Upconversion Luminescence Transients

Inhomogeneous active-ion distributions in laser materials lead to strong deviations of upconversion versus direct luminescence transients from the quadratic law of energy-transfer upconversion. Measured luminescence decay curves in $LaSc_{3}(BO_{3})_{4}:Nd^{3+}$ and $GdVO_{4}:Nd^{3+}$ confirm experimentally the predicted deviations. Differences in energy migration within the metastable level of $Nd^{3+}$ are indentified

Impact of the spectroscopic properties of rare-earth ions on solid-state laser systems

The electronic energy level schemes within the 4f subshells of rare-earth ions give rise to a number of fluorescence transitions ranging from the near-UV to the mid-IR spectral region. A large variety of laser lines have been demonstrated based on these fluorescence transitions. Depending on the energy level scheme of the individual rare-earth ion, the characteristics of the host material chosen, the specifications of the pump source and resonator configuration, and the resulting population mechanisms within the energy level scheme, these lasers may operate in quite different regimes. Specifically, additional absorption of pump photons at transitions originating in highly populated excited states as well as energy-transfer processes between neighboring rare-earth ions can have a significant impact on the population dynamics of a rare-earth-ion laser system.\ud Two examples shall be discussed in my presentation. First, the Nd3+ laser at 1 µm was considered as an almost ideal four-level laser system for many years. However, with increasing pump powers available, it has turned out that energy-transfer-upconversion processes from the 4F3/2 upper laser level may lead to a reduced storage time and subsequent multiphonon relaxations can generate significant extra heat dissipation in the crystal with its undesired consequences of thermal lensing and rod fracture, when the system is operated in a regime of higher excitation density such as a Q-switched laser or as an amplifier [1]. Second, the erbium 3-µm laser suffers from the long lifetime of its lower laser level. Depending on the host geometry, pump source, and dopant concentration, this laser was operated in three regimes [2] in which the problem of the depopulation of the lower laser level was solved in different ways: a) pump excited-state absorption and cascade lasing, b) energy-transfer to a co-doped rare-earth ion, and c) energy-transfer upconversion and energy recycling to the upper laser level.\ud \ud [1] M. Pollnau, P.J. Hardman, M.A. Kern, W.A. Clarkson, D.C. Hanna, Phys. Rev. B 1998, 58, 16076.\ud [2] M. Pollnau, S.D. Jackson, IEEE J. Select. Topics Quantum Electron. 2001, 7, 30

Broadband waveguided light sources

In recent years, broadband fiber interferometers have become very popular as basic instruments used in optical low-coherence reflectometry for diagnostics of fiber and integrated optics devices or in optical coherence tomography (OCT) for imaging applications in the biomedical field. The longitudinal resolution of such instruments is inversely proportional to the optical bandwidth of the light source. Broadband luminescence from transition-metal-ion or rare-earth-ion doped materials can significantly improve the longitudinal resolution compared to superluminescent diodes, but the low brightness of its luminescence typically leads to a low dynamic range in OCT. Femtosecond lasers based on, e.g., Ti:sapphire have, therefore, been used as large-bandwidth high-brightness light sources, and subcellular imaging has been demonstrated in this way. Since current femtosecond light sources do not necessarily meet the requirements of compactness, ease of use, and low cost, a suitable light source for OCT is still not available.\ud We have demonstrated the suitability of a superluminescent Ti:sapphire crystal as a light source in the wavelength region 700-1000 nm for OCT. Single spatial mode, fiber coupled output powers of ~40 µW can be generated using a 5 W pump, and OCT with ~2 µm axial resolution has been performed [1]. Guiding of the fluorescence in planar-waveguide geometry can further increase the single-mode fluorescence output powers [2]. Ultimately, in a channel-waveguide geometry, the coupling efficiency of fluorescence emission into a single-mode fiber is expected to further increase to the mW level. The significantly improved sensitivity that will result at this fluorescence power may allow for rapid in vivo ultrahigh-resolution OCT with a simple broadband light source.\ud We have successfully created Ti:sapphire channel waveguides by rib fabrication in pulsed-laser-deposition grown Ti:sapphire planar waveguides by reactive ion etching [3] or Ar+ beam milling [4] through polyimide contact masks. We are also currently investigating ion beam implantation as a tool for producing sapphire and Ti:sapphire planar waveguides directly from bulk material [5].\ud Comparison of the output characteristics between transition-metal-ion and rare-earth-ion doped waveguide structures and its consequences for interferometry will be given at the conference.\ud \ud [1] A.M. Kowalevicz, T. Ko, I. Hartl, J.G. Fujimoto, M. Pollnau, R.P. Salathé, Opt. Express 10, 349 (2002).\ud [2] M. Pollnau, R.P. Salathé, T. Bhutta, D.P. Shepherd, R.W. Eason, Opt. Lett. 26, 283 (2001).\ud [3] A. Crunteanu, M. Pollnau, G. Jänchen, C. Hibert, P. Hoffmann, R.P. Salathé, R.W. Eason, C. Grivas, D.P. Shepherd, Appl. Phys. B 75, 15 (2002).\ud [4] C. Grivas, D.P. Shepherd, T.C. May-Smith, R.W. Eason, M. Pollnau, A. Crunteanu, M. Jelinek, IEEE J. Quantum Electron. 39, 501 (2003).\ud [5] L. Laversenne, A. Crunteanu, P. Hoffmann, M. Pollnau, P. Moretti, J. Mugnier, in Conference on Lasers and Electro-Optics Europe, Munich, Germany, 2003, paper CG3-2-WED, accepted

Rare-earth-ion-doped continuous-wave 3μm lasers

This paper reviews the progress on rare-earth-ion doped lasers in the wavelength range near 3μm, with an emphasis on fiber-based devices. $Er^{3+}$ and $Ho^{3+}$ lasers are discussed

Er-doped aluminium oxide waveguide amplifiers

Within the EU STREP project "Photonic integrated devices in activated amorphous and crystalline oxides" (PI-OXIDE, http://pi-oxide.el.utwente.nl/), 6 partners are developing integrated optical devices based on erbium-doped layers of amorphous $Al_2O_3$ and crystalline $Y_2O_3)$. In $Al_2O_3$:Er channel waveguides structured by chlorine-based reactive ion etching [1], we have recently achieved gain with a maximum of 0.7 dB/cm at 1533 nm and a tuneability of 35 nm [2]

Layer growth of high-quality BaSO4:Mn6+ using liquid phase epitaxy

Single-crystalline host materials doped with transition-metal ions are of high interest for applications as tunable lasers. Mn6+ ions exhibit broadband luminescence, however, Mn6+-doped crystals or waveguide structures could as yet not be grown in sufficient quality. The active material has to be free of inclusions or defects larger than λ/10, with λ, the wavelength of the porpagating beam. The interface between active layer and substrate must be optically flat to receive low-loss guiding properties. Finally, in the case of homo-epitaxy of BaSO4, the doped layer has to be arranged on the substrate (001) direction, because … .\ud The growth temperature of BaSO4:Mn6+ is limited by the decomposition of BaSO4 at 1590°C, its phase transition above 1010°C, and especially the chemical reduction of the manganese dopant from Mn6+ to Mn5+ above 620°C. Therefore, the growth of BaSO4:Mn6+ from a solution at lower temperatures is the most suitable method. Liquid phase growth is close to the thermodynamic equilibrium and has enabled us to grow high-quality layers.\ud First, we prepared undoped BaSO4 crystals of 10 x 5 x 1 mm3 in a, b, and c-direction, respectively, using the flux method with LiCl as solvent. Subsequently, growth of high-quality undoped BaSO4 was performed by liquid phase epitaxy (LPE), using the additive ternary CsCl-KCl-NaCl solution. We obtained crystalline layers free of inclusions, grown in the Frank-Van der Merwe mode (layer-by-layer growth). Finally, layers of BaSO4:Mn6+ were fabricated with thicknesses up to 150 μm, at growth rates of 3 μm/h and temperatures of 500–580°C. The thickness was controllable with a precision of 0.1 μm. The Mn6+ concentration in the doped layer was up to 1 mol.% with respect to S6+.\ud In collaboration with the University of Hamburg, absorption and emission spectra were measured, which confirmed that the manganese ion was incorporated in the layer solely in its sextavalent oxidation state. Room-temperature luminescence in the wavelength range 850-1600 nm was observed

Modulation-frequency encoded multi-wavelength fluorescence analysis

We introduce a principle of parallel optical processing: modulation-frequency-encoded multi-wavelength laser excitation, fluorescence detection with a single detector, and Fourier analysis decoding. As an example, we demonstrate simultaneous detection of DNA fragments from different origins

Sub-quadratic dependence of visible upconversion on infra-red direct luminescence decay owing to static energy-transfer upconversion

Because of their broadband luminescence, TM-ion-doped materials are of high interest for applications as tunable and short-pulse lasers. Systems with a d1 electron configuration possess only one excited 3d level and excited-state absorption into higher-lying 3d levels is impossible. One of these d1 systems, Ti:sapphire has become the most successful tunable and short-pulse laser system to date. Mn6+ is a promising d1 ion for a tunable laser system. In BaSO4, near-infrared emission from Mn6+ was observed. The room-temperature stimulated-emission cross section is larger than the excited-state-absorption cross section in the spectral range 920-1600 nm [1], i.e., as a laser material BaSO4:Mn6+ can offer a broad tuning range.\ud Here we report on the first epitaxial growth of Mn6+-doped BaSO4 layers. Growth techniques such as the melt growth fail, because barium sulphate has a phase transition at 1090°C and exhibits thermal decomposition at 1590°C. Therefore, we grew BaSO4 substrate crystals by the flux method. The Mn6+ ions tend to reduce to Mn5+ at T  620°C. We used a CsCl-KCl-NaCl solvent [2] for the LPE of BaSO4:Mn6+. This solvent has a low solidification temperature of 480°C and the growth process could be performed at temperatures well below 620°C. The nominal Mn6+ concentration was up to 0.8 mol%. High quality, lack of large-size inclusions, and low defect concentration were achieved. These Mn6+-doped BaSO4 layers were investigated spectroscopically by absorption, emission, and luminescence-excitation measurements at room temperature. Excitation at 800 nm leads to broadband Mn6+ emission between 850 and 1600 nm. Currently, we investigate the lasing potential of our BaSO4:Mn6+ layers.\ud [1] T.C. Brunold, H.U. Güdel, S. Kück, G. Huber, JOSA B 14, 2373 (1997).\ud [2] D. Ehrentraut, M. Pollnau, J. Cryst. Growth 234, 533 (2002)

Stimulated emission and excited-state absorption at room temperature on the 550 nm-laser transition in Er3+ doped YAlO3

A pump- and probe-beam technique is used for measuring time-resolved and cw-pumped excited-state absorption (ESA) and stimulated-emission (SE) spectra of Er3+:YAlO3 with high resolution. In combination with absorption and fluorescence spectra, detailed information on the wavelengths and cross-sections of ESA and SE at the 550 nm laser transition is provided