Laser Linewidth

A JEOL 100 CXII transmission electron microscope with a scanning attachment has been modified for electron-beam writing. A fully software controllable scan generator has been developed which includes a facility for digital rotation, necessary since the modified stage cannot provide rotation. Gold palladium dots have been fabricated on GaAs with dimensions down to 20 nm, and 60 nm pitch gratings with a 30 nm linewidth.

As optical lithography advances toward the 10 nm mark, much effort is being expended to push electron beam lithography into the deep sub-10-nm regime. A significant issue at this length scale is the ability to accurately measure and compare linewidths. Measurements using secondary electron micrographs have a bias of a few nanometers and are therefore difficult to interpret in the sub-10-nm regime. Transmission electron microscopy can give greater accuracy but requires significant effort. This article shows that the use of a backscattered electron image together with a metal coating where appropriate can yield better measurement results than by using secondary electrons. With the use of a suitable model, linewidths for sub-10-nm hydrogen silsesquioxane lines were extracted with an estimated error of 1 nm.

In this article the authors report on a new optical linewidth metrology method which uses measured changes in the resonant wavelength of fabricated ring resonator structures to establish the linewidth of waveguides. The technique has a precision of better than 0.5nm for 500nm linewidths and can be used for process control although it cannot be used with arbitrary optical structures. To demonstrate the technique they fabricated ring resonator structures on silicon on insulator substrates several times over a period of 70days, using electron beam lithography with hydrogen silsesquioxane (HSQ) negative tone resist followed by dry etching. They show that waveguide width variations of less than 0.5nm can consistently be achieved when the HSQ is diluted immediately prior to use but highly irreproducible results are soon obtained if a single dilution is prepared and used for an extended period.

We propose a novel approach for real-time generation of random walk noise. Our approach aims to generate natural laser phase noise and construct a linewidth tunable laser. By utilizing fieldprogrammable gate array circuits and digital-to-analog converters, we generate noise that is subsequently combined with low-frequency analog noise. This combined noise is then fed into a lithium niobate phase modulator to emulate phase fluctuations. Our method facilitates accurate control over the laser linewidth spectrum, closely approximating actual noise characteristics.

Amorphous FexSi1−x films have been prepared on Si substrates in order to fabricate submicrometric magnetic structures with soft magnetic behavior. The magnetic properties compositional dependence of the unpatterned samples has been analyzed to select the Fe content (x=0.7) with the lowest coercive and anisotropy fields values. Arrays of Fe0.7Si0.3 lines have been fabricated by electron beam lithography combined with a liftoff technique, with typical dimensions of 200 nm linewidth and 1 μm line spacing. These arrays present coercive fields parallel to the line direction as small as 9 Oe.

New in situ Raman and synchrotron X-ray diffraction data (between 300 and 400 K) in conjunction with separate temperature-dependent Raman data (between 7 and 400 K) are presented. The low-frequency Raman spectra show good agreement with theoretical values obtained previously using periodic DFT calculations. The temperature-dependent spectra reveal the presence of significant anharmonicity of librational modes neither predicted theoretically nor noted in previous experiments. The splitting of the internal deformation mode ? 2 (of E symmetry in the free ion) decreases continuously with increasing temperature, but drops abruptly at the first-order orthorhombic to hexagonal phase transition observed at 381 K. The temperature dependence of the linewidth of the internal deformation mode ? 2 reveals coupling to reorientational motions of the borohydride ion in the orthorhombic phase. The thermal evolution of both crystal structure and vibration frequencies agree with the phase diagram suggested by the Landau theory.

Monolayers of transition metal dichalcogenides are direct gap semiconductors, which have attracted much attention in the recent past. Due to a strong Coulomb interaction, they possess strongly bound electron-hole pairs, with binding energies of hundreds of meV which is an order of magnitude larger than in conventional materials. Here, we investigate the microscopic origin of the homogeneous linewidth and coherence lifetime of excitonic resonances in monolayer molybdenum disulfide, taking exciton phonon scattering and radiative recombination into account. We find a superlinear increasing homogeneous linewidth from 2 meV at 5 K to 14 meV at room temperature corresponding to a coherence lifetime of 160 fs and 25 fs.

Atomically thin transition metal dichalcogenides are direct-gap semiconductors with strong light–matter and Coulomb interactions. The latter accounts for tightly bound excitons, which dominate their optical properties. Besides the optically accessible bright excitons, these systems exhibit a variety of dark excitonic states. They are not visible in the optical spectra, but can strongly influence the coherence lifetime and the linewidth of the emission from bright exciton states. Here, we investigate the microscopic origin of the excitonic coherence lifetime in two representative materials (WS2 and MoSe2) through a study combining microscopic theory with spectroscopic measurements. We show that the excitonic coherence lifetime is determined by phonon-induced intravalley scattering and intervalley scattering into dark excitonic states. In particular, in WS2, we identify exciton relaxation processes involving phonon emission into lower-lying dark states that are operative at all temper...

In this paper, we investigate the influence of the linewidth enhancement factor and active region volume on the switching characteristics of bistable injection-locked multi-quantum-well Fabry-Perot lasers. In this analysis we start from the full scale model of the multimode rate equation system describing dynamics of injection-locked lasers. On the basis of this model, we derive a simple analytical procedure for calculation of switching time and energy with respect to the injection power, and for different values of frequency detuning between the master and the slave laser. We find that the higher values of linewidth and active region volume can improve switching characteristics of injection-locked Fabry-Perot lasers, providing either shorter switching times (10 ps) or lower switching energies (1 fJ).

Coherent population trapping is studied in 87 Rb in a buffer gas. Experimental results are reported on the contrast of the field independent hyperfine resonance line and on its linewidth. A physical model, which includes an optical pumping process that traps atoms in a Zeeman sublevel of the ground state, is developed. The model explains the general behavior of the experimental observations.

Samples of (BiPb)2Sr2Ca2Cu3Ol0+~ (BSCCO-2223) prepared by the sol-gel route were fluorinated in C1F3, resulting in approximately 3 fluorine atoms per formula as measured by fluoride selective-electrode. The samples were characterized by x-rays diffraction (XRD) and magnetization measurements. Neither the lattice parameters nor T¢ (114K) changed after the fluorination. Nuclear magnetic resonance (NMR) measurements of 19F in the temperature range of 20-400K show that for T>270K there are 2 types ofF nuclei environment. At T<270K only one line is resolved. At T<T¢, the fluorine linewidth increases with decreasing temperature. This is attributed to the formation of vortices. At very low temperatures, a new structure appears. This is attributed to an enhanced NQR signal from the Cu nuclei.

This study delves into the linguistic and stylistic dimensions of Yoruba advertisements for products, concentrating on oral data collected from different states. The primary aim is to identify and analyze the distinctive linguistic patterns and stylistic devices that are employed to captivate and persuade the consumer. Utilizing a qualitative approach, this research examines various elements such as language use, rhetorical strategies, cultural references, and persuasive techniques within these advertisements. Furthermore, the study highlights the role of tone, rhythm, and phonological patterns in enhancing the appeal and memorability of the advertisements. By exploring the interplay between language and culture, this research underscores the importance of cultural context in shaping advertising messages. The findings suggest that effective advertising in Yoruba not only relies on the linguistic proficiency of the advertisers but also on their ability to weave cultural nuances into their narratives. This approach not only enhances the relatability of the advertisements but also fosters a sense of identity and pride among the Yoruba-speaking populace. This linguistic and stylistic analysis of Yoruba advertisements offers valuable insights into how local languages and cultural nuances influence consumer behavior and marketing strategies in Nigeria. It emphasizes the need for advertisers to adopt culturally sensitive and linguistically rich approaches to connect more effectively with their target audiences. This study contributes to the broader field of linguistic stylistics and advertising, providing a framework for future research on the intersection of language, culture, and marketing.

The behaviour of the 63 Cu nuclear magnetic resonance signals in tetrahedrally coordinated copper(I) complexes has been investigated as a function of temperature, solvent and anion. Frcm the strong dependence of the linewidth and the chemical shift on these parameters, exchange processes with complexes of lower symmetry are concluded.

We use classical electrodynamics calculations to investigate the plasmonic properties of the post-transition metals Al, Bi, Ga, In, and Sn, which are active in the ultraviolet, focusing in particular on the material-and resonance-dependent origins of plasmon broadening. Analytic Mie theory, the modified-long wavelength approximation, and the quasistatic dipole approximation together show that radiative processes dominate plasmon dephasing and damping in small (5-25 nm radius) Al, Bi, Ga, In, and Sn spheres. For Al, Ga, In, and Sn, the radiative contribution (~1-2 eV) to the plasmon linewidth is more than 100-fold greater than the non-radiative contribution (0.001-0.02 eV) derived from the bulk dielectric function. This is significantly different than what is observed for Ag spheres, where non-radiative contributions (~0.1 eV) are the primary source of broadening up to a radius of 25 nm. Overall, these data suggest that the plasmonic properties, dephasing, and lifetimes for Al, Ga, In, and Sn-and to a lesser extent Bi-spheres are qualitatively similar. To develop a more general understanding of the relationship between plasmon energy and linewidth, we use a model for ideal free-electron Drude metals. It is seen that the linewidth increases at higher energies even for lossless Drude metals, suggesting that the increased broadening observed in UV-active metals is a generalizable observation. These data have important implications for the use of these metals for ultraviolet plasmonics. The increased importance of radiative damping for post-transition metals could influence the ability to harvest photons, generate hot carriers, and enhance spectroscopy in the ultraviolet while providing new opportunities for manipulating high-energy photons.

Fullerene C, samples are characterized by a very weak EPR spectrum of g = 2.6925 and linewidth 2AB,, x 2 Gs. Like graphite, it is related to the C& holes in the defect structure. For fullerene doped with potassium, a strong two-component EPR spectrum was recorded. The narrow line (N) was of width 2ABz = 2 Gs while the wide line (W) width was 2ABz = 14Gs. The fact that the g-value for the narrow line g" = 2.6625 does not change with temperature indicates that it must be determined by an unpaired electron delocalized on the w C& ion. For the wide line, g = 2.6617 at room temperature decreases to g" = 2.6666 at the temperature of liquid helium, which suggests that the line originates from W& anions of K,C,. A strong delocalization of an electron in the complex is indicated by a pronounced temperature dependence of ABE which at liquid-helium temperature decreases to 2ABE = 1Gs. Applying the microwave absorption method, the critical temperature of K,C, complexes, equal to about 16K, was determined. EPR spectra of the systems K,M,C, (M = Li, Cs) are more complex. EXPERIMENTAL Sample preparation To synthesize fullerene, the method described by Kratschmer et al. [l] was applied. The reaction chamber where the soot was generated by the contact between graphite electrodes was filled with helium up to 0.7atm. The fullerenes were extracted from the soot with a mixture of hexane and

Nearly defect-free InN microcrystals grown on Si͑111͒ substrates have been realized by plasma-assisted molecular beam epitaxy. High-resolution transmission electron microscope images reveal that these microcrystals exhibit single-crystalline wurtzite structure. Low temperature photoluminescence ͑PL͒ shows a strong emission peak at 0.679 eV with a very narrow linewidth of 17 meV at excitation power density of 3.4 W / cm 2. Temperature-dependent PL spectra follow the Varshni equation well, and peak energy blueshifts by ϳ45 meV from 300 to 15 K. Power-densitydependent PL spectroscopy manifests direct near-band-edge transition. A low carrier density of 3 ϫ 10 17 cm −3 has been estimated from PL empirical relation, which is close to the critical carrier density of the Mott transition of 2 ϫ 10 17 cm −3 .

We report on two fully-fiberized, input polarization independent depolarizer configurations that do not need any alignment once fabricated. This makes them suitable for real-world applications. The first configuration uses polarizationmaintaining (PM) fibers and relatively few components. The second configuration uses standard (non-PM) single mode fibers (SMFs) to introduce the large time delays needed to enable narrow-linewidth signal depolarization, albeit at the cost of a slightly increased component count and insertion loss. We theoretically analyze the depolarization performance of the proposed configurations as a function of various depolarizer parameters. Subsequently, we design the depolarizers considering the imperfections of standard commercially available components. Finally, we fabricate prototypes of both configurations, characterize their depolarization performance using sources of variable linewidth, and compare the experimental results with theoretical analysis. The fabricated depolarizers are shown to be capable of the depolarization of 20 MHz and 1 MHz linewidth signals with residual degree of polarization (DOP) below 4%.

The growth and characterization of Bismuth (Bi) substituted YIG (Bi-YIG, Bi0.1Y2.9Fe5O12) thin films are reported. Pulsed laser deposited (PLD) films with thicknesses ranging from 20 to 150 nm were grown on Gadolinium Gallium Garnet substrates. Two substrate orientations of (100) and (111) were considered. The enhanced distribution of Bi 3+ ions at dodecahedral site along (111) is observed to lead to an increment in lattice constant from 12.379 Å in (100) to 12.415 Å in (111) oriented films. Atomic force microscopy images showed decreasing roughness with increasing film thickness. Compared to (100) grown films, (111) oriented films showed an increase in ferromagnetic resonance linewidth and consequent increase in Gilbert damping. The lowest Gilbert damping values are found to be (1.06±0.12) ×10-4 for (100) and (2.30±0.36) ×10-4 for (111) oriented films with thickness of ≈150 nm. The observed values of extrinsic linewidth, effective magnetization, and anisotropic field are related to thickness of the films and substrate orientation. In addition, the in-plane angular variation established four-fold symmetry for the (100) deposited films unlike the case of (111) deposited films. This study prescribes growth conditions for PLD grown single-crystalline Bi-YIG films towards desired high frequency and magneto-optical device applications.

We have developed a hyperspectral imaging scheme that involves a combination of dual-comb spectroscopy and Hadamard-transform-based single-pixel imaging. The scheme enables us to obtain 12,000 hyperspectral images of amplitude and phase at a spatial resolution of 46 µm without mechanical scanning. The spectral resolution is 20 MHz, as determined by the linewidth of a single comb mode, and the spectral interval is 100 MHz over a spectral range of 1.2 THz centred at 191.5 THz. As an initial demonstration of our scheme, we obtained spectroscopic images of a standard test chart through an etalon plate. The thickness of an absorptive chromium-coated layer on a float-glass substrate was determined to be 70 nm from the hyperspectral phase images in the near-infrared wavelength region.

We present the diode pumping of a Nd-doped strontium and lanthanum (Nd:ASL) crystal Sr1-xLax-yNdyMgxAl12-xO19 (0.05 ≤ x ≤ 0.5; y = 0.05) for second harmonic generation around 450 nm. In order to fulfill the pumping requirements of this crystal, we have developed a high-brightness pump source based on a tapered amplifier in an extended cavity with a volume Bragg grating for wavelength stabilization. A pump brightness of 110 MW.cm-2 sr-1 has been obtained with a linewidth lower than 80 pm at 798 nm. This laser source has been used to pump a Nd:ASL crystal to obtain 300 mW at 906 nm and 53 mW at 453 nm by intracavity doubling with a LBO crystal.

We report on the formation of ultra-cold Na 2 molecules using single-photon photoassociation of a Bose-Einstein condensate. The photoassociation rate, linewidth and light shift of the J = 1, v = 135 vibrational level of the A 1 Σ + u molecular bound state have been measured. We find that the photoassociation rate constant increases linearly with intensity, even where it is predicted that many-body effects might limit the rate. Our observations are everywhere in good agreement with a two-body theory having no free parameters.

Rotational and lattice phonon excitations in the Raman spectrum of solid molecular deuterium have been measured from 1.8 GPa to-200 GPa at 77-295 K to study pressure-induced changes in structural and dynamical properties of the dense solid. Continuous and discontinuous changes in three distinct pressure ranges are observed. At lower pressures ((30 GPa), there is a gradual increase in the linewidth of the So(0) band, together with gradually decreasing resolution of the higher-energy So(1) and So(2) features. At intermediate pressures (60-100 GPa), a change in the pressure dependence and linewidth of So(0), and a linewidth and intensity decrease in the E2~p honon band occur. This is interpreted as evidence for a phase transformation in the molecular solid beginning at-65 GPa (77 K). At the highest pressures (-160 GPa), abrupt changes in the low-frequency excitations suggest either an expansion of the molecular bond or a change in ordering at the vibron discontinuity. Evidence from the lowfrequency spectra for interaction between the deuterium sample and diamond anvil is also examined. ' ' In addition, a distinct isotope effect is observed in the pressure of the low-temperature 150-GPa

The effect of the source FM noise on the lineshape in sub-Doppler saturation spectroscopy is examined both theoretically and experimentally. We show that the line profile is simply given by the convolution of the homogeneous profile with an "effective" spectrum, depending on the relative values of the sample relaxation time and of the source frequency correlation time but not departing significantly from the source field power spectrum in the usual cases. This result is well supported by a millimeter wave experiment involving a nearly perfect source, frequency modulated by a Gaussian noise.

We report on random lasing observed with 5-m-long Er-doped fiber comprising an array of weak fiber Bragg gratings (FBGs) inscribed in the fiber core and uniformly distributed over the whole fiber length. The laser design ensures domination of dynamical population inversion grating over the FBGs in total distributed feedback enabling laser stabilization and nonlinear laser line filtering of the natural laser Lorentzian linewidth down to sub-300-Hz range, both observed in the experiment.

We develop a theoretical framework for modeling of semiconductor laser coupled to an external fiber-optic ring resonator. The developed approach has shown good qualitative agreement between theoretical predictions and experimental results for particular configuration of a self-injection locked DFB laser delivering narrow-band radiation. The model is capable of describing the main features of the experimentally measured laser outputs such as laser line narrowing, spectral shape of generated radiation, mode-hoping instabilities and makes possible exploring the key physical mechanisms responsible for the laser operation stability.

The narrow linewidth fiber laser is useful in applications such as fiber sensing, optical communications, and spectroscopy. This paper presents an investigation of the model and an experiment of a stable, wavelength-selective, narrow linewidth, ring cavity erbium-doped fiber laser incorporating two fiber Bragg gratings (FBG) at 1530.18 nm and 1550.08 nm, respectively. An F-P tunable filter was used to select a specific wavelength after optimizing the spectral output from the two FBGs to measure their respective linewidths. The erbium-doped ring fiber laser was optimized by adjusting the optical cavity loss using a variable optical coupler at a coupling ratio of 95%. The variable coupler was set to an optimal coupling ratio of 95%, where the spectral output powers of 3.4 mW at 1530.18 nm and 3.1 mW at 1550.08 nm were achieved as the optimal fiber laser output powers. The balanced output power had an optical signal-to-noise ratio of (OSNR) of 61 dB for each wavelength. The linewidth w...

We demonstrate a fiber in-line single photon source based on a hybrid system of colloidal single quantum dots deposited on an optical nanofiber and cooled down to cryogenic temperature (3.7 K). We show that a charged state (trion) of the single quantum dot exhibits a photo-stable emission of single photons with high quantum efficiency, narrow linewidth (3 meV FWHM) and fast decay time (10.0 ± 0.5 ns). The single photons are efficiently coupled to the guided modes of the nanofiber and eventually to a single mode optical fiber. The brightness (efficiency) of the single photon source is estimated to be 16 ± 2% with a maximum photon count rate of 1.6 ± 0.2 MHz and a high single photon purity (g 2 (0) = 0.11 ± 0.02). The device can be easily integrated to the fiber networks paving the way for potential applications in quantum networks.

We study the optical excitation spectrum of an atom in the vicinity of a dielectric surface. We calculate the rates of the total scattering and the scattering into the evanescent modes. With a proper assessment of the limitations, we demonstrate the portability of the flat-surface results to an experimental situation with a nanofiber. The effect of the surface-induced potential on the excitation spectrum for free-to-bound transitions is shown to be weak. On the contrary, the effect for bound-to-bound transitions is significant leading to a large excitation linewidth, a substantial negative shift of the peak position, and a strong long tail on the negative side and a small short tail on the positive side of the field-atom frequency detuning.

We show that single-atoms can be trapped on the surface of a subwavelength-diameter silica-fiber, an optical nanofiber, without any external field, and that single photons spontaneously emitted from the atoms can be readily detected through the single guided-mode of the nanofiber. A key point of the work is our finding that atom trapping sites are created on the nanofiber surface by irradiating the atom cloud around the nanofiber with a violet laser radiation.

Ferromagnetic resonance spectra of La,_xCaxMnO3 powders (0.1 < x 5 0.9) have been investigated over a range of temperatures. The spectra could be fitted to a sum of two Lorentzians for all the compositions. The intense line with a nearly constant g shows a linear decrease in linewidth with increase in temperature, while the weaker line with a variable g shows a maximum in linewidth in the T, region. The latter is also associated with a geff which depends on the composition.

Interspin distances between 0.8 nm and 2.0 nm can be measured through the dipolar broadening of the continuous wave (cw) EPR spectrum of nitroxide spin labels at X-band (9.4 GHz, 0.35 T). We introduce Gd 3+ as a promising alternative spin label for distance measurements by cw EPR above 7 Tesla, where the |−1/2〉 to |1/2〉 transition narrows below 1 mT and becomes extremely sensitive to dipolar broadening. To estimate the distance limits of cw EPR with Gd 3+ , we have measured spectra of frozen solutions of GdCl 3 at 8.6 T (240 GHz) and 10 K at concentrations ranging from 50 mM to 0.1 mM, covering a range of average interspin distances. These experiments show substantial dipolar broadening at distances where line broadening cannot be observed with nitroxides at X-band. This data, and its agreement with calculated dipolar-broadened lineshapes, show Gd 3+ to be sensitive to distances as long as ~3.8 nm. Further, the linewidth of a bis-Gd 3+ † Electronic supplementary information (ESI) available: Details of the calculations of reflections from the sample in the presence of refractive broadening, as well as the method of eliminating the contribution of refractive broadening from the linewidth are discussed. Further, we provide an example of the temperature dependence of the Pake patterns for the high-spin system, and describe the details of calculating the correlation times and averaging of the dipolar interaction for the 260 K experiments. See

A theory of gain, refractive index, and linewidth enhancement factor in the quantum dots (QDs) in the nonthermal coupling (NTC) case is discussed. The ground and excited state transitions are included. Effect of parameters, like recombination time and radiative transition time, is studied. The study shows that the QDs can have zero linewidth enhancement factor. The present review covers important patents useful in this field.

Four wave mixing analysis is stated for quantum dot semiconductor optical amplifiers (QD SOAs) using the propagation equations (including nonlinear propagation contribution) coupled with the QD rate equations under the saturation assumption. Long wavelength III-nitride InN and AlInN QD SOAs are simulated. Asymmetric behavior due to linewidth enhancement factor is assigned. P-doping increases efficiency. Lossless efficiency for InAlN QDs for longer radii is obtained. Carrier heating is shown to have a considerable effect and a detuning dependence is expected at most cases. InN QD SOAs shown to have higher efficiency.

By applying a prepared 3 nm-thick tungsten diselenide (WSe 2) saturable absorber to an electro-optic (EO) modulated fundamental-light laser, an active and passive Q-switched intra-cavity optical parametric oscillator (IOPO) has been experimentally realized. Benefiting from the saturable absorption property of WSe 2 , the output characteristics of Q-switched IOPO with signal-light oscillating are obviously improved, including the 93.7%increased peak power, 54.6%-compressed pulse width, 30.1%-improved conversion efficiency, 60.1%-improved pulse-train stability, 17.8%-improved beam quality, and 27.4%-shortened linewidth. With the values of absorption parameters calculated from the measured nonlinear-transmittance curve of WSe 2 , the rate equations for dual-loss Q-switched IOPO with WSe 2 saturable absorber and electro-optic modulator are numerically solved. The theoretical values correspond to the experimental data well.

We report high-resolution photoemission results for the 4f levels in YbCu2Si2 and CeSi2. The data are compared to the predictions of the Anderson impurity model for valence band photoemission. These predicted spectra are obtained both in an approximate fashion by broadening crystal field (CF) states whose parameters are determined from existing neutron scattering data, as well as from a Gunnarson-Schonhammer calculation. Inclusion of crystal fields does not affect our two fundamental conclusions: that there is a natural linewidth in the problem (~-130meV) which is much greater than predicted by the Anderson model, and that the expected temperature dependence of the Kondo resonance is absent. Due to the large natural linewidth, experimental resolution (60-95meV) is not a limiting factor.

Linewidth control is a critical issue for yield enhancement in semiconductor manufacturing. Most of the existing techniques such as run-to-run control have been developed to control the critical dimension (CD) in photolithography and etching process. However, few studies have addressed the tool behavior that would also affect the result of CD in etching process and the etch bias that is the CD difference between photolithograph and etching process. This study aims to propose a manufacturing intelligence (MI) approach to develop dispatching rules for etching tool in order to reduce the variation of critical dimension measured after etching process and determine the machine subgroups for compensating the etching bias. An empirical study was conducted to estimate the validity of proposed approach and the results showed practical viability of this approach.

The purpose of this study was to determine whether Ricco's diameter, the spatial extent within which sensitivity demonstrates a perfect reciprocity between contrast and area, enlarges as the stimulus velocity increases. Detection thresholds were measured for a single line of length 10 arcmin as a function of linewidth that varied between 0.31 and 21.7 arcmin and for velocity ranging from 0 to 6 deg/s. We fitted the detection threshold versus linewidth data with two power functions of slope 0 and 1 and defined the intersection of these two functions as Ricco's diameter. For an increase in velocity from 0 to 6 deg/s, Ricco's diameter increases in dimension by approximately a factor of 4. Similar results were obtained when Ricco's diameter was estimated by comparing detection threshold of a thin line to that of an edge. The increase in Ricco's diameter with stimulus velocity suggests that the spatial-frequency mechanism that mediates line detection shifts progressively toward lower spatial frequencies for faster moving stimuli.

We report our first results on broadband electrical feedback applied to tunable lead-salt diode lasers. We use a heterodyne technique in combination with electronic frequency discrimination to reduce the diode-laser linewidth. The loop band width is about 10 MHz. We present results on high-frequency modulation to study the frequency response of the diode laser. From this experiment we are able to estimate the carrier tuning rate of the used lead-salt diode laser. Preliminary results in linewidth reduction by fast electrical feedback promise the possibility to apply the fast electrical feedback technique to lead-salt diode lasers.

We report a systematic increase of the linewidth of luminescence, absorption and excitation spectra of undoped GaAs-Gal_xA~xAs Multi-Quantum Well structures with decreasing layer thickness. This broadening is interpreted as due to the island-like structure of the interfaces. A very simple mod~l assuming islands with a one monolayer height and a lateral size ~ 300 A gives very good agreement with the reported linewidths.