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.
Recent work from our Photonics Research Laboratory has shown that
a new class of hydroxy amino styryl
pyridinium derivatives exhibit both one- and two-photon pumped lasing.
Because of the exciting prospect of
up-conversion lasing by direct two-photon excitation, we have conducted
a thorough study of the spectroscopic
and lasing properties of a group of related compounds with the
objective of understanding the structure−spectroscopic properties relationship. These dyes were found to
have two mesomeric forms, one predominant
in the ground-state and the other in the excited-state, leading to a
large Stokes shift. A low fluorescence
quantum yield was observed for all the studied dyes and could be
possibly attributed to two factors: (1) the
presence of a counterion, iodide, which increases the
singlet-to-triplet intersystem crossing transition, and
(2)
a twisted intramolecular charge-transfer (TICT) geometry derived from
the rotation of the amino moiety.
However, significant lasing efficiencies were observed under pulse
pump conditions possibly because the
dyes are being stimulated to emit at a faster rate than the
nonradiative processes. From the one-photon
pumped laser loss calculations, we found that the losses are only due
to the cavity effect. Solvent effect
studies for the hydroxy amino styryl pyridinium derivatives showed that
the chromophore is very sensitive
to hydrogen bonding donor (HBD) solvents. In addition, the
dye-doped sol-gel:PMMA composite glass solid
matrix exhibits a behavior close to the dye dissolved in water,
suggesting a microenvironment of pure silica.
These results indicate that the dye is attached to the silica
skeleton of the composite glass through hydrogen
bonding.
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.
A newly synthesized hemicyanine dye,
trans-4-[p-[N-ethyl-N-(hydroxyethyl)amino]styryl]-N-methylpyridinium
tetraphenylborate (ASPT), has recently been shown to be an excellent
dye for both one- and two-photon-pumped lasing and for applications in optical power limiting. We
report on the absorption, one- and two-photon-induced emission, quantum yield, and the excited-state lifetime
of ASPT in different solvents of varying
polarity. We also present the spectral characteristics of ASPT
within a reverse-micelle and β-cyclodextrin
cavity. Our results demonstrate that ASPT is one of the few dyes
which exhibits highly solvent-polarity-dependent fluorescence in the useful orange-red region of the spectrum
(560−620 nm). Therefore, it offers
many applications in the biomedical field as a fluorescent probe
molecule. In addition, ASPT is fluorescent
upon excitation with an infrared (IR) source (two-photon-induced
emission) making it even more attractive,
since the background fluorescence from biological fluids upon IR
excitation is highly reduced compared to
ultraviolet or visible excitation. The solvatochromic effect of
ASPT is proposed to be due to a change in the
dipole moment and reduced hydrogen bonding effects of the dye upon
excitation. On the basis of our results,
we propose that ASPT exists in two mesomeric forms. One is
predominant in the ground-state and the other
in the excited-state. We observe a low quantum yield of the dye
(<10%) in all the investigated solvents.
The excited-state lifetime of ASPT is also observed to be
solvent-dependent (30−400 ps). The calculated
radiative decay rate is observed to remain constant but the
nonradiative decay rate increases in solvents of
increasing polarity. Finally, we report on the spectral feature of
ASPT within a reverse-micelle cavity with
varying water loadings and upon binding to a β-CD
cavity.
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%.
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