Andreas Hemmerich

A compact grating-stabilized diode laser system for atomic physics

We describe a compact, economic and versatile diode laser system based on commercial laser diodes, optically stabilized by means of feedback from a diffraction grating. We offer detailed information which should enable the reader to copy our set-up which uses only easily machined mechanical parts. Our system offers single-mode operation with a linewidth of a few 100 kHz, continuous scans over 25 GHz, high chirp rates (up to 9 GHz/ms) and FM-modulation up to the GHz range. We discuss radiofrequency phase-locking of two independent lasers systems, allowing well controlled fast frequency switching which overcomes the limitations imposed by acousto-optic modulators.

All solid state laser source for tunable blue and ultraviolet radiation

Tunable blue and ultraviolet single mode laser light has been generated by frequency quadrupling the output of a semiconductor laser with two successive frequency doubling stages. The laser source is based on a commercial high power semiconductor laser near 972 nm which combines a low power single mode master oscillator with a high power amplifier. The doubling stages consist of nonlinear crystals which are placed inside compact optical buildup resonators. Up to 156 mW tunable blue radiation near 486 nm and 2.1 mW ultraviolet light near 243 nm have been produced.

Second-harmonic generation and optical stabilization of a diode laser in an external ring resonator

The second harmonic of the 842-nm output of a GaAlAs diode laser is generated in a KNbO(3) crystal in a resonant, external ring cavity. The diode laser is optically stabilized to the ring cavity through feedback from the counterpropagating fundamental wave, which is weakly excited in the resonator. We have produced 6.7 mW of tunable, narrowband radiation at 421 nm and have used that light to perform saturation spectroscopy on narrow transitions in rubidium.

Collective atomic motion in an optical lattice formed inside a high finesse cavity

We report on collective nonlinear dynamics in an optical lattice formed inside a high finesse ring cavity in a so far unexplored regime, where the light shift per photon times the number of trapped atoms exceeds the cavity resonance linewidth. We observe bistability and self-induced squeezing oscillations resulting from the retroaction of the atoms upon the optical potential wells. We can well understand most of our observations within a simplified model assuming adiabaticity of the atomic motion. Nonadiabatic aspects of the atomic motion are reproduced by solving the complete system of coupled nonlinear equations of motion.

Optically stabilized narrow linewidth semiconductor laser for high resolution spectroscopy

Abstract A narrow bandwidth tunable semiconductor laser system operating near 780 nm is described. The commercial laser diodes are frequency stabilized by optical feedback from an external, confocal Fabry-Perot resonator. The feedback phase is electronically stabilized to improve the frequency stability. A beat signal with 30 kHz linewidth between two identical, independent systems is recorded. The performance of this system is demonstrated in a laser cooling experiment with rubidium.

Normal mode splitting and mechanical effects of an optical lattice in a ring cavity

A novel regime of atom-cavity physics is explored, arising when large atom samples dispersively interact with high-finesse optical cavities. A stable far-detuned optical lattice of several million rubidium atoms is formed inside an optical ring resonator by coupling equal amounts of laser light to each propagation direction of a longitudinal cavity mode. An adjacent longitudinal mode, detuned by about 3 GHz, is used to perform probe transmission spectroscopy of the system. The atom-cavity coupling for the lattice beams and the probe is dispersive and dissipation results only from the finite photon-storage time. The observation of two well-resolved normal modes demonstrates the regime of strong cooperative coupling. The details of the normal mode spectrum reveal mechanical effects associated with the retroaction of the probe upon the optical lattice.

Dynamical phase transition in the open Dicke model

Significance Nonequilibrium phenomena in quantum many-body systems are not well understood to date. This applies in particular for open systems, coupled to an external bath. We use a Bose–Einstein condensate in a high-finesse optical resonator with ultralow bandwidth to emulate the open Dicke model. In well-controlled sweeps across the Hepp–Lieb–Dicke phase transition, we observe hysteretic dynamics showing power-law scaling with respect to the transition time, which suggests an interpretation in terms of a Kibble–Zurek mechanism. Our observations indicate the possibility of universal behavior in the presence of dissipation. The Dicke model with a weak dissipation channel is realized by coupling a Bose–Einstein condensate to an optical cavity with ultranarrow bandwidth. We explore the dynamical critical properties of the Hepp–Lieb–Dicke phase transition by performing quenches across the phase boundary. We observe hysteresis in the transition between a homogeneous phase and a self-organized collective phase with an enclosed loop area showing power-law scaling with respect to the quench time, which suggests an interpretation within a general framework introduced by Kibble and Zurek. The observed hysteretic dynamics is well reproduced by numerically solving the mean-field equation derived from a generalized Dicke Hamiltonian. Our work promotes the understanding of nonequilibrium physics in open many-body systems with infinite range interactions.

Reducing the linewidth of a diode laser below 30 Hz by stabilization to a reference cavity with a finesse above 10(5).

An extended-cavity diode laser operating in the Littrow configuration emitting near 657 nm is stabilized through its injection current to a reference cavity with a finesse of more than 10(5) and a corresponding resonance linewidth of 14 kHz. The laser linewidth is reduced from a few megahertz to a value below 30 Hz. The compact and robust setup appears ideal as a portable optical frequency standard that uses the calcium intercombination line.

Topological semimetal in a fermionic optical lattice

Experimental progress has made it possible to load fermionic atoms into higher orbital bands. Such systems provide a platform for studying quantum states of matter that have no prior analogues in solid-state materials. This theoretical study predicts a semimetallic topological state in these systems, which can be turned into a topological insulating phase. Optical lattices have an important role in advancing our understandingof correlated quantum matter. The recent implementation of orbital degrees of freedom in chequerboard1,2 and hexagonal3 optical lattices opens up a new avenue towards discovering novel quantum states of matter that have no prior analogues in solid-state electronic materials. Here, we predict that an exotic topological semimetal emerges as a parity-protected gapless state in the orbital bands of a two-dimensional fermionic optical lattice. This new quantum state is characterized by a parabolic band-degeneracy point with Berry flux 2π, in sharp contrast to the π flux of Dirac points as in graphene. We show that the appearance of this topological liquid is universal for all lattices with D4 point-group symmetry, as long as orbitals with opposite parities hybridize strongly with each other and the band degeneracy is protected by odd parity. Turning on inter-particle repulsive interactions, the system undergoes a phase transition to a topological insulator whose experimental signature includes chiral gapless domain-wall modes, reminiscent of quantum Hall edge states.

Surface-plasmon mirror for atoms

We demonstrate specular reflection of a thermal rubidium beam with low-power laser light. The atoms are reflected by the gradient force of an evanescent wave enhanced by surface plasmons excited in a thin silver layer. With only 6 mW of diode laser power we achieve a deflection angle of 2.5 mrad. Analysis of the velocity distribution of the reflected atoms yields an enhancement factor of 60 +/- 20 for the amplitude square of the evanescent wave.