Xiyuan Liu

A single-atom detector integrated on an atom chip: fabrication, characterization and application

We describe a robust and reliable fluorescence detector for single atoms that is fully integrated on an atom chip. The detector allows spectrally and spatially selective detection of atoms, reaching a single-atom detection efficiency of 66%. It consists of a tapered lensed single-mode fiber for precise delivery of excitation light and a multi-mode fiber to collect the fluorescence. The fibers are mounted in lithographically defined holding structures on the atom chip. Neutral 87Rb atoms propagating freely in a magnetic guide are detected and the noise of their fluorescence emission is analyzed. The variance of the photon distribution allows us to determine the number of detected photons per atom and from there the atom detection efficiency. The second-order intensity correlation function of the fluorescence shows near-perfect photon anti-bunching and signs of damped Rabi oscillations. With simple improvements, one can increase the detection efficiency to 95%.

Simple integrated single-atom detector

We present a reliable and robust integrated fluorescence detector capable of detecting single atoms. The detector consists of a tapered lensed single-mode fiber for precise delivery of excitation light and a multimode fiber to collect the fluorescence. Both are mounted in lithographically defined SU-8 holding structures on an atom chip. 87Rb atoms propagating freely in a magnetic guide are detected with an efficiency of up to 66%, and a signal-to-noise ratio in excess of 100 is obtained for short integration times.

Fabrication of alignment structures for a fiber resonator by use of deep-ultraviolet lithography

We present a novel method to mount and align an optical-fiber-based resonator on the flat surface of an atom chip with ultrahigh precision. The structures for mounting a pair of fibers, which constitute the fiber resonator, are produced by a spin-coated SU-8 photoresist technique by use of deep-UV lithography. The design and production of the SU-8 structures are discussed. From the measured finesses we calculate the coupling loss of the SU-8 structures acting as a kind of fiber splice to be smaller than 0.013 dB.

Minimal optical decomposition of ray transfer matrices

The properties of first-order optical systems are described paraxially by a ray transfer matrix, also called the ABCD matrix. Here we consider the inverse problem: an ABCD matrix is given, and we look for the minimal optical system that consists of only lenses and pieces of free-space propagation. Similar decompositions have been studied before but without the restriction to these two element types or without an attempt at minimalization. As the main results of this paper, we found that general lossless one-dimensional optical systems can be synthesized with a maximum of four elements and two-dimensional optical systems can be synthesized with six elements at most.

Efficient reconstruction of two-dimensional complex amplitudes utilizing the redundancy of the ambiguity function

In a previous paper [Opt. Commun.225, 19-30 (2003)] we presented a method to reconstruct two-dimensional complex amplitudes by using the ambiguity function of one-dimensional intensity scans, obtained from two optical setups involving cylindrical lenses. We demonstrate that the internal redundancy of the ambiguity function can be utilized to improve the efficiency of this method even further. We show that the phase reconstruction errors can be minimized with an appropriate algorithm, and we present experimental data that illustrate the efficient reconstruction of a two-dimensional phase element.

Diffractive micro lens arrays with overlapping apertures

In this paper we outline the design method and the fabrication of diffractive overlapping lens arrays, which are applied to wave front measurement, which is one of our research topics. Other possible applications of these diffractive phase elements will also be reported.

High Resolution Wavefront Measurement with Phase Retrieval Using a Diffractive Overlapping Micro Lens Array

For the measurement of wave fronts there is a variety of methods, which can be divided into interferometric techniques, gradient-based methods, iterative phase retrieval and phase space methods. Each of these methods has its advantages and disadvantages. The proposed technique is a combination of iterative phase retrieval and a diffractive overlapping micro lens array. Unlike interferometric or holographic methods, phase retrieval methods require no reference wave. Thus no laser is required and the measurement set-up is very simple.

Phase Retrieval with a Diffractive Micro Lens Array

Multi-plane phase retrieval is a well established technique for reconstructing both, amplitude and phase of an object wave. This standard technique works best, if the intensity of the object wave changes rapidly along the optical axis. For slowly varying intensities, the iterative procedure may not converge at all. To overcome this limitation we combined the standard technique with a periodic phase element. We demonstrate that a binary diffractive micro lens array with overlapping aperture significantly improves the convergence of phase retrieval and thus the quality of reconstruction. Thus multi-plane phase retrieval can be applied for both rough and smooth phase distributions.

High resolution wavefront sensing with non-interferometric techniques

For the measurement of wave fronts there are is a variety of methods, which can be coarsely divided into interferometric and non-interferometric techniques. Each of these methods has its advantages and disadvantages. Here we combine the Hartmann-Shack (HS) method with the multi-plane phase retrieval method by Pedrini, thus obtaining a robust technique for measuring amplitude and phase with much higher spatial resolution than the classical HS-method.