Ground State Absorption Cross Section Research Articles

Reverse saturable absorption of platinum ter/bipyridyl polyphenylacetylide complexes

Transition metal poly-ynes are very interesting nonlinear optical materials for optical limiting applications because of their low linear absorption at low intensities, but high nonlinear absorption at high intensities in the entire visible spectral range. The reverse saturable absorption (RSA) of seven platinum ter/bipyridyl polyphenylacetylide complexes has been investigated at 532 nm with nanosecond pulses. In an f/214 geometry, with sample linear transmission of 90% in a 2 mm cell, the reverse saturable absorption thresholds for these complexes are 8–30 mJ/cm2. The maximum output fluence of these complexes are limited to 0.45–1.16 J/cm2 for incident fluence as high as 3.6 J/cm2. The RSA is strongly influenced by the nature of the triaryl coordination ligand and the conjugation length of the polyphenylacetylide ligand in these complexes. This RSA is mainly attributed to the triplet excited state absorption, with a ratio of effective excited state to ground state absorption cross sections as high as ∼20. These results suggest that platinum ter/bipyridyl polyphenylacetylide complexes could be promising candidates for nanosecond optical limiting applications; and it is possible to improve the reverse saturable absorption by appropriate structural modifications.

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.

The primary investigation of optical-limiting effects of optical active phthalocyanine derivative TMBO-SiPc

The optical-limiting property of an optically active phthalocyanine derivative with different configuration in chloroform solution was measured at 532 nm with 10 ns pulses. The excited state absorption cross-section σ ex of the derivative was determined with the Z-scan technique. The results showed that the phthalocyanine derivative possesses larger σ ex than the ground state absorption cross-section σ 0, indicating that it is a material for reverse saturable absorption. And it is specially noticed that the change in the optically active configuration affects its optical-limiting property significantly.

Spectroscopic properties of magnesium aluminosilicate glass-ceramics doped with divalent cobalt ions

Electronic absorption and luminescence spectra and nonlinear optical properties of magnesium aluminosilicate glass-ceramics doped with divalent cobalt ions are studied in relation to their synthesis conditions. The lifetimes of the 4T1(4P) and 4T1(4F) excited levels of tetracoordinate Co2+ ions in these glass-ceramics are found to depend on the cobalt concentration and lie within the ranges from 25 to 40 and from 120 to 450 ns, respectively. Using the absorption saturation curves, the ground state absorption cross section for the wave-length of 1.54 μm is estimated to be σGSA=3.3×10−19cm2. The induced emission of the tetrahedrally coordinated Co2+ ion, which dominates over the absorption from the excited state in the spectral range from 640 to 690 nm (4T1(4P)→4A2 transition), is found.

Picosecond laser saturable absorption studies on F 2− colour centres in LiF crystal

The saturable absorption behaviour of F 2 − colour centres in LiF is studied by picosecond laser pulse excitation (wavelength λ L =1054 nm , duration Δt L =6 ps ). The colour centre number density, N 0, the ground-state absorption cross-section, σ L, and the single-photon excited-state absorption cross-section, σ ex,L, are determined by numerical fitting the energy density dependent energy transmission. A decrease of energy transmission at high excitation energy densities (w 0 L >1 J cm −2) is attributed to two-photon absorption of other colour centres (N 1 and N 2 bands of F 4 centres) present in the sample.

Optical limiting and picosecond relaxation of carbocyanines upper electronic states

Nonlinear absorption, anomalous fluorescence and relaxation of high-lying electronic states in six carbocyanine dyes, including cryptocyanine (DCI), DDI, DTDCI, DTTCI, DOTCI and HDIDCI, in solution and in polymer, were studied by means of picosecond transient absorption spectroscopy and nonlinear transmission experiments. Absorption cross-sections of the S 1→S n transition, and decay rates of the second singlet excited state, S 2, were measured. All dyes showed strong reverse saturable absorption in the 450–600 nm region with large excited to ground state absorption cross-section ratios. For DTDCI this ratio, at 470 nm, was measured to be 350, which is the largest ever reported. All molecules have shown strong optical limiting effect. However, in all molecules except DCI a saturation of the optical limiting process was observed as expected, owing to relatively long, up to 17.5 ps, lifetime of the S 2 state. The S 2 state fluorescence quantum yields were also measured.

Accurate method for the measurement of absorption cross sections of solid-state saturable absorbers

A rigorous approach taking into account both the temporal and spatial distributions of the pulsed probe beam used for transmission measurements of solid-state saturable absorbers is applied to characterise these absorbers accurately. The use of this method can significantly influence the resulting value for the ground-state absorption cross section. It also permits us to avoid the introduction of an artificial excited-state absorption in systems where, according to energy-level diagrams, no such loss should occur. This is illustrated in the case of passive Q-switchers for 1.5-μm lasers such as Co2+-doped LaMgAl11O19 and MgAl2O4 and Co2+- or Cr2+-doped ZnSe and ZnS.

Nanosized glass-ceramics doped with transition metal ions: nonlinear spectroscopy and possible laser applications

Absorption, luminescence and saturation of absorption for cobalt-doped glass-ceramics were studied. Characteristic times of luminescence decay of 4T1(4P)→4A2 transition for tetrahedrally coordinated Co2+ ions in lithium-, zinc-, magnesium- and magnesium–gallium–aluminosilicate glass-ceramics were evaluated to be in tens of ns time scale. Kinetics of bleaching relaxation of 4A2→4T1(4F) transition for tetrahedrally coordinated Co2+ ions in magnesium–aluminosilicate glass-ceramics was found to be 450±150 and 120±40 ns for 0.1 and 0.3 wt% cobalt concentration, respectively. Ground-state absorption cross-section for Co2+ ions at 1.54 μm was estimated to be ∼3×10−19 cm2. Passively Q-switched pulses from Er:glass laser of 6 mJ in energy and 80 ns in duration using the Co-doped magnesium–aluminosilicate glass-ceramics sample as saturable absorber were demonstrated.

Cobalt-doped transparent glass ceramic as a saturable absorber Q switch for erbium:glass lasers

A new saturable absorber Q switch for 1.54-mum Er:glass lasers is presented. The saturable absorber is a transparent glass ceramic that contains magnesium-aluminum spinel nanocrystallites doped with tetrahedrally coordinated Co(2+) ions. We obtained Q-switched pulses of up to 5.5 mJ in energy and 80 ns in duration at 1.54 mum. The relaxation time of (4)A(2) ?(4)T(1)((4)F) transition bleaching was measured to be (450 ? 150) ns. Ground-state and excited-state absorption cross sections at 1.54-mum wavelength were estimated to be (3.2 ? 0.4) x 10(-19) cm(2) and (5.0 ? 0.6) x 10(-20) cm(2), respectively.

Optical limiting properties of optically active phthalocyanine derivatives

The optical limiting properties of four optically active phthalocyanine derivatives in chloroform solutions and epoxy resin thin plates were measured at 532 nm with 10 ns pulses. The excited state absorption cross-section σ ex and refractive-index cross-section σ r were determined with the Z-scan technique. These chromophores possess larger σ ex than the ground state absorption cross-section σ 0, indicating that they are the potential materials for reverse saturable absorption (RSA). The negative σ r values of these chromophores add to the thermal contribution, producing a larger defocusing effect, which may be helpful in further enhancing their optical limiting performance. The optical limiting responses of the thin plate samples are stronger than those of the chloroform solutions.

Cr 2+:Cd 0.55Mn 0.45Te crystal as a new saturable absorber for 2 μm lasers

For the first time, to our knowledge, passive Q-switching of 2.09 μm Ho,Tm,Cr:YAG laser has been demonstrated using Cr 2+:Cd 0.55Mn 0.45Te as the saturable absorber. A pulse duration of about 200 ns and an output pulse energy up to 8 mJ were obtained with a Cr 2+:Cd 0.55Mn 0.45Te passive shutter. The ground-state absorption cross-sections for the Cr 2+:Cd 0.55Mn 0.45Te crystal was estimated at 2.09 μm to be 11×10 −19 cm 2, while excited-state absorption was shown to be negligible. Computer simulations of Q-switching exhibits satisfactory agreement with experimental data.

Amplified spontaneous emission in neat films of arylene-vinylene polymers

Amplified spontaneous emission (travelling-wave lasing) was achieved for a set of 18 poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene-vinylene) (MEH-PPV) related arylene-vinylene copolymers, a part of which substituted with additional phenyl groups at the vinyl double bond. Wave-guiding neat thin films on glass substrates were used. The samples were transversally pumped with single second harmonic pulses of a mode-locked ruby laser (wavelength 347.15 nm). Travelling-wave emission occurred in the wavelength region between 485 and 650 nm. Repeat unit based absorption cross-section spectra, normalized fluorescence quantum distributions, and amplified spontaneous emission spectra are presented. Ground-state absorption cross-sections at the wavelengths of peak amplified emission are extracted from effective gain length measurements. Effective stimulated emission cross-sections are derived from pump pulse energy densities necessary for optical narrowing.

Ultrafast charge photogeneration in conjugated polymer thin films

We report femtosecond transient absorption spectroscopy on thin films of blue-emitting derivatives of polyindenofluorene (PIF). Probe wavelength and pump intensity dependence measurements allow the separation of the broadly overlapped 1B u exciton and charge pair absorption spectra. We find that charge pairs are produced within the instrument resolution (∼100 fs) followed by exciton–exciton bimolecular annihilation on a picosecond timescale. Two possible mechanisms for ultrafast charge generation are considered: direct charge separation of nascent 1B u excitons by static quenching at intrinsic defects and sequential transitions to produce highly energetic excitons that dissociate efficiently. Photophysical modelling of intensity-dependent data reveals that sequential absorption followed by charge separation is the most likely mechanism for ultrafast charge pair generation, with the ratio of excited-state to ground-state absorption cross section times the effective yield of charge pairs≈0.1. These observations are consistent with previous photocurrent measurements and quantum chemical calculations assigning highly excited states to greatly delocalised excitons [Köhler et al., Nature 392 (1998) 903].

Co 2+ :LiGa 5 O 8 saturable absorber passive Q switch for 1.34 μm Nd3+:YAlO3 and 1.54 μm Er3+:glass lasers

Passive Q switching of a Nd3+:YAlO3 laser at 1.34 μm and an Er3+: glass laser at 1.54 μm was demonstrated using a Co2+:LiGa5O8 crystal as a saturable absorber. Q-switched pulses at 1.34 μm of 13 mJ energy and 90 ns duration and at 1.54 μm of 8 mJ energy and 70 ns duration were obtained. The ground state absorption cross section of the Co2+:LiGa5O8 was estimated to be (2.6±0.4)×10−19 cm2 and (2.5±0.5)×10−19 cm2 at 1.34 and 1.54 μm, respectively. The4T1(4F)→4A2 relaxation time of the Co2+ ion in the LiGa5O8 crystal was measured to be ∼130 ns.

Optic nonlinearities of a copper complex of pyrazinoporphyrazine

For the copper complex of pyrazinoporphyrazine AzaPhcCu(C(CH 3 ) 3 ) 8 in CH 2 Cl 2 , we use nanosecond pulses to measure its third-order nonlinear optic effects at 532 nm. The results show that its effective third-order nonlinear refractive index n 2 eff (-7.85 x 10 -10 esu) is larger than that of the AzaPhcCu(C 12 H 25 ) 4 and the CuPc(OC 5 H 11 ) 8 . The reverse saturable absorption (RSA) of AzaPhcCu-(C(CH 3 ) 3 ) 8 is demonstrated by the ratio of effective excited state to ground state absorption cross sections. The observed intersystem crossing lifetime τ isc of AzaPhcCu(C(CH 3 ) 3 ) 8 (∼15 ns) and AzaPhcCu(C 12 H 25 ) 4 (∼45 ns) are longer than that of CuPc(OC 5 H 11 ) 8 (∼5.0 ns). The aza-substituents are suggested to form a S 1 (n,π * ) state as their lowest excited singlet-states, and their radiationless intersystem crossing rates (1/τ isc ) are discussed with the spin-orbit coupling (ψ s |H SO |ψ T ) and the vibronic coupling (X S |X T ) effects between S 1 and T 1 states.

Co 2+ : ZnS and Co2+:ZnSe saturable absorber Q switches

The materials, Co2+:ZnS and Co2+:ZnSe, serving as Q switches with an Er3+: glass laser at 1.54 μm have been demonstrated and analyzed. The output pulses were typically 15–65 ns in duration and of 1–4 mJ depending on the doping concentration and the crystal quality. The ground-state absorption cross section of Co:ZnS has been determined to be 7.7×10−19 cm2 and 7.6×10−19 cm2 for Co:ZnSe, by bleaching experiments.

Saturable absorber Co^2+:MgAl_2O_4 crystal for Q switching of 134-µm Nd^3+:YAlO_3 and 154-µm Er^3+:glass lasers

Saturable-absorber Q switching of a neodymium-doped YAlO(3) laser at 1.34 microm and an erbium-doped glass laser at 1.54 microm with a Co(2+):MgAl(2)O(4) crystal was demonstrated. Q-switched 1.34-microm pulses of 19-mJ energy and 60-ns duration and Q-switched 1.54-microm pulses of 2.7-mJ energy and 75-ns width were obtained. The ground-state absorption cross sections of the Co(2+):MgAl(2)O(4) crystal were estimated to be (2.8 +/- 0.4) x 10(-19) and (3.5 +/- 0.6) x 10(-19) cm(2) at 1.34 and 1.54 microm, respectively. The (4)T(1)((4)F)-->(4)A(2) relaxation time of the Co(2+) ion in the MgAl(2)O(4) crystal was measured to be approximately 350 ns.

Z-scan measurement of the ground and excited state absorption cross sections of Cr/sup 4+/ in yttrium aluminum garnet

Z-scan measurements of the ground and excited state absorption cross sections of Cr/sup 4+/-doped yttrium aluminum garnet are reported. These quantities are found to depend on crystal preparation but are independent of temperature from -60/spl deg/C to 98/spl deg/C. Their ratio is shown to weakly determine the energy output of a passively Q-switched Nd:glass laser using this material as the saturable absorber.

Polymethine and squarylium molecules with large excited-state absorption

We study nonlinear absorption in a series of ten polymethine dyes and two squarylium dyes using Z-scan, pump-probe and optical limiting experiments. Both picosecond and nanosecond characterization were performed at 532 nm, while picosecond measurements were performed using an optical parametric oscillator (OPO) from 440 to 650 nm. The photophysical parameters of these dyes including cross sections and excited-state lifetimes are presented both in solution in ethanol and in an elastopolymeric material, polyurethane acrylate (PUA). We determine that the dominant nonlinearity in all these dyes is large excited-state absorption (ESA), i.e. reverse saturable absorption. For several of the dyes we measure a relatively large ground-state absorption cross section, σ 01, which effectively populates an excited state that possesses an extremely large ESA cross section, σ 12. The ratios of σ 12/ σ 01 are the largest we know of, up to 200 at 532 nm, and lead to very low thresholds for optical limiting. However, the lifetimes of the excited state are of the order of 1 ns in ethanol, which is increased to up to 3 ns in PUA. This lifetime is less than optimum for sensor protection applications for Q-switched inputs, and intersystem crossing times for these molecules are extremely long, so that triplet states are not populated. These parameters show a significant improvement over those of the first set of this class of dyes studied and indicate that further improvement of the photophysical parameters may be possible. From these measurements, correlations between molecular structure and nonlinear properties are made. We propose a five-level, all-singlet state model, which includes reorientation processes in the first excited state. This includes a trans– cis conformational change that leads to the formation of a new state with a new molecular configuration which is also absorbing but can undergo a light-induced degradation at high inputs.

Continuous-wave power transmission and thermal lensing of a saturable absorber subject to excited-state absorption

Rate-equation analysis has been used in an investigation of the role of saturation and excited-state absorption in the power transmission characteristics and thermal lensing of an absorber. Use of an iterative approach gives explicit analytical results for power transmission and thermal focal length in the presence of excited-state absorption. Sample calculations indicate that pump absorption can increase or decrease with increasing incident pump power, depending on the relative strength of the excited-state absorption cross section with respect to the ground-state absorption cross section. In the case of thermal lensing, results further indicate that saturation and excited-state absorption act as two competing effects, the former reducing the strength of the thermal lens and the latter causing the opposite effect. The analytical formulas derived in this analysis should prove useful to experimentalists in determination of ground-state and excited-state absorption cross sections from experimental power transmission and lensing data.