This thesis discusses the possibility to modify the spontaneous emission rate in molecules. The modification is achieved by preparing the molecule initially in a superposition of vibrational eigenstates of the first electronic state. Thus the spontaneous emission rate depends on the coefficients of the superposition. A set of coefficients that maximizes the spontaneous emission rate will be found. These coefficients as well as the corresponding spontaneous emission rate, however, will turn out to depend crucially on the Franck Condon factors of the molecule of concern. These Franck Condon factors can be quite different for different molecules. For this reason a part of the thesis is devoted to numeric calculations on the basis of data of specific real molecules. Finally, the results will be discussed in the greater context.
This thesis discusses the possibility to modify the spontaneous emission rate in molecules. The modification is achieved by preparing the molecule initially in a superposition of vibrational eigenstates of the first electronic state. Thus the spontaneous emission rate depends on the coefficients of the superposition. A set of coefficients that maximizes the spontaneous emission rate will be found. These coefficients as well as the corresponding spontaneous emission rate, however, will turn out to depend crucially on the Franck Condon factors of the molecule of concern. These Franck Condon factors can be quite different for different molecules. For this reason a part of the thesis is devoted to numeric calculations on the basis of data of specific real molecules. Finally, the results will be discussed in the greater context.
It was first realized by Purcell (1) that the spontaneous emission rate of a quantum system can be enhanced or suppressed by placing it in a resonant radiofrequency cavity. Spontaneous emission as it was first theoretically described by Einstein (2) is not a pure property of matter. It is described by matter coupled to quantized radiation field modes, which can be manipulated in an artificial structure such as a cavity. Cavity quantum electrodynamics (QED) is used to describe such effects that intimately depend on the fact that light is made of photons. Cavity QED has been extensively studied in atoms and the experimental proof was awarded with the Nobel Prize in 2012 (3). It has been applied to cooling of single atoms and for the detection of single atoms and creating and studying states with few photons and few atoms (4). In more recent developments these ideas have been extended to manipulate electronic states (5) and vibrations in molecules (6, 7). In PNAS, Flick et al. (8) introduce recent theoretical developments for the application of cavity QED to molecules and suggest possible novel applications to photochemistry. These include nonadiabatic dynamics of molecules in cavities, the modification of molecular properties, and the integration of QED with density functional theory.
A quantized electromagnetic field mode can be described as a harmonic oscillator whose coordinate is the electric field displacement (Fig. 1 A ). The zero point energy translates into a nonvanishing field intensity ⟨ e 2 ⟩ in the ground state | 0 ⟩ (vacuum fluctuations). The electric vacuum field increases for a small cavity volume e c = ℏ ω c V ϵ 0 , [1]
Fig. 1.
( A ) Illustration of the standing wave cavity mode and the resulting field quantization (Fock states). ( B ) Combined matter-field states (dressed states, | ± , n ⟩ …
[↵][1]1To whom correspondence should be addressed. Email: smukamel{at}uci.edu.
[1]: #xref-corresp-1-1
The control of the spontaneous emission of quantum emitters is of fundamental importance for the development of future quantum technologies, like quantum cryptography or quantum computing. Such applications rely on the manipulation of atoms, molecules, or "artificial" atoms, as elementary sources of light, and on the exploitation of the quantum nature of the emitted light, single photons. With the recent developments in nanofabrication techniques and nanotechnologies, the modification of the dynamics of the spontaneous emission by the environment is being investigated at the level of a few emitters, allowing for unprecedented control and manipulation of the spontaneous emission. In parallel to the experimental efforts, theoretical understanding of the fundamental interaction mechanisms between quantum emitters and their environment also becomes more and more essential.In this thesis, we tackle three different paradigms of the spontaneous emission phenomenon, all dealing with modifications of the spontaneous emission induced the environment. Firstly, we tackle the problem of monitored spontaneous emission, that is how the processus of emission is modified when the emitting system is being frequently monitored by an external observer, and which is closely related to the problem of measurementin quantum mechanics. Secondly, we consider the interaction between quantum emitters and optical resonances supported by nearby nanostructures. Finally, we study the remote interaction between quantum emitters and surfaces engraved with nanostructures which are designed and arranged in specific patterns, so-called metasurfaces.We present and deal with different formalisms to model such different situations, interfacing different fields of physics like quantum optics and nanophotonics. In each of these situations, we illustrate with realistic theoretical predictions how the spontaneous emission is modified: in the first case, how the lifetime of the quantum emitter is altered,in the second case how the frequency of the emitted photon is altered, and in the last situation how the environment may induce quantum coherence in the emitter. For each case, for provide with experimental proposals for future confirmations of these predictions, to bring a better understanding and control over these fundamental processes.
This thesis describes the design and construction of a novel molecule-surface scattering apparatus. The setup combines a Stark decelerator with a state-of- the-art surface scattering chamber, and allows background-gas free scattering of packets of velocity-controlled polar molecules in selected quantum states from well-defined surfaces with unprecedented energy resolution. Results for scattering of CO (a3Π) in different vibrational quantum states from a clean Au(111) surface are presented. CO is co-expanded with Xe or Ar to yield a supersonic molecular beam which produces fast, internally cold molecules. Excitation of CO to the long-lived electronically excited a3Π1 state makes it accessible to manipulation with electric fields due to the state’s large dipole moment. The complete removal of both carrier gas and remaining ground- state CO is achieved by focusing and deflecting the metastable CO with a switched electrostatic hexapole state selector. The 131-stage Stark decelerator is used to tune the velocity of a part of the metastable CO pulse, with an initial velocity of 360 m/s, in the range from (512 ± 4) m/s – (50 ± 4) m/s, corresponding to an incident translational energy between (307 ± 5) cm-1 and (3.0 ± 0.5) cm-1, respectively.This range can be extended up to (1300 ± 25) m/s, corresponding to an energy of(1980 ± 80) cm-1, by simple modifications to the setup. Prior to collision, the molecules can be laser- prepared in different vibrational and rotational quantum states. State- selective detection of the scattering products makes it possible to determine the quantum state, the scattering angle, and the kinetic energy of the scattered molecules. Therefore, detailed information about the scattering process can be obtained. The design of the machine allows for flexible laser access near the surface, such that a variety of different preparation and detection schemes can be employed. Ultra-high vacuum conditions in the scattering chamber are achieved by baking the entire apparatus, including the decelerator and the hexapole. Because of the carrier gas removal and a total of three differential pumping stages, no pressure rise is detected in the scattering chamber due to the incident molecular beam. Scattering experiments of CO (a3Π) on Au(111) have been performed. The scattering process leads to electron emission because the metastable excitation energy exceeds the work function of the surface. The electron emission yield, i.e. the number of electrons emitted per incident metastable particle, has been measured to be 0.13 ± 0.05. A laser ionization-depletion method is used which makes it possible to directly compare the intensity of the metastable CO ionization signal to the intensity decrease of the electron emission signal. It was also shown that vibrationally excited metastable CO leads to a higher electron emission yield. By preparing different vibrational distributions of CO in the a3Π electronic state via Franck-Condon pumping, the electron emission yield was determined for groups of vibrational levels. The first three vibrationally excited levels have an increased electron emission yield which on average is a factor of 1.5 larger than that of the vibrational ground state. Averaged over the vibrational levels with quantum number v = 4, 5, 6 , this factor is approximately 2.6 . In the range between 93 m/s and 512 m/s, the electron emission was observed to be independent of the velocity of the incident molecules. When the surface temperature is raised from 250 K to 900 K, the intensity of the electron emission signal increases by 27 %. These results are not in agreement with the models currently used to describe the de-excitation of electronically excited molecules at metal surfaces. This demonstrates the need for more theoretical work to incorporate the role of vibration into the description of the de-excitation mechanism.
Abstract A method for manipulating molecules without suitable electronic properties to be viable candidates for laser cooling is described. Only a few natural molecules have a highly diagonal Franck–Condon factor (FCF) for the laser cooling scheme. Thus, we manipulate the FCFs by using an electric field identified by an optimized method based on linear regression. The details and the validity of the method are demonstrated using the CsH molecule. The different angles between the electric field and the molecular axis are considered, and the potential energy surfaces dependent on the electric field and angle are constructed. The vibrational energies and FCFs dependent on the angles are obtained, and the corresponding laser cooling schemes relative to the angles are built, which induce the accumulated FCFs of each scheme to exceed 0.999. The schemes can exert work at the angle between the molecular axis and the electric field is 1080° with five pumping lasers with center wavelengths of 473.74, 499.58, 527.72, 540.22, and 571.62 nm. In principle, the present method can be extended to any diatomic molecules without highly diagonal FCFs under natural conditions, providing a theoretical basis for studying physical and chemical problems involving cold molecules.
Starting from an ultracold sample of ground-state 23 Na 87 Rb molecules, we investigate the lowest ro-vibrational level of the b 3 Π state with high resolution laser spectroscopy. This electronic spin-forbidden X 1 Σ + ↔ b 3 Π transition features a nearly diagonal Franck–Condon factor and has been proposed useful for probing and manipulating the ultracold molecular gas. We measure the transition strength directly by probing the ac Stark shift induced by near resonance light and determine the total excited-state spontaneous emission rate by observing the loss of molecules. From the extracted branching ratio and the theoretical modeling, we find that the leakage to the continuum of the a 3 Σ + state plays the dominant role in the total transition linewidth. Based on these results, we show that it is feasible to create optical trapping potentials for maximizing the rotational coherence with laser light tuned close to this transition.