Jayant D. Bhawalkar

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.

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.

TWO-PHOTON-PUMPED CAVITY LASING IN A DYE-SOLUTION-FILLED HOLLOW-FIBER SYSTEM

Two-photon-pumped cavity lasing has been achieved in a dye-solution-filled hollow-fiber configuration that provides both a longer gain length and a higher local pump intensity. The gain medium is a solution of the new dye 4-[N-(2-hydroxyethyl)-N-(methyl)amino phenyl]-4′-(6-hydroxyhexyl sulfonyl)stilbene in dimethyl sulfoxide, which shows a strong two-photon-absorption-induced fluorescence when excited with near-infrared radiation. When the system is pumped with 800-nm 5-ns laser pulses, frequency-upconverted cavity lasing can be observed with a peak lasing wavelength near 565 nm and a 2–3.5-ns pulse duration. One of the major advantages of using a hollow-fiber geometry is the low two-photon pump threshold, which was measured to be as low as 20–30 μJ for a 15-cm-long hollow-fiber sample of 100-μm internal diameter. The conversion efficiency from the absorbed pump energy to the upconverted lasing output was 2.3%.

Upconversion dye‐doped polymer fiber laser

Two‐photon pumped frequency upconversion cavity lasing at ∼610 nm is accomplished in a dye‐doped polymer fiber system, pumped with ∼12 ns and 1.06 μm IR laser pulses. The dopant is a novel dye, trans‐4‐[p‐(N‐hydroxyethyl‐N‐methylamino)styryl]‐N‐methylpyridinium iodide, abbreviated as ASPI, which possesses a greater two‐photon absorption cross section and stronger upconversion fluorescence emission compared to common commercial dyes (such as rhodamine 6G). Using a Q‐switched Nd:YAG pulse laser as the pump source, cavity lasing could be achieved in a 3‐cm‐long ASPI‐doped poly(2‐hydroxyethyl methacrylate) solid fiber of 100 μm diameter. The experimental results of spectral, temporal, spatial, and input–output characteristics of the cavity lasing are presented. The slope efficiency of upconversion lasing was 0.9%.

Properties of two-photon pumped cavity lasing in novel dye doped solid matrices

Two-photon pumped frequency upconversion cavity lasing at /spl sim/600 nm is accomplished in three types of dye-doped solid rods pumped with /spl sim/10 ns and 1.06-/spl mu/m IR laser pulses. The dopant is a new dye, trans-4-[p-(N-ethyl-N-(hydroxyethyl)amino)styryl]-N-methylpyridinium tetraphenylborate, abbreviated as ASPT, which possesses a greater two-photon absorption cross section and stronger upconversion fluorescence emission than common commercial dyes (such as rhodamine). Three different materials were chosen as solid matrices: poly(2-hydroxyethyl methacrylate), VYCOR porous glass, and sol-gel glass. Using a Q-switched Nd:YAG pulse laser as the pump source, strong cavity lasing could be achieved in these three ASPT doped solid rods as well as in ASPT solution in a liquid cell. The spectral, temporal, and spatial characteristics of the cavity lasing output have been systematically investigated. The measured output-input characteristics, lasing lifetime, and damage threshold for the three different rods are presented.

Solid-state tunable cavity lasing in a poly(para-phenylene vinylene) derivative alternating block co-polymer

There is a great deal of interest in developing π-conjugated polymeric semiconductor based diode lasers which can yield lasing under electrical excitation. Towards this goal, tunable cavity lasing was achieved in a poly(para-phenylene vinylene) derivative alternating block copolymer, BMPPV, dispersed in an active solid matrix of poly(9-vinylcarbazole), with which this block co-polymer also exhibits strong electroluminescence. The laser operated in the blue region of the spectrum over the range 480 to 510 nm, with an optical efficiency approaching 10%. Linewidth narrowing and a narrowing of the temporal profile are reported. Operating lifetime of the solid-state laser was in excess of 5000 pulses at the same pumping position on the sample. Lasing properties of the same block copolymer in the solution phase were also investigated and the results compared with the solid state laser performance.

Optical limiting, pulse reshaping, and stabilization with a nonlinear absorptive fiber system.

Optical limiting, pulse reshaping, and stabilization effects have been demonstrated based on a two-photon absorption mechanism with a dye-solution-filled hollow fiber system. The nonlinear absorptive medium is the solution of a new dye, trans-4-[p-(N-hydroxyethyl-N-methylamino)styryl]-N-methylpyridinium iodide (ASPI) in dimethyl sulfoxide, with which we filled a 20-cm-long quartz hollow fiber of 100-mum internal diameter. The input optical signal was a laser pulse train that contained ~30 pulses of 130-ps pulse width. When the input peak intensity reached 400-1000-MW/cm(2) levels, obvious optical limiting could be observed and the envelope of the transmitted pulse train became flatter and broader. By using another new dye solution, 4-[N-(2-hydroxyethyl)-N-(methyl)amino phenyl]-4?-(6-hydroxyhexyl sulfonyl)-stilbene (APSS) in benzyl alcohol, which interacted with a series of ~800-nm laser pulses of ~8-ns pulse width, we obtained a much higher nonlinear absorption coefficient and a superior optical peak-power stabilization effect.

Three-photon induced upconverted fluorescence from an organic compound: application to optical power limiting

Visible blue emission has been observed in a solution of an organic molecule, 6-propionyl-2-dimethylaminonaphthalene (PRODAN) when excited by laser radiation at 1064 nm. This upconverted emission has been shown to be due to three-photon absorption followed by fluorescence with a peak at 450 nm. The three-photon absorption cross section is estimated to be 5.2 × 10−77 cm6 s2. Optical power limiting based on three-photon absorption has been demonstrated.