Continuous-wave excitation

Much of the previous section dealt with two-level systems. Real molecules, however, are not two-level systems for many purposes there are only two electronic states that participate, but each of these electronic states has many states corresponding to different quantum levels for vibration and rotation. A coherent femtosecond pulse has a bandwidth which may span many vibrational levels when the pulse impinges on the molecule it excites a coherent superposition of all tliese vibrational states—a vibrational wavepacket. In this section we deal with excitation by one or two femtosecond optical pulses, as well as continuous wave excitation in section A 1.6.4 we will use the concepts developed here to understand nonlinear molecular electronic spectroscopy. 

Let N(j,Ni,N2, and Nj, be the equilibrium population densities of the states 0, 1,2, and 3, respectively (reached under continuous wave excitation intensity Iq), and let N = NQ + Ni+N2 + N3he the total density of optical absorbing centers. The up-converted luminescence intensity ho (corresponding to the transition 2 0) depends on both N2 and on the radiative emission probability of level 2, A2. This magnitude, which is dehned below, is proportional to the cross section a20 (called the emission cross section and equal to the absorption cross section ao2, as shown in Chapter 5). Thus we can write... 

Two-Photon Absorption Rates Under Continuous-Wave Excitation. 189... 

Figure 8.10 Fluorescence spectra of nanoaggregates from the symmetrically functionalized oligomers CLP4, MOP4, CNP4, and NMeP4 after continuous-wave excitation at 325 nm.

The interval 235i — l3,S i has been measured by the method of two-photon, Doppler-free excitation in two experiments [13] [14], We will detail the latter experiment, which employs continuous-wave excitation. 

We report on polarization sensitive microphotoluminescence studies of wurtzite type GaN/AlN quantum dots grown on (0001) sapphire substrates by molecular beam epitaxy. Typical linearly polarization degree of photoluminescence is about 0.06. Photoluminescence intensity of some particular samples decreases over time under continuous-wave excitation on a timescale of minutes. Emission of such samples is linearly polarized with polarization degree of about 0.15. Possible reasons of the observed behavior are discussed. 

The PL spectra of our QDs samples (Fig. 1) contain a bright ultraviolet band, which position varies in the range of 3.2-3.S eV with variation of the QDs size. A typical value of full width at half maximum (FWHM) of these PL bands is about 400 meV. We found that photoluminescence intensity of the sample 417 with the smallest amount of deposited GaN decreases under continuous-wave excitation in more than two orders of magnitude during of about 30 minutes and then remains constant. Photoluminescence intensity of the other samples is stable. 

Havrilla G. J., Weeks S. J. and Travis J. C. (1982) Continuous wave excitation in laser enhanced ionization spectrometry, Anal Chem 54 2566-2570. 

Continuous wave Excitation is continuous resonator Q is held constant at some moderate value... 

P.C. Rausch, F. Rolfs, M.R. Winkler, A. Kottysch, A. Schauer, W. Steiner (1993). Pulsed versus continuous wave excitation mechanisms in photodynamic therapy of differently graded squamous cell carcinomas in tumor-implanted nude mice. Eur. Arch. OtorhinolaryngoL, 250, 82-87. 

S.W. Hell, M. Booth, S. Wilms, Two-photon near- and far-field fluorescence microscopy with continuous-wave excitation, Opt. Lett. 23, 1238-1240 (1998)... 

Under continuous-wave excitation of a fluorescent molecular system, typically in solution or in solid samples, it is possible to record absorption, fluorescence (both emission and excitation spectra) and phosphorescence spectra. 

The fluorescence properties of a given molecular system, namely the intensity (or eflEciency) and the spectral shape, recorded under continuous-wave excitation, are sensitive to the medium in which they are embedded (namely solution versus solid state) and the neighboring molecules (chromophores and/or fluorophores). It is this sensitivity that allows information to he obtained on the sample s morpholep. 

Resonance spectroscopy is a low power, CW application used for quality inspection. It is used to test the quality of mass-produced components because it is fast and the cost per test is low. A test sample is injected with CW ultrasound or sound at one transducer and the response at a second transducer is measured. A spectmm is built-up by stepping over many frequencies. Continuous wave excitation fills the sample under test with ultrasoimd, allowing waves from relatively distant parts of the sample to reach the receiver and contribute to the spectmm. A whole sample canbe tested without any scanning and interpretation is done automatically by computer, typically using a trained artificial neural network, resulting in short testing times. The time to complete a full test of, 100 test frequencies on an automotive component could be as short as 1 sec so that more than 1000 parts per hour can be tested. Resonance spectroscopy is suitable for 100% quality assurance of mass-produced parts. 

Figure 2.1 Typical absorption and emission spectra for Ln ions in dilute aqueous solution (black lines) absorption data (red lines) emission data (blue lines) emission excitation data, (a, b) Tb. (c, d) Nd. [Tb ]= l.3mM for (b) [Nd ] = 27.6mM for (d). Emission and emission excitation data were collected using continuous wave excitation from a Xe arc lamp. Wavelengths for exciting the emission (Xex) 3.nd monitoring the emission excitation (Xem) spectra are indicated. The detector in B was a photomultiplier tube. The detector in d was a single-element inCaAs device. David E. Morris, Los Alamos National Laboratory...

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