https://link.aps.org/pdf/10.1103/RevModPhys.70.721

Nobel Lecture: Laser cooling and trapping of neutral atoms

The authors review progress in understanding the nature of atomic collisions occurring at temperatures ranging from the millidegrees Kelvin to the nanodegrees Kelvin regime. The review includes advances in experiments with atom beams, light traps, and purely magnetic traps. Semiclassical and fully quantal theories are described and their appropriate applicability assessed. The review divides the subject into two principal categories: collisions in the presence of one or more light fields and ground-state collisions in the dark.

Experiments and theory in cold and ultracold collisions

The lure of lower temperatures has attracted physicists for the past century, and with each advance towards absolute zero, new and rich physics has emerged. Laypeople may wonder why ‘‘freezing cold’’ is not cold enough. But imagine how many aspects of nature we would miss if we lived on the surface of the sun. Without inventing refrigerators, we would only know gaseous matter and never observe liquids or solids, and miss the beauty of snowflakes. Cooling to normal earthly temperatures reveals these dramatically different states of matter, but this is only the beginning: many more states appear with further cooling. The approach into the kelvin range was rewarded with the discovery of superconductivity in 1911 and of superfluidity in helium-4 in 1938. Cooling into the millikelvin regime revealed the superfluidity of helium-3 in 1972. The advent of laser cooling in the 1980s opened up a new approach to ultralow-temperature physics. Microkelvin samples of dilute atom clouds were generated and used for precision measurements and studies of ultracold collisions. Nanokelvin temperatures were necessary to explore quantum-degenerate gases, such as Bose-Einstein condensates first realized in 1995. Each of these achievements in cooling has been a major advance, and recognized with a Nobel prize. This paper describes the discovery and study of BoseEinstein condensates (BEC’s) in atomic gases from my personal perspective. Since 1995, this field has grown explosively, drawing researchers from the communities of atomic physics, quantum optics, and condensedmatter physics. The trapped ultracold vapor has emerged as a new quantum system that is unique in the precision and flexibility with which it can be controlled and manipulated. At least 30 groups have now created condensates, and the publication rate on Bose-Einstein condensation has soared following the discovery of the gaseous condensates in 1995 (see Fig. 1).

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Nobel lecture: When atoms behave as waves: Bose-Einstein condensation and the atom laser*

We give a comprehensive overview of the development of micro traps, from the first experiments on guiding atoms using current carrying wires in the early 1990's to the creation of a BEC on an atom chip.

Microscopic atom optics: from wires to an atom chip

Publisher Summary This chapter focuses on the concept of evaporative cooling of trapped neutral atoms. The recent observations of Bose–Einstein condensation have shown dramatically the potential of evaporative cooling. Through evaporative cooling, phase-space density could be increased by six orders of magnitude in these experiments. In such experiments, evaporative cooling was used to reach temperature and densities that are unprecedented for trapped atoms and greatly exceeded what had been reached before by laser cooling. Laser cooling has recently broken the recoil limit in three dimensions (3D) and reached extremely cold temperatures of 3 nK in 1D. However, none of these optical sub-recoil techniques has been realized so far at high densities. The current density limitations are caused by the absorption of light, radiation trapping, and excited state collisions. An often-mentioned disadvantage of evaporative cooling is the loss of atoms. However, as discussed in the chapter, the efficiency of evaporative cooling is quite high.

Evaporative Cooling of Trapped Atoms

Cadmium sulfide (CdS) is an interesting material for applications in optoelectronic and pho- tovoltaic devices. In particular, thin films of n-type CdS are widely used as a window layer in heterojunction solar cells. The optical bandgap energy is the most interesting parameter for these applications. For spray-deposited indium-doped cadmium sulfide (CdS:In) thin films prepared on glass substrates the transmittance measurements were used to estimate the optical bandgap energy. Tailing in the bandgap was observed and found to obey Urbach rule. The effects of film thickness and the substrate temperature on the bandgap energy and the width of the tail were investigated. A linear relation between bandgap energy and width of the tail was found.

https://cyberleninka.org/article/n/1221045.pdf

A study of the optical bandgap energy and Urbach tail of spray-deposited CdS:In thin films

Neutral sodium atoms have been continuously loaded into a 0.1-K-deep superconducting magnetic trap with laser light used to slow and stop them. At least 1 \ifmmode\times\else\texttimes\fi{} ${10}^{9}$ atoms were trapped with a decay time of 2\textonehalf{} min. The fluorescence of the trapped atoms was studied as a function of time; possible loss mechanisms from the trap are discussed.

Continuous stopping and trapping of neutral atoms

There are several metals that form ohmic contacts for ZnO thin films, such as copper, aluminum and silver. The aim of this work is to make a comparison between these ohmic contacts. To achieve this purpose, polycrystalline ZnO thin films were prepared by the spray pyrolysis technique, and characterized by the I–V measurements at room temperature. Two strips of each metal were thermally evaporated on the surface of the film and measurements were first recorded in the dark and room light, then in the dark before and after annealing for Al, which was found to be the best in the set. Films with aluminum contacts gave the smallest resistivity, best ohmicity and they are slightly affected by light as required. On the other hand, copper was found to be the worst, and films with copper contacts gave the largest resistivity, worst ohmicity and they are the most affected by light. Annealing improved the aluminum contacts due to alloying and doping.

https://iopscience.iop.org/article/10.1088/1674-4926/36/3/033005/pdf

Riyad N. Ahmad-Bitar

Papers

A study of the optical bandgap energy and Urbach tail of spray-deposited CdS:In thin films

Continuous stopping and trapping of neutral atoms

A comparison between different ohmic contacts for ZnO thin films

Effect of doping and heat treatment on the photoluminescence of CdS films deposited by spray pyrolysis

The influence of the substrate temperature on the photovoltaic properties of spray-deposited CdS:In thin films