We report on light trapping by a moving refractive index front inside a silicon waveguide, the so-called optical push broom effect. The front generated by a fast pump pulse collects and traps the energy of a signal wave with smaller group velocity tuned near to the band gap of the waveguide with hyperbolic dispersion. The energy of the signal wave is accumulated inside the front and distributed in frequency. The presented effect can be utilized to compress signals in time and space.
Dynamic manipulation of light has received considerable attention in recent years. The process of an optical signal undergoing a transition between two modes of a photonic structure is referred to as a photonic transition. We show that a signal wave interacting with a free carrier front in a slow light waveguide experiences indirect photonic transitions leading to reflection from the moving front. Theory and experimental results are presented. The front induced dynamic frequency conversion is also compared to the frequency shifting based on other nonlinear effects like cross-phase modulation and four wave mixing.
The process of an optical signal undergoing a transition between two modes of a photonic structure is referred to as a photonic transition. We show that a signal wave interacting with a free carrier front in a slow light waveguide experiences indirect photonic transitions leading to transmission or reflection from the moving front. Theory and experimental results are presented. The front induced dynamic frequency conversion is also compared to the frequency shifting based on other nonlinear effects like cross-phase modulation and four wave mixing.
Ultrathin crystalline silicon (c-Si) solar cells, which are of several micrometers thick, have attracted much attention in recent years, since it can greatly save raw materials than the traditional ones. To enhance the absorption, as well as to improve the cell efficiency, of the ultrathin c-Si, light trapping nanostructures are used to increase the effective absorption length to close to the 4n2 of the materials thickness, which is determined by the Lambertian limit. Here, we propose a novel interlaced semi-ellipsoid nanostructures (ISENs) to improve the performance of ultrathin c-Si solar cells. In this structure, the large and small periods in x and y direction can improve the light trapping capability at long and short wavelengths respectively. Meanwhile, the graded refractive index of the surface can act as the antireflection coating. By optimizing the ISENs, the short circuit current density of 30.15mA/cm2 was achieved by simulations for a 2 μm thick c-Si solar cell with rx = 500 nm, ry = 200 nm, rz= 550 nm and without antireflection coating and metal back reflector. The absorption is close to 87% of the Lambertian limit with equivalent thickness. We expect this structure can be fabricated by low cost nanosphere lithography technology and used to improve the efficiency of the ultrathin c-Si solar cells.
The dynamic manipulation of light can be achieved by the interaction of a signal pulse propagating through or reflected
from a refractive index front. Both the frequency and the wave vector of the signal are changed in this case, which is
generally referred to as an indirect transition. We have developed a theory to describe such transitions in integrated
photonic crystal waveguides. Through indirect transitions, the following effects can be envisaged: large frequency shifts
and light stopping and order of magnitude pulse compression and broadening without center frequency shift. All effects
can be potentially realized with a refractive index modulation as small as 0.001.
For the experimental realization, we have used slow light photonic crystal waveguides in silicon. The refractive index
front was obtained by free carriers generation with a switching pulse co-propagating with the signal in the same slow
light waveguide. The group velocities of the signal and the front could be varied arbitrarily by choosing the right
frequencies of the signal and switching pulses. The indirect transition was unambiguously demonstrated by considering
two situations: a) the front overtaking the signal and b) the signal overtaking the front. In both cases, a blue shift of the
signal frequency was observed. This blue shift can only be explained by the occurrence of the expected indirect
transition and not by a direct transition without wave vector variation.
Photonic bandgap effects in reflectance and photoluminescence of silica opal-based photonic crystal with embedded europium (III) salt of ethylenediaminetetraacetic acid (HEuEDTA) were studied. The position of the bandgap was accurately fitted to the wavelengths of visible lines of Eu3+ photoluminescence by proper choice of the size of SiO2 spheres. High spatial anisotropy of photoluminescence was observed by spatially- and spectrally-resolved laser spectroscopic measurement. The influence of crystal structure quality on spatial anisotropy of photonic bandgap and photoluminescence is discussed.