Giant optical activity in dielectric planar metamaterials with 2D chirality

For the first time, all-dielectric planar chiral metamaterials consisting of arrays of silicon nitride gammadions on fused silica substrates have been fabricated, and shown to be capable of inducing large changes to the polarization states of transmitted light in a manner that is dependent on the two-dimensional chirality of the microstructured silicon nitride film. The polarization response is found to reverse for opposite enantiomers, and also for the same enantiomer when it is illuminated from opposite sides of the structure. In addition, the polarization states of the various diffracted beams are found to be non-reversible. These structures therefore appear to display elements of non-reciprocal behaviour. The polarization responses of complementary designs, different chiral geometries and various silicon nitride film thicknesses have also been studied. As a result we conclude that multiple reflections within the patterned silicon nitride layer play an important role in defining the mechanism by which these structures are able to modify the polarization states of diffracted light

Silicon and Germanium Nanostructures for Photovoltaic Applications: Ab-Initio Results

Actually, most of the electric energy is being produced by fossil fuels and great is the search for viable alternatives. The most appealing and promising technology is photovoltaics. It will become truly mainstream when its cost will be comparable to other energy sources. One way is to significantly enhance device efficiencies, for example by increasing the number of band gaps in multijunction solar cells or by favoring charge separation in the devices. This can be done by using cells based on nanostructured semiconductors. In this paper, we will present ab-initio results of the structural, electronic and optical properties of (1) silicon and germanium nanoparticles embedded in wide band gap materials and (2) mixed silicon-germanium nanowires. We show that theory can help in understanding the microscopic processes important for devices performances. In particular, we calculated for embedded Si and Ge nanoparticles the dependence of the absorption threshold on size and oxidation, the role of crystallinity and, in some cases, the recombination rates, and we demonstrated that in the case of mixed nanowires, those with a clear interface between Si and Ge show not only a reduced quantum confinement effect but display also a natural geometrical separation between electron and hole

Current and future photovoltaics

Photovoltaics, now a billion-dollar industry, is experiencing staggering growth as increased concerns over fuel supply and carbon emissions have encouraged governments and environmentalists to become increasingly prepared to offset the extra cost of solar energy. Three 'generations' of photovoltaics have been envisaged that will take solar power into the mainstream. Photovoltaic production is currently 90% 'first-generation' or '1G' solar cells that rely on expensive bulk multi-crystalline or single-crystal semiconductors. Dominated by silicon wafers, they are reliable and durable but expensive. Half of the cost of 1G devices is the silicon wafer and efficiencies are limited to around 20%. Instead of using wafers, cheaper 'second-generation' (2G) solar cells would use cheap semiconductor thin-films deposited on low-cost substrates to produce devices of similar efficiencies. A number of thin-film device technologies account for around 5–6% of the market. As 2G technology reduces the active material cost, eventually the substrate will be the cost limit, and higher efficiency will be needed to maintain the $/W cost-reduction trend. 'Third-generation' devices (3G) will utilise new technologies to produce high-efficiency devices. Recently, tremendous advances outside the photovoltaic industry in nanotechnologies, photonics, optical metamaterials, plasmonics and semiconducting polymer sciences offer the prospect of cost-competitive photovoltaics based on new science and 3G concepts. Within the next 20 years, it is reasonable to expect that cost reductions, a move to 2G technologies and the implementation of some new technologies and 3G concepts can lead to fully cost-competitive solar energy

Bio-mimetic subwavelength surface for near-zero reflection sunrise to sunset

We present a study of antireflective schemes and their operation over a full day. We compare simulation results for single and double layer antireflective coatings with biomimetic moth-eye structures, taking into account the full range of wavelengths and incident angles experienced by fixed solar cells from sunrise to sunset. We show that solar cells incorporating antireflective moth-eye arrays could produce up to 12% more energy than those employing single layer antireflective coatings

Double-polysilicon self-aligned lateral bipolar transistors

A new lateral bipolar junction transistor that utilises a double-polysilicon self-aligned structure to maximise high-frequency performance is introduced. Silicon-on-oxide (SOI) wafers are used to isolate devices from the substrate and to minimise parasitic substrate capacitances(CJCS0) around 1.3–2.6 fF (substrate is ground). A SOI thickness of 0.2–0.5 μm combined with 0.13–0.25 μm lithography could allow a reduction of transistor dimensions down to (0.2–0.5) · (0.13–0.25) lm2 and give an estimated minimum emitter/base junction capacitance(CJE0) of 0.54–1.36 fF. Simple device isolation is predicted to produce a small collector/base junction capacitance (CJC0) of 0.42–2.00 fF. Furthermore, use of a double base contact can help reduce base resistance (RB) to 0.43–1.17 kW and a wide collector window directly contacted to the collector is estimated to result in around 0.66–1.58 kW collector resistance (RC). By taking all parameters into account a cut-off frequency (fT) of 69–116 GHz and maximum oscillation frequency (fmax) of 61–128 GHz is predicted for this design, in addition a gain of 47–101(using minimum gain enhancement) and roughly 10.6–21.0 ps ECL propagation delay time, at a current of 0.4–1.0 mA could be achieved. Our simulations indicate that this new doubled-polysilicon self-aligned structure could outperform all other silicon bipolar transistors that have been reported

A new model of geometric chirality for two-dimensional continuous media and planar meta-materials

We have, for the first time, identified ten tenets of two-dimensional (2D) chirality that define and encapsulate the symmetry and scaling behaviour of planar objects and have used them to develop three new measures of geometric 2D chirality. All three models are based on the principle of overlap integrals and can be expressed as simple analytical functions of the two-dimensional surface density, rho(r). In this paper we will compare the predicted behaviour of these models and show that two of them are fully integrable and scalable and can therefore be applied to both discrete and continuous 2D systems of any finite size, or any degree of complexity. The only significant difference in these two models appears in their behaviour at infinite length scales. Such differences could, however, have profound implications for the analysis of chirality in new generations of planar meta-materials, such as chiral arrays, fractals, quasi-periodic 2D crystals and Penrose tiled structures

Si/SiGe near-infrared photodetectors grown using low pressure chemical vapour deposition

Near-infrared photodetectors have been fabricated using standard CMOS processes in conjunction with the multilayer growth of Si/SiGe0.06 using low-pressure chemical vapor deposition (LPCVD). Cross-section scanning electron microscopy (SEM) indicates the existence of quantum dot like corrugations in devices with particularly thick SiGe0.06 quantum wells. With an accumulation of germanium atoms at the crest of such features and commensurate high germanium concentration we see a considerable enhancement of the long wavelength detection sensitivity of photodetectors in the range 1100–1300 nm. By fitting experimental data the minimum energy gap of the structure is found to be 0.88 eV corresponding to a germanium concentration of around 15%

Nonreciprocal diffraction through dielectric gratings with two-dimensional chirality

It is generally assumed that the propagation of light through a dielectric medium is always reciprocal under reversal of the propagation direction. We show that in an all-dielectric diffractive system nonreciprocal polarization changes are possible for individual diffracted beams when the diffractive medium possesses two-dimensional chirality. This nonreciprocity is characterized by different eigenstates for the system for the forward and reverse directions respectively, yet it is also entirely consistent with the predictions of the Lorentz reciprocity lemma