TY - BOOK

T1 - Scattering & absorption of light by a single molecule under a subwavelength aperture

AU - Gerhardt, Ilja

PY - 2006

Y1 - 2006

N2 - In this dissertation an experiment is introduced, in which a single dye molecule is excited in the optical near field of a subwavelength aperture under cryogenic conditions. The response of the molecule is observed as a fingerprint on the excitation beam and by its red-shifted fluorescence. The experiment reveals an efficient coupling between the excitation light and the molecule. Single molecule spectroscopy was developed at the end of the 1980s. The first experiments were performed in an absorptive configuration. The detection of molecules was achieved by focusing a laser beam down to a few micrometers and cooling down the sample to liquid helium temperatures. In order to record the weak fingerprint of the molecule on the laser beam, a double lock-in detection was used. Today single molecule spectroscopy is usually performed as fluorescence excitation spectroscopy. The molecules are detected by their red-shifted fluorescence and the excitation light is blocked by an optical long-pass filter. This allows a high signal-to-noise ratio in the detection and spectroscopy of single molecules, although at the cost of sacrificing the information on the narrow-band emission of the zero-phonon line. In many applications it is desirable to detect a quantum emitter directly in transmission. Textbook formulae suggest that in order to observe the effect of a single molecule on a laser beam, one has to focus it tightly on a region comparable to the absorption cross-section. It turns out, that in practice, this is more subtle. In this thesis the main issues are discussed and experimental results in the method of targeting the absorption cross-section of a single molecule are shown. In our configuration we use a pulled and metal coated glass fiber with a subwavelength aperture at its end to excite single DBATT (dibenzanthanthrene) molecules in the near field. By cooling the sample to below 2K, we achieve narrow zero-phonon transitions and therefore large absorption cross-sections. Our experiments show the first extinction measurements of light in the near field of a single molecule with no further noise suppressing elements. The results show an effect that is more than 3 orders of magnitude larger than in other absorption type experiments on a single molecule, demonstrating the efficient coupling of light with a single emitter in the near field. This technique might also be applied to quantum dots or systems with very small Stokes shifts.

AB - In this dissertation an experiment is introduced, in which a single dye molecule is excited in the optical near field of a subwavelength aperture under cryogenic conditions. The response of the molecule is observed as a fingerprint on the excitation beam and by its red-shifted fluorescence. The experiment reveals an efficient coupling between the excitation light and the molecule. Single molecule spectroscopy was developed at the end of the 1980s. The first experiments were performed in an absorptive configuration. The detection of molecules was achieved by focusing a laser beam down to a few micrometers and cooling down the sample to liquid helium temperatures. In order to record the weak fingerprint of the molecule on the laser beam, a double lock-in detection was used. Today single molecule spectroscopy is usually performed as fluorescence excitation spectroscopy. The molecules are detected by their red-shifted fluorescence and the excitation light is blocked by an optical long-pass filter. This allows a high signal-to-noise ratio in the detection and spectroscopy of single molecules, although at the cost of sacrificing the information on the narrow-band emission of the zero-phonon line. In many applications it is desirable to detect a quantum emitter directly in transmission. Textbook formulae suggest that in order to observe the effect of a single molecule on a laser beam, one has to focus it tightly on a region comparable to the absorption cross-section. It turns out, that in practice, this is more subtle. In this thesis the main issues are discussed and experimental results in the method of targeting the absorption cross-section of a single molecule are shown. In our configuration we use a pulled and metal coated glass fiber with a subwavelength aperture at its end to excite single DBATT (dibenzanthanthrene) molecules in the near field. By cooling the sample to below 2K, we achieve narrow zero-phonon transitions and therefore large absorption cross-sections. Our experiments show the first extinction measurements of light in the near field of a single molecule with no further noise suppressing elements. The results show an effect that is more than 3 orders of magnitude larger than in other absorption type experiments on a single molecule, demonstrating the efficient coupling of light with a single emitter in the near field. This technique might also be applied to quantum dots or systems with very small Stokes shifts.

KW - Single Molecule Spectroscopy

KW - Near-Field Optics

KW - Applied Optics

KW - Light Scattering

KW - Optics

KW - Low-temperature Physics

KW - Production Of Low Temperatures

KW - Thermophysics

M3 - Doctoral thesis

T3 - Dissertation

ER -