Two Photon Absorption

The effect of fluctuations on the nonlinear response of an underdamped oscillator to an external periodic field at a subharmonic frequency has been investigated theoretically, numerically, and with an analog electronic circuit model. The system studied has often been analyzed in nonlinear optics in the context of twophoton absorption and second-harmonic generation. We consider its nonlinear spectroscopy. Its resonant nonlinear response is described over a broad range of the fluctuation intensities. It is shown that the fluctuation intensity can be used to ''tune'' the oscillator so as to maximize the nonlinear response. The dependence of the absorption cross section on the fluctuation intensity displays a clearly resolved maximum. If the eigenfrequency of the oscillator is a nonmonotonic function of its energy, the signal at the second harmonic displays a resonant peak at one of two different frequencies, depending on the noise intensity.

The American Physical Society Two-photon induced photocurrent imaging in CdS nanosheets 1 P. KUMAR, A. WADE, H.E. JACKSON, L. M. SMITH , University of Cincinnati, J. YARRISON RICE, Miami University, YOUNG-JIN CHOI, JAE-GWAN PARK, Korea Institute of Science and Technology -We study the photocurrent from photoexcitation of charged carriers in CdS nanosheet (NS) structures at room temperature. Schottky type contact pads separated by ∼ 4 µm across a ∼ 4 µm wide single NS were fabricated using photolithography followed by Ti/Al (20 nm/200 nm) metal evaporation and lift-off. Ar + bombardment before the metal deposition was used to create Ohmic contacts. For Schottky type contacts, spatial imaging of the photocurrent exhibits peak photocurrents near the reversed bias contact confirming the confinement of the electric field within the space charge region due to the applied bias voltage. For devices with the Ar + bombarded contacts, we observe the peak photocurrent distinctly away from the contacts. We find a nearly quadratic power dependence of photocurrent in the laser power range ∼0.1-10 GW/cm 2 for sub band gap excitation (λ = 800 nm), while linear power dependence of photocurrent for above band gap excitation (λ = 488 nm). Simple model calculations are used to obtain a two-photon absorption coefficient β ∼100 cm/GW in these CdS nanosheets.

In this paper, novel poly(esterimide)s with attached as a side group azobenzene chromophores are presented. The polymers differing in substituents on the azobenzene moieties and chromophore concentration. Azopolymers were obtained by a twostep synthetic approach. This include the preparation of a precursor poly(esterimide) with benzene moiety pendant from main chains, followed by the covalent bonding of the NLO chromophores onto the polyimide backbone by a post-azo-coupling reaction using the following coupling components: p-nitroaniline, 2-cyan-4-nitroanilin, 2,4-dinitroaniline and Disperse Orange 3. The precursor poly(esterimide) was obtained from synthesized 2,2 0 [N-phenylethyloaniline-di(4-estro-1,2dicarboxylic)]anhydride and 4,4 0 -methylene bis(2,6-dimethylaniline). The degree of functionalization of the poly(esterimide)s was estimated by UV-vis spectroscopy. The obtained polymers were characterized and evaluated by FT-IR, 1 H NMR, X-ray, UV-vis, DSC and TGA. The results showed the azopolymers exhibited high glass transition temperatures (T g ¼ 159{224 C), high thermal stability with decomposition temperatures in the range of 311-334 C and good solubilities in common organic solvents. First time two-photon absorption (TPA) coefficient in cast films of amorphous azopoly(esterimide)s was measured using Z-scan experiment which was performed with femtosecond laser pulses 100 fs at 800 nm. The TPA coefficient was determined to be in the range of 1.54-11.69 cm/GW.

Nonlinear optics has become an essential area of study for understanding and harnessing the complex light-matter interactions that occur in molecules, nanostructures, and advanced materials. This review introduces a summary of the latest advancements, with a particular focus on the nonlinear optical characteristics of noble metal nanoparticles, especially silver nanoparticles (AgNPs). These nanomaterials have gained significant interest due to their typical plasmonic features and size-dependent behavior. Silver nanoparticle colloids demonstrate enhanced light absorption at high intensities, known as reverse saturable absorption, along with a selfdefocusing optical response characterized by a negative nonlinear refractive index. It has been observed that the size of noble metal nanoparticles significantly influences their nonlinear optical properties. The thermal lens phenomenon was addressed utilizing a unique approach that considers the

Strong molecule-light interaction enables the control of molecular structures and dynamical processes. A molecular model is proposed to greatly enhance the intermolecular distance of resonant energy transfer, where the molecules are strongly driven by an optical cavity. The optimal intermolecular distance and quantum yield of energy transfer are observed, resulting from the balance between dipole-dipole interaction and molecule-cavity coupling. Our approach is {\textit{non-perturbative}}, going much beyond the Forster mechanism of resonant energy transfer. Our work sheds the light on the spectroscopic study of the cooperative energy transfer in molecular polaritons.

Following a first study on a late afterglow in flowing pure nitrogen post discharge, we report new two-photon absorption laser-induced fluorescence (TALIF) measurements of the absolute ground-state atomic nitrogen density N(4S) and investigate the influence of methane introduced downstream from the discharge by varying the CH4 mixing ratio from 0% up to 50%. The N (4S) maximum density is about 2.2 × 1015 cm−3 in pure N2 for a residence time of 22 ms and does not change significantly for methane mixing ratio up to ∼15%, while above, a drastic decrease is observed. The influence of the residence time has been studied. A kinetic model has been developed to determine the elementary processes responsible for the evolution of the N (4S) density in N2/CH4 late afterglow. This model shows the same decrease as the experimental results even though absolute density values are always larger by about a factor of 3. In the late afterglow three-body recombination dominates the loss of N (4S) atoms...

We provide a compendium of the quantum mechanical equations for photon emission rates, photon absorption cross sections, and photon scattering cross sections. For each case, the different equations that apply for discrete or continuous electron states of the emitting, absorbing, or scattering material are given.

Substantial refractive index changes in clinical grade poly(methyl methacrylate) (PMMA) with both femtosecond NIR (800 nm) and NUV (387 nm) laser pulses are demonstrated, enabling volume gratings and waveguides in bulk PMMA. In the NIR, temporal pulse duration is shown to be critical, with modification efficiency increasing rapidly as pulse duration decreases below 200 fs. On the other hand, in the NUV, efficient index modification can be accomplished with longer, 200 fs pulses. The successful direct modification of ultra-pure, clinical grade PMMA demonstrates an alternative to processes that require doping to increase sensitivity. Grating diffraction efficiency and refractive index profile measurements infer a maximum positive refractive index change of n max = 3x10 -3 in exposed regions. Chemical analysis of the modified structures in the NUV suggests direct polymer back bone cleavage and monomer production as photomodification pathway. Holographic writing at 387 nm with a high NA objective produced weak periodic features of period = 0.42 m. The combined results in the NIR and NUV demonstrate the importance of peak intensity for inducing non-linear absorption and suggest that three and two photon absorption are responsible for modification of pure PMMA at 800 nm and 387 nm, respectively.

Stable films containing CdS quantum dots of mean size 3.4 nm embedded in a solid host matrix are prepared using a room temperature chemical route of synthesis. CdS/synthetic glue nanocomposites are characterized using high resolution transmission electron microscopy, infrared spectroscopy, differential scanning calorimetry and thermogravimetric analysis. Significant blue shift from the bulk absorption edge is observed in optical absorption as well as photoacoustic spectra indicating strong quantum confinement. The exciton transitions are better resolved in photoacoustic spectroscopy compared to optical absorption spectroscopy. We assign the first four bands observed in photoacoustic spectroscopy to 1s e -1s h , 1p e -1p h , 1d e -1d h and 2p e -2p h transitions using a non interacting particle model. Nonlinear absorption studies are done using z-scan technique with nanosecond pulses in the off resonant regime. The origin of optical limiting is predominantly two photon absorption mechanism.

We have studied the nonlinear optical properties of nanolayered Se/As2S3 film with a modulation period of 10 nm and a total thickness of 1.15 μm at two [1064 nm (8 ns) and 800 nm (20 ps)] wavelengths using the standard Z-scan technique. Three-photon absorption was observed at off-resonant excitation and saturation of two-photon absorption at quasiresonant excitation. The observation of the saturation of two-photon absorption is because the pulse duration is shorter than the thermalization time of the photocreated carriers in their bands and three-photon absorption is due to high excitation irradiance.

We report measurements of optical loss in GaAs/Al 2 O 3 nonlinear waveguides over the spectral range 1.3-2:1 lm in the infrared using a scattering technique. The optical source was a tunable femtosecond optical parametric oscillator (OPO) generating output pulses with durations of 200-250 fs at a repetition rate of $90 MHz and an average power of $50 mW. Loss coefficients of $1.15-2.55 cm À1 , corresponding to losses of $5-11 dB/cm were obtained from the measured data. The loss decreases with increasing wavelength due to the Rayleigh scattering contribution.

UV-Laser interactions with wide bandgap insulators and semiconductors has generated a number of examples of point defect production, surface and bulk modification, etching and re-deposition processes, as well as numerous PLD related applications involving the emitted particles. In this talk we examine these phenomena and modifications in oriented single crystals of the transparent semiconductor ZnO which as a band-gap of ~3.4 eV. This material is of high interest for production of transparent conductors and transistors, solar cells, lasers, sensors, and in catalysis (including potential use in the photo-dissociation of water). We explore exposure of single crystal ZnO to UV laser radiation under ultra high vacuum conditions. We examine the underlying mechanisms for nanoparticle growth on the exposed surface, the emissions of ions, the intense and energetic emissions of neutral Zn, O, and O 2 , and the extraordinary clean etching of this very brittle but highly important technological material.

The spectroscopic diagnostic technique of two photon absorption laser-induced fluorescence (TALIF) of atomic species has been applied to single-point measurements of velocity and static temperature in the NASA Ames Interaction Heating Facility (IHF) arc jet. Excitation spectra of atomic oxygen and nitrogen were recorded while scanning a tunable dye laser over the absorption feature. Thirty excitation spectra were acquired during 8 arc jet runs at two facility operating conditions; the number of scans per run varied between 2 and 6. Curve fits to the spectra were analyzed to recover their Doppler shifts and widths, from which the flow velocities and static temperatures, respectively, were determined. An increase in the number of independent flow property pairs from each as-measured scan was obtained by extracting multiple lower-resolution scans. The larger population sample size enabled the mean property values and their uncertainties for each run to be characterized with greater confidence. The average € ±2σ uncertainties in the mean velocities and temperatures for all 8 runs were ±1.4% and ±11 %, respectively.

The new nonlinear crystal for the mid-IR CdSiP 2 was discovered only very recently but the interest in this chalcopyrite is enormous because it possesses most of the attractive properties of the related ZnGeP 2 but allows in addition pumping at 1064 nm without two-photon absorption and uncritical phase-matching for 6 µm generation with maximized effective nonlinearity. The last feature is due to the fact that this crystal is negative uniaxial in contrast to ZnGeP 2 which shows positive birefringence. We now measured its nonlinear coefficient using SHG of femtosecond pulses generated near 4.6 µm from a seeded KNbO 3 optical parametric amplifier. The SHG efficiency was compared for uncoated samples of CdSiP 2 and ZnGeP 2 , both ≈0.5 mm thick, in the low conversion limit (<10% internal conversion efficiency) which justifies the use of the plane wave approximation. Taking into account the experimentally determined phase-matching angles for type-I SHG (oo-e type in CdSiP 2 and ee-o type in ZnGeP 2 ), which were in good agreement with the existing Sellmeier approximations, we arrived at d 36 (CdSiP 2 )~d 36 (ZnGeP 2 ) which is rather unexpected having in mind the larger band-gap of CdSiP 2 . The reliability of the measurement was tested at the same wavelength by comparing ZnGeP 2 with HgGa 2 S 4 which led to the result d 36 (ZnGeP 2 )~3d 36 (HgGa 2 S 4 ), in very good agreement with previous estimations.

Herein we report the experimental and theoretical study of the temperature dependence of a thiacarbocyanine dye in its monomer, H- and J-aggregates states. We demonstrate the ability to control the ratio of monomer, H- and/or J-aggregates with heat. We link such a control to the conformation dependence of the molecule. An alternative way to gain access to the dominating species without changing the concentration as a complete switching mechanism between all the present species is proposed. The results presented in this work lead to a better understanding of thiacarbocyanine dye’s behavior.

Fluorescent intercalating dyes have previously been used to form a selfassembled photonic wire at the nano scale, employing long range energy transfer. Here we present a mimic of a photosynthetic reaction centre antenna complex using porphyrin-modified DNA and fluorescent intercalating dyes. Visible light is used to excite an intercalated dye followed by fast energy transfer between dyes due to their large spectral overlap. A redox active porphyrin moiety absorbing at higher wavelength acts as an energy sink, thereby transferring energy from the DNA strand to the porphyrin. Effectively this energy transfer process has increased the absorption coefficient of the porphyrin. We will study the efficiency of energy transfer to the porphyrin as a function of intercalating dye concentration, and study the effect of lipid vesicles on the system.

Fluorescent intercalating dyes have previously been used to form a selfassembled photonic wire at the nano scale, employing long range energy transfer. Here we present a mimic of a photosynthetic reaction centre antenna complex using porphyrin-modified DNA and fluorescent intercalating dyes. Visible light is used to excite an intercalated dye followed by fast energy transfer between dyes due to their large spectral overlap. A redox active porphyrin moiety absorbing at higher wavelength acts as an energy sink, thereby transferring energy from the DNA strand to the porphyrin. Effectively this energy transfer process has increased the absorption coefficient of the porphyrin. We will study the efficiency of energy transfer to the porphyrin as a function of intercalating dye concentration, and study the effect of lipid vesicles on the system.

We demonstrate the generation of near-single cycle longitudinally polarized terahertz radiation using a large-area radially biased photoconductive antenna with a longitudinal field amplitude in excess of 2 kV/cm. The 76 mm diameter antenna was photo-excited by a 0.5 mJ amplified near-infrared femtosecond laser system and biased with a voltage of up to 100 kV applied over concentric electrodes. Amplitudes for both the transverse and longitudinal field components of the source were measured using a calibrated electro-optic detection scheme. By tightly focusing the radiation emitted from the photoconductive antenna, we obtained a maximum longitudinal field amplitude of 2.22 kV/cm with an applied bias field of 38.5 kV/cm.

All-inorganic CsPbBr 3 perovskite colloidal quantum dots have recently emerged as a promising material for a variety of optoelectronic applications, among others for multiphoton-pumped lasing. Nevertheless, high irradiance levels are generally required for such multiphoton processes. One strategy to enhance the multiphoton absorption is taking advantage of high local light intensities using photonic nanostructures. Here, we investigate two-photonexcited photoluminescence of CsPbBr 3 perovskite quantum dots on a silicon photonic crystal slab. By systematic excitation of optical resonances using a pulsed near-infrared laser beam, we observe an enhancement of two-photon-pumped photoluminescence by more than 1 order of magnitude when comparing to using a bulk silicon film. Experimental and numerical analyses allow relating these findings to near-field enhancement effects on the nanostructured silicon surface. The results reveal a promising approach for significantly decreasing the required irradiance levels for multiphoton processes being of advantage in applications such as biomedical imaging, lighting, and solar energy.

All-inorganic CsPbBr 3 perovskite colloidal quantum dots have recently emerged as a promising material for a variety of optoelectronic applications, among others for multiphoton-pumped lasing. Nevertheless, high irradiance levels are generally required for such multiphoton processes. One strategy to enhance the multiphoton absorption is taking advantage of high local light intensities using photonic nanostructures. Here, we investigate two-photonexcited photoluminescence of CsPbBr 3 perovskite quantum dots on a silicon photonic crystal slab. By systematic excitation of optical resonances using a pulsed near-infrared laser beam, we observe an enhancement of two-photon-pumped photoluminescence by more than 1 order of magnitude when comparing to using a bulk silicon film. Experimental and numerical analyses allow relating these findings to near-field enhancement effects on the nanostructured silicon surface. The results reveal a promising approach for significantly decreasing the required irradiance levels for multiphoton processes being of advantage in applications such as biomedical imaging, lighting, and solar energy.

All-inorganic colloidal perovskite quantum dots (QDs) based on cesium, lead, and halide have recently emerged as promising light emitting materials. CsPbBr3 QDs have also been demonstrated as stable two-photon-pumped lasing medium. However, the reported two photon absorption (TPA) cross sections for these QDs differ by an order of magnitude. Here we present an in-depth study of the TPA properties of CsPbBr3 QDs with mean size ranging from 4.6 to 11.4 nm. By using femtosecond transient absorption (TA) spectroscopy we found that TPA cross section is proportional to the linear one photon absorption. The TPA cross section follows a power law dependence on QDs size with exponent 3.3 ± 0.2. The empirically obtained power-law dependence suggests that the TPA process through a virtual state populates exciton band states. The revealed power-law dependence and the understanding of TPA process are important for developing high performance nonlinear optical devices based on CsPbBr3 nanocrystals.

Cell damage in UV and NIR laser microscopes by highly focused micromanipulation and fluorescence excitation microbeams has been studied. Damage in erythrocytes, spermatozoa and Chinese hamster ovary cells was detected by monitoring morphology changes, autofluorescence detection, cloning assay, and viability screening. It was found that 364nm/365 nm UVA radiation induced irreversible cell damage at radiant exposures as low as <10 J/cm2. NIR CW microradiation used in laser tweezers was also able to damage cells via a two-photon excitation process, in particular, when using <800nm trapping beams. Non-destructive two-photon excitation in femtosecond NIR microscopes is possible within a narrow intensity window. The lower limit is determined by two-photon absorption coefficients and detector efficiency, the higher by intracellular optical breakdown in the extranuclear region. Above certain wavelength-dependent intensity thresholds in femtosecond microscopy, cells were completely destroyed by fragmentation concomitant with plasma generation. The influence of excitation and micromanipulation microbeams should be considered when studying physiology and metabolism of vital cells.

We are using a femtosecond pump-probe apparatus to study heat transfer when a pulsed jet of liquid water impinges on a hot Pt-coated Si surface (Leidenfrost Effect). The light source in the experiment is a 100 mW Er:fiber laser operating at a wavelength of λ=1550 nm; the total volume of the pulsed water jet is ∼0.9 mm 3 . The temperature change within the Si substrate at a distance of 50 microns from the interface is measured by a novel time-resolved thermometry based on two-photon absorption. We measure the thermal conductance of the water layer within 50 nm of the interface by time-domain thermo-reflectance; changes in the thermal conductance provide a direct measurement of the contact time of the liquid. We convert the integral of the temperature excursion to the energy transferred using a Green's function solution of heat conduction in the Si substrate. Both the energy transferred and contact time show a smooth evolution from high values at 110C to low values at 210C without any clear indication of a Leidenfrost point.

A detailed performance comparison of new interesting Ybdoped crystals in the same oscillator setup, with a single-mode fibercoupled diode laser pump, is reported. We intended to assess the shortest pulses achievable with available SESAM technology, running a fair comparison with laser crystals Yb:KLuW, Yb:SSO, Yb:CALGO, Yb:CALYO and Yb:CaF 2 , very likely including the most promising choices for the next generation of commercial bulk ultrafast solid-state systems.

espanolEn el presente trabajo se estudia la respuesta en fluorescencia de un vapor de Rubidio confinado en los intersticios de una celda de vidrio poroso, sistema altamente difusivo para la luz. El interes es el de estudiar las consecuencias espectroscopicas asociadas al confinamiento espacial de los atomos. La transicion seleccionada posee un tiempo de vida medio del estado excitado mayor al tiempo medio de vuelo de los atomos en el poro. Este estado atomico es alcanzado mediante la excitacion del atomo por dos campos de diferentes frecuencias. Los espectros de fluorescencia obtenidos presentan formas inusuales, y son bien descriptos por un modelo teorico sencillo que contempla el caracter difuso de la luz. Si bien los mencionados espectros no dan informacion del confinamiento 3D de los atomos, la accion del confinamiento se ve claramente reflejada al medir el tiempo de decaimiento de la senal de fluorescencia, en donde se observa un acortamiento del tiempo con respecto a la senal ...

The production of Er 3+ -doped willemite glass-ceramic using waste soda lime silica (SLS) glass was reported in this study. The precursor glass and glass-ceramics were prepared using conventional melt-quenching technique with empirical formula (Er 2 O 3 ) x [(ZnO) 0.5 (SLS) 0.5 ] 1-x where x = 1 wt.% and sintered at various temperatures. The trend of density and the linear shrinkage of the glass-ceramic samples were increased with the progression of sintering temperature. The crystallization behaviour and surface morphology of the glass-ceramics was studied using X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). It was revealed that at a lower sintering temperature (700°C), the stable state of αwillemite (α-Zn 2 SiO 4 ) phase was formed. Fourier transform infrared (FTIR) spectroscopy reveals that the presence of different functional groups in the samples. Three transition states of excitation were shown in Uv-visible (UV-Vis) spectra which arise from the ground state 4 I 15/2 to the excited states 4 F 7/2 , 2 H 11/2 , 4 F 9/2 . Broad green emission at 557 nm under excitation 385 nm was obtained. These spectra reveal that the luminescence of the samples was increased with the progression of sintering temperature due to the presence of Er 3+ ions into the willemite crystal. Such luminescence of glass and glass-ceramics are expected to have potential applications in optoelectronic devices.

Electroabsorption (EA) spectra associated with overlapping vibronics of even-and odd-parity excitons are related in the Condon approximation to the linear absorption of discrete localized states. The EA intensity and profile depend strongly on the linewidth, F, with F -2 scaling of features due to Stark shifts and F-1 scaling of induced absorption and bleaching. Static disorder in polymer films broadens the linear absorption and reduces EA intensity compared to polydiacetylene (PDA) crystals, whose resolved spectra indicate overlapping vibrational sidebands. Similar excitations and transition moments in films conspicuously alter EA profiles and allow estimates of symmetry-breaking perturbations. EA spectra of PDA crystals and films are consistent with Pariser-Parr-Pople theory of linear and nonlinear optical spectra. The high sensitivity of EA also provides direct tests of excited-state displacements at induced transitions of even-parity excitons.

This paper reviews recent advances in the nanotube and graphene polymer composites synthesized in our laboratories for laser protection application, i.e., OL. We first discuss the mechanisms involved in the polymer nanocomposites for OL and the Z-scan technique used to measure the nonlinear optical properties of the materials in question. Subsequently, the design, synthesis, characterization, and OL properties of the representative nanotube and graphene polymer composites are introduced, followed by a prospect for use of carbon nanomaterial polymer composites for optical limiting.

Nonlinear refraction, nonlinear absorption and optical limiting in photorefractive crystals Bi 12 SiO 20 (BSO) and Bi 12 GeO 20 (BGO) at the wavelengths of 1064 and 532 nm were investigated. It was shown that both BSO and BGO crystals possess by positive nonlinear refraction in two investigated spectral ranges (n 2 BSO =(2.5 ± 0.5) • 10 )12 esu, n 2 BGO = (6.3 ± 1.3) • 10 )12 esu at k = 1064 nm; n 2 BSO = (4.4 ± 0.9) • 10 )12 esu, n 2 BGO = (7.4 ± 1.5) • 10 )12 esu at k = 532 nm). The nonlinear absorption was due to three-photon absorption at the wavelength of 1064 nm (b (3x ) and two-photon absorption at the wavelength of 532 nm (b (2x) BSO = (2 ± 0.4) • 10 )9 cm W )1 , b (2x) BGO = (3.7 ± 0.7) • 10 )9 cm W )1 ).

Several substituted difuranonaphthalenes have been identified as being viable sensitizers for the production of singlet molecular oxygen (a 1 ∆ g ) upon two-photon nonlinear excitation with a focused laser beam. The two-photon absorption cross sections of these molecules are comparatively large and depend significantly on the functional groups attached to the chromophore. To facilitate the further development of such sensitizers, computational tools have been employed to model the two-photon absorption cross sections of some difuranonaphthalenes as well as distyryl benzenes that likewise can be viable singlet oxygen precursors. Ab initio calculations using response theory yield cross sections that reproduce experimental data well. Specifically, for these comparatively large molecules, the calculations not only model relative substituent-dependent changes well but also yield reasonably accurate cross sections. Thus, ab initio computational methods can indeed be used as a predictive tool in the design of potentially useful, two-photon singlet oxygen sensitizers.

Singlet molecular oxygen (a 1 ∆ g ) has been produced and optically monitored in time-resolved experiments upon nonlinear two-photon excitation of photosensitizers that contain triple bonds as an integral part of the chromophore. Both experiments and ab initio computations indicate that the photophysical properties of alkyne-containing sensitizers are similar to those in the alkenecontaining analogues. Most importantly, however, in comparison to the analogue that contains double bonds, the sensitizer containing alkyne moieties is more stable against singlet-oxygenmediated photooxygenation reactions. This increased stability can be advantageous, particularly with respect to two-photon singlet oxygen imaging experiments in which data are collected over comparatively long time periods.

Deposition technique have been characterized by Raman spectroscopy and Electron Spin Resonance (ESR). This study focuses on the correlation between carbon cluster structure and sp 2 related defects. The ESR spectrum of as prepared and thermally annealed carbon clusters is a single line located near the free electron g-factor. Resonance lines were accurately fitted by a single line symmetric and narrow Lorentzian line. The resonance line parameters (line position, line amplitude, and line width) were found to be extremely sensitive to the thermal treatment of the as deposited samples. The paramagnetic centres are randomly distributed and correlated with the nanosized nature of the investigated system. The absence of the resonance line asymmetry, typical for the ESR spectra of conducting carbaceneous materials, confirms the small size of carbon nanoclusters and indicates poor electrical contacts between them. The resonance line parameters (position, intensity, and width) are not affected by the orientation of the carbon cluster film relative to the direction of the external magnetic field. The evolution of the ESR signal upon thermal treatments is sensitive to the increase of the sp 2 average domain size as revealed by Raman spectroscopy. The Raman spectrum of as obtained and thermally annealed carbon clusters, in the domain 1000 cm -1 to 2000 cm -1 shows two Lorentzian lines located at 1300 cm -1 and 1550 cm -1 representing the D and G bands. The position and amplitude of these lines were found to be affected by thermal annealing.

A π‐conjugated twelve‐porphyrin tube is synthesized in 32 % yield by a template‐directed coupling reaction that joins together six porphyrin dimers, forming twelve new CC bonds. The nanotube has two bound templates, enclosing an internal volume of approximately 4.5 nm3. Its UV/Vis/NIR absorption and fluorescence spectra resemble those of a previously reported six‐porphyrin ring, but are red‐shifted by approximately 300 cm−1, reflecting increased conjugation. Ultrafast fluorescence spectroscopy demonstrates extensive excited‐state delocalization. Transfer of electronic excitation from an initially formed state polarized in the direction of the nanotube axis (z axis) to an excited state polarized in the xy plane occurs within 200 fs, resulting in a negative fluorescence anisotropy on excitation at 742 nm.

Photoionization of a cold atomic sample offers intriguing possibilities to observe collective effects at extremely low temperatures. Irradiation of a rubidium condensate and of cold rubidium atoms within a magneto-optical trap with laser pulses ionizing through 1-photon and 2-photon absorption processes has been performed. Losses and modifications in the density profile of the remaining trapped cold cloud or the remaining condensate sample have been examined as function of the ionizing laser parameters. Ionization cross-sections were measured for atoms in a MOT, while in magnetic traps losses larger than those expected for ionization process were measured.

Multiphoton absorption polymerization has been demonstrated by a number of groups to occur considerably more efficiently near noble metal nanostructures. It is generally assumed that the mechanism underlying this increased efficiency is an increase in the effective two-photon absorption cross-section due to field enhancement from the nanostructure. However, we have recently demonstrated that for small aggregates of gold nanoparticles, multiphoton absorption drives broadband luminescence that can expose a photoresist directly through single-photon absorption. In this talk we will show that the same mechanism is responsible for metal-enhanced multiphoton absorption polymerization (MEMAP) for a range of different metal nanostructures. In fact, we have yet to find a system in which field enhancement leading to enhanced two-photon absorption of a photoinitiator is responsible for MEMAP. These observations suggest that we need to rethink the mechanisms of other nonlinear optical phenomena...

Reduced graphene oxides with varying degrees of reduction have been produced by hydrazine reduction of graphene oxide. The linear and nonlinear optical properties of both graphene oxide as well as the reduced graphene oxides have been measured by single beam Z-scan measurement in the picosecond region. The results reveal both saturable absorption and two-photon absorption, strongly dependent on the intensity of the pump pulse: saturable absorption occurs at lower pump pulse intensity (~1.5 GW/cm2 saturation intensity) whereas two-photon absorption dominates at higher intensities (≥5.7 GW/cm2). Intriguingly, we find that the two-photon absorption coefficient (from 1.5 cm/GW to 4.5cm/GW) and the saturation intensity (from 1 GW/cm2 to 2 GW/cm2) vary with chemical reduction, which is ascribed to the varying concentrations of sp2 domains and sp2 clusters in the reduced graphene oxides. Our results not only provide an insight into the evolution of the nonlinear optical coefficient in redu...

In this contribution, the entangled two-photon absorption (ETPA) process on naturally occurring flavoproteins was studied. Low temperature responsive protein (LOT6P) and b-Type dihydroorotate dehydrogenase (DHOD B) which possess flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) chromophores embedded in the protein environment were investigated. ETPA cross-section was measured and we found that increases when going from aqueous solution of the free flavin chromophore to the chromophore embedded in the protein. This enhancement is particularly evident when entangled photons are used as excitation light compared to classical light. Our results prove the potential of ETPA as a sensing technique for fluorescent proteins even for those whose classical TPA cross-section is small compared to well-known fluorescent proteins. Two-photon absorption (TPA) has been used to excite fluorescent proteins since shortly after the discovery of the green fluorescent protein. Experimental techniques like two-photon laser scanning microscopy (TPLSM) 2 and TPA-Förster resonance energy transfer (TPA-FRET) 3 , have utilized the fluorescence induced in proteins when excited by TPA. Some of the advantages of TPA excitation are photobleaching reduction, deep penetration and three-dimensional high resolution. Classical TPA activity of a molecule is quantified by the TPA cross-section, δTPA. Flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), shown in Figure , are usual cofactors of proteins having enzymatic, 4-6 photosensing 7-8 and photorepairing activity. 9 Two common flavoproteins are DNA photolyase 10 and cryptochromes. 11 Most of the research on the photophysics of flavoproteins have focused on the steady-state and time resolved (fs to ns) UV-Vis spectroscopy. Currently, the fluorescence lifetime, the excited state lifetime and the photoinduced electron transfer rate constants are well-known properties of FMN and FAD, free and protein-encaged, in a variety of oxidation states and at different pH values in solution. Nevertheless, two-photon absorption on flavins and flavoproteins has not been studied extensively. On the search of new light sources to induced TPA, entangled photons were proposed theoretically and later probe experimentally to be efficient in two-photon excitation of different organic chromophores. Entangled two-photon absorption (ETPA) has emerged as a new technique to study non-linear properties of organic molecules. In addition to a dramatic decrease of the input photon flux necessary to induce TPA, ETPA also presents the advantage of a linear dependence on the signal, contrary to the quadratic dependence on classical (random) TPA. Because of the ultralow intensity of entangled photons, this light source fills the requirements to be a powerful tool for the two-photon excitation of biological samples

The linear and nonlinear optical properties of two thiophene-based cyclic molecules have been investigated. These molecules represent nanometer sized cavities which may be useful for novel photonic devices. By virtue of long-range interactions, these chromophores serve as novel architectures for enhanced two-photon absorption (TPA) properties. Measurements of the different size ring structures showed a 550% increase in the TPA cross-section for the larger macrocycle. Electronic structure calculations have suggested an increase in coupling of the excited states in these systems as the ring size is increased. Measurements of the ultrafast transient absorption and fluorescence were carried out with these systems in order to probe the interaction between the chromophores. The results of the transient decays as well as fluorescence anisotropy decay times gives stronger proof to the suggestion of delocalized states in the cyclic macrocycles. These results provide information regarding the optical properties of these novel systems useful for potential applications in photonics.

The use of nonclassical states of light to probe organic molecules has received great attention due to the possibility of providing new and detailed information regarding molecular excitations. Experimental and theoretical results have been reported which show large enhancements of the nonlinear optical responses in organic materials due to possible virtual electronic state interactions with entangled photons. In order to predict molecular excitations with nonclassical light, more detailed investigations of the parameters involved must be carried out. In this report we investigate the details of the state-to-state parameters important in calculating the contribution of particular transitions involved in the entangled two-photon absorption process for diatomic molecules. The theoretical discussion of the entangled two-photon process is described for a set of diatomic molecules. Specifically, we provide detailed quantum chemical calculations which give accurate energies and transition moments for selection-rule allowed intermediate states important in the entangled nonlinear effect for the diatomic molecules. These results are used to estimate in a more accurate manner the nonmonotonic behavior of the entangled two-photon absorption cross-section. We also derive accurate approximations that can be used to predict the period between entanglement-induced

Photoionization of a cold atomic sample offers intriguing possibilities to observe collective effects at extremely low temperatures. Irradiation of a rubidium condensate and of cold rubidium atoms within a magneto-optical trap with laser pulses ionizing through 1-photon and 2-photon absorption processes has been performed. Losses and modifications in the density profile of the remaining trapped cold cloud or the remaining condensate sample have been examined as function of the ionizing laser parameters. Ionization cross-sections were measured for atoms in a MOT, while in magnetic traps losses larger than those expected for ionization process were measured.

A flowing microwave post-discharge source sustained at 2.45 GHz in pure nitrogen has been investigated by optical emission spectroscopy (OES) and two-photon absorption laser-induced fluorescence (TALIF) spectroscopy. Variations of the optical emission along the post-discharge (near, pink and late afterglow) have been studied and the gas temperature has been determined. TALIF spectroscopy has been used in the late afterglow to determine the absolute ground-state nitrogen atomic densities using krypton as a reference gas. Measurements show that the microwave flowing post-discharge is an efficient source of N ( 4 S) atoms in late afterglow. In our experimental conditions, the maximum N ( 4 S) density is about 2.2 × 10 15 cm -3 for a pressure of 22 Torr, at 300 K. The decay of N ( 4 S) density as a function of the time spent in the quartz tube has been modelled and a wall recombination probability γ of (2.1 ± 0.3) × 10 -4 is obtained.

We describe a novel pump-probe optical method, based on a Sagnac (anti-resonant ring) interferometer, which removes over 99% of the DC background light inherent in normal, two beam pump-probe measurements. Suppression of the DC component is critical for pumpprobe imaging applications. In addition, this method greatly increases the signal-to-noise ratio compared with normal, two beam pump-probe measurements. The technique is demonstrated by detection of Rb atoms aspirated into a flame. The placement of the flame near the center of the ring interferometer reduces noise caused by flame turbulence.

The carrier lifetime of a photonic crystal all-optical switch is optimized by controlling the surface of GaAs by Atomic Layer Deposition. We demonstrate an all optical modulation capability up to 100GHz at Telecom wavelengths, with a contrast as high as 7dB. Wavelength conversion has also been demonstrated at a repetition rate of 2.5GHz with average pump power of about 0.5mW