Optoelectronics and Nanostructure Science
Graduate School of Science and Technology, Shizuoka University, Japan
Department of Physics, University of Dhaka
Department of Physics, University of Dhaka
Slow and first image propagation through single and coupled image resonators
We applied Fourier space analysis to a comprehensive study of the propagation of pulsed two-dimensional images through single and coupled image resonators. The Fourier method shows that the image can propagate through the resonator successfully as long as the spatial and temporal Fourier components of the image are within the bandwidth of the amplitude and phase transfer functions. The relevant steep dispersion of the cavity can yield delayed or advanced images. The Fourier method reproduces characteristic aspects of the experimental observations of the image propagation, and also predicts new aspects, such as the spatial image profile dependence on the observation time and the coupling strength. To demonstrate the time evolution of the experiment, space- and time-resolved image propagations were performed using a streak camera.
Optical precursors in coupled-resonator-induced transparency
We experimentally examined the propagation of temporally square modulated optical pulses through a coupled ring resonator. Sharp transient spikes appeared as the square pulses entered the system. The main signal gradually grew up through coupled-resonator-induced transparency (CRIT), with the time constant determined by a second resonator. Transient spikes were attributed to the higher and lower spectral components of the incident pulse, to which the resonators cannot respond; hence, they were interpreted as optical precursors. The experiments, therefore, demonstrated that precursors and the main signal can be observed separately, with amplitudes comparable to that of the incident step in CRIT.
Causal information velocity in fast and slow pulse propagation in an optical ring resonator
We examined the propagation of nonanalytical points encoded on temporally Gaussian-shaped optical pulses in fast and slow light in an optical ring resonator at λ = 1.5 μm. The temporal peak of the Gaussian pulse was either advanced or delayed, reflecting anomalous or normal dispersions in the ring resonator, relevant to under- or overcoupling conditions, respectively. The nonanalytical points were neither advanced nor delayed but appeared as they entered the ring resonator. The nonanalytical points could be interpreted as information; therefore, the experimental results suggested that information velocity is equal to the light velocity in vacuum or the background medium, independent of the group velocity. The transient behaviors at the leading and trailing edges of the nonanalytical points are discussed in terms of optical precursors.
Slow optical pulse propagation in an amplifying ring resonator
We experimentally investigated optical pulse propagation through an amplifying ring resonator, in which a semiconductor optical amplifier was used as a loss or gain medium. Initially, the transmission intensity showed resonance dips as a function of laser frequency. These dips were gradually filled and transformed into resonance peaks as the amplification gain increased. A Gaussian-shaped temporal pulse was either advanced by −7.3 ns−7.3 ns at low amplification gain or delayed up to 45 ns45 ns at high gain. System operation was classified into a complete set of the four regions depending on the gain parameter, and experimental delay times were analyzed based on relevant system dispersion.
Delayed optical images through coupled-resonator-induced transparency
We propagated transverse two-dimensional images encoded on optical pulses through a frequency window of a coupled-image-resonator-induced transparency. The optical images are stored and delayed by 10.6ns10.6ns, reflec ting the tunable dispersion of the coupled resonator. The k-space bandwidth of the amplitude transfer func tion of the system is discussed in the presence of the off-resonance Fano interference effect between the two resonators.
Advanced and delayed images through an image resonator
We performed optical image propagation experiments in an image resonator consisting of a Fabry-Perot resonator in reflection geometry. Two-dimensional images encoded on optical pulses of 32ns were stored, and either advanced, −6.0ns, or delayed, 10.9ns, using the dispersion relation relevant to the image resonator, in the under- or over- coupling condition, respectively. The overall images are propagated through the resonator clearly, while the diffraction effects were analyzed both in real-space and in k-space.
Power dependence of size of laser ablated colloidal silver nanoparticles
Silver nanoparticles have been produced by laser ablation of silver metal in nanopure water without any chemical additives. It has been observed that laser power has a control over the size of the nanoparticles. Increasing laser power shows a clear blue shift in the absorption peak of fabricated nanoparticles indicating that the average size of the particles decreases with increasing laser power. Ablation for longer period reduces the average size of nanoparticles which is attributed to the re-ablation of fabricated nanoparticles. A good correlation has been observed between the peak of the absorption spectrum measured by UV-VIS spectroscopy and the average particle size measured by scanning electron microscope imaging method. The value of the coefficient of correlation is determined to be 0.965.