Arthur K. Mills

XUV frequency combs via femtosecond enhancement cavities

We review the current state of tabletop extreme ultraviolet (XUV) sources based on high harmonic generation (HHG) in femtosecond enhancement cavities (fsEC). Recent developments have enabled generation of high photon flux (1014 photons s?1) in the XUV, at high repetition rates (>50?MHz) and spanning the spectral region from 40 to 120?nm. This level of performance has enabled precision spectroscopy with XUV frequency combs and promises further applications in XUV spectroscopic and photoemission studies. We discuss the theory of operation and experimental details of the fsEC and XUV generation based on HHG, including current technical challenges to increasing the photon flux and maximum photon energy produced by this type of system. Current and future applications for these sources are also discussed.

Near-threshold harmonics from a femtosecond enhancement cavity-based EUV source: effects of multiple quantum pathways on spatial profile and yield

We investigate the photon flux and far-field spatial profiles for near-threshold harmonics produced with a 66 MHz femtosecond enhancement cavity-based EUV source operating in the tight-focus regime. The effects of multiple quantum pathways in the far-field spatial profile and harmonic yield show a strong dependence on gas jet dynamics, particularly nozzle diameter and position. This simple system, consisting of only a 700 mW Ti:Sapphire oscillator and an enhancement cavity produces harmonics up to 20 eV with an estimated 30-100 μW of power (intracavity) and > 1μW (measured) of power spectrally-resolved and out-coupled from the cavity. While this power is already suitable for applications, a quantum mechanical model of the system indicates substantial improvements should be possible with technical upgrades.

Compact laser cooling apparatus for simultaneous cooling of lithium and rubidium

We report on a dual species laser cooling apparatus capable of collecting over 108 87Rb or 85Rb atoms from an atomic vapor or up to (8±2)×107 6Li atoms directly into a magneto-optic trap (MOT) from an effusive oven without the need for a Zeeman slower. The use of a miniature atomic oven placed close to the trapping region yields a compact vacuum system with a captured flux of more than 4×106 lithium atoms per second and a high quality vacuum in the 10−10 Torr range. The atomic sources, laser system, and vacuum system are described. In addition, we use this system to study atom loss from the MOT due to interspecies collisions between 6Li and 85Rb or 87Rb. We report for the first time the heteronuclear loss coefficients for 6Li-85Rb mixtures.

Simple method to determine dispersion of high-finesse optical cavities

We present a simple and quick, yet accurate method to measure the dispersion of high finesse optical cavities. By exciting the cavity with a femtosecond frequency comb and measuring the resonance condition as a function of optical frequency, the cavitys dispersion can be determined with minimal uncertainty. Measurement results are presented from an evacuated reference cavity with low group delay dispersion as well as several differential, intra-cavity measurements of well known optical materials demonstrating the dynamic range and accuracy of this technique.

Miniature fiber-optic multiphoton microscopy system using frequency-doubled femtosecond Er-doped fiber laser

We report on a miniature fiber-optic multiphoton microscopy (MPM) system based on a frequency-doubled femtosecond Er-doped fiber laser. The femtosecond pulses from the laser source are delivered to the miniature fiber-optic probe at 1.58 µm wavelength, where a standard single mode fiber is used for delivery without the need of free-space dispersion compensation components. The beam is frequency-doubled inside the probe by a periodically poled MgO:LiNbO3 crystal. Frequency-doubled pulses at 786 nm with a maximum power of 80 mW and a pulsewidth of 150 fs are obtained and applied to excite intrinsic signals from tissues. A MEMS scanner, a miniature objective, and a multimode collection fiber are further used to make the probe compact. The miniature fiber-optic MPM system is highly portable and robust. Ex vivo multiphoton imaging of mammalian skins demonstrates the capability of the system in imaging biological tissues. The results show that the miniature fiber-optic MPM system using frequency-doubled femtosecond fiber laser can potentially bring the MPM imaging for clinical applications.

Collapse of superconductivity in cuprates via ultrafast quenching of phase coherence

The possibility of driving phase transitions in low-density condensates through the loss of phase coherence alone has far-reaching implications for the study of quantum phases of matter. This has inspired the development of tools to control and explore the collective properties of condensate phases via phase fluctuations. Electrically gated oxide interfaces1,2, ultracold Fermi atoms3,4 and cuprate superconductors5,6, which are characterized by an intrinsically small phase stiffness, are paradigmatic examples where these tools are having a dramatic impact. Here we use light pulses shorter than the internal thermalization time to drive and probe the phase fragility of the Bi2Sr2CaCu2O8+δ cuprate superconductor, completely melting the superconducting condensate without affecting the pairing strength. The resulting ultrafast dynamics of phase fluctuations and charge excitations are captured and disentangled by time-resolved photoemission spectroscopy. This work demonstrates the dominant role of phase coherence in the superconductor-to-normal state phase transition and offers a benchmark for non-equilibrium spectroscopic investigations of the cuprate phase diagram.Pump–probe, time-resolved ARPES experiments with underdoped cuprates reveal the transient enhancement of the density of phase fluctuations, eventually leading to the collapse of superconductivity.

An XUV Source using a Femtosecond Enhancement Cavity for Photoemission Spectroscopy

Recent development of extreme ultraviolet (XUV) sources based on high harmonic generation (HHG) in femtosecond enhancement cavities (fsEC) has enabled generation of high photon ux ( ̴ 1013-1014 photons/sec) in the XUV, at high repetition rates (> 50 MHz) and spanning the spectral region from 40 nm - 120 nm. Here we demonstrate the potential offered by this approach for angle-resolved photoemission spectroscopy by measuring the photoemission spectrum of Au using 8.3 and 25 eV photons with excellent resolution at rapid data rates.

Femtosecond enhancement cavity EUV source with high energy resolution

A table-top EUV source based on a femtosecond enhancement cavity is seeded by a 1040-nm Yb-doped fiber amplifier system with 185-fs pulses and produces >;10 microwatts/harmonic at 80 MHz out to 55 nm.

Compact multiphoton microscopy system based on frequency-doubled femtosecond erbium-doped fiber laser

We report on a compact multiphoton microscopy system based on a frequency-doubled, femtosecond erbium-doped fiber laser. It has a great potential to transform the bench-top multiphoton microscopy to a portable system for ;in vivo imaging.

Studying Correlated Electron Systems With a New Tunable (<25 eV) Tabletop XUV Source

We characterize a new table-top, tunable XUV source spanning 8 to 25 eV based on a femtosecond enhancement cavity. This source is designed to investigate correlated electron systems with angle and time resolved photoemission spectroscopy.