![Calculated emission rate images [R(x, y)] show that for different polar orientations of a molecule in the x-z plane, maximum fluorescence is seen when the dipole axis and the beam axis are aligned (θ = 0).](https://img.laserfocusworld.com/files/base/ebm/lfw/image/2016/01/th_68530.png?auto=format%2Ccompress&w=250&width=250 "Calculated emission rate images [R(x, y)] show that for different polar orientations of a molecule in the x-z plane, maximum fluorescence is seen when the dipole axis and the beam axis are aligned (θ = 0).")
In the world of molecular interaction, the positioning of the "poles" determines whether or not two molecules are likely to interact. This molecular dipole moment is the axis along which two molecules must align to emit or absorb energy. Researchers at the University of Rochester (Rochester, NY) have found a way to identify the alignment of a molecule's axis, which may help biochemists understand protein folding, communication between cells, and cell growth. The results, presented by Lukas Novotny, assistant professor of optics, and his team, represent the first time direct experimental measurements of the longitudinal field of a molecule have been performed.1
The new method images the molecular dipole moment using radially polarized light. In linear polarization, light vibrates within a plane. In radial polarization, pioneered by Thomas Brown, associate professor of optics at the university, the vibration radiates outward from the light beam in several planes and generates a strong longitudinal field at the focus.
To achieve radially polarized light, the team converted the fundamental laser mode of an argon ion laser (l = 488 nm) into two perpendicularly polarized modes, which were then superimposed. Expanding the beam and tightly focusing it onto the molecule created a very small electric field of equal strength in all three dimensions of cylindrical coordinates. An atom will absorb energy when one of the radial planes aligns with the direction of a molecule's axis, causing a slight burst of fluorescence.
Scanning of molecules with known dipole orientation showed that the fluorescence rate is a function of the lateral cylindrical coordinate of the radial beam, r = (x, y), and that longitudinal field strength is more intense than that of the transverse field. A molecule with its dipole transverse (q = 90°) to the beam axis fluoresces only in response to the transverse field, Er (see figure). The resulting image shows two lobes aligned along the dipole moment. A molecule with its dipole aligned to the longitudinal field, Ez, (q = 0°) results in an image with a single lobe at the center. If a molecule has dipole orientations with both transverse and longitudinal components, the image will have two lobes of varying intensity. Once the excitation field is experimentally confirmed, an arbitrary dipole orientation can be effectively determined based on the fluorescence.
One important result, say the researchers, is that the longitudinal field is not associated with momentum or energy transport. Because the magnetic field is zero along the optical axis, there is no energy transport at the center of the beam. The energy transport and field momentum are associated only with the transverse field. The fact that the field strength is maximized just above the dielectric surface may be important in imaging and data storage applications based on solid immersion lenses.
REFERENCES
- L. Novotny, M. Beversluis, K. Youngworth, and T. Brown, Phys. Rev. Lett. (June, 2001).
