Single-molecule nano-optics at low temperature

The work presented in this thesis involves experiments on single molecules at low
temperature. At low temperature, many disturbing temperature activated processes
are frozen out, resulting in extremely sharp zero-phonon lines in the fluorescence ex-
citation spectra of certain molecule-matrix systems. The presence of sharp absorp-
tion lines is accompanied by the fact that the absorption cross-section is significantly
increased, approaching a value of about 10% of the theoretical limit for an oscillating
dipole.
The first part of this thesis provides background information for understanding
the experimental results. It introduces the energy level scheme of a single mole-
cule in a solid matrix at low temperature, the methods to get down to the single-
molecule level, the requirements for a molecule-matrix system for single-molecule
spectroscopy, the absorption cross-section and saturation. After this theoretical
part, the experimental techniques used for the experiments in this thesis, confocal
microscopy and aperture scanning near-field optical microscopy, are discussed.
This theoretical part is followed by an experimental part, which describes the
combined home-built confocal and aperture scanning near-field optical microscope
working at 1.8 K in a helium bath cryostat. It starts with a general overview,
followed by detailed descriptions of the most important parts of the set up. The
last part of the experimental section describes the preparation of the two different
samples used in these studies: terrylene doped into crystals of p-terphenyl and
terrylene in a stretched film of linear low-density polyethylene.
The second part of this thesis describes the experimental results. It starts
with a study of the imaging properties of single molecules at low temperature as a
function of excitation frequency and excitation intensity. Molecules are imaged at
several spectral positions on their resonance curve. The spot sizes of single molecules
appear increased in resonance compared to the out-of-resonance values and increase
with excitation intensity. With the help of Monte Carlo simulations, addressing the
measured spot size as a function of detuning (i.e. decreasing signal-to-background
ratio) and as a function of intensity, the observed effects could be attributed to
pronounced saturation effects in single-molecule imaging. In fact, the spot size of a
single molecule turns out to increase with intensity, even below saturation.
After this first experiment, the main experiment of this thesis is described. It
starts with the characterisation of a new sample for single-molecule spectroscopy
at low temperature, terrylene in a stretched film of linear low-density polyethylene.
Terrylene molecules in a stretched film of linear low-density polyethylene are all ori-
ented along the stretching direction and show su±cient spectral stability. The degree
of orientation turns out to include all three dimensions. The molecules all have their
transition dipole moments aligned in the sample plane, which leads to a maximised
absorption cross-section. The maximised absorption cross-section of the terrylene
molecules makes this sample a good candidate for single-molecule detection by ab-
sorption. Single molecule detection by absorption is the main experiment described
in this thesis. Despite the conceptual ease of performing a bulk absorption exper-
iment, single-molecule detection by absorption is often considered hardly possible:
the excitation and emission wavelength are exactly the same and the detector is di-
rectly exposed to high intensity laser light. Single-molecule detection by absorption
exploits the fact that the coherent part of light emitted by the molecule can interfere
in the far field with the excitation light. Due to a phase difference between the light
scattered resonantly by the molecule and the reflected excitation light, a dispersive
signal is observed on scanning the excitation frequency over the resonance of a single
molecule. Single molecules are thus detected in absorption as dispersive features.
Absorption and fluorescence excitation spectra are recorded simultaneously. From
fluorescence excitation spectra, the exact spectral position and line width of single
molecules were determined, which facilitated the analysis of the absorption spectra.
Dynamical features, like blinking and spectral jumping, are observed. From the
amplitudes of the absorption signal and the line width of the signals, a negative
correlation between line width and amplitude was found. This confirms the physical
principle that the amount of coherently scattered light decreases upon saturation
or due to dephasing. Unfortunately, the signal-to-background ratio of absorption
spectra is still rather low compared to fluorescence excitation spectra. A method to
improve the signal-to-background ratio could be to go to near-field excitation.