Search

We report on the size dependent of up-converted lasing threshold for CsPbBr 3 perovskite quantum dots, excited via two-photon absorption, showing threshold as low as 1.6 mJ/cm2 under femtosecond excitation.

We study two-photon absorption in CuInS2 quantum dots. We show that the maximum two-photon absorption cross-section strongly dependents on quantum-confinement, and that the material is a promising candidate as label for multi-photon bio-imaging.

We report a method for producing, from the raw materials, high optical and geometrical quality glass tubes and photonic crystal fiber (PCF) preforms, without using extrusion or drilling at any stage. A thermal glass study was carried out in order to choose the appropriate glass composition to avoid crystallization problems during the tube, preform and fiber fabrication. A two period PCF was fabricated in addition to a co-doped Erbium and Thulium photonic crystal fiber. In the latter, a 187 nm wide amplified spontaneous emission (ASE) spectrum was obtained when pumping a 15 cm long fiber at a wavelength of 790 nm.

We present absorption spectra as a function of temperature for CdTe quantum dots in doped glasses. We calculate the absorption spectra considering the transition energies from a spherical model. We consider also the inhomogeneous broadening due to the quantum-dot size distribution and the homogeneous broadening, due mainly to electron-phonon interactions. The samples grown with a two-step heat treatment present very thin size distributions, with a standard deviation of 5.8%, which is comparable to the best CdSe samples in the literature. Also, the results show that the homogeneous width increases as the quantum-dot size decreases, indicating an enhancement of the electron-phonon interaction for small dots.

CdTe quantu m dots in borosilicate glasses were produced and their growth kinetics, the effects of quantum confinement for electrons and phonons and the time resolved optical transmission were studied. Quantum dots in the range of 10 to 25 with a 5% size dispersion were grown. The growth kinetics were studied by small angle X-ray scattering using synchrotron light. A spherical k .p model for the electron confinement explains absorption data and selection rules, and a dielectric continuum model for the phonon confinement could explain the resonant longitudinal and surface optical phonons, as well as their overtone combinations. The time response of absorption was studied by a pump-probe measurement on the femtosecond time scale. For samples showing a large amount of surface traps the recovery time can be as fast as 300 fs, while for an almost surface trap free sample it remains on the 1-3 ps time scale. © 1997 Elsevier Science B.V.

Electron-hole recombination in CdTe quantum dots was studied by photoluminescence and resonant femtosecond pump–probe measurements. A dependence of recombination times with pump pulse intensity was observed and we attribute this to the Auger recombination process. The overall kinetic processes that we observed are a fast decay from the initial excited state to surface trap states, the Auger recombination, the recombination of electrons from the surface states, and a longer time recombination which we attribute to electrons in the deep traps states.

We present results obtained from a numerical simulation of a colliding-pulse mode-locked dye laser and study the influence of group-velocity dispersion on the pulse characteristics. It is found that positive third-order dispersion can help to shorten the pulse duration and to reduce the dependence of energy and pulse duration on pump power.

We demonstrate that a combination of prisms and diffraction gratings can provide not only quadratic but also cubic phase compensation of ultrashort optical pulses. We obtain compressed pulses as short as 6 fsec.

We present preliminary measurements of the optical properties of Rhodamine B molecules adsorbed or trapped in porous matrices such as Vycor 7930 glass, silica xerogels and aerogels. The electronic transfer between donor and acceptor molecules adsorbed on such fractal structures is also discussed; a fast sub-picosecond resolution experiment is proposed to study this process.

PbTe doped tellurite glass photonic optical fiber for non linear application were developed using rod in tube method in a draw tower. We follow the growth kinetics of the quantum dots in the optical fiber by High Resolution Transmission Electron Microscopy giving some results related with the growth kinetic of the same in function of time so much for optical fiber as for the glass bulk. Absorption peak near 1500 nm as observed and it was attributed the optical resonance due PbTe quantum dots in the core fiber.

We have been able to produce soft glass conventional core-clad and micro-structured fibers using rod-and-tube and stack-and-draw method respectively. The stack-and-draw technique shows several difficulties when used with soft glasses, that we managed to avoid using two different lead and alkaline glasses. Non commercial glasses and fibers were thermo-mechanically and optically characterized.

Femtosecond pump-probe studies at 1.5 μm of PbTe quantum dots in a glass matrix have revealed oscillations due to a spheroidal acoustic mode. The wavelength dependence of the observed frequencies indicates size selectivity. The frequency and damping of the mode as a function of wavelength agree with the predictions of an acoustic continuum model that suggests that the dominant damping mechanism is radiative loss from the dots to the surrounding glass.

Micro-structured Er 3+ - Tm 3+ co-doped tellurite fiber with three rings of holes was fabricated using a soft glass drawing tower by a stack-and-draw technique. Amplified spontaneous emission (ASE) around 1550nm band were observed when pumped with both, 980nm and 790nm, lasers.

Er 3+ -Tm 3+ co-doped tellurite photonic crystal fiber was fabricated via a stack-and-draw procedure and without using extrusion in any stage. The final fiber presents a 187 nm bandwidth of amplified spontaneous emission (ASE) intensity around 1550nm when pumped with 790nm. In this manuscript a soft-glass tube fabrication technique, using the centrifugation method, is also shown.

Side access to the holes of photonic crystal fibers is demonstrated with a novel technique. Liquid or gases to be sensed can, thus, be inserted via the fiber lateral while the tips are optically monitored.

We report degenerate and non-degenerate two-photon absorption spectra of II-VI semiconductor quantum-dots. We find that these spectra depend on quantum dot size and size distribution. We propose a theory for explaining the non-degenerate case.

Electron-hole recombination was theoretically and experimentally studied in CdTe quantum dots. A dependence of recombination times with pump pulse intensity was observed dots and it was attributed to the Auger recombination process. The rate equation model can describe completely the experimental data.

We show the possibility to synthesize a structural quantum dot (SQD) in the glasses by a double annealing process. We have synthesized two groups of CdTeS-doped glasses treated with two annealing processes. Both groups have the same composition of CdTeS and the same glass matrix. Each of them was first treated at a lower temperature for an extra long time (several days) and then treated again at a higher temperature for a shorter time. The two groups have the same second treatment but different initial annealing. The glass matrix contains (in wt%) 47.66% SiO 2 , 16.55% B 2 O 3 , 30.57% Na 2 O and 5.22% ZnO mixed with CdO, Te and S in the CdTe 0.4 S 0.6 stoichiometry

We present femtosecond resolution measurements that demonstrate the occurrence of the ac Stark effect in glass samples doped with CdSSe nanocrystallites. The sample is excited by a 50 fs pulse 80 meV below the band edge. The absorption edge shifts tracking the profile of the pumping pulse. At intensities below 0.5 GW/cm2 the edge shift is linear in the pump intensity. Above this value the shift tends to saturate. We also studied CdTe quantum dots in a glass matrix excited resonantly with the 1S - 1S transition. In this case we observe the dynamics of hole relaxation in the valence band. The hole population equilibrates with a time constant of less than 100 fs.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A modified Imacon type streak camera with temporal resolution of 1.5 psec was used for the measurement of pulses generated by a mode locked Er fiber laser operating at 1.55 micron wavelength. The specially developed S1-PV001 image tube had an initial sensitivity of 26 mu A/W at 1.3 micron. To improve the signal to noise ratio for recording low level CW laser radiation additional shutter circuitry was added to the Imacon camera electronics. The camera was operated at 15 Hz repetition rate, triggered by a sample of the RF signal used for driving the acoustooptic mode locker. A very sensitive and simple recording system was used for recording the time dispersed pictures generated in the ICT output screen using a high gain PMT and a 150 micron slit mounted on a translation stage. The signal from the PMT was read with a box car averager and sent to a PC type computer which controlled the translation stage. The acoustooptically mode locked Er fiber laser can generate pulses of 80 psec in CW operation with a peak power of 0.6 W.

Nonlinear interaction between spectral components in two different photonic bandgaps is experimentally demonstrated by launching femtosecond pulses near a zero-dispersion wavelength of a hybrid photonic crystal fiber, which guides by a combination of total internal reflection and bandgap effects. It is demonstrated that the initial pulse becomes spectrally broadened, and narrowband resonant radiation is generated in a different bandgap from the one responsible for guiding at the pump wavelength. The spectral intensity of the resonant radiation peaks at 2.7 dB below that of the broadened pulse in the pump-guiding bandgap.

With the growing importance of erbium-doped fiber amplifiers for telecommunications applications, considerable effort has been devoted to minimizing the pump power requirements for producing high gain. The best reported results are for λp = 0.98 μm wavelength pumping1,2 with gain coefficient up to 4.9 dB/mW, and for λp = 1.48 μm pumping,3,4 with a lower maximum gain coefficient of 2.6 dB/mW. We report here a low threshold erbium-doped fiber amplifier which, when pumped at 1.48 μm, has a gain coefficient is 5.9 dB/mW, and, when pumped at 0.972 μm, an almost equal gain coefficient of 6.1 dB/mW.

A new, simple, technique is demonstrated to laterally access the cladding holes of solid-core photonic crystal fibers (PCFs) or the central hole of hollow-core PCFs by blowing a hole through the fiber wall (using a fusion splicer and the application of pressure). For both fiber types material was subsequently and successfully inserted into the holes. The proposed method compares favorably with other reported selective filling techniques in terms of simplicity and reproducibility. Also, since the holes are laterally filled, simultaneous optical access to the PCFs is possible, which can prove useful for practical sensing applications. As a proof-of-concept experiment, Rhodamine fluorescence measurements are shown.

The development of microstructured fibres offers the prospect of improved fibre sensing for low refractive index materials such as liquids and gases. A number of approaches are possible. Here we present a new approach to evanescent field sensing, in which both core and cladding are microstructured. The fibre was fabricated and tested, and simulations and experimental results are shown in the visible region to demonstrate the utility of this approach for sensing.

This letter describes a principle demonstration of an ultrafast optical switch operating at 1Tbit∕s using CdTe-quantum-dots-doped glasses. Using a three-beam pump and probe experiment, we showed that thermal effects are responsible for a baseline in the pump and probe graphs and the nonexistence of carrier accumulation effects. After eliminating the thermal effects, we showed that, when two pump pulses are delayed by 1 ps, each pump pulse modulates the probe pulse independently, making this material highly promising for ultrafast all optical switching.

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.

We investigate the role of walk-off in the evolution of a weak cw wave in the anomalous-dispersion regime modulated by an intense picosecond pulse through cross-phase modulation. The cw wave can evolve into a short pulse whose duration and amplitude will depend on the degree of walk-off. When the walk-off length is shorter than necessary for short-pulse formation, the signal wave becomes amplitude modulated with a modulation depth of a few percent. In this case a frequency chirp exists in the signal wave, which can be used to generate short pulses on propagation through an appropriate dispersive element.

We use a rate equation model to analyse a technique for the generation of picosecond duration laser pulses in argon pumped dye lasers. The system is based on passive mode-locking of the Ar+ laser by saturable absorption of the dye which is inserted in the Ar+ laser cavity. Simultaneously, the dye is forced to oscillate in a regime of synchronously pumped mode-locking. The system is relatively simple, does not require the use of an acousto-optical light modulator and can be easily implemented in commercially available lasers. Pulses shorter than 100 ps and having an average power of 20 mW were obtained at a repetition rate of 110 MHz. Good agreement between the model and experimental results was also obtained.

We describe the simultaneous application of the self-injection and cavity dumping techniques to a flash pumped two-frequency dye laser. Short pulses with 1.5 ns fwhm were obtained with peak power in excess of 100 kW. A peak intensity gain by a factor 3 was observed with respect to normal cavity dumping operation. A simple model is presented to explain the power gain under self injected conditions and it is shown that for long cavities the power gain is limited by the upper laser level lifetime.

We present a quantitative experimental study of the effects of pump power and saturable absorber concentration on the performance of a colliding pulse modelocked dye laser. Pulses as short as 47 fs were generated without the use of any additional (intracavity or extracavity) pulse compression scheme. The laser has a well defined stability region in the pump power × saturable absorber concentration plane, and exhibits a strong bistable behavior in the limits of this stability region.

Optical pulses with a duration of 10 fs are used to investigate dynamic spectral hole burning and the equilibration of an optically induced non-thermal population distribution of large molecules in solution.

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.

The efficiency, duration, and spectral content of the emission from laser‐produced Ta plasmas in the 10–71 nm spectral region have been measured for laser pulse durations ranging from 100 fs to 70 ps. Efficiencies from 0.3% for 100 fs pulses to 2.65% for 70 ps pulses, x‐ray pulse durations from less than 10 ps for 100 fs excitation pulses to 105 ps for 70 ps excitation pulses, and spectral content peaking in the 17–35 nm region were measured for 1 mJ pulses at intensities from 1011 to 5×1014 W/cm2.

We report the first observation of femtosecond photon echoes from the band-to-band transitions in a bulk semiconductor. The time decay of the echo, found to vary from 3.5 to 11 fs, has allowed us to determine the polarization dephasing rate in GaAs. This rate was found to depend on the carrier density in the experimental range covered, 1.5\ifmmode\times\else\texttimes\fi{}${10}^{17}$ to 7\ifmmode\times\else\texttimes\fi{}${10}^{18}$ ${\mathrm{cm}}^{\ensuremath{-}3}$, indicating the dominance of carrier-carrier scattering as the principal dephasing mechanism. The observed functional dependence of the dephasing rate on the carrier density has yielded previously unavailable information on Coulomb screening in a nonequilibrium carrier distribution.

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.

Femtosecond optical measurement techniques have been used to study the primary photoprocesses in the light-driven transmembrane proton pump bacteriorhodopsin. Light-adapted bacteriorhodopsin was excited with a 60-femtosecond pump pulse at 618 nanometers, and the transient absorption spectra from 560 to 710 nanometers were recorded from -50 to 1000 femtoseconds by means of 6-femtosecond probe pulses. By 60 femtoseconds, a broad transient hole appeared in the absorption spectrum whose amplitude remained constant for about 200 femtoseconds. Stimulated emission in the 660- to 710-nanometer region and excited-state absorption in the 560- to 580-nanometer region appeared promptly and then shifted and decayed from 0 to approximately 150 femtoseconds. These spectral features provide a direct observation of the 13-trans to 13-cis torsional isomerization of the retinal chromophore on the excited-state potential surface. Absorption due to the primary ground-state photoproduct J appears with a time constant of approximately 500 femtoseconds.

The generation of ultrashort light pulses in a colliding pulse mode-locked dye laser is studied by carrying out a numerical simulation of the pulse propagation in the laser cavity. The phenomena considered in this simulation were saturable amplification, saturable absorption, bandwidth limiting element, self-phase modulation, group velocity dispersion, and linear loss.

The generation of far-infrared radiation (FIR) by optically pumping molecular gases was first accomplished by Chang and Bridges in 1970 [1]. They made use of CO2 laser to pump specific vibrationalrotational states of CH3F molecule at low pressure, producing population inversion between rotational levels in the excited vibrational state. The importance of this pumping scheme for obtaining FIR laser lines is due to the existence of a great number of polar molecules with vibrational absorption in the 9–11μm region where the CO2 laser emits, combined with the high efficiency of the CO2 laser pump. So far more than 50 molecules have been found to lase, yielding over 1000 lines in the 5–250 cm-1 spectral range [2] and an intensive search for new and efficient lines is now taking place in several laboratories. A good review of the FIR laser field has been given by Hodges [3].

Two pulse photon echoes were observed in organic dye molecules in solution using 6-fs optical pulses. The results show an initial rapid dephasing of the echo due to level multiplicity followed by quantum beaks. The temporal decay of the quantum beats is characterized by a ${\mathrm{T}}_{2}$ of about 60 fs.

AOptical second-harmonic studies show that the electronic structure in the top 75 - 130 A of a crystalline Si surface loses cubic order only 150 fsec after the Si is excited by an intense 100 fsec optical pulse. This suggests that atomic disorder can be induced directly by electronic excitation, before the material becomes vibrationally excited. In contrast, the electronic properties of the equilibrium molten phase are not obtained for several hundreds of fsec.

We report on the dynamics of nonequilibrium distributions of dye molecules, such as, Nile Blue, Cresyl Violet, sulforhodamine 640, and rhodamine B, excited by pump pulses of 65-fs duration and probed by pulses compressed to durations of 10 fs. We examine a spectral region centered at the pump wavelength, 618 nm, and ranging from 570 to 670 nm. Differential absorption spectra are taken at intervals of 25 fs following the initial excitation of the dye. We observe optically induced changes which reach equilibrium within a few hundred femtoseconds following excitation. Of special interest is hole burning at the pump wave-length as well as bleaching of replica holes at energies shifted from the pump energy by a vibrational quantum energy. We also observe transient excited state absorption in near-infrared dyes due to pump-induced nonequilibrium population of the vibrational levels of the upper electronic state and transient-induced transmission in dyes such as rhodamine B where our 618-nm pump pulse lies below the absorption edge of the dye. We discuss the relationship of these optically induced dynamics with ultrashort pulse formation in lasers.

A principal problem in obtaining optical pulses with bandwidths approximating that of the visible spectrum and durations approaching the transform limit has been the uncompensated cubic phase distortion of pulses compressed by grating pairs. We describe the use of combined prism and grating sequences to compensate not only the quadratic, but also the cubic, phase distortion of optical pulses frequency broadened in an optical fiber. We obtain pulses of 6-fs duration which can be used on a routine basis for time-resolved spectroscopy. We have used these pulses to study the dynamics of spectral hole burning in organic dye molecules and in semiconductor materials. Examples are given of recent work on molecules, such as cresyl violet and Nile blue, and on semi-conductor materials, such as GaAs. We have also used these 6-fs pulses to examine the optical Stark effect in organic dye molecules such as rhodamine B and rhodamine 6G. We obtain, in particular, direct observations of the dynamic shift of the rhodamine B absorption spectrum on a femtosecond time scale. We also use the intensity dependence of these shifts as an alternate means of measuring the molecular dipole moment.

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

Cross-phase modulation (XPM) is important whenever two optical fields copropagate and at least one of them is intense. In this work we investigate the role of walk-off between two optical fields copropagating in an optical fiber. We consider a cw signal in the anomalous dispersion regime and an intense picosecond pulse in the normal dispersion regime. The continuous wave is modulated through XPM and can evolve into a short pulse whose duration and amplitude depend on the degree of walk-off. The signal pulses are longer and less intense than in the no walk-off case, and a longer fiber is necessary to form it. When the walk-off length is shorter than the necessary length for short pulse formation the signal wave becomes amplitude modulated, with a modulation depth of a few percent. In this case a frequency chirp exists in the signal wave and can be used to generate short pulses on propagation through an appropriate dispersive element.