Upconversion photonic

The main cost of this enlianced time resolution compared to fluorescence upconversion, however, is the aforementioned problem of time ordering of the photons that arrive from the pump and probe pulses. Wlien the probe pulse either precedes or trails the arrival of the pump pulse by a time interval that is significantly longer than the pulse duration, the action of the probe and pump pulses on the populations resident in the various resonant states is nnambiguous. When the pump and probe pulses temporally overlap in tlie sample, however, all possible time orderings of field-molecule interactions contribute to the response and complicate the interpretation. Double-sided Feymuan diagrams, which provide a pictorial view of the density matrix s time evolution under the action of the laser pulses, can be used to detenuine the various contributions to the sample response [125]. 

Wenger OS, Giidel HU (2003) Influence of Crystal Field Parameters on Near-Infrared to Visible Photon Upconversion in Ti2+ andNi2+ Doped Halide Lattices 106 59-70 Wheatley AEH, see Linton DJ (2003) 105 67-139 Wilhelm M, see Haubner R (2002) 102 1-46... 

Lanthanide (Ln) - or rare-earth-doped upconverting nanocrystals usually have similar optical properties as their bulk counterparts [45]. Upconversion is characterized by the successive absorption of two or more photons via intermediate... 

Soukka T, Rantanen T, Kuningas K (2008) Photon upconversion in homogeneous fluorescence-based bioanalytical assays. Ann N Y Acad Sci 1130 188-200, Fluorescence Methods and Applications Spectroscopy, Imaging, and Probes... 

In upconversion systems absorption takes place in two stages. An ion absorbs a photon of the incident radiation and goes to an excited state. It then transfers most of the energy either to another state of that ion or to the excited state of another ion. If this second excited state is metastable, it has time to absorb another photon before it spontaneously... 

Fluorescence upconversion and time correlated single photon counting... 

Sample preparation was given elsewhere [2]. Femtosecond fluorescence upconversion and picosecond time-correlated single-photon-counting set-ups were employed for the measurement of the fluorescence transients. The system response (FWHM) of the femtosecond fluorescence up-conversion and time-correlated single-photon-counting setups are 280 fs and 16 ps, respectively [3] The measured transients were fitted to multiexponential functions convoluted with the system response function. After deconvolution the time resolution was 100 fs. In the upconversion experiments, excitation was at 350 nm, the transients were measured from 420 nm upto 680 nm. Experiments were performed under magic angle conditions (to remove the fluorescence intensity effects of rotational motions of the probed molecules), as well as under polarization conditions in order to obtain the time evolution of the fluorescence anisotropy. 

For alcohol solvents, measurements were made with time-correlated single photon counting. The remaining measurements were made with the fluorescence upconversion system. The transients in alcohol solvents were fitted with a single exponential kinetic function. The kinetics in acetone is also well described by a single exponential, but in benzonitrile, dimethyl-sulfoxide, and propylene carbonate the kinetics were modeled with a biexponential decay. 

Upconversion lanthanide-containing nanophosphors, which emit higher-energy photons when excited by lower-energy photons have stirred increasing research interest in recent years. The predominant mechanisms of upconversion in nanophosphors are excited-state absorption (ESA), energy-transfer upconversion (ETU) and photon avalanche (PA) (Prasad, 2004 Auzel, 2005). In the ESA process, two photons are sequentially absorbed by the same ion,... 

If an ion possesses two excited states with approximately same energy separation, energy transfer can occm between two ions in the first excited state (Figme 4c). One ion returns uomadiatively to the ground state while the second is promoted to the second excited state, then decays with emission of a photon whose energy is about twice that of incident photons. There are other upconversion processes two-step absorption, cooperative sensitization, cooperative luminescence. Upconversion by energy transfer is the most efficient process. 

Fig.l. Schematic representation of three common nonlinear processes that convert low-energy photon pump sources into higher-energy output SHG = Second Harmonic Generation, STPA = Simultaneous Two-Photon Absorption, UC = Upconversion. Adapted from [17]... 

A wide variety of upconversion mechanisms have been proposed and observed. The majority of these involve some combination of absorption and nonradiative energy-transfer (ET) steps. Absorption may come in two basic forms. Ground-state absorption (GSA, Fig. 2 a) results in promotion of an ion from its ground state to an excited state. Excited-state absorption (ESA, Fig. 2 b) involves absorption of a photon by an excited ion, and results in promotion of that ion to a higher excited state. 

The energy- and time-dependence of an upconversion process are extremely useful for identification of the upconversion mechanism. A third variable parameter that has received relatively little attention is the excitation power. It is often stated in the literature that two-photon excitation processes such as upconversion must follow a quadratic power dependence, or more generally. 

All of the cases treated in this section have been for the simplest three-level upconversion systems, under both GSA/ETU and GSA/ESA two-photon excitation conditions. For higher order multiphoton excitation processes, a number of... 

Table 1. Double-logarithmic slopes of the steady-state excited-state population densities, N, of levels i = 1... and luminescences from these states for n-photon excitation. The investigated limits are 1. small upconversion or 2. large upconversion by A) ETU or B) ESA, and decay predominantly i) into the next lower-lying state or ii) by luminescence to the ground state. Adapted from [27]...

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