Search

We report on the electromagnetic response in metamaterials constituted by 2-dimensional array of metallic elements. The metamaterial transmittance presents a single dipolar resonance which can be easily shifted in the spectral region from 1 to 5 THz through the invariance properties of Maxwell equations under a scale transformation. Basing on these studies, we have designed and tested both direct (bandstop) and complementary (bandpass) filters fabricated by electron beam lithography, investigating the behavior of their figures of merit under the scale transformation. (C) 2010 Elsevier B.V. All rights reserved.

The electronic measurement of the angle between two planes is generally performed by using the so-called electrogoniometers. The major drawback in using such devices is the presence of a fixed hinge that imposes a fixed center of rotation. This can cause problems when measuring the bending angle in some joints, such as Cardan or human joints, which have a variable rotation center. Based on an optical fiber, a sensor measuring the relative angle in a rotating joint has been developed. This joint makes use of the intensity modulation of a laser beam propagating in a single-mode optical fiber, due to the changes of its polarization status originated by the rotation of contiguous portions of the fiber, where controlled birefringence has been induced by the joint rotation. A prototype of this sensor has been developed with a range of the relative angle of 90deg , a resolution of less than 0.01deg, and a standard deviation of 0.1deg. The main advantages of this innovative sensor are lightness, flexibility, high speed of reaction, and high accuracy. This paper describes the development of the proposed sensor, with particular reference to the applications of human joint movement monitoring. Additionally, the equipment implemented for the test is illustrated, and results from laboratory tests are reported and discussed.

The nonlinear refractive index, n2, and the two photon absorption coefficient, β, of semiconductor doped glasses were experimentally investigated in the region of two photon absorption. We demonstrated that the magnitude and sign of the measured n2 strongly depend on the previous exposure of the sample to the laser radiation. For fresh (not exposed) samples the nonlinearity is negative and large. After repeated measurements in the same spot, the magnitude of n2 decreases, eventually becomes positive, and saturates to a value that approximates the value expected from bulk semiconductor theory.

Transport characterizations of wires obtained by electron beam lithography and etching of (100) Si/SiGe heterostructures with a high-mobility two-dimensional electron gas are reported. Depending on the wire width, two different regimes for the electrical transport are found. Wires with a width larger than ~200 nm exhibit metallic behaviour in the quasi-ballistic regime. The conductance dependence on the wire width reveals the presence of a depletion layer, ~100 nm thick, on each etched side of the wire. The wires of width smaller than 200 nm have very large resistance and two different behaviours. The first kind of wires exhibit a zero-current region, compatible with a Coulomb blockade effect involving multiple tunnel junctions or with a space-charge limited current. Other wires are insulating up to applied voltages larger than 5-6 V and their I-V characteristics can be fitted by the functional dependence of voltage-induced tunnelling of Fowler-Nordheim type.

Some relevant features of a particular type of multimirror laser cavity, i.e., those cavities terminating in high-finesse reflective multipass interferometers, are theoretically investigated with the aim of highlighting the dependence of the level of spectral control achievable with these cavities on their geometrical and optical characteristics. It is shown that cavities equipped with suitably designed reflective multipass interferometers are not only capable of forcing a laser operation on a single longitudinal mode but can act at the same time as effective emission frequency stabilizers because of their inherently low sensitivity to any perturbation affecting the cavity optical length. This feature is particularly attractive for intrapulse and pulse-to-pulse stabilization of high power pulsed lasers. Criteria are given also for the design of optimized cavities that could simultaneously provide narrow linewidth laser emission and minimize the effects of perturbations.

We present the design, fabrication and experimental tedt of terahertz bandpass filters made of two-dimensional arrays of Al slot dipoles on Si substrate. The filters are obtained through an inexpensive process with full freedom of design and can be easily implemented for applications in THz spectroscopy.

A novel multipass grating interferometer is presented that is capable of operating as an efficient, very-narrow-band output coupler for high-power, large-bandwidth lasers. Its application for tunable (up to 3.5 GHz), single-mode operation of a TEA CO(2) laser is discussed.

This paper illustrates the design steps of a printed circuit board used as test vehicles for the measurement of the electrical properties of carbon nanotube's deposit. The board has been built and used for the measurements. The measured data are post-processed in order to extract the required properties of the only nanotubes. An equivalent circuit is extracted for further use in EDA simulators and the IEEE P1597 Standard is used to compare the results.

The longitudinal-mode-selection and frequency-control properties of a novel multi-interferometric laser cavity configuration have been experimentally investigated in a high-power TEA CO 2 laser. The cavity has been obtained by adding a fourth mirror to a cavity terminating in a Fox-Smith-type interferometer. We demonstrate that the use of four-mirror cavities of this type can result in effective mode control and stabilization of the emitted frequency, and can be of practical use where the more conventional three-mirror cavities present some limitations.

Various techniques for the spectroscopy of interfaces are discussed such as core level spectroscopy, near-edge absorption, and optical second harmonic generation. The CaF2/Si(111) interface serves as a model for determining electronic states at an interface. A pair of interface states is found, one occupied, the other empty. They represent the bonding (antibonding) combinations of the orbitals that hold the interface together. The interface layer exhibits new properties, e.g., more than twice the band gap of bulk Si. The bonding at complex interfaces, such as SiO2/Si, can also be analyzed using the simple relation between core level shift and oxidation state. Only models with an extended interface are compatible with the observed distribution of intermediate oxidation states.

The electronic measurement of the angle between two planes is generally performed by using of the so called electrogoniometers. The major drawback in using such devices is the presence of a fixed hinge that imposes a fixed centre of rotation. This can cause problems to measure the bending angle in some joints, like Cardan or human joints, which have a variable rotation centre. Based on an optical fiber, a sensor measuring the relative angle in a rotating joint has been developed. It makes use of the intensity modulation of a laser beam propagating in a single-mode optical-fiber, due to the changes of its polarization status originated by the rotation of contiguous portions of fiber, where controlled birefringence has been induced by a fixed radius fiber loop. A prototype of this sensor has been developed with a range of the relative angle of 90deg with a resolution of less than 0.01deg and a standard deviation of 0.1deg. Main advantages of this innovative sensor are lightness, flexibility, high speed of reaction and high accuracy. This paper describes the implemented sensor, the equipment implemented for the test and the results of laboratory activities are reported and discussed.

The presence of misfit and threading dislocations formed by strain relaxation in multilayers of Ge islands grown on Si(100) is investigated by transmission electron microscopy and atomic force microscopy. We find that the dislocations are generated inside the islands and propagate into the silicon spacers remaining confined within the columnar regions above the islands themselves. Thus the dislocations drive the vertical ordering of the stacked relaxed islands in a way similar to the strain induced self-ordering mechanism in the strained islands.

In the present work we investigate photo-detectors based on thick relaxed Ge layers, epitaxially grown on silicon after insertion of a low-temperature-grown Ge buffer layer. By using this procedure it was possible to grow films having thicknesses comparable with light penetration depth in the 1.3–1.6μm spectral range. The films exhibited flatness on the atomic scale. Two kinds of detectors were investigated: vertical heterojunction diodes and a planar Metal-Semiconductor-Metal structure. The detectors show a good responsivity at normal incidence at both 1.3 and 1.55μm. The photocurrent increases with the voltage applied, reaching a maximum responsivity of 0.24 A/W at 1.3μm under a bias of IV. A complete optoelectronic characterization of the fabricated devices is performed. The results confirm the feasibility of the proposed approach for the fabrication of 1.3–1.55μm near infrared photodetectors integrated on silicon chips.

We experimentally investigate the origin of the nonlinear refractive index, n2, of dye-doped polymeric films. We observe large positive or negative values of n2 for different dyes and conclude that it is due to two photon absorption resonant enhancement of the nonlinearity.

CISE is carrying out since 1989 a programme of work for the development of a transportable CO2 Doppler Wind LIDAR system. Up to now the main effort has been spent in the design, the construction and the testing of a suitable pulsed laser source, as well as in the definition of system specifications and in the preliminary system design.

The surface-sensitive nonlinear optical processes of second-harmonic generation (SHG) and sum-frequency generation (SFG) are powerful tools for investigating the structural and electronic properties of semiconductor surfaces and interfaces. Information about the symmetry and structural properties of the surface or interface can be obtained from an analysis of the polarization dependence of the nonlinear optical response, as has been illustrated for clean Si(111) surfaces prepared with different surface reconstructions. The approach has been applied to real-time, in-situ studies of thin film growth and annealing, from which barriers for adatom motion can be inferred. Information concerning the electronic properties of surfaces and interfaces can be obtained from spectroscopic measurements of resonant three-wave mixing (SHG or SFG). The method has been utilized, for example, to probe electronic transitions at the insulator/semiconductor interface formed by CaF2/Si(111). These measurements led to a determination of the “interface” band gap, i.e. the minimum energy separation between filled and empty interface states. The interface band gap is found to differ strongly from that of either of the bulk materials, highlighting our ability to probe the distinctive properties of this quasi-two-dimensional region.

The authors have grown CdTe quantum dots in glass, studied the relaxation dynamics in the femtosecond time scale, and measured the non-linear refractive index in the infrared. Their observations, of very large non-linear refractive index in the transparency region and femtosecond recovery times of the resonant non-linearity, indicate that these materials are most promising for all-optical photonic switching applications.

The authors present a fast, simple and nondestructive method to characterise rare-earth-doped fibres (REDF). They measure the response of the fibre to a step-like pulse excitation and obtain, from a single measurement, the intrinsic saturation power, the excited-state lifetime and the linear absorption coefficient, which are fundamental for a complete modelling of REDF devices. They demonstrate the method using different erbium-doped fibres (EDF).

The effect of optical cavity length perturbations on the intrapulse frequency stability of laser cavities terminating in reflective multipass interferometers is investigated. We demonstrate both theoretically and experimentally, in a long-pulse single-mode TEA CO(2) laser, that intrapulse frequency chirping can be strongly reduced by using relatively long high-finesse interferometers. These results can be applied to the design of single-mode lasers with negligible chirping.

We demonstrate experimentally that optical four-wave mixing can be used to study surface polaritons in solids. The dispersion characteristics of surface polaritons in GaP are measured and compared with theoretical calculations.

A detailed analysis of a self-injected Nd: YAG laser with an unstable cavity is presented. With respect to the normal Q-switched laser-mode operation the pulse peak power is increased by a factor of eight with a pulse length of about 1 ns. Moreover single transverse mode operation is achieved, with divergence near the diffraction limit. An evaluation of the most significant parameters useful for the design of an unstable resonator for self-injection applications is also presented.

A new type of laser which generates pulses with amplification band-limited picosecond duration is presented. 15 ps, 0.9 GW, 3 pps, λ = 1.06 μm in a single pulse are obtained with good stability. Accurate timing of the pulses can be achieved and no external pulse selection is required.

An electro‐optical ring modulator, used as a cavity output coupler, has permitted simultaneous active mode locking, Q switching, and cavity dumping of a continuous‐pumped neodymium:yttrium aluminum garnet laser. We have obtained output pulses of 4 MW peak power and 35 ps duration at repetition rates up to 2.5 kHz.

The authors describe the design, construction and operating characteristics of a fast-risetime high-voltage pulser for Pockels cell driving. The risetime is less than 0.5 ns and the jitter less than 1 ns while the trigger delay time is approximately 20 ns. As an application the authors show that the use of this pulser in a self-injected Nd:YAG laser allows the production of subnanosecond duration high peak power impulses with low jitter and good stability.

Adiabatic pulse compression by a large factor (~10) has been achieved simply and reliably in conventional lasers by suitable driving of the cavity Q-factor with a Pockels cell (PC), This is the effect of an unconventional technique of self-injection (or cavity flipping).1,2

Conventional Q-switching (QS) of pulsed solid- state lasers is achieved by means of electrooptic modulators (EOM) which give a defined polarization status. This technique can be applied to solid-state lasers as Nd:YAG, Nd:glass, etc. when the pumping rate is low enough to consider as negligible the polarization losses due to the rod thermal birefringence. Nd:glass lasers require slab configurations even in the case of moderate output average power. Continuous wave high-power Nd:YAG lasers can operate successfully in the QS mode only when acoustooptic modulators (AOM) are used, since they introduce low losses independent of the polarization distribution.

We have chosen CaF2/Si(111) as a prototype to study the two‐dimensional band structure of an ordered interface using angle‐resolved photoemission and optical second harmonic generation. A pair of interface state bands is found, one occupied, the other empty. The former disperses from EF−0.8 eV at Γ to EF−1.4 eV (−1.6 eV) at M (K). An interface band gap of 2.4 eV is determined via resonant second harmonic generation using a truly buried interface with 500 A of CaF2 on top of Si. Therefore, the interfacial gap is twice as large as the gap in Si and five times smaller than the gap in CaF2. The pair of interface bands can be understood as bonding/antibonding combinations of the Si dangling bond orbital and the Ca 4s orbital, with Ca in the 1+oxidation state.

We present a detailed analysis of the self-injected Nd:YAG laser which is capable of generating subnanosecond duration pulses with peak powers as high as 200 MW. The model predicts that pulses with durations much shorter than the electrical rise time of the electrooptical switch driver can be obtained. Application of this technique to a conventionally Q-switched Nd:YAG laser allowed us to obtain 550 ps pulses with 120 mJ in a very stable and reliable way.

The first ’’self‐injected’’ unstable resonator laser is reported. 1‐ns, high peak power, single‐mode pulses are generated with neodymium‐yttrium aluminum garnet (Nd‐YAG) laser made with the same relevant optical components of the commercial DCR1 Quanta Ray oscillator.