This paper presents an experimental demonstration of a visible light communications link with an light emitting diode and a low-bandwidth organic photodetector as transmitter and receiver, respectively, that achieves sub 4 Mbits/s speeds. An artificial neural network (ANN) equalizer is required in order to achieve such high data rates because of the influence of intersymbol interference. The digital modulation formats tested in this paper are nonreturn-to-zero on–off keying (OOK), and fourth-order pulse position modulation (4-PPM). Without equalization, data rates of 200 and 300 kbits/s can be achieved for 4-PPM and OOK, respectively. With ANN equalization, data rates of 2.8 and 3.75 Mbits/s can be achieved for the first time for OOK and 4-PPM, respectively.

Germanium tin (GeSn) is a group IV semiconductor with a direct band-to-band transition below 0.8 eV. Nonequilibrium GeSn alloys up to 20% Sn content were realized with low temperature (160°C) molecular beam epitaxy. Photodetectors and light emitting diodes (LEDs) were realized from in situ doped pin junctions in GeSn on Ge virtual substrates. The detection wavelength for infrared radiation was extended to 2 μm with clear potential for further extension into the mid-infrared. GeSn LEDs with Sn content of up to 4% exhibit light emission from the direct band transition, although GeSn with low Sn content is an indirect semiconductor. The photon emission energies span the region between 0.81 and 0.65 eV. Optical characterization techniques such as ellipsometry, in situ reflectometry, and Raman spectroscopy were used to monitor the Sn incorporation in GeSn epitaxy.

We revisit the electrodynamics of resonant high-Q interactions in atomic systems with a view to gaining insights into the design of meta-atoms and hence bulk metamaterials with profoundly different electromagnetic responses. The relevance of phase coherence and nonlinearity in charged systems is emphasized, as is the need to take care over defining how one specifies effective boundaries and cavities that ultimately determine light–matter interactions. Radically new material properties become apparent once one designs organized clusters of small numbers of atoms or meta-atoms for which the usually applied random phase approximation (RPA) does not apply. The RPA relies on averages in sufficiently large volumes consisting of large numbers of interacting systems, while our model assumes a small volume with averages in time, i.e., ergodicity. New meaning is given to the concept of effective and practically useful constitutive parameters, based on this very fundamental point of view, which is important to metamaterials.

The factors that influence the generation of a high-quality optical frequency comb (OFC) based on a recirculating frequency shifter (RFS) due to the maximum output power and noise figure of Er-doped fiber amplifier (EDFA) are studied theoretically and experimentally. Based on the theoretical analysis, numerical simulations and experiments under different EDFA parameters have been carried out. The results show that the performance of the OFC based on a RFS can be improved effectively by optimizing the maximum output power and the noise figure of the EDFA.

In this paper, we describe a new type of digital-to-analog converter (DAC) for optical wireless communication. Conversion occurs in the optical rather than the electrical domain. The overall intensity of the light transmitted by an array of light-emitting diodes (LEDs) is varied by changing the number of LEDs that are switched on. A number of different structures are described, and their compatibility with light dimming and overall energy efficiency are discussed. The linearity of the new DAC depends on the geometry of the system and on the variability in light output between individual LEDs.

We present imaging experiments in focusing Kerr media using digital holography and digital reverse propagation (DRP) of the wave. For moderate power, the nonlinear DRP algorithm can be used to improve the quality of images over the linear DRP. We discuss the limits of the method at high power, the role of small-scale filaments, and the problem of time-dependent self-phase modulation.