Recent successes in developing two-photon absorption (2PA) materials and applications have now created significant interest in exploring three-photon absorption (3PA) based novel optical materials and new applications. 3PA-based techniques may exhibit two major advantages: (1) much longer IR wavelengths (1.2-1.7 μm) can be used, and (2) much better beam confinement (resolution) can be achieved owing to the cubic dependence of nonlinear absorption on the local intensity of the excitation IR light. We have demonstrated efficient three-photon excitation in a number of nonlinear organic materials developed at our Institute or in cooperation with other research groups. The 3PA capability of a given material can be estimated by measuring the 3PA coefficient (or cross-section) at a given excitation wavelength or as a function of the wavelength. The 3PA-active materials can be either highly fluorescent or non-fluorescent. Potential applications of novel and highly efficient three-photon absorbing materials include (i) three-photon pumped (3PP) and frequency upconverted lasing, (ii) 3PA-based optical power limiting and stabilization, (iii) 3PA-based bio-imaging via IR to visible conversion, and (iv) 3PA-associated 3D data storage and microfabrication. Some recent experimental results of 3PP lasing as well as 3PA-based power limiting are briefly presented.

We measure third-order optical response of two organometallic compounds using the degenerate four wave mixing method. From measurements of DFWM efficiency, we deduce the values of third-order susceptibilities χ^<3>. From measurements of χ^<3>, we deduce the values of the second-order hyperpolarisabilities γ. The merit factor for each compound is given and the value obtained for the most efficient compound in terms of γ(second order hyperpolarizability) is 10⁴ larger than the value of CS₂, which is a reference material. The obtained optical nonlinearities are compared to those of other compounds previously studied. A preliminary correlation between structure and third-order optical properties is proposed.

We have performed nonlinear spectroscopic measurements to investigate the chemical structure/nonlinear optical property relations for a set of alkyl fluorene derivatives. The characterization method we have utilized is a femtosecond white-light continuum (WLC) pump-probe spectrometer that can rapidly characterize an organic sample’s nondegenerate two-photon absorption (2PA) spectrum. The nature of these experiments requires sophisticated data analysis. In particular, the relative group velocity mismatch between the pump and probe, which are at different frequencies, makes these pulses walk through each other within the thickness of the sample. For widely different frequencies, this can severely diminish the 2PA signal strength. However, given careful analysis, we have found good agreement with well-known semiconductor samples. Confidence in this method has allowed us to investigate the effects of solvism, electron-withdrawing character, conjugation length, and symmetry on the two-photon absorbing properties of these molecules. We have found an optimum solvent polarity as well as electron-withdrawing character which serves to maximize the strength of the 2PA in these materials. Different synthesis avenues have provided us with two different methods of extending the conjugation length that increases the nonlinearity as well. Finally, investigations of molecules with disparate symmetry have allowed us to identify the symmetry of the excited states. In addition, we present the first experimental study of the intermediate state resonance enhancement of nondegenerate 2PA in organic molecules. Using a simplified sum-over-states expression, we make comparisons between experiment and theory.

A detailed theoretical study of nonlinear molecular photonic processes accompanying the propagation of short intense laser pulses through a thin organic liquid cell and an organic liquid cored fiber array is presented. A model is proposed to account for the measurements of a recently developed organic liquid, and a comparison with pure two-photon and excited state absorption mechanisms is performed.

At present a special attention is concentrated on increasing of the efficiency of multi-photon absorption of organic systems because of new emerging applications based on this effect. In our experiments we use strong two-photon absorbing chromophore, 4,4’-bis(diphenylamino)stilbene (BDPAS), to design new dendrimer molecules, in such a way that the branching center allows for pi-electronic conjugation between branches. Here we present, for the first time, unambiguous spectroscopic evidence of strong cooperative enhancement of two-photon and three-photon absorption in a series of these dendritic macromolecules. Maximum two-photon cross section increases in proportion to N², where N = 2, 4, 6 is the number of constituent identical chromophore units in the parent BDPAS and lowest, G-0 dendrimer generation. Almost the same scaling law is observed for three-photon absorption. For higher generations, G-1 and G-2, comprising N = 14 and 30 chromophores, respectively, the cooperativity in multiphoton response starts to saturate. We show that three-photon absorption provides important complementary information, which we use for evaluation of the size of domains where chromophores are coherently coupled.

Transmission spectra and photoinduced transmission change are observed in periodic waveguide which consist of a quartz grating substrate and a thin protein film of bacteriorhodopsin. We propose a scheme to achieve all optical switching using the photoinduced refractive index change of bacteriorhodopsin.

Highly purified deoxyribonucleic acid (DNA) was isolated from salmon and scallop sperm by an enzymatic isolation process. Characterization of the optical and electromagnetic properties of DNA suggested suitability for optical waveguide applications. One of the characteristic features of DNA we discovered was an intercalation of aromatic compounds into stacked layers within the double helix of DNA molecules. We found that various optical dyes inserted into the double helix of DNA molecules render optical waveguide films of dye-intercalated DNA suitable for active photonic devices. Our investigation includes intercalation of fluorescent dyes, photochromic dyes, nonlinear optic chromophores, two photon dyes and rare earth compounds into DNA comparing results with poly(methyl methacrylate) (PMMA) based materials.

Porphyrins and related molecules with strong two-photon absorption (TPA) are extremely called for because of several emerging applications, including 3D optical memory, high-resolution fluorescence microscopy and photodynamic therapy. In this paper we demonstrate for the first time that an asymmetric meso-substitution of porphyrin macrocycle with electron-donating diphenylamino-stilbene or bis-(diphenylamino)-stilbene groups results in a drastic enhancement of intrinsic TPA cross section in the near-IR region. The cross section value amounts to 500 - 900 GM depending on substituent group and link structure, which is about 10² times the corresponding value for the unsubstituted parent molecule. Compared to symmetrical porphyrins, the TPA spectra of this series follow qualitatively the corresponding one-photon spectra. Therefore, we describe the observed TPA spectra and absolute cross section values by taking into account the change of permanent dipole moment upon excitation. A new zeroth-generation dendrimer, consisting of a porphyrin core, symmetrically tertakis-meso-substituted with strong TPA dendrons, reveals 7 times increase of the cross section (740 vs 110 GM) as compared to its mono-meso-substituted analogue. We also demonstrated an efficient singlet oxygen generation upon two-photon excitation of these new molecules, which makes them particularly attractive for photodynamic therapy.

Organic materials with large two-photon absorption are desired for numerous photonics applications, such as optical limiting,upconversion lasing, three-dimensional data storage,and photodynamic therapy. Stilbazolium derivatives are interesting two-photon absorbers for these applications. In this work,the nonlinear transmissivities of trans -4-[4-(dimethylamino)styryl ]-1-methylpyridinium iodide (DASPI),trans -4-(4-aminostyryl)-1-methylpyridinium iodide (ASPI),trans -4-[2-(1-methylpyrryl)vinyl ]-1-methylpyridinium iodide (MPVPI),trans -4-(2-pyrrylvinyl)-1-methylpyridinium iodide (PVPI),and trans -4-styryl-1-methylpyridinium iodide (SPI)at 800,850,900,950 and 1000 nm have been studied respectively using 21 ps laser pulses. The two-photon induced fluorescence of these compounds at 872,900,and 940 nm has also been investigated.All of these compounds exhibit two-photon absorption and two-photon induced up-converted fluorescence in the near-IR wavelengths,and the two-photon absorption cross section and two-photon induced fluorescence intensity vary with the wavelength and with the chemical structure changes. These preliminary results suggest that it is possible to increase the two-photon absorption cross-sections by proper structure modifications

Multi-Photon Laser Scanning Microscopy (MPLSM) requires efficient two-photon absorbing fluorescent (TPF) probes. In particular, probes exhibiting bio-functionality are very attractive for MPLSM studies of biological samples. We have synthesized and studied a new class of TPF probes capable of caging metal ions, such as Ca⁺² and Na⁺, which play an important role in neuronal mechanisms. The TPF probes are based on a tetraketo derivative with a symmetric Donor-Acceptor-Donor (D-A-D) structure. The donor is an azacrown moiety, which also serves as a metal ion-caging unit. We studied the linear and the non-linear spectroscopic properties of these TPF probes as a function of conjugation length and the size of the crown ring. We find that this new class of TPF probes possesses very large two-photon excitation cross-section coefficients (~1000GM) at near IR wavelengths as well as affinity to metal ions. In the presence of changing sodium ion concentration the dye spectra reveals four distinguishable forms and the TPF efficiency changes strongly. We therefore conclude that the dye can perform as a sensitive metal ion TPF probe.

Two-photon fluorescence microscopy is a prominent tool in biological imaging analysis. Many commercially available fluorescent dyes currently being used have sufficed for multiphoton based imaging of biological samples. While measured two-photon cross-sections (in Goppert Meyer, GM units) of some of the dyes are available, many exhibit relatively low two-photon cross-section values in the tunability range of Ti:sapphire lasers commonly used in multiphoton microscopy imaging. For example, Bodipy FL exhibits a maximum GM unit of 18 at 925 nm, compared to a range of 2-4 GM units from 775 - 875 nm. Furthermore, available fluorophores may be plagued with either low fluorescence quantum yield and/or the additional problem of rapid photobleaching upon exposure to the high peak power provided by the fs laser source. In order to address the demand for better performing dyes for two-photon based imaging, we have prepared a new series of reactive fluorophores tailored for multiphoton imaging. These fluorophores are based upon the fluorene ring system, known to exhibit high fluorescence quantum yields, typically > 0.7, and possess high photostability. They have been functionalized with various moieties to act, e.g., as efficient amine-reactive fluorescent probes for the covalent attachment onto amine-containing biomolecules. Single-photon spectral characteristics, as well as measured two-photon cross sections of a reactive fluorophore and its model conjugate in solution, as well as spectral characterizations of a bovine serum albumin (BSA) conjugate are presented.

Near-field multiphoton optical lithography is demonstrated by using ~120 fs laser pulses at 790 nm in an apertureless near-field optical microscope, which produce the lithographic features with ~ 70 nm resolution. The technique takes advantage of the field enhancement at the extremity of a metallic probe to induce nanoscale multiphoton absorption and polymerization in a commercial photoresist, SU-8. Even without optimization of the resist or laser pulses, the spatial resolution of this technique is as high as λ/10, nearly a factor of two smaller than the previous multiphoton lithography in the far field.

We report here the caracterization of efficient photoinitiators for radicalar polymerization by two-photon absorption (TPA). Symmetric molecules bearing tertiary amines as a donor group D and a biphenyl or a fluorene for the transmitting electron group π were proposed for the visible. For IR, the selected phoinitiator presents the general structure D-π-A-π-D, in which A is an acceptor group. The initiation efficiencies of these systems were evaluated by the determination of the threshold intensities for a given exposure duration. Molecules for the visible are more sensitive than those designed for IR. Comparing to the commercial resins for UV photopolymerization generally involved for TPA, these optimized intiators led to a significant increase of the sensitivity during fabrication. Weaker incident intensities and faster scanning speeds could be used. This approach led the fabrication of tridimensional micro-objects with a less onerous nanosecond pulses microlaser.

We investigate the four-wave mixing (FWM) effect in a dispersion-managed transmission line. The dispersion-managed line consists of many repeated fiber spans, each of which includes a positive- and a negative-dispersion fiber. The analytical expression of the FWM is obtained. The influence of the channel spacing, the fiber spans’ number and the compensating dispersion parameter on the FWM effect is analysed. The dispersion-managed transmission line is optimized to depress the generated FWM noise.

Two-photon induced photopolymerization allows fabrication of complex three-dimensional structures with sub-micron resolution in a single exposure/development cycle. We analyze the kinetics of two-photon polymerization in multi-functional acrylic photopolymers and SU-8 epoxy resins as a function of two-photon photosensitizer and electron acceptor concentrations. The rate of polymerization is observed to be directly proportional to the concentration of the two-photon photosensitizer, directly proportional to the square of the intensity, and varies more weakly with the concentration of electron acceptor. A transition from high aspect ratio to low aspect ratio structures is observed that depends on both the energy absorbed per unit area and the concentration of the photoinitiator system. In acrylates, quantitative estimates of the photoinitiation rates suggest that the concentration of radicals at threshold is much higher under two-photon exposure conditions than under one-photon exposure conditions. Diffusion of radicals or inhibitors into or out of the illuminated region may have important effects on the overall reaction rate for two-photon induced polymerization.

Nowadays, it seems evident that a unique nonlinear optical (NLO)material cannot offer simultaneously linear transparency,colour neutrality and broadband optical limiting efficiency at the performance levels required for sensor and eye protection against all laser threats.Several combinations of NLO materials were investigated last few years, including multicell or multilayer geometries. The approach presented here combines multiphoton absorption with nonlinear scattering. For that purpose, singlewall carbon nanotubes are suspended in various solutions of multiphoton absorbing chromophores. Such combinations allow us to obtain optical limiters of high linear transmittance and excellent colour neutrality. Broadband optical limiting is expected from the association of these two broadband materials,and enhanced optical limiting efficiency is expected from cumulative effects in the nanosecond regime. We report here on the optical limiting studies performed with nanosecond laser pulses on several families of multiphoton absorbers in chloroform,with carbon nanotubes suspended in the solutions. The performances of these samples are compared with those of simple multiphoton absorber solutions and carbon nanotube suspensions, and the differences observed are interpreted in terms of cumulative NLO effects and adverse aggregation phenomenon. Ways to optimise stability of the suspensions are also experimented and discussed.

The two-photon polymerizations of metal ions doped acrylate monomers and oligomers have been investigated, which has been applied for three-dimensional (3-D) micro/nano-structure fabrication. Titanium (IV) ions doped urethane acrylate photopolymerisable resins were synthesized, and their properties of optical and polymerization were investigated. The resolution of two-photon polymerization for micro/nanofabrication was evaluated. Titanium oxide (TiO₂) nanoparticles were generated in the polymer matrix of micro-sized polymer structures. A 3-D diamond photonic crystal structure, which consisted of polymer composite materials of TiO₂ nanoparticles, was successfully fabricated by direct laser writing technique, and its photonic bandgap was confirmed. This work would give us a new solution for producing 3-D micro/nanodevices of functional polymer composite materials.