Saturation Energy Density Research Articles

Axially modified gallium phthalocyanines and naphthalocyanines for optical limiting

Phthalocyanines (Pcs) offer a high architectural flexibility in structure, which facilitates the tailoring of their physical, optoelectronic and chemical parameters over a very broad range. This tutorial review describes the recent advances in the synthesis of soluble axially substituted or bridged gallium phthalo- and naphthalocyanine compounds, and their photophysical and nonlinear optical properties. The exploitation of the chemical reactivity of the Ga-Cl bond can allow the preparation of a series of highly soluble axially substituted and bridged Pc complexes. Axial substituents in Pcs influence favourably nonlinear optical absorption for the presence of a dipole moment perpendicular to the macrocycle in the axially substituted phthalocyanines. All Z-scans performed exhibit a decrease of transmittance about the focus typical of an induced positive nonlinear absorption of incident light. Substitution and dimerization of the phthalocyanine monomer resulted in significant reductions in the saturation energy density of the material displaying clear evidence of the usefulness of structurally modifying the gallium phthalocyanine unit. Similar to indium phthalocyanines, gallium phthalocyanines are also among the most promising materials that have been investigated as limiters of intense light and the current series presents a selection of structural modifications useful for varying their nonlinear optical properties.

Gain and saturation energy measurements in low pressure longitudinally excited N 2-lasers

A flat-plate Blumlein circuit has been used for operating a low pressure longitudinally excited oscillator–amplifier N 2-laser at 14 kV input voltage (LE–LE type). For investigating the effect of the excitation length on the laser performances, various amplifiers made of glass tubes of different lengths ranging from 15.5 to 35 cm with 4 mm inner bore diameters have been used. The measurements have been carried out for the laser parameters: small signal gain, and saturation energy density; and the laser beam divergence. Details of our measurements are presented. The results of our measurements have also been compared with the reported values of laser parameters in TE–TEA and LE N 2-laser configurations.

Ultra-fast photonic crystal/quantum dot alloptical switch for future photonic networks

We demonstrated a novel two-dimensional photonic crystal (PC) based Symmetric Mach Zehnder type all-optical switch (PC-SMZ) with InAs quantum dots (QDs) acting as a nonlinear phase-shift source. The 600- ?m-long PC-SMZ having integrated wavelength-selective PC-based directional couplers and other PC components exhibited a 15-ps-wide switching-window with 2-ps rise/fall time at a wavelength of 1.3 ?m. Nonlinear optical phase shift in the 500-?m-long straight PC waveguide was also achieved at sufficiently low optical-energy (e.g., ??phase shift at ~100-fJ control-pulse energy) due to the small saturation energy density of the QDs, which is enhanced in the PC waveguide, without using conventional measures such as SOAs with current-injected gain. The results pave the way to novel PC- and QD-based photonic integrated circuits including multiple PC-SMZs and other novel functional devices.

InGaAs-AlAs-AlAsSb coupled quantum well intersubband transition all-optical switch with low switching energy for OTDM systems

We report a novel intersubband transition all-optical switch with low switching energy using InGaAs-AlAs-AlAsSb coupled double quantum wells (C-DQWs) with AlAs spacer layers. Very low saturation energy density of 34 fJ//spl mu/m/sup 2/ was observed for bulk transmittance with wavelength of 1.62 /spl mu/m. Using a waveguide, whose core is 93 periods of the C-DQWs, an all optical switch with a low switching energy of 10 pJ for 10-dB extinction ratio was realized.

Strong Optical Limiting of Soluble Axially Substituted Gallium and Indium Phthalocyanines

Optical limiting in In‐ and Ga‐phthalocyanines exhibits a range of saturation densities and absorption cross‐section ratios. Substitution of the p‐trifluoromethylphenyl group at the central metal in tBu4PcGaCl and tBu4PcInCl reduces the saturation energy density by a factor of >3. Tailorable optical limiting properties are shown in the Figure, which plots the nonlinear absorption coefficient as a function of intensity for the five compounds studied.

Estimation of the saturation energy density in a 2.92-μm Q-switched single-frequency Cr3+:Yb3+:Ho3+:YSGG laser

An equation is derived for calculating the TEM00q-mode energy density in a laser cavity using experimentally measured quantities (e.g., the reflectivity of the output mirror, the pump excess over the threshold, the output energy, and the TEM00q-mode radius). The equation is used for estimating the saturation energy density in at 2.92-μm Cr3+:Yb3+:Ho3+:YSGG laser

Nonlinear absorption properties of some 1,4,8,11,15,18,22,25-octaalkylphthalocyanines and their metallated derivatives

The third-order nonlinear optical properties of a series of 15 unmetallated and metallated 1,4,8,11,15,18,22,25-octaalkylphthalocyanines have been investigated. The palladium-metallated compound is the strongest nonlinear absorber of the series, but, due to its comparatively high linear absorption coefficient, it exhibits a relatively low ratio of excited- to ground-state absorption cross-sections (κ) when compared to the other compounds. The highest values for κ were found for derivatives metallated with indium and lead. The nickel-metallated compounds are the weakest nonlinear absorbers, indicating that they are unsuitable as potential materials for practical passive optical limiters. Phenomenologically, the data for κ and the saturation energy density (Fsat) were found to follow trends dependent upon the linear absorption coefficient. This may have some implications for the design of phthalocyanines for nonlinear optical applications.

New axially aryloxy substituted gallium phthalocyanines for nonlinear optics

The ability to control reductions in the saturation energy density is important from the point of view of realizing applications where one would require successive layers in the practical limiter to be fabricated from materials of decreasing saturation energy density. In this paper, two new axially aryloxy substituted gallium phthalocyanine compounds: tBu4PcGa(p-CPO) and [tBu4PcGa]2·SDPO, have been prepared and structurally characterized. Their nonlinear optical (NLO) properties are described.

Near-infrared intersubband absorption in GaN/AlN quantum wells grown by molecular beam epitaxy

GaN/AlN multiple-quantum-well structures were grown by molecular beam epitaxy. Abrupt interfaces and good periodicity were confirmed. Absorption measurements indicated that intersubband absorptions occurred at wavelengths of 1.3–2.2 μm. Spectral fits by Lorentzians suggested that the well thicknesses fluctuated by two monolayers. The linewidths of the individual fits were as narrow as 80–120 meV. The characteristics of the absorption saturation were investigated at a wavelength of 1.46 μm. A relaxation time of 400 fs and saturation energy density of 0.5 pJ/μm2 were obtained. These results are promising for realizing ultrafast optical switches with energy consumption of the picojoule order.

Laser Thomson scattering measurements of negative ion density in a glow discharge plasma

Laser Thomson scattering (LTS) is shown to provide a useful means for measurements of negative ion density in a glow discharge plasma. Because the saturation energy density is so low that complete photo-detachment occurs at a very early stage of the probing laser pulse for the LTS measurement, the main part of the laser pulse is Thomson scattered by the photo-detached electrons as well as those already generated in the discharge. The energies of photo-detached electrons are below the energy difference between the photon energy and the electron affinity of the negative ions, and Thomson scattering from these appears below the difference energy. The principle has been tested using the second harmonic of a YAG laser light (532 nm) against an inductively coupled plasma operated in a mixture of Ar(80%)/O2(20%) at a pressure of 20 mTorr, and yielded the negative ion density of 1.5×1017 m-3, which amounted to 37% of the electron density of 4.1×1017 m-3 or 27% of the negative charges (electrons and negative ions).

Observation of intensity dependent, non-exponential vibrational relaxation in liquid bromoform

The intensity dependence of the vibrational relaxation in liquid bromoform after excitation of the CH stretching vibration was investigated using a picosecond infrared double resonance spectrometer. Varying the excitation energy density in the range from 2% to 50% of the saturation energy density a 20% decrease of the measured decay time with increasing pump intensity is observed. Based on the assumption of binary interactions, this dependence can be explained even quantitatively by the increased relaxation probability due to bottlenecked population on low-lying vibrational modes of CHBr 3.

Transient gain enhancement for a train of picosecond pulses in a large-aperture KrF laser amplifier

The heavily depleted steady-state gain in a large-aperture amplifier can be transiently enhanced by temporally suppressing the intra-cavity amplified spontaneous emission (ASE). In our previous experiments of amplifying two 10-ps pulses in a 29-cm-diameter KrF amplifier, we showed that output fluences of three times the saturation energy density Esat were obtained at a pulse separation of 1.7 ns for full-filling beams, whereas it took 4 ns for 5-cm-diameter beams. Our recently developed time-dependent ASE code almost reproduced this observed quick gain recovery. In this paper, we report the experimental results of amplifying six 10-ps pulses in series to the saturation level in the large-aperture amplifier. The quick gain recovery was observed also for the pulse train. Short-pulse outputs of 3Esat were obtained at a reduced pulse separation of only 1.5 ns.

Triplet quantum yield determination by picosecond laser double-pulse fluorescence excitation

The quantum yields of S 1 → T 1 triplet formation of some organic dyes are determined by picosecond double-pulse laser excitation and time-resolved fluorescence detection. The second pulse fluorescence signal is reduced due to first pulse induced singlet-triplet intersystem crossing. Disturbances due to higher excited state triplet-singlet back transfer are minimized by proper laser pulse duration and pulse energy density adjustment. The saturation energy density of triplet ground state depopulation has to be higher than the saturation energy density of singlet ground state depopulation, and the energy density of the second excitation pulse should be comparable to or less than the saturation energy density of ground state depopulation.

Optimization of passively Q-switched lasers

In optimizing passively Q-switched lasers, there is a unique choice of output coupler and unsaturated absorber transmission which maximizes the laser output energy and efficiency for each three-way combination of laser gain medium, absorber medium, and pump intensity (i.e, inversion density). In the present paper, we generalize and solve the three coupled differential equations which describe the passively Q-switched laser to obtain closed form solutions for key laser parameters such as the output energy and pulsewidth. We then apply the Lagrange multiplier technique to determine the optimum mirror reflectivities and unsaturated absorber transmissions as a function of two dimensionless variables. The first variable, z, corresponds to the ratio of the logarithmic round-trip small signal gain to the round-trip dissipative (nonuseful) optical loss and is identical to that which was used in previous treatments to optimize the rapidly Q-switched laser. The second variable, /spl alpha/, is unique to the passively Q-switched laser and is equal to the saturation energy density of the amplifying medium divided by the saturation energy density of the absorber. It is largely determined by the ration of the absorber to stimulated emission cross sections, but also depends on the speed of relaxation mechanisms in the amplifying and absorbing media relative to the resonator photon decay time. Several design curves, valid for all four level amplifying and absorbing media, are then generated. These permit the design of an optimum passively Q-switched laser and an estimate of its key performance parameters to be obtained quickly with the aid of a simple hand calculator. In the limit of large /spl alpha/ (>10), the design curves are virtually indistinguishable from the rapidly Q-switched case. The curves can also be used to perform rapid tradeoff studies of different absorbing materials. The theory can also be applied to CW-pumped, repetitively Q-switched systems through a simple multiplicative factor for the laser gain. The theory is applied to the analysis of a passively Q-switched Nd:YAG laser previously reported in the literature and shown to give excellent agreement with the experimental results.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

SRS-amplification of picosecond stokes pulses in the field of counterpropagating pump pulses

with the duration of the amplified pulses. At the same time the use of backward SRS removes this limitation. In the present work results of theoretical as well as experimental study of amplification of picosecond Stokes pulses with backward SRS in methane in the field of counterpropagating nano- and subnanosecond pumping pulses arc given. A modified installation from 131 was used in the experiments, in which the SRS-mirror in pulse laser pumping unit II was also used for compressing the pump pulses when necessary, and in amplification unit III instead of Ti:S-amplifiers SRS-cuvettes of length LI = 120 and L2 = 25 cm were placed. We present here only the main parameters of this laser installation. At the output of the second cascade of the SS-compressor (unit I) the Stokes pulses (,t S = 0.63 F4m) had an energy of Ws < 8 mJ with a pulse length of T S < 20 ps. The pulse laser pumping unit (2 L = 0.532/~m) made it possible to obtain energies W1 <_ 300 mJ in the nanosecond (rl < 4 nsc) pulse mode and W2 < 110 mJ in the subnanosecond (r2 < 0.65 ns) pulse mode. A delay line additionally inserted between units I and III made it possible to change the moment of arrival of the pump pulses by Ato = +--1.3 ns relative to the arrival time of the signal pulse. Furthermore, the meeting point of the pulses in the SRS-cuvette was adjusted by moving it longitudinally. Thus, by changing the focusing, the meeting point of the pulses, and the methane pressure in the cuvette, the conditions of interaction were optimized to achieve the maximum output energy of the first Stokes pulse for specified input energies of the pump signal pulses. To transform the energy of the pump pulses efficiently with backward SRS, as in the case of laser amplifiers 13 I, it is necessary that the energy density of the Stokes pulses be close to the saturation energy density. In the quasistationary case, where the duration of timewise smooth laser and Stokes pulses TL, S is much more than the time of transverse relaxation T2 of the excited molecules, the saturation energy flux density is found by Fsa t = ;tL(nL+ns)/gc S = 2/gc [61, where nL, S are the refractive indices of the SRS-medium at the laser and Stokes frequencies, g is the specific gain (in intensity) of the Stokes component [7 l, which depends on the pressure; c is the velocity of light. At a methane pressure of p -- 30 aim the time of dephasing of the oscillations is T2 = 16 ps, the gain g = 0.86 cm/GW [71, and the saturation energy density is Fsa t = 78 mJ/cm 2. Thus, assuming by analogy with laser amplifiers [3 ] that in the nonstationary mode of amplification of short Stokes pulses the saturation energy Fsa t has the same (relatively low) order of magnitude, one can hope to achieve rather high efficiencies of

Short-pulse amplification in an F_2 gain module

We report on what is to our knowledge the first subpicosecond-pulse amplification experiments in an F2 gain module at 157 nm. 500-fs seed pulses were produced by a four-wave mixing process in xenon by excitation with KrF short-pulse radiation. The amplification experiments yield a small-signal gain coefficient of ~5%/cm and an estimate for the saturation energy density of approximately 180 μJ/cm2. For a single-pass amplification, an output energy of 60 μJ has been measured so far. From the spectral width of the amplification spectra, a minimum possible duration of ~200 fs for the amplification of short pulses can be estimated.

Short-pulse amplification and gain dynamics of an ArF excimer amplifier

Subpicosecond pulses at 193 nm, generated by a nonresonant difference-frequency mixing process in xenon, have been amplified in an ArF excimer gain module to output energies of as much as 10 mJ. Investigations on the gain characteristics of the ArF excimer amplifier yield a saturation energy density of 1.9 mJ/cm(2) and a maximum small-signal gain coefficient of 0.15 cm(-1). Gain recovery measurements have been performed. A dominant gain recovery time of ~3 ns with indications of a small fast contribution in the picosecond range have been observed.

Theoretical Study of Subpicosecond Pulse Amplification in XeF(C-A)

The feasibility of using XeF(C-A) excimer media as an amplifier for subpicosecond short pulse lasers is theoretically investigated. Numerical caculations by using a coherent interaction model show that as a result of both a large saturation energy density and bleaching of absorbers output energy densities of >100 mJ/cm2 are easily obtained. Influence of narrow band absorption of excited xenon metastable atoms on the amplifier gain and pulse waveform is also predicted.

Relationship between the effective saturation energy and the amplification diagram of GLS1 and GLS2 neodymium glasses

An analysis is made of the transition from an amplification diagram to the saturation energy density relating directly to each amplifier cross section. On the basis of a comparison between the experimental results and an empirical formula for the effective cross section of stimulated transitions, expressions are derived for the degree of release of the stored energy and for the amplification diagrams. These expressions can allow for an inhomogeneous broadening of a luminescence line in calculations of the pulse deformation in the active medium and in determination of the decay integral. The relationship is identified between the effective saturation energy density and the parameters of the spectral inhomogeneity of the luminescence line profile of the active medium.

Gain measurement of a CuBr laser by means of modified amplified spontaneous emission

A modified amplified spontaneous emission technique has been introduced to measure the radial distributions of the gain and the saturation energy density of the output of a double-discharge pulsed CuBr laser. An asymmetric distribution of the gain was obtained. With the laser tube temperature at 420 °C, the peak value of the gain and the saturation energy density are 70 db/m and 85 μJ/cm3, respectively.