Leonardo de S. Menezes

Random fiber laser

We investigate the effects of two-dimensional confinement on the lasing properties of a classical random laser system operating in the incoherent feedback (diffusive) regime. A suspension of 250 nm rutile (TiO2) particles in a rhodamine 6G solution was inserted into the hollow core of a photonic crystal fiber generating the first random fiber laser and a novel quasi-one-dimensional random laser geometry. A comparison with similar systems in bulk format shows that the random fiber laser presents an efficiency that is at least 2 orders of magnitude higher.

Infrared‐to‐visible CW frequency upconversion in Er3+‐doped fluoroindate glasses

We report on efficient frequency upconversion in Er3+‐doped fluoroindate glasses. The process is observed under 1.48 μm laser diode excitation and results in the generation of strong blue (∼407 nm), green (∼550 nm), and red (∼670 nm) radiation. The main mechanism that allows for upconversion appears to be the energy transfer among Er3+ ions in excited states. The results illustrate the large potential of this new class of glasses for photonic applications.

Surface-plasmon-enhanced frequency upconversion in Pr3+ doped tellurium-oxide glasses containing silver nanoparticles

A frequency upconversion process in Pr3+ doped TeO2–ZnO glasses containing silver nanoparticles is studied under excitation with a nanosecond laser operating at 590 nm, in resonance with the H34→D12 transition. The excited Pr3+ ions exchange energy in the presence of the nanoparticles, originating efficient conversion from orange to blue. The enhancement in the intensity of the luminescence at ∼482 nm, corresponding to the P30→H34 transition, is due to the influence of the large local field on the Pr3+ ions, which are located near the metallic nanoparticles.

Upconversion luminescence in Er3+ doped and Er3+/Yb3+ codoped zirconia and hafnia nanocrystals excited at 980 nm

Frequency upconversion (UC) luminescence in nanocrystalline zirconia (ZrO2) and hafnia (HfO2) doped with Er3+ and Yb3+ was studied under continuous-wave excitation at 980 nm. Samples of ZrO2:Er3+, ZrO2:Er3+/Yb3+, and HfO2:Er3+/Yb3+ were prepared by the sol-gel technique and characterized using x-ray diffraction and electron microscopy. A study of the infrared-to-green and infrared-to-red UC processes was performed including the analysis of the spectral and the temporal behavior. The mechanisms contributing to the UC luminescence were identified as excited state absorption and energy transfer among rare-earth ions.Frequency upconversion (UC) luminescence in nanocrystalline zirconia (ZrO2) and hafnia (HfO2) doped with Er3+ and Yb3+ was studied under continuous-wave excitation at 980 nm. Samples of ZrO2:Er3+, ZrO2:Er3+/Yb3+, and HfO2:Er3+/Yb3+ were prepared by the sol-gel technique and characterized using x-ray diffraction and electron microscopy. A study of the infrared-to-green and infrared-to-red UC processes was performed including the analysis of the spectral and the temporal behavior. The mechanisms contributing to the UC luminescence were identified as excited state absorption and energy transfer among rare-earth ions.

Infrared-to-green and blue upconversion in Tm3+-doped TeO2-PbO glass

The infrared-to-visible upconversion (UC) in Tm3+-doped TeO2–PbO glass was investigated by exciting the samples with a Q-switched neodymium doped yttrium aluminum garnet laser operating at 1064nm. UC emission bands at ∼454, ∼482, and ∼518nm corresponding, respectively, to the transitions D21→F43, G41→H63, and D21→H53 were observed. Based on the dependence of the UC signals as a function of the Tm3+ concentration and excitation intensity, we concluded that the generation of the UC emissions involves excited state absorption and cross relaxation among Tm3+ ions.

In-fiber modal Mach-Zehnder interferometer based on the locally post-processed core of a photonic crystal fiber

We demonstrate a novel, compact and low-loss photonic crystal fiber modal Mach-Zehnder interferometer with potential applications to sensing and WDM telecommunications. By selectively collapsing a ~1-mm-long section of a hole next to the solid core, a pair of modes of the post-processed structure are excited and interfere at its exit. A modulation depth of up to ~13 dB and an insertion loss as low as 2.8 dB were achieved. A temperature sensitivity of -53.4 pm/°C was measured, making the device suitable for temperature sensing.

Efficient and short-range light coupling to index-matched liquid-filled hole in a solid-core photonic crystal fiber

A photonic crystal fiber (PCF) with a section of one of the holes next to the solid core filled with an index-matched liquid is studied. Liquid filling alters the core geometry, which locally comprises the original silica core, the liquid channel and the silica around it. It is demonstrated that when light reaches the filled section, it periodically and efficiently couples to the liquid, via the excitation of a number of modes of the composite core, with coupling lengths ranging from tens to hundreds of microns. The resulting modal-interference-modulated spectrum shows temperature sensitivity as high as 5.35 nm/°C. The proposed waveguide geometry presents itself as an interesting way to pump and/or to probe liquid media within the fiber, combining advantages usually found separately in liquid-filled hollow-core PCFs (high light-liquid overlap) and in solid-core PCFs (low insertion losses). Therefore, pumping and luminescence guiding with a PCF filled with a Rhodamine solution is also demonstrated.

Spectroscopy, energy transfer, and frequency upconversion in Tm3+-doped TeO2-PbO glass

Absorption and luminescence experiments were performed in thulium (Tm3+)-doped TeO2-PbO glass samples. Spectroscopic quantities such as transition probabilities, branching ratios, and radiative lifetimes related to the Tm3+ ions were determined using the Judd-Ofelt theory. Luminescence properties were studied by exciting the samples using a pulsed laser operating at 590 nm. Stokes luminescence centered at 792 nm and frequency upconversion (UC) to the blue (≈457 nm) were observed. The intensity and the time evolution of both emissions were studied for different concentrations of Tm3+ ions. The results indicate that the dipole-dipole interaction provides the most important contribution for energy transfer among the Tm3+ ions. The origin of the UC process is attributed to energy transfer among Tm3+ ions in pairs.

Stokes luminescence and frequency upconversion in Pr3+ doped TeO2-PbO glass

Optical properties of Pr3+ doped TeO2–PbO glass were investigated by resonant excitation of H43→D21 transition at ∼586nm. The decay time of D21 level, the energy transfer (ET) rates from the D21 level to other Pr3+ states, and the frequency upconversion process which leads to emission at 484nm (P03→H43 transition) were studied. The ET rates between Pr3+ ions were determined and their interaction was identified as due to dipole-dipole potential. A model used to describe the emission P03→H43 provided information on the dynamics of Pr3+ pair states. The upconversion ET rate was also determined.

Optically Detected Thermal Effects in Rare-Earth Doped Materials for Host Characterization, Thermometric Devices, Nanothermometry and Biothermometry

We present a review of recent works on optically investigated thermal effects in crystalline and amorphous materials doped with trivalent rare-earth (RE) ions. The paper describes how the frequency upconversion (UC) photoluminescence (PL) technique is used to investigate the thermal behavior of samples and how to perform optical measurements of temperature. The UC technique is based on the sequential multiphoton absorption phenomenon that leads to anti-Stokes type emission. By measuring the relative intensity between UC emissions from thermally coupled RE energy levels, the absolute temperature of a sample can be determined. Research in this area is motivated by the possible uses of UC for basic characterization of materials and for noncontact thermometry using nanoscale devices as well as for biological and medical studies. Examples based on the application of bulk materials and nanopowders doped with several RE ions are presented.