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. Twophoton 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.

The efficient two-photon absorption of certain chromophores can be used to extend photodynamic therapy to the near infrared spectral region. Whereas the photosensitizer itself may not exhibit two-photon absorption, it could be used in conjunction with one of these new chromophores. The chromophores can act as "photon harvesters" whereby they absorb two photons of near infrared light and transfer the energy to the photosensitizer, which can generate singlet oxygen in the presence of atmospheric oxygen.

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.

Background and ObjectivesFractional treatment with a dual wavelength 1,064 and 532 nm picosecond‐domain laser, delivering a 10 × 10 array of highly focused beamlets via a holographic optic, was investigated for the treatment of acne scars.StudyTwenty‐seven of 31 subjects completed the study, 19 were treated using 1,064 nm and 8 were treated at 532 nm, all having four‐monthly treatments. Blinded evaluation of digital images by three physician evaluators comparing pre‐ and 3‐month post‐treatment images measured efficacy using a 10‐point scale. Subject self‐assessment of treatment effects were also recorded. Safety was measured by recording subject discomfort scores and adverse effects.ResultsBlinded reviewers correctly identified the baseline image in 61 of the 81 image sets (75%), and baseline acne scar scores were 1.8 ± 0.7 and 1.8 ± 0.5 for the 1,064 and 532 nm cohorts, and decreased to 1.1 ± 0.5 (P < 0.001) and 1.1 ± 0.0 (P < 0.005), respectively. Post‐treatment erythema, mild edema, and petechiae were the only side effects noted.ConclusionThe 1,064 and 532 nm picosecond‐domain laser incorporating a 10 × 10 holographic beam‐splitting handpiece was found to be safe and effective for the treatment of facial acne scars. The treatments were well tolerated and the subjects experienced little to no downtime. Lasers Surg. Med. 49:796–802, 2017. © 2017 The Authors. Lasers in Surgery and Medicine Published by Wiley Periodicals, Inc.

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