High-resolution thermal sensing and bioimaging at the cellular level and in animal models is interesting for both early diagnosis and controlled treatment via photothermal conversion of several diseases. Despite excellent in vitro results have been obtained with visible emitting luminescent nanothermometers, their application for in vivo studies is very limited due to the reduced penetration depth of visible light in biological tissues. This can be overcome if materials with emitting in the so-called biological windows (650-1350 nm) are used. Despite all this work, the number of studies exploring the possibilities of longer emission wavelengths in luminescence thermometry are scarce. This includes those lying in the so called short-wavelength infrared (SWIR) that extends from 1.35 to 2.3 μm. SWIR light transmits more effectively (up to three times) through specific biological tissues (oxygenated blood and melanin-containing tumors), achieving higher penetrations depths. Due to the reduced tissue absorbance and scattering within this region. Here, we analyze the possibilities for temperature sensing purposes of emissions in the SWIR region generated by Er3+, Tm3+ and Ho3+ ions in KLu(WO4)2 nanoparticles. The thermometric responses of these particles are compared with those shown by other Ln3+-doped nanoparticles of the same family of materials operating in the other biological windows, and demonstrate the potentiality of SWIR emitting nanoparticles for temperature measurements in biological tissues. The results indicate that SWIR emitting nanoparticles are good candidates for luminescent thermometry in biomedical applications.
In this work, we have demonstrated the use of different technologies to fabricate straight channel waveguides, S-bend waveguides, Y-splitter and Mach-Zehnder (MZ) structures on RbTiOPO4 crystals and its isomorphs. We used reactive ion etching (RIE), inductively coupled plasma-RIE (ICP-RIE), femtosecond pulse laser micro-fabrication and ion diffusion techniques to structure these crystals. Computer simulations have been carried out and compared with the optical characterization of the waveguides which are in agreement with each other.
We have studied different strategies of use of luminescence thermometry with upconverting nanoparticles in the biological range of temperatures, among them, the thermal sensing ability of fluoresncent lifetime of Er,Yb:NaY2F5 nanoparticles. Er,Yb:NaY2F5O nanocrystals show great potentiality as thermal sensors at the nanoscale for biomedical applications due to the incorporation of additional non-radiative relaxation mechanisms that shorten the emission lifetime generated by the oxygen present in the structure. Here we report ex-vivo temperature determination by laser induced heating in chicken breast using lifetime-based thermometry in these up-conversion nanoparticles.
We studied up-conversion emission of triply doped (Ho,Tm,Yb):KLu(WO4)2 (KLuW) nanocrystals at the range of temperature 296-673 K at different excitation wavelengths. The intensity ratio between two emission lines was used for monitoring the temperature. Pumping Yb3+ at 980 nm provides a good response at relatively high temperatures, while pumping Tm3+ at 802 nm provides an excellent sensitivity in the biological range of temperatures., which make the material also attractive for biological temperature sensors.
Continuous-wave (CW) lasing of Ho in KRE(WO4)2 (RE=Y, Gd, Lu) crystals has been compared using in-band
pumping by a diode pumped Tm:KLu(WO4)2 laser under identical conditions at room temperature. The three monoclinic
double tungstate hosts perform similarly with the maximum output power obtained for Ho:KY(WO4)2, 406 mW with
slope efficiency of 59.9%. For Ho:KLu(WO4)2 the maximum power reached 392 mW but the slope efficiency was
slightly higher, 61.6%. Ho:KGd(WO4)2, for which the ionic radius difference between dopant and substituted ions is
maximum, generated an output power of 368 mW with slope efficiency of 53.2%.
Passive Q-switching of a diode-pumped (Tm,Yb):KLu(WO4)2 laser, operating between 1910 and 1950 nm, has been
achieved using polycrystalline Cr2+:ZnS as a saturable absorber in a hemispherical L-shaped cavity. The dependence on
the doping levels of Tm and Yb and the low-signal absorption of the saturable absorber has been studied. The highest
average output power reached 272 mW at a repetition rate of 2.04 kHz corresponding to a pulse energy of 133 μJ. This
was achieved with (8.8 at. % Tm, 2.3 at. % Yb):KLu(WO4)2 active element and a saturable absorber with 85% lowsignal
transmission. The highest pulse energy, 148 μJ, was achieved with the same combination at lower (0.94 kHz)
repetition rate. The obtained pulse energy with the different samples is compared with theoretical analysis and the pulse
duration and peak power are estimated from the calculations.
Monoclinic crystals of Tm-doped KLu(WO4)2 were used to demonstrate pulsed laser operation near 2 μm. Passive Qswitching
and passive mode-locking were the techniques employed to produce such laser pulses. For passive Qswitching
we used an AlGaAs -based diode laser to pump the active elements and Cr:ZnSe and Cr:ZnS crystals as
saturable absorbers. For passive mode-locking we used a Ti:sapphire laser as pump source and single-walled carbon
nanotubes as saturable absorbers. In the former case, maximum pulse energies of 200 μJ for a pulse duration of 70 ns
were achieved at a repetition rate of 3 kHz with Cr:ZnS saturable absorber, while in the latter case, ultrashort pulse
durations of ~10 ps were measured with a maximum average power of 240 mW. In both laser regimes the oscillation
wavelength was ~1945 nm.
We demonstrate here that it is possible to fabricate 1D and 2D diffraction gratings on the (001) surface of RbTiOPO4
(RTP) and KTiOPO4 (KTP) single crystals. We analyzed the linear and nonlinear optical properties of 1D and 2D
nonlinear photonic crystals. We show enhanced second harmonics when the samples were illuminated with a pulsed
Nd:YAG laser, when compared to non-structured surface of the same materials and mainly there exists an asymmetry on
the diffraction patterns of the second harmonic generated light, showing higher intensity in diffraction orders different to
the zero order in the reflection configuration.
We demonstrate the first thin disk epitaxial Tm-laser based on the monoclinic KLu(WO4)2 with 15 at. % doping. The
doped epitaxial layer serving as an active medium is only 80 μm thick. The large absorption enables efficient pumping
with only a single double pass of the pump radiation from the diode-laser. For output coupling between 0.4% and 2.8 %,
the slope efficiency in the continuous-wave regime is in the 7-11% range and the laser threshold is 1.5...2.5 W of
absorbed pump power. The laser emission spectra are centered at ~1850, 1915 and 1940 nm for output couples of 2.8%,
1.6% and 0.4% transmission, respectively. In all cases the emission spectra are "structured", consisting of a number
(typically 5...10) of narrow emission lines spread irregularly over 15...30 nm.
High-quality crystals of KLu(WO4)2, shortly KLuW, were grown with sizes sufficient for characterization of the thermomechanical and optical properties, and substantial progress was achieved in the field of spectroscopy and laser operation with Yb3+- and Tm3+-doping. We review the properties of flux grown KLuW, the Yb3+ and Tm3+ spectroscopy, and present laser results obtained in several operational regimes both with Ti:sapphire and direct diode laser pumping using InGaAs and AlGaAs diodes near 980 and 800 nm, respectively. The slope efficiencies with respect to the absorbed pump power achieved with continuous-wave (CW) bulk and epitaxial Yb:KLuW lasers under Ti:sapphire laser pumping were ≈57 and ≈66%, respectively. Output powers as high as 3.28 W were obtained with diode pumping in a simple two-mirror cavity where the slope efficiency with respect to the incident pump power reached ≈78%. Passively Q-switched laser operation of bulk Yb:KLuW was realized with a Cr:YAG saturable absorber resulting in oscillation at ≈1031 nm with a repetition rate of 28 kHz and simultaneous Raman conversion to ≈1138 nm with maximum energies of 32.4 and 14.4 &mgr;J, respectively. The corresponding pulse durations were 1.41 and 0.71 ns. Passive mode-locking by a semiconductor saturable absorber mirror (SESAM) resulted in bandwidth-limited pulses with duration of 81 fs (1046 nm, 95 MHz) and 114 fs (1030 nm, 101 MHz) for bulk and epitaxial Yb:KLuW lasers, respectively. Slope efficiency as high as 69% with respect to the absorbed power and an output power of 4 W at 1950 nm were achieved with a diode-pumped Tm:KLuW laser. The tunability of this laser, under Ti:sapphire laser pumping, extended from 1800 to 1987 nm. An epitaxial Tm:KLuW laser provided slope efficiency as high as 64% and a tuning range from 1894 to 2039 nm when pumped by a Ti:sapphire laser.
The strongly anisotropic monoclinic double tungstates are known for their large absorption and emission cross sections and broader spectral lines of the rare earth dopants which makes them preferable for diode pumping. In the case of Tm the position of the absorption peak near 800 nm is very suitable for pumping with AlGaAs laser diodes. For the first time to our knowledge we grew Tm-doped KLu(WO4)2 crystals with high optical quality and obtained cw laser oscillation with a commercial 20 W diode bar. Only simple beam shaping optics was used for the 802 nm pump beam. The 2.9 mm thick, uncoated, 3 at. % Tm-doped KLu(WO4)2 was studied in a nearly hemispherical 50 mm long cavity with
longitudinal pumping. Room temperature was maintained by water cooling the crystal. The sample was Ng-cut and the
oscillating polarization was parallel to the Nm optical axis. With a 3% output coupler the polarized output at 1950 nm reached 4 W for 15 W of incident pump power. The slope efficiency with respect to the absorbed pump power amounted to 69% and the maximum optical efficiency reached 47%. It is the first time such high powers were generated with Tm-doped monoclinic double tungstates.
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