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We have established a thermal distribution model of the multilayer dielectric grating with laser irradiation based on the Beer–Lambert theory and the thermal conduction theory. The temperature change on the surface and inside of the grating is simulated according to the laser irradiation time, laser power, substrate size, and substrate material. The actual thermal behavior of a 50 × 50 × 10 mm3 multilayer dielectric grating with fused silica substrate is measured with 10 kW fiber laser irradiation. The results show a dynamic equilibrium between the energy absorbed in the laser irradiation area and the energy transferred in the thermal transfer area when the maximum temperature of the grating is kept stable. The simulation results are in good agreement with the experimental data, which provides a reference for understanding the thermal dynamics process of the multilayer dielectric grating with high power density laser irradiation.

A facile, low-cost and low-temperature fabrication approach of counter electrode is essential for pursuing robust photovoltaic devices. Herein, we develop a hydrothermal in situ growth of Cobalt–Ruthenium (Co–Ru) alloy nanofiber electrode for quantum dot solar cell (QDSC) applications. Colloidal CdS QDs with tunable absorption band edge are synthesized and used as light absorber. After optimizing the QDs with the highest photoluminescence quantum yield accompanied by considerable solar light absorption ability, QDSC based on Co–Ru alloy electrode delivers a much higher power conversion efficiency than its counterparts, i.e., either pure Co or Ru metal electrodes. In detail, Co–Ru alloy electrode exhibits high specific area, excellent electrical behavior, intimate interface contact, and good stability, thus leading to notable improved device performances. The impressive robust function of Co–Ru alloy with simple manufacturing procedure highlights its potential applications in robust QDSCs.

Metal oxide nanocrystals have been pursued for various applications in photovoltaics as a buffer layer. However, it remains a challenging task to adjust their energy levels to achieve a better match of the donor–acceptor system. Herein, we report the fabrication of graphene quantum dots (GQDs) with bright blue photoluminescence by a top-down strategy based on laser fragmentation with posthydrothermal treatment. The GQDs demonstrate appropriate energy level positions and are used as an intermediate buffer layer between TiO2 and P3HT to form a cascade energy level architecture. The introduction of the GQDs into a bulk heterojunction hybrid solar cell has led to an enhancement of the power conversion efficiency.

An effective energy level regulation of acceptor by doping samarium phosphate nanophosphors (SmPO4 NPs) was reported for inorganic/organic hybrid solar cell applications. SmPO4 NPs doped TiO2/P3HT bulk heterojunction (BHJ) solar cell shows an enhanced power conversion efficiency of approaching 3% as compared with that of its counterpart without SmPO4 NPs (1.98%). The underlying photophysical mechanism was probed by applying femtosecond transient absorption spectroscopy and the results show that the efficiency enhancement was ascribed to the improved hot electron, less energetic electron, hole transports at the interface of BHJ apart from down-conversion photoluminescence of SmPO4 NPs. It has been evidenced that the hot electron transfer life time was shortened by more than 40% (i.e., from τhot-e=30.2 to 17.9 ps) than pure TiO2 acceptor while the hole transfer lifetime was boosted by almost 20% (i.e., from 6.92 to 5.58 ns). Such charge carrier improvements stem from the efficient energy level regulations by SmPO4 NPs. In detail, the conduction band (CB) edge of TiO2 has been elevated by 0.57 eV while the valence band (VB) edge has been elevated by 0.32 eV, thus not only narrowing down the energy offset between CB energy levels of acceptor TiO2 and donor P3HT, but also meanwhile enlarging the band gap of TiO2 itself that permits to inhibit electron-hole recombination within TiO2. This work demonstrates that samarium ions can efficiently facilitate exciton generation, dissociation and charge transport and have an important role in enhancing photovoltaic performance.

Photovoltaic performances are critically dependent on efficient photoexcited charge carrier generation. Vanadium ions are introduced into titanium dioxide (TiO2) as a photocatalyst to tailor the energy level alignment of TiO2 and the conjugated polymer poly(3-hexylthiophene) (P3HT). Incorporation of vanadium ions into TiO2 yields a remarkable decrease of the energy offset between the conduction band edge of TiO2 and the lowest unoccupied molecular orbital of P3HT. The reduction of the ‘excess’ energy offset leads to a faster electron transfer at the interface of the bulk heterojunction with the lifetime decreasing from 30.2 to 19.7 ps, thus giving rise to a notable enhancement of the photovoltaic performance. That is, a power conversion efficiency of 3.01% for the vanadium-doped TiO2/P3HT solar cell at 5.0 wt% vanadium-doping is obtained as compared to 2.00% for pure TiO2.

We investigate the nonlinear optical properties of Cu 9 S 5 nanocrystals using the open aperture Z-scan method with 20 ps and 4 ns laser pulses at 532 nm. We found that optical nonlinearity arises from free carrier absorption in the former case, whereas in the latter it occurs mostly due to nonlinear scattering. Parameters characterizing free carrier absorption and nonlinear scattering were determined. The dynamics of the nonlinear processes was confirmed by pump-probe measurements with 20 ps pulses. Observation of optical limiting at both excitation time regimes indicates the potential of Cu 9 S 5 nanocrystals in laser safety devices.

We study a CW Ne–Ne laser-induced thermal diffusion and convection in a chloroaluminum phthalocyanine–ethanol solution by measuring the transmittance and deflection of the beam normally incident on the sample with the entrance surface parallel and normal to the optical table, respectively. In the former geometry, we find a monotonous increase of transmittance with time and no transmitted beam deflection. We quantitatively simulate the observed transmittance by involving thermal diffusion alone. In the latter geometry, we find a transition of transmittance from ascent to descent with time and a downward transmitted beam deflection. We additionally invoke convection to explain the results.

Using the Z-scan technique with 532 nm 19 ps laser pulses separated by two time intervals τp-p’s (0.1 s and 1.0 s) sandwiching the mass diffusion time constant of the C49H43ClO6 + 1,2 dichloroethane solution, we investigate short-pulse-induced solute migration in the sample by measuring its transmittance change with τp-p variation. Preparing the sample at two concentrations, we find that τp-p reduction, from 1.0 s to 0.1 s, increases its transmittance when input pulse energy e1 exceeds a threshold eT, which is lower for the dilute solution than the concentrated one. At two e1’s above eT for the dilute solution, τp-p-reduction-induced transmittance increase in the dilute solution, as compared to that in the concentrated solution, is more at the lower e1 and less at the higher e1. This differs from continuous-wave-driven thermal diffusion which always causes a larger transmittance increase in the concentrated solution by inducing a larger temperature gradient. From this study, we predict that solute migration induced by short pulses at 1064 nm is one of the undesired heating effects occurring when this solution is used to simultaneously Q-switch and mode-lock Nd:YAG lasers.

We investigate nonlinear refraction of chloroaluminum phthalocyanine dissolved in ethanol using the Z-scan technique with 532-nm 19-ps laser pulses, separated by various time intervals with respect to the solution’s mass diffusion time constant. As a result, we verify that a negative lensing effect is induced by the outward solute migration, in agreement with our theoretical inference.

By chopping 820 nm 18 femtosecond (fs)-laser pulses, continuously generated by a self-mode locked Ti:Al2O3 laser at 82 MHz, into trains with both train-width and train-to-train separation considerably longer than the thermal diffusivity time constant τth of CS2, we conducted Z-scan measurements on it at various times relative to the leading pulse of each train (T's). As a result, we observed negative nonlinear refraction strengthening with T within τth and gradually stabilizing with T exceeding τth. We quantitatively explain the experimental results in terms of the thermal lensing effect. In particular, we attribute the heat generation to non-radiative relaxation of libration excited by individual 18 fs-pulses via stimulated Raman scattering. In contrast to the commonly held view of multi-photon excitation, we propose and verify a new heat-generating mechanism for the thermal lensing effect in CS2.

We experimentally verified fs laser pulse induced thermal lensing effect in CS2 with the Z-scan technique and explain this effect as a result of relaxation of stimulated Raman scattering excited libration.

Transient optical Kerr effect of liquids C2H4Cl2 and C2H4Br2 is investigated, for the first time to our knowledge, with a femtosecond (fs) probe laser delayed with respect to a coherent fs pump laser. Coherent coupling and electronic Kerr signals are observed around zero delay when pump and probe overlap. Persisting after the pump-probe overlap are Kerr signals arising from the torsional and other intramolecular vibrations of the trans and gauche conformations; Kerr signals arising from the intermolecular motion are also observed. Vibrational quantum interference is only observed in liquid C2H4Br2 and the related beats data are fitted with the torsional vibrations, 91 cm−1 (gauche) and 132 cm−1 (trans), and the CCBr angle-bending vibrations, 231 cm−1 (gauche) and 190 cm−1 (trans), with dephasing times, 0.45 ps, 0.45 ps, 2 ps, and 1.5 ps, respectively. These vibrational frequencies agree with those obtained in the frequency-domain. That no vibrational mode is observed for C2H4Cl2 might be attributed to ineffective Raman-pumping. Kerr signals observed after the pump-probe overlap are Fourier transformed to give the spectra of the intermolecular motion and the vibrational spectrum, which agrees with the one observed in the infrared absorption and/or Raman scattering heretofore.

CuI embedded in borosilicate glasses are prepared by a melt and heat-treatment method. We obtain the free carrier absorption cross section σ ≈ 1.0 × 10−17 cm2 and the two-photon absorption coefficient β ≈ 0.14 cm/GW by means of the Z-scan method using 6-ns laser pulses at 532 nm, with an irradiation I0 ≈ 16 MW/cm2. Upon excitation with 355 nm laser pulses at room temperature, very bright luminescence is observed, while the peak position shifts to lower energy as the band gap of CuI decreased by crystal growth. We also observed that the peak intensity varies linearly with the input intensity. The results could be attributed to the radiative recombination associated with traps or defects from impurities in the CuI microcrystallites.

The pump–probe Raman-induced optical Kerr effect (RIKE) of simple molecular liquids, studied with femtosecond laser pulses, exhibit long lasting beats ascribable to vibrational quantum interference (QI). While energy conservation entails vibrational resonances in RIKE, momentum conservation boils down to wave vector-matching in the pump and probe processes, which calls for the participation of a vibrational excitation wave. The refractive index dispersion around vibrational resonances is intimately related to the focusing angle of the pump (also probe) beam. The larger the focusing angle, the greater the excitation wave number, i.e. the more energetic the vibration in resonance; if the focusing angle is too small, energetic vibrations cannot be observed in vibrational QI, even if energy conservation is fulfilled. If the pump and probe beams are perfectly collimated, then all beams must be collinear in order to conserve momentum.

Nonlinear absorptive and refractive properties of C60/toluene solution are studied using the Z-scan technique and a 532 nm laser delivering single picosecond (ps) pulses and trains of ps pulses separated by 7 nanoseconds (ns). With single ps pulses, reverse saturable absorption and positive nonlinear refraction are observed and attributed to population transitions among the singlet states. With ps pulse trains, a sign change of nonlinear refraction relative to that of single ps-pulse excitation is observed and is attributed to heat-induced temperature rise and population relaxing to triplet states.

Using the Z -scan technique with 532 nm laser pulses of dual widths in the picosecond regime, we distinguish between two-photon absorption (TPA) and excited state absorption (ESA). To account for reverse saturable absorption in chloro-aluminum phthalocyanine dissolved in methanol both TPA and ESA, rather than either one, need be invoked.

A short laser pulse with energy exceeding a threshold can induce the outward solute migration. In this study we further verify that application of a temperature gradient along the light propagation direction impedes this migration.

We report a novel method of determining the lifetimes of the high-lying excited singlet states of dye molecules in solution. The method is based on simulating the Z-scan technique with ground-state and excited-state absorptions of an incident laser pulse having Gaussian temporal and spatial distributions. This leads to the important finding that the use of laser pulses as long as tens of picoseconds can resolve subpicosecond lifetime.

We present an effective method for enhancing the refractive index sensitivity of a fiber-optic particle plasmon resonance sensor based on spatial light modulation technology. The sensor was realized by modification of spherical gold nanoparticles in the fiber sensing zone. Three different beam shapes were tested and one of the relatively high refractive index sensitivity (∼12.67/RIU) was found. A 5% enhancement was found over sensors without beam modification.

Use was made of a novel Fabry–Perot interferometer technique to determine the variation of refractive index with temperature (∂n/∂T) of a potassium niobate crystal with a GaAlAs diode laser. The transverse intensity distributions of the laser transmitted through the crystal were found to be temperature dependent, and the ∂n/∂T values along crystallographic b and c axes were determined to be −4.5×10−5 and 6.1×10−5 °C−1, respectively. The temperature changes required for tangential tuning from transverse mode TEMqmn to TEMqm+1n and sagittal tuning from TEMqmn to TEMqmn+1 were found to be 0.29 and 1.27 °C, respectively, when the laser was polarized along the b axis. When the laser polarization was along the c axis, the tangential and sagittal tunings were found to be 0.10 and 0.54 °C, respectively. These differences between the tangential and sagittal directions indicated that the monolithic ring resonator used was indeed asymmetric.

The transmittive and reflective Z-scan technique is used with a 10 Hz, frequency doubled, Q-switched, and mode-locked Nd:YAG laser to verify that the reflectivity of the super-resolution near-field structure of an SiN/Sb/SiN thin film increases as incident intensity decreases. This intensity-dependent reflection, called nonlinear reflection, reflects a TEM(00) mode laser beam more strongly at its periphery than at its center and so shrinks the transmitted laser beam. The observed nonlinear reflection is attributed to laser-induced change of carrier densities in Sb, to justify quantitatively the experimental results.

Nonlinear refraction of liquids C 6 H 6 , C 6 H 5 Cl, C 6 H 5 Br, and C 6 H 5 I was investigated, for the first to our knowledge, using the Z-scan technique with 82 MHz 35 fs laser pulses at 800 nm. For all samples studied a negative lensing effect was observed, opposite to that obtained with 10 Hz 19 picosecond laser pulses. The negative lensing effect was found to increase with the pulse train width and gradually turn steady on the time scale of thermal diffusivity. The sign difference is thus considered as a thermal lensing effect which sustains across femtosecond pulses, but not the picosecond pulses. In light of previous Raman induced Kerr effect (RIKE) studies on a variety of simple transparent molecular liquids, the negative nonlinear refraction observed with 35 fs pulses is attributed to electronic or resonant nuclear motions (librational and vibrational motions). In this study, results show that the shape and bond strength of molecules seems to have strong impact on the thermal lensing effects.

We present results of optical limiting experiments designed to study optical geometries for increasing the dynamic range over which limiters function without incurring optical damage. Specifically, we investigate a tandem geometry with two passive nonlinear elements, one placed in the focal plane of a lens and the second placed “upstream” of the focal position to protect the material at focus from damage. To provide a proof-of-principle demonstration of this geometry, simple limiters consisting of combinations of reverse saturable absorber dyes and a carbon black suspension in thin cells were tested. Our results show that a substantial increase in device performance can be achieved by use of a tandem limiter geometry. Simple modelling predicts that the dynamic range of a separate-element tandem limiter is given by the product of the dynamic ranges of the individual component limiting elements, in agreement with our experimental results.

We present recent studies of negative lensing effect observed in several molecular liquids using Z-scan technique and 28 femtosecond 800 nm laser pulses. Raman induced Kerr effect and thermal lensing effect are invoked to explain the sign of nonlinear refraction.

Using the Z-scan technique, we verify the threshold energy for a 19 ps laser to induce outward solute migration decreases with the increase of the solution temperature, well agreeing with our proposed potential well model.

We present a long period grating sensor fabricated in a large mode area photonic crystal fiber. This device is insensitive to temperature change but highly sensitive to refractive index variation (167.5 nm/RI or 2.57×10−4 limiting resolution).

Using the Z-scan technique with an 800-nm 28 femtosecond laser running at 82-MHz, we verify thermal lensing effect accumulated across pulses in a transparent (both linear and nonlinear) liquid: methanol.

We report a sensitive single beam technique for measuring both the nonlinear refractive index and nonlinear absorption coefficient for a wide variety of materials. We describe the experiment and present a brief analysis including cases where nonlinear refraction is accompanied by nonlinear absorption. In these experiments the transmittance of a sample is measured through a finite aperture in the far-field as the sample is moved along the propagation path (z) of a focused Gaussian beam. The sign and magnitude of the nonlinear refraction are easily deduced from such a transmittance curve (Z-scan). Employing this technique a sensitivity of better than A/300 wavefront distortion has been achieved using picosecond frequency doubled N&YAG laser pulses. In cases where nonlinear refraction is accompanied by nonlinear absorption, it is possible to separately evaluate the nonlinear refraction as well as the nonlinear absorption by performing a second Z-scan with the aperture removed. We demonstrate this me...

A negative lensing effect is observed in transparent liquids C2H4Cl2 and C2H4Br2 using the Z-scan technique and 28 femtosecond titanium: sapphire laser pulses. Two possible mechanisms, Raman induced Kerr effect and thermal lensing effect, are invoked to account for the sign change of nonlinear refraction.

A sensitive single-beam technique for measuring both the nonlinear refractive index and nonlinear absorption coefficient for a wide variety of materials is reported. The authors describe the experimental details and present a comprehensive theoretical analysis including cases where nonlinear refraction is accompanied by nonlinear absorption. In these experiments, the transmittance of a sample is measured through a finite aperture in the far field as the sample is moved along the propagation path (z) of a focused Gaussian beam. The sign and magnitude of the nonlinear refraction are easily deduced from such a transmittance curve (Z-scan). Employing this technique, a sensitivity of better than lambda /300 wavefront distortion is achieved in n/sub 2/ measurements of BaF/sub 2/ using picosecond frequency-doubled Nd:YAG laser pulses. >

We observed a negative lensing effect in transparent molecular liquids (methanol and acetic acid) with 28 femtosecond laser pulses. In this paper, we propose two possible mechanisms to explain why the sign is opposite to that observed with picosecond and nanosecond laser pulses.

Opposite signs of nonlinear refraction in chloroaluminum (ClAl) phthalocyanine (Pc) dissolved in methanol are observed using the Z-scan technique with two output modes of a frequency-doubled, Q-switched, mode-locked Nd:YAG laser operating at 10 Hz. Using the single picosecond (ps) pulses switched out of the pulse trains, positive nonlinear refraction is observed and attributed to population transitions among ClAlPc’s singlet states. Using the full nanosecond pulse trains, a sign change of nonlinear refraction relative to that of single ps-pulse excitation is observed, and is attributed to the heat-induced temperature rise and the population relaxing to ClAlPc’s triplet states.

Transition from reverse-saturable absorption to saturable absorption of the chloroaluminum phthalocyanine solution excited by a giant laser pulse is ascribed not just to the saturation of excited state absorption, but also to the outward migration of the solute molecules at the laser beam center. While the saturation of excited state absorption occurs within a single picosecond laser pulse, the beam center population decrease is sustained much longer than the pulse duration. We distinguish these two mechanisms with the Z-scan technique, utilizing picosecond pulses with pulse-to-pulse separations ranging from 0.1 to 5.0 s.

Using a femtosecond (fs) laser we have studied the momentum conservation of Raman-induced Kerr effect (RIKE) in CHBr/sub 3/ liquid, where two intramolecular vibrations are Raman-excited. It is learned that in order to observe a given vibration in RIKE the fs pump laser must be focused to converge at a sufficiently large angle.

Transient optical Kerr effect of liquids C/sub 2/H/sub 4/Cl/sub 2/ and C/sub 2/H/sub 4/Br/sub 2/ is studied with trains of 40-femtosecond laser pulses in the pump/delayed-probe experiment. Around delay zero coherent coupling and electronic Kerr signals are observed. Persisting after delay zero are Kerr signals arising from the torsional motion and other intramolecular vibrations of the trans and gauche conformations.

We report a method of determining the lifetimes of the high‐lying excited singlet states of dye molecules in solution. The method is based on the Z‐scan technique and the observation of saturation of excited state absorption in dye solutions using laser pulses having Gaussian temporal and spatial distributions. This leads to the important finding that the use of laser pulses as long as tens of picosecond can resolve subpicosecond lifetimes. The method is demonstrated by measuring an upper excited singlet state lifetime of 900 fs using 30 ps full width at half maximum input pulses on a chloroaluminum phthalocyanine dye.

Using the pump-probe with phase object technique with 20 ps laser pulses at 532 nm, we investigated the carrier relaxation process subsequent to two-photon absorption (TPA) in ZnO. As a result, we found that an additional subnanosecond decay pathway is activated when the pump beam intensity surpasses 0.4 GW/cm2. We attributed this intensity-dependent pathway to a TPA induced bulk defect state and our results demonstrate that this photo induced defect state has potential applications in ZnO based optoelectronic and spintronic devices.

The thermal lensing effect in transparent (linear and nonlinear) molecular liquids can be modulated by disrupting continuously output 82 MHz 28 fs laser pulses at 800 nm to form trains of various widths (τt) with respect to the thermal diffusivity time τth (~2 ms). We present nonlinear refraction results for CHBr3 (bromoform) obtained by the Z-scan technique. The results show that the thermal lensing effect increases with τt when τt is less than τth, but becomes steady when τt exceeds τth and reaches 30 ms. The proposed technique of modulating the thermal lensing effect by varying τt has great application potential in information photonics and optoelectronic devices, such as variable optical attenuators, holographic recording media, optical limiters and optical switches.

Using the Z-scan technique, we find that migration of chloroaluminum phthalocyanine in liquid ethanol can be induced by the absorption of a 19 ps laser pulse with energy exceeding a threshold but not by that of a 2.8 ns pulse depositing more energy at the solute molecules. Considering each solute molecule as an oscillator confined within a potential well, we explain, in accordance with the five-energy-band model, that solute molecules excited by a 19 ps pulse retain more translational excess energy to overcome the potential well barrier compared with those excited by a 2.8 ns pulse of equal energy. Therefore, they are more likely to migrate out of the laser beam center, weakening the solution's absorption that we detect in the Z-scan measurements. Furthermore, we theoretically infer that the 19 ps pulse-induced solute migration tends to be nonquasistatic and experimentally verify that it cannot be attributed to the Soret effect, a quasistatic process.

We describe a sensitive technique for measuring nonlinear refraction in a variety of materials that offers simplicity, sensitivity and speed. The transmittance of a sample is measured through a finite aperture in the far-field as the sample is moved along the propagation path (z) of a focused Gaussian beam. The sign and magnitude of the nonlinearity is easily deduced from such a transmittance curve (Z-scan). Employing this technique a sensitivity of better than λ/300 wavefront distortion is achieved in n2 measurements of BaF2 using picosecond frequency doubled Nd:YAG laser pulses.

We present nonlinear refraction results for liquids methanol and acetic acid obtained with the Z-scan technique and 28 femtosecond (fs) 800 nm laser pulses. In contrast to the positive lensing effect obtained previously with picosecond and nanosecond laser pulses, a negative lensing effect is observed. The associated mechanism features the third-order polarization arising from the nonlinear response of the molecular skeletal motion that is driven into resonance through its electrostatic coupling to the valence electron cloud distorted by the fs laser field.

The principles of operation of semiconductor optical limiters which utilize two-photon absorption and free-carrier induced defocusing are described. We present a review of early work using psec pulses at 532 nm in ZnSe, in which the problem of damage in solid state limiters is overcome by optimizing the focusing geometry. Limiting energies as loW as 10 nJ are seen, and a dynamic range (damage energy divided by limiting energy) in excess of 104 is demonstrated. The somewhat complicated propagation theory is simplified into a set of scaling rules which are used to predict operating characteristics of semiconductor limiters at longer wavelengths and for shorter pulses. We present new limiting data obtained with longer pulses in ZnSe, in CdTe at 1.06 μm and InSb at 10.6 μm, and we compare these results with the scaling rules.

Optical limiting measurements have been made on solutions of several metal containing phthalocyanines and naphthalocyanines. Measurements at 532nm using nanosecond pulses from a Q-switched Nd:YAG laser show limiting throughputs of 1-10 millijoules with mild focussing in alcohol solutions with nominal transmissions of 30-70 percent. Measurements on chloro-aluminum-phthalocyanine solutions utilizing individual 30 psec pulses or trains (spanning about 100nsec) of modelocked pulses have shown even lower limiting throughputs. Thus, the dynamic range of the limiting behavior has been shown to cover at least three orders of magnitude. Prompt limiting is attributed to strong singlet-singlet (S1-Sn) absorption, whereas the longer time limiting behavior is postulated to result from strong triplet-triplet (T1-Tn) absorption. In addition to these studies, efforts have been underway to identify materials with reduced limiting throughput and improved optical transmission characteristics.