Christopher Corder

Laser cooling without spontaneous emission using the bichromatic force

It is widely believed that spontaneous emission (SE) is necessary to remove entropy from an atomic sample during laser cooling. In fact, SE is needed for energy removal when laser cooling is done with single-frequency light, but with more than one frequency, both energy and entropy can be removed using only stimulated processes. Our experimental demonstration of this phenomenon works by restricting the atom–light interaction to a time short compared to a cycle of absorption followed by natural decay [Phys. Rev. Lett.114, 043002 (2015)PRLTAO0031-900710.1103/PhysRevLett.114.043002]. We present here additional information on these results, in particular, simulations of the motion of atoms under the bichromatic force that compare well with our data. This accomplishment is of interest to direct laser cooling of molecules or in experiments where working space or time is limited.

Atom lithography with metastable helium

A bright metastable helium (He∗) beam is collimated sequentially with the bichromatic force and three optical molasses velocity compression stages. Each He∗ atom in the beam has 20 eV of internal energy that can destroy a molecular resist assembled on a gold coated silicon wafer. Patterns in the resist are imprinted onto the gold layer with a standard selective etch. Patterning of the wafer with the He∗ was demonstrated with two methods. First, a mesh was used to protect parts of the wafer making an array of grid lines. Second, a standing wave of λ=1083 nm light was used to channel and focus the He∗ atoms into lines separated by λ/2. The patterns were measured with an atomic force microscope establishing an edge resolution of 80 nm. Our results are reliable and repeatable.

High-power ultrafast Yb:fiber laser frequency combs using commercially available components and basic fiber tools

We present a detailed description of the design, construction, and performance of high-power ultrafast Yb:fiber laser frequency combs in operation in our laboratory. We discuss two such laser systems: an 87 MHz, 9 W, 85 fs laser operating at 1060 nm and an 87 MHz, 80 W, 155 fs laser operating at 1035 nm. Both are constructed using low-cost, commercially available components, and can be assembled using only basic tools for cleaving and splicing single-mode fibers. We describe practical methods for achieving and characterizing low-noise single-pulse operation and long-term stability from Yb:fiber oscillators based on nonlinear polarization evolution. Stabilization of the combs using a variety of transducers, including a new method for tuning the carrier-envelope offset frequency, is discussed. High average power is achieved through chirped-pulse amplification in simple fiber amplifiers based on double-clad photonic crystal fibers. We describe the use of these combs in several applications, including ultrasensitive femtosecond time-resolved spectroscopy and cavity-enhanced high-order harmonic generation.

Development of a tunable high repetition rate XUV source for time-resolved photoemission studies of ultrafast dynamics at surfaces

The characterization of surfaces using photoelectron spectroscopy or photoemission electron microscopy provides sensitive probes of surface structure and electronic properties. Conventional extreme ultraviolet (XUV) light sources used for photoemission do not have ultrafast time resolution, which inhibits applying these techniques to the study of surface dynamics on their natural time scale. The high harmonics (HHG) of intense femtosecond laser pulses are capable of providing ultrashort XUV pulses for photoemission. However, for pulse-based photoemission measurements it is necessary to limit the density of electrons emitted by each pulse to prevent detrimental spacecharge effects. Therefore, to maintain reasonable data acquisition rates, the pulses must occur at a high repetition rate. Since the HHG process requires high peak fundamental laser powers, repetition rates have typically been limited to well below 1 MHz. In our lab, we can perform time-resolved XUV photoemission experiments at an 87 MHz repetition rate using a cavity-enhanced HHG source. Harmonics are generated at 87 MHz by resonantly enhancing a Yb:fiber laser capable of 1 μJ pulses in a passive optical cavity to pulse energies > 100 μJ. Average photon fluxes of up to 7x1011 photons/s in a single isolated harmonic are delivered to a surface science end station. This delivered flux and repetition rate are comparable to a synchrotron light source, but with pulse durations nearly 1000 times shorter. In this paper, we discuss critical details of the source and its performance.

Atomic nanofabrication of periodic structures

Metastable helium has 20 eV of internal energy that destroys a resist assembled on a wafer. An optical standing wave was used to channel and focus the He* atoms into lines separated by λ /2.