Saturation optical transition

S.W. Hell, S. Jakobs, L. Kastrup, Imaging and writing at the nanoscale with focused visible light through saturable optical transitions. Appl. Phys. A 77, 859-860 (2003)... 

In this section we will study time-resolved laser spectroscopy and generally discuss radiative properties of atoms and molecules, and methods of studying these properties. Since very short laser pulses with a power density sufficient to well saturate optical transitions can be obtained, a large fraction of the irradiated ground-state atoms can be transferred to the excited state. Using step-wise excitations with synchronized lasers a large number of atoms can be excited into very highly excited states. When the laser pulse ceases the exponential decay of the excited state can be monitored. Note, that primarily the population number N(t) decays exponentially, i.e.. 

The analysis of molecular spectra is complicated because of the very large number of lines that is obtained simultaneously in normal excitation or absorption experiments. With narrow-band laser excitation an individual excited rotational-vibrational level can be populated selectively and only the decays originating in the excited state are observed. A similar simphfica-tion in absorption measurements is very desirable. Through the possibility of saturating optical transitions, a certain lower level can be labelled by depleting the population with a laser pump laser). If this laser is switched on and off repetitively, all absorption lines originating in the labelled level will be modulated when induced with a second (probe) laser [9.69, 9.70]. A number of schemes for modulation detection are indicated in Fig. 9.6. Several schemes can be used to ascertain that absorption has ocemred, as discussed... 

The appearance of the optical absorption bands (Q and B) has a clear threshold at a low non-zero coverage, implying that the electronic structure of the first adsorbed molecules is different from that of the bulk ones. Thus, a clear distinction between molecules directly bonded to the aluminium substrate d- < 0.3 run) and molecules not directly bonded to the substrate d- >0.3 nm) can be made. In the latter case the electronic structure, as revealed by EELS, is identical to that of bulk CuPc, while in the former case modification of the electronic structure prevents transitions toward the LUMO orbital. Above 1.0 nm the Q and B band intensities saturate. The optical transitions are inhibited for molecules directly bonded to the alumiiuum substrate... 

The fluorescence technique, like other methods based on scatter (elastic or inelastic), has been shown by us - and others to be a reliable unperturbing method of measuring spatial/ temporal flame temperatures and species concentrations. To avoid the dependency of the fluorescence signal on the environment of the emitting species, it has been shown by several workers that optical saturation of the fluorescence process (i.e., the condition occurring when the photoinduced rates of absorption and emission dominate over the spontaneous emission and colli sional quenching rates) is necessary. Pulsed dye lasers have sufficient spectral irradiances to saturate many transitions. Our work has so far been concerned with atomic transitions of probes (such as In, Pb, or T1) asoirated into combustion flames and plasmas. 

Table 3.7), i.e., at first glance the broad line approximation works unsatisfactorily. One must, however, keep in mind the instability of the modes due to the phase change under conditions of freely running modes (without synchronization). Broadening of the Bennet dips due to saturation of optical transition also takes place [268]. This makes the broad line approximation more realistic considering that the width of the amplification contour of the laser is 6 109 s-1. 

Several far-field light microscopy methods have recently been developed to break the diffraction limit. These methods can be largely divided into two categories (1) techniques that employ spatially patterned illumination to sharpen the point-spread function of the microscope, such as stimulated emission depletion (STED) microscopy and related methods using other reversibly saturable optically linear fluorescent transitions (RESOLFT) [1,2], and saturated structured-illumination microscopy (SSIM) [3], and (2) a technique that is based on the localization of individual fluorescent molecules, termed Stochastic Optical Reconstruction Microscopy (STORM [4], Photo-Activated Localization Microscopy (PALM) [5], or Fluorescence Photo-Activation Localization Microscopy (FPALM) [6]. In this paper, we describe the concept of STORM microscopy and recent advances in the imaging capabilities of STORM. 

The fluorescence rate is then Rf = Ptk1Q. By comparing this expression with the fluorescence rate introduced in Section II.A. as Rf = a(I/hv), and realizing that xl -kl0 = klo/(kl0 + kl3) = f, we can define an intensity dependent absorption cross section which accounts for saturation of the optical transition, as... 

Saturation of an optical transition ( hole-burning ) and subsequent analysis of the time and frequency-dependence of the recovery of the ground-state species has become a well-known technique for the study of the picosecond photophysics of radiationless transitions in stable molecules, transient species, and laser dyes in particular. " ... 

Figure 13 shows two kinetic traces of the formation and decay of e in methanol, (a) in the absence and (b) in the presence of a 20 ns ruby laser pulse. Saturating the e optical transition at 694 nm leads to a significant permanent loss of absorbers during the 0.4 J cm laser pulse. Figure 14(a) shows the spectra of e in methanol and 2-propanol before and during bleaching at 694 nm, and it is clear that there is a uniform loss (A.4) of intensity across the band. This loss, however, has a nonlinear dependence... 

The second method is based on a time-of-flight measurement. The laser beam is again split, but now both partial beams cross the molecular beam perpendicularly at different positions zi and Z2 (Fig- 4.12). When the laser is tuned to a molecular transition /) k) the lower level /> = (Vi, Ji) is partly depleted due to optical pumping. The second laser beam therefore experiences a smaller absorption and produces a smaller fluorescence signal. In the case of molecules even small intensities are sufficient to saturate a transition and to completely deplete the lower level (Sect. 2.1). 

Even at moderate laser powers the optical transition may become saturated (Sect. 2.1) and the lower level i ) can be appreciably depleted. This considerably increases the population difference AA = N ij) — N i ) and thus the absorption of the RF field on the transition ij) i ), which is proportional to AN. In the conventional Rabi technique the population N(E) follows a Boltzmann distribution, and for AE < kT the difference AN becomes very small. 

---