On-axis Spectrum Research Articles

CONICAL EMISSION OF TI:SAPPHIRE FEMTOSECOND LASER PULSES PROPAGATING IN WATER

Conical emission is investigated for Ti :sapphire femtosecond laser pulses propagating in water. The colored rings can be observed in the forward direction due to the constructive and destructive interference of transverse wavevector, which are induced by the spatio-temporal gradient of the free-electron density. With increasing input laser energy, due to filamentation and pulse splitting induced by the plasma created by multiphoton excitation of electrons from the valence band to the conduction band, the on-axis spectrum of the conical emission is widely broadened and strongly modulated with respect to input laser spectrum, and finally remains fairly constant at higher laser energy due to intensity clamping in the filaments.

Spectral switches of vector Gaussian Schell-model beams passing through a spherically aberrated lens

On the basis of the polarization matrix and propagation equation of the cross-spectral density matrix, the spectral changes for the on-axis spectrum of vector Gaussian Schell-model (GSM) beams focused by a spherically aberrated lens are studied, where the influence of the spherical aberration, correlation and polarizer on the behaviour of the spectral switch is stressed. It is shown that the spectrum of vector GSM beams passing through a spherically aberrated lens may be blueshifted and redshifted in comparison with the source spectrum, and a spectral switch may also take place. However, there are only blue shifts for two kinds of cases passing through an aberration-free lens and an astigmatic lens. The spectral switch arises from the oscillation of the spectral modifier with frequency.

Temporal and spectral properties of ultrashort pulsed Laguerre–Gaussian beams in dispersive media

The temporal and spectral properties of ultrashort pulsed Laguerre–Gaussian (LG) beams propagating in dispersive media are studied both theoretically and numerically. It is shown that the on-axis pulse broadens with increasing propagation distance and the on-axis spectrum is blueshifted. For the off-axis spectrum the blueshifts decrease and then the spectrum becomes redshifted as the radial distance increases. The propagation of ultrashort pulsed LG beams in free space and of ultrashort pulsed Gaussian beams can be regarded as special cases of our results, and some interesting properties are discussed.

The spectral properties of partially coherent flat-topped light beams propagating in linear gain or loss media

Based on the flat-topped beams model proposed by Li and the propagation theory of partially coherent light, the spectrum analytical expression of partially coherent flat-topped beams propagating in the linear gain or loss media was derived and the spectral properties were studied quantitatively by numerical calculation. The variation characteristics of the on-axis spectrum in near- and far-field regions have been discussed in details. The effects of the property of the linear gain or loss media and some parameters of the light source, such as the central frequency, spectrum width and spatial coherence parameter on the spectral shift have also been discussed. It can be shown that the light source parameters have greater effects on the value of the frequency shift than on the total changing trend of the spectrum along the propagation distance, whereas the latter is determined by the beam order of flat-topped beam.

The far-field properties of ultrashort pulsed beams with constant waist width in free space

By using the angular spectrum and complex analytic signal representations, the analytic propagation expression and its Fourier spectrum for ultrashort Gaussian pulsed beams with constant waist width in the far field are derived. The far-field properties of few- and single-cycle ultrashort Gaussian pulsed beams with constant waist width are studied, and compared with those of ultrashort Gaussian pulsed beams with constant diffraction length. The properties of pulse broadening, spectrum narrowing, off-axis spectrum redshift and on-axis spectrum blueshift are found for ultrashort Gaussian pulsed beams with constant waist width in the far field.

Spectral shifts and spectral switches in diffraction of ultrashort pulsed beams passing through a circular aperture

Based on the Fourier transform method, a simple closed-form expression for the on-axis power spectrum of ultrashort Gaussian pulsed beams in diffraction at a circular aperture is derived, which permits us to study spectral changes both analytically and numerically. It is shown that for diffracted pulsed beams there exist spectral red and blue shifts, spectral narrowing, and spectral switches in the near field. The aperture diffraction plays an important role in spectral switching, but both the truncation parameter and bandwidth (or equally, Fourier transform limited pulse duration) affect the behavior of spectral switches.

Spectral changes of Gaussian Schell-model beams passing through a dispersive lens

Based on the propagation law of partially coherent light, the spectral changes of Gaussian Schell-model beams passing through a dispersive lens are studied, and the material dispersion of first and second order is taken into consideration. As compared with the dispersion-free case, where there is only a blue shift of the on-axis spectrum in the neighborhood of the focus, the first-order dispersion may result in the on-axis spectral splitting, and both blue and red shifts take place near the focus. The second-order dispersion gives rise to a further increase of spectral splitting. The spectral splitting increases as the spatial correlation increases.

Experimental observation of the phenomenon of spectral switching for a class of partially coherent light

Experimental observations of the phenomenon of spectral switching are made for a class of partially coherent light incident on a circular aperture. The on-axis spectrum of the light in the far-field is different from the spectrum of the light at the aperture. It is shown that, depending on the value of the parameter a/L~(/spl omega/) (a is the radius of the aperture and L~(/spl omega/) is the effective correlation length of the light at the aperture at the central frequency /spl omega//sub 0/ of the source spectrum), the spectral shift shows a gradual change but for a particular value of a/L~(/spl omega/) the spectral shift exhibits a rapid transition and the phenomenon of spectral switching occurs. The generation of many spectral lines, from a single spectral line, in the far-field is also demonstrated.

Spectral changes and 1 X N spectral switches in the diffraction of partially coherent light by an aperture.

The off-axis and on-axis spectra in the far zone of an aperture for the case in which a particular class of partially coherent light with a broad spectrum is diffracted by an aperture are studied. It is shown that the spectrum in the far zone is generally different from that at the aperture; i.e., the spectrum is split into two or more peaks. Moreover, the spectrum varies with the diffractive angle. For a fixed diffractive angle, the spectral shift, defined as the difference between the frequencies at which the observed spectrum and the spectrum at the aperture take their maximum, shows a gradual change with the change in the coherence at the aperture. However, as the coherence reaches some critical values, the spectral shift exhibits a rapid transition; i.e., spectral switch occurs. The coherence that causes the spectral switch to take place is different for different diffractive angles. Therefore we propose a new kind of 1 x N spectral switch, where N detectors (output ports) are placed at different diffractive angles in the far zone, and the spectral shifts at different detectors are measured. By adjusting the coherence of the aperture (input port) to the desired values, we obtain a rapid transition of the spectral shift in the desired output ports.

The spectral switch of partially coherent light in Young's experiment

The spectral switch in Young's two-slit experiment with partially coherent light is studied based on the propagation law of partially coherent light. It is shown that, by varying the source correlation, slit parameter and observation distance, the on-axis spectrum changes gradually, and the spectral switch can take place, where the spectral shift exhibits a rapid transition from red to blue shift or vice verse.

Spectral shifts and spectral switches in diffraction of partially coherent light by a circular aperture

When a particular class of partially coherent light is incident upon a circular aperture, the on-axis spectrum of the light in the far zone is different from the spectrum of the light at the aperture. An explicit expression describing the on-axis spectrum of the light in the far zone is obtained and the numerical calculation is performed. It is shown that by measuring the spectral shifts against some parameter, such as a/L~(a is the radius of the aperture and L~ is the effective correlation length of the light at the aperture at the central frequency of the source spectrum); we find that the spectral shifts show a gradual change. However when a/L~ is equal to particular values, the spectral shifts exhibit a rapid transition, i.e., spectral switches occur. Some parameters, such as the half-width at half-maximum of the source spectrum /spl Gamma/ and the ratio of the central obstruction of the source /spl epsiv//sub s/ affect the spectral switch positions and the spectral switch performance. The generation of multi-spectral lines in the far zone from a single spectral line is also shown.

Undoped and doped GaN thin films deposited on high-temperature monocrystalline AlN buffer layers on vicinal and on-axis α(6H)–SiC(0001) substrates via organometallic vapor phase epitaxy

Monocrystalline GaN(0001) thin films have been grown at 950 °C on high-temperature, ≈ 100 nm thick, monocrystalline AlN(0001) buffer layers predeposited at 1100 °C on α(6H)−SiC(0001)Si substrates via OMVPE in a cold-wall, vertical, pancake-style reactor. These films were free of low-angle grain boundaries and the associated oriented domain microstructure. The PL spectra of the GaN films deposited on both vicinal and on-axis substrates revealed strong bound excitonic emission with a FWHM value of 4 meV. The near band-edge emission from films on the vicinal substrates was shifted slightly to a lower energy, indicative of films containing residual tensile stresses. A peak attributed to free excitonic emission was also clearly observed in the on-axis spectrum. Undoped films were too resistive for accurate Hall-effect measurements. Controlled n-type, Si-doping in GaN was achieved for net carrier concentrations ranging from approximately 1 × 1017 cm−3 to 1 × 1020 cm−3. Mg-doped, p-type GaN was achieved with nA−nD ≈ 3 × 1017 cm−3, ρ ≈ 7 Ω · cm, and μ ≈ 3 cm2/V · s. Double-crystal x-ray rocking curve measurements for simultaneously deposited 1.4 μm GaN films revealed FWHM values of 58 and 151 arcsec for deposition on on-axis and off-axis 6H−SiC(0001)Si substrates, respectively. The corresponding FWHM values for the AlN buffer layers were approximately 200 and 400 arcsec, respectively.

Analyses of molecular and neutral atomic emission spectra from an electrothermal launcher plasma

Analyses of neutral atomic copper line and C/sub 2/ Swan Band (d/sup 3//spl Pi/-a/sup 3//spl Pi/) emission in the 420 to 570 nm spectral region from the experimental electrothermal launcher device SIRENS have been performed to determine edge and core plasma densities and temperatures. Time-averaged core arc temperatures of 10,000 to 22,000 /spl deg/K and arc densities of 7/spl times/10/sup 23/ to 2/spl times/10/sup 24/ m/sup -3/ have been deduced by measurements along the axis of the device using the relative intensities and the Stark broadening of the copper lines. The observed C/sub 2/ band spectra were compared with synthetic spectra calculated using a molecular spectra-radiative transport code. At a discharge energy of 1.56 kJ, for 90/spl deg/ aspect, the C/sub 2/ temperature was determined to be about 3500 /spl deg/K, with a column density of 2/spl times/10/sup 21/ m/sup -2/. On-axis spectra, at a discharge energy of 1.07 kJ, were found to correspond to a C/sub 2/ temperature of about 7500 /spl deg/K and a column density of 7/spl times/10/sup 20/ m/sup -2/. Because of the lower temperatures, it is believed that the C/sub 2/ emission comes from the plasma edge. >

Growth via Mocvd and Characterization Of GaN and AlxGa1−xN(0001) Alloys for Optoelectronic and Microelectronic Device Applications

ABSTRACTMonocrystalline GaN(0001) thin films have been grown at 950°C on high-temperature, 100 nm thick, monocrystalline AIN(0001) buffer layers previously deposited at 1100°C on α(6H)-SiC(0001)si substrates via MOCVD in a cold-wall, vertical, pancake-style reactor. AlxGa1−xN films (0≤x≤1) were grown directly on the same SiC surface at 1100°C. Abrupt heterojunctions among the alloy composition were demonstrated. All films possessed a smooth surface morphology and were free of low-angle grain boundaries and associated oriented domain microstructures. Double-crystal x-ray rocking curve measurements for the GaN(0004) reflection for simultaneously deposited 1.4 μm films revealed FWHM values of 58 and 151 arcsec for materials grown on on-axis and off-axis material, respectively. The corresponding values for the AIN(0004) buffer layers were ≈ 200 and ≈400 arc sec, respectively. A similar relationship was found for the alloys for 0≤x≤0.2. The PL spectra of the GaN films deposited on both vicinal and on-axis substrates revealed strong bound exciton emission with a FWHM value of 4 meV. The spectra of these films on the vicinal substrates were shifted to a lower energy, indicative of films containing residual tensile stresses. A peak believed to be associated with free excitonic emission was also observed in each on-axis spectrum. Rutherford backscattering, Auger depth profiling and energy dispersive analysis were used to determine the AIN/GaN ratios in the alloys. Cathodoluminescence of solutions with x<0.5 exhibited strong near band edge emission with a FWHM as low as 31 meV. The band gaps were determined via spectral ellipsometry. Undoped GaN and A1xGa1−xN films were too resistive for accurate Hall-effect measurements. Controlled n-type Si-doping in GaN and AlxGa1−xN (for x≤0.4) was achieved for net carrier concentrations ranging from approximately 2×1017 cm−3 to 2×1019 (AlxGa1−xN) or to l × 1020 (GaN) cm−3. Mg-doped, p-type GaN was achieved with nA−nD = 3 × 1017 cm−3, p ≈ 7 Ω cm and μ ≈ 3 cm2/V·s.

Estimation of turbulence spectra parameters from log-amplitude fluctuations spectra of an optical wave

From the general formula for the temporal frequency log-amplitude fluctuations spectra the asymptotic expressions are found, valid for the geometric and wave approximations. A comparison of the plane and spherical wave formulas with those valid for the beam wave is made in all frequency regions. The difference between the on-axis and off-axis beam spectra is presented. The possibility of estimating from the log-amplitude scintillations the inner and outer scales of turbulence together with the exponent of turbulence spectra in the subrange is briefly discussed.

Sound radiation patterns in the frequency domain of cries from a vespertilionid bat

Ultrasonic cries from an immobilized vespertilionid batMyotis daubentoni, were recorded simultaneously in front of the bat (on-axis), and at various off-axis angles. The differences between the on-axis and off-axis spectra were computed and related to the theory of directional emission from a baffled rigid-piston radiator. This theory implies zero radiation at a specific frequency for a given piston diameter and off-axis angle (Fig. 1). The bat's difference spectra showed notches of ca. 25 dB with properties as the zeros of the piston theory (Fig. 7). The structure of off-axis cries was found to be qualitatively predictable from the on-axis cries, using the piston theory (Fig. 4). The difference spectra had a complex, oscillatory fine-structure that could not be accounted for by the basic piston theory.

Asymptotic Theory for Beam Propagation in a Random Medium*

In a previous paper, we derived expressions for the irradiance distribution of a finite beam. Here we derive expressions for the full coherence function. We find an asymptotic analytic expression for the coherence function for very large propagation distances. In this region, the on-axis angular spectrum of the finite beam is one fourth that of an initial plane wave and three fourths that of an initial spherical wave. When the scattering dominates diffraction, we find overlapping asymptotic representations for the coherence function for all distances. We present some explicit results for an initially coherent gaussian wave.