A single atom emitting single photons is a fundamental source of light. But the characteristics of this light depend strongly on the environment of the atom. For example, if an atom is placed between two mirrors, both the total rate and the spectral composition of the spontaneous emission can be modified. Such effects have been observed using various systems: molecules deposited on mirrors, dye molecules in an optical cavity, an atom beam traversing a two-mirror optical resonator, single atoms traversing a microwave cavity and a single trapped electron. A related and equally fundamental phenomenon is the optical interaction between two atoms of the same kind when their separation is comparable to their emission wavelength. In this situation, light emitted by one atom may be reabsorbed by the other, leading to cooperative processes in the emission. Here we observe these phenomena with high visibility by using one or two single atom(s), a collimating lens and a mirror, and by recording the individual photons scattered by the atom(s). Our experiments highlight the intimate connection between one-atom and two-atom effects, and allow their continuous observation using the same apparatus.
Resonance fluorescence of a single trapped ion is spectrally analyzed using a heterodyne technique. Motional sidebands due to the oscillation of the ion in the harmonic trap potential are observed in the fluorescence spectrum. From the width of the sidebands the cooling rate is obtained and found to be in agreement with the theoretical prediction.PACS: 32.80. Pj, 42.50.Lc, 42.50.Vk Since the first preparation of a single atom in a Paul trap and observation of its resonance fluorescence [1], investigation of this light has revealed a range of unique properties. Examples are its nonclassical nature [2] and the highly nonlinear response, in the form of sudden intensity jumps, of a multi-level atom to continuous laser excitation [3]. The fluorescence is, at the same time, a unique tool for determining the state of the atom. This is particularly obvious for a single particle where each photon emission marks the respective projection of the atomic wave function into the final state of the corresponding transition. It is also of great interest to study, through its resonance fluorescence, the motion of a single laser-excited particle, e.g. for investigating laser cooling schemes or in connection with proposals for quantum state manipulation or quantum information processing with trapped particles [4].
The presence of mirrors modifies both the coherent coupling of an atom to a light mode and its spontaneous emission into the mode [1]. We study such cavity QED effects experimentally with single ions and optical cavities. We focus on two examples which are equally interesting as fundamental systems and for application in quantum information processing. (i) By retroreflecting the fluorescence of a single trapped Ba+ ion with a mirror 25 cm away, we observe inhibition and enhancement of the atom's spontaneous emission. When two ions are trapped, the distant mirror creates super‐ and subradiance. (ii) With a single trapped Ca+ ion we demonstrate coherent coupling of its narrow S1/2 – D5/2 “qubit” transition to a mode of a high‐finesse optical cavity. We also achieve deterministic coupling of the cavity standing wave to the ion's vibrational state by controlling the ion's position with nanometer‐precision and selectively exciting vibrational state‐changing transitions.
Laser welding of polymers increasingly finds application in a large number of industries such as medical technology, automotive, consumer electronics, textiles or packaging. More and more, it replaces other welding technologies for polymers, e. g. hot-plate, vibration or ultrasonic welding. At the same rate, demands on the quality of the weld, the flexibility of the production system and on processing speed have increased.Traditionally, diode lasers were employed for plastic welding with flat-top beam profiles. With the advent of fiber lasers with excellent beam quality, the possibility to modify and optimize the beam profile by beam-shaping elements has opened.Diffractive optical elements (DOE) can play a crucial role in optimizing the laser intensity profile towards the optimal M-shape beam for enhanced weld seam quality. We present results on significantly improved weld seam width constancy and enlarged process windows compared to Gaussian or flat-top beam profiles. Configurations in which the laser beam diameter and shape can be adapted and optimized without changing or aligning the laser, fiber-optic cable or optical head are shown.
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