Guang S. He

Multiphoton Absorbing Materials: Molecular Designs, Characterizations, and Applications

4. Survey of Novel Multiphoton Active Materials 1257 4.1. Multiphoton Absorbing Systems 1257 4.2. Organic Molecules 1257 4.3. Organic Liquids and Liquid Crystals 1259 4.4. Conjugated Polymers 1259 4.4.1. Polydiacetylenes 1261 4.4.2. Polyphenylenevinylenes (PPVs) 1261 4.4.3. Polythiophenes 1263 4.4.4. Other Conjugated Polymers 1265 4.4.5. Dendrimers 1265 4.4.6. Hyperbranched Polymers 1267 4.5. Fullerenes 1267 4.6. Coordination and Organometallic Compounds 1271 4.6.1. Metal Dithiolenes 1271 4.6.2. Pyridine-Based Multidentate Ligands 1272 4.6.3. Other Transition-Metal Complexes 1273 4.6.4. Lanthanide Complexes 1275 4.6.5. Ferrocene Derivatives 1275 4.6.6. Alkynylruthenium Complexes 1279 4.6.7. Platinum Acetylides 1279 4.7. Porphyrins and Metallophophyrins 1279 4.8. Nanoparticles 1281 4.9. Biomolecules and Derivatives 1282 5. Nonlinear Optical Characterizations of Multiphoton Active Materials 1282

Two-photon absorption and optical-limiting properties of novel organic compounds

The optical-limiting behavior and two-photon absorption properties of four novel organic compound solutions in tetrahydrofuran have been investigated. An ultrashort laser source with 0.5-ps pulse width and 602-nm wavelength was employed. The transmissivities of the various 1-cm-thick solution samples have been measured as a function of the beam intensity as well as of the solute concentration. The measured results can be fitted on the assumption that two-photon absorption is the only predominant mechanism causing the observed opticallimiting behavior. Based on the intensity-dependent transmissivity measurements, the molecular two-photon absorption coefficients for the four compounds are presented.

NONLINEAR MULTIPHOTON PROCESSES IN ORGANIC AND POLYMERIC MATERIALS

For several decades there has been extensive research in the area of multiphoton spectroscopy. However, multiphoton processes have not found widespread applications due to the relatively low multiphoton absorption cross sections of most materials. A new generation of multifunctional organic materials with large multiphoton absorption cross sections has opened up a number of unique applications in photonics and biophotonics. Two-photon pumped upconversion lasing, multiphoton absorption-induced optical power limiting, multiphoton laser scanning microscopy, and two-photon three-dimensional optical data storage are some of the recent photonic applications of multiphoton processes highlighted in this article. Two-photon photodynamic therapy is another promising application to biophotonics which is also discussed here. .

Optical limiting effect in a two‐photon absorption dye doped solid matrix

We recently reported a new lasing dye, trans‐4‐[p‐(N‐ethyl‐N‐hydroxylethylamino)styryl]‐N‐methylpyridinium tetraphenylborate (ASPT), which has also been shown to possess a strong two‐photon absorption (TPA) and subsequent frequency upconversion fluorescence behavior when excited with near infrared laser radiation. Based on the TPA mechanism, a highly efficient optical limiting performance has been demonstrated in a 2 cm long ASPT‐doped epoxy rod pumped with 1.06 μm Q‐switched laser pulses at 50–250 MW/cm2 intensity levels. The measured nonlinear absorption coefficient reached 6 cm/GW for the tested sample of dopant concentration d0=4×10−3 M/L. The molecular TPA cross section of ASPT in the epoxy matrix is estimated as σ2=2.5×10−18 cm4/GW or σ2′=4.7×10−46 cm4/photon/s, respectively. Two‐photon pumped cavity lasing is also observed in an ASPT‐doped polymer rod.

Observation of stimulated emission by direct three-photon excitation

Multiphoton processes, predicted theoretically in 1931, were for a long time considered to be mainly of academic interest. This view changed when it was shown that a two-photon absorption process could, because of a quadratic dependence of excitation on intensity, produce a spatially confined excitation useful for three-dimensional data storage and imaging. Two-photon absorption has received considerable attention recently because of the development of highly efficient two-photon-sensitive materials, leading to numerous technological applications. These successes have created interest in exploring applications based on three-photon excitations. For a three-photon process, a longer excitation wavelength such as those common in optical communications can be used. Also, the cubic dependence of the three-photon process on the input light intensity provides a stronger spatial confinement, so that a higher contrast in imaging can be obtained. Here we report the observation of a highly directional and up-converted stimulated emission as an amplified spontaneous emission, produced in an organic chromophore solution by a strong simultaneous three-photon absorption at 1.3 µm. This achievement suggests opportunities for a three-photon process in frequency-upconversion lasing, short-pulse optical communications, and the emerging field of biophotonics.

Two‐photon pumped cavity lasing in novel dye doped bulk matrix rods

Trans‐4‐[p‐(N‐ethyl‐N‐hydroxyethylamino)styryl]‐N‐methylpyridi that possesses a much greater two‐photon absorption cross section and much stronger upconversion fluorescence emission than common organic dyes (such as rhodamine), when excited with near infrared laser radiation. Utilizing ASPT doped bulk polymer rods, two‐photon pumped frequency upconverted cavity lasing has been accomplished using a Q‐switched Nd:YAG laser as the pump source. The wavelength and pulse duration were ∼600 nm and 3–6 ns, respectively, for the cavity lasing; whereas the corresponding values for pump pulses were 1.06 μm and ∼10 ns, respectively. For a 7 mm long sample rod with a dopant concentration d0=8×10−3 M/L, the conversion efficiency from the absorbed pump energy to the cavity lasing output was ∼3.5% at a pump energy level of 1.3 mJ. The lasing lifetime, in terms of pulse numbers, was more than 4×104 pulses at 2 Hz repetition rate and room temperature.

Efficient, two-photon pumped green upconverted cavity lasing in a new dye

Abstract Two-photon pumped green cavity lasing has been observed in a solution of a new dye in N,N-dimethyl formamide (DMF). The dye, 4-[N-(2-hydroxyethyl)-N-(methyl) amino phenyl]-4′-(6-hydroxyhexyl sulfonyl) stilbene, abbreviated as APSS, shows a strong two-photon absorption induced fluorescence when excited at 800 nm. The molecular two-photon absorption cross section was measured to be σ 2 = 1.5 × 10 −19 cm 4 /GW or σ ′ 2 = 3.8 × 10 −47 cm 4 /photon/s, and is 1 to 2 orders of magnitude higher than Rhodamine 6G. The peak of the lasing spectrum was around 555 nm, and is to the best of our knowledge, the shortest wavelength obtained from a two-photon pumped dye laser. The conversion efficiency from the absorbed pump energy to the lasing output energy was as high as 2.8%. The absorption and the fluorescence spectra of the new dye, along with the spectral and temporal behavior of the two-photon pumped lasing output is presented.

Studies of two-photon pumped frequency-upconverted lasing properties of a new dye material

The two-photon absorption (TPA), TPA-induced frequency upconversion emission, and two-photon-pumped (TPP) lasing properties of a new dye, trans-4[p-(N-hydroxyethylN-methylamino)stryryl]-N-methylpyridinium iodide (abbreviated as ASPI) were experimentally investigated. This new dye has a moderate TPA cross section (σ2≈3.9×20−20 cm4/GW in benzyl alcohol), but exhibits a low lasing threshold and high lasing efficiency when pumped with a 1064 nm pulsed laser beam. Furthermore, the TPA-induced fluorescence yield is strongly dependent on the polarity of the solvent, making it a promising dye for sensing applications. The spectral, temporal, and spatial structures as well as the output/input characteristics of the TPP cavity lasing and the superradiant (cavityless) lasing are systematically measured using a 1 cm path quartz cuvette filled with the ASPI solution or a doped polymer rod. The net conversion efficiency from the absorbed 1064 nm pump pulse energy to the ∼615 nm upconverted cavity lasing energy was foun...

Two-photon absorption based optical limiting and stabilization in organic molecule-doped solid materials☆

Abstract Optical limiting and stabilization via two-photon absorption (TPA) in organic molecule-doped solid materials have been investigated. The nonlinear materials are epoxy rod and a composite glass rod doped with the same organic dopant (2,5-benzothiazole 3,4-didecyloxy thiophene). An ultrashort laser source with 0.5 ps pulsewidth and 602 nm wavelength was employed. The transmissivity of these two materials has been measured as a function of the input beam intensity. The measured results can be well fitted based on the assumption that TPA is the predominant mechanism producing the observed optical limiting behavior. Also, optical stabilization behavior is observed, and, at ∼930 MW/cm 2 input intensity level, the output intensity fluctuation is three times less than the input intensity fluctuation.

Multi-photon excitation properties of CdSe quantum dots solutions and optical limiting behavior in infrared range.

Multi-photon absorption and excitation properties of CdSe quantum dots in hexane with different dot-sizes have been investigated. The two- and three-photon absorption (2PA and 3PA) coefficients were measured by using ~160-fs laser pulses at wavelengths of ~775-nm and ~1300-nm, respectively. The dependence of one-, two- and three-photon induced fluorescence spectra as well as their double-exponential decay on the dot-sizes was studied. Based on the fluorescence emission spectra and temporal decay constants for a given sample solution excited by one-, two-and three-photon absorption, it can be concluded that the transition pathways for fluorescence emission and decay under one-, two- and three-photon excitation are nearly identical. The optical power limiting capabilities based on 2PA and 3PA mechanisms are demonstrated separately. In addition, a saturation behavior of 3PA at ~1300 nm was observed.