide. We show that the maximal power of thermal radiation into a propagating single mode is limited only by the temperature of the thermal emitter and does not depend on other parameters of the waveguide. Furthermore, we show that the power of the thermal emitter cannot be increased by resonant coupling. For a given temperature, the enhancement of the total emitted power is only possible if the number of excited modes is increased. Either a narrowband or a broadband thermal excitation of the mode is possible, depending on the properties of the emitter. We finally discuss an example system, namely a thermal source for silicon photonics.

Pulse compression and broadening by reflection from a moving front of a photonic crystal.

Previously, the effect of pulse bandwidth compression or broadening was observed in reflection from a moving front together with the Doppler shift. In this letter, an approach is presented, which alters pulse bandwidth without change in the central frequency. It occurs when light is reflected from a moving front of an otherwise stationary photonic crystal. This means that the photonic crystal lattice as such is stationary and only its boundary to the environment is moving, thus extruding (or shortening) the photonic crystal medium. The compression (broadening) factor depends on the front velocity and is the same as for the conventional Doppler shift. Complete reflection and transformation of the pulse can be achieved even with weak refractive index contrast, what makes the approach experimentally viable.

Optical three-port circulators made with ring resonators.

We propose a circulator consisting of a ring resonator coupled to three waveguides with Bragg reflectors at one end of each waveguide. A magneto-optically active material placed inside the ring resonator causes the two counter-propagating modes to split in resonance frequency, which can be exploited for perfect circulation by properly adjusting the coupling between the three waveguides and the ring. Such a device features a transmission spectrum that is similar to three-port photonic crystal circulators but is much simpler to build as it only contains elements that have already been experimentally realized.

Comment on "Nonreciprocal light propagation in a silicon photonic circuit".

We show that the structure demonstrated by Feng et al. (Reports, 5 August 2011, p. 729) cannot enable optical isolation because it possesses a symmetric scattering matrix. Moreover, one cannot construct an optical isolator by incorporating this structure into any system as long as the system is linear and time-independent and is described by materials with a scalar dielectric function.

Resonance splitting in gyrotropic ring resonators.

We present the theoretical concept of an optical isolator based on resonance splitting in a silicon ring resonator covered with a magneto-optical polymer cladding. For this task, a perturbation method is derived for the modes in the cylindrical coordinate system. A polymer magneto-optical cladding causing a 0.01 amplitude of the off-diagonal element of the dielectric tensor is assumed. It is shown that the derived resonance splitting of the clockwise and counterclockwise modes increases for smaller ring radii. For the ring with a radius of approximately 1.5µm, a 29GHz splitting is demonstrated. An integrated optical isolator with a 10µm geometrical footprint is proposed based on a critically coupled ring resonator.