Photonics optical dichroism

Two-photon circular dichroism (Meath and Power 1987) was first predicted over a decade ago (Tinoco 1975 Power 1975 Andrews 1976), and is closely related to two-photon resonance effects in optical rotatory dispersion, the... 

Systems involving two centers with fixed mutual orientation have been shown to exhibit other circular differential effects with a similar linear dependence on the separation. Examples include circular differential Rayleigh and Raman scattering (Barron and Buckingham 1974 Andrews and Thirunamachandran 1978), optical rotation (Barron 1975) and two-photon circular dichroism in which only one chromophore is excited (Andrews 1976). As might be expected, on performing subsequent rotational averages for the case where A and B are randomly oriented, the odd-j terms in the rate equations vanish and consequently no circular dichroism is displayed. 

Vibrational optical activity (VOA) is a relatively new area of natural optical activity. It consists of the measurement of optical activity in the spectral regions associated with vibrational transitions in chiral molecules. There are two basic manifestations of VOA. The first is simply the extension of electronic circular dichroism (CD) into the infrared region where fundamental one-photon vibrational transitions are located. This form of VOA is referred to as vibrational circular dichroism (VCD). It was first measured as a property of individual molecules in 1974 [1], and was independently confirmed in 1975 [2]. Within the past twelve years, VCD has been reviewed on a number of occasions from a variety of perspectives [3-15], and two more reviews are currently in press [16,17], The second form of VOA has no direct analog in classical forms of optical activity. Optical activity in Raman scattering, known simply as Raman optical activity (ROA), was measured successfully for the first time in 1973 [18], and confirmed independently in 1975 [19], ROA has been described in detail and reviewed several times in the past decade from several points of view [20-24], and two additional reviews [25,26], one with a view toward biological applications [25] and the other from a theoretical perspective [26], are currently in press. In addition, two articles of a pedagogical nature are in press that have been written for a general audience, one on infrared CD [27] and the other on ROA [28],... 

The isotropic signal delivers (rotation-free) information on the temporal evolution of the population numbers of the investigated vibrational transition(s). The induced dichroism is governed by the time constant ror (second-order reorientational correlation time, 1 = 2) and possibly population redistribution that may contribute to the loss of induced optical anisotropy. The zero-setting of the delay time scale (maximum overlap between pump and probing pulses) is determined by a two-photon absorption technique in independent measurements with an accuracy of better than 0.2 ps (67). 

Polarization is a very important property of synchrotron radiation. It provides the capability for the study of magnetic and optical circular and linear dichroism, for polarization dependent EXAFS, and a variety of other experiments. On-axis, (in the case of a single electron in the orbit plane, i.e., zero emittance) the photon is 100% polarized with the E vector parallel to the plane. Above and below the plane the radiation is elliptically polarized and the degree of linear polarization is defined as ... 

For single-photon absorption in isotropic media, there is essentially no dependence on beam polarization. Such dependence as does exist arises only for optically active (chiral) compounds, and is associated with quantum interference between electric dipole and magnetic dipole (or electric quadrupole) interactions. These weak effects produce the characteristic polarization dependence that is manifest in the phenomena of circular dichroism and optical rotation. 

Besides absorption and emission measurements, other techniques (optical, magnetic or magneto-optical) can also be used for determination and assignment of crystal-field energy levels inside the 4f shell two-photon absorption (TPA), Zeeman effect spectroscopy, electron paramagnetic resonance (EPR) and magnetic circular dichroism (MCD). 

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