Chad A. Meserole

Terahertz emission via ultrashort-pulse excitation of magnetic metal films

We observe terahertz emission by optical rectification of an intense 1.5-eV, 50-fs pulse in single-crystal iron thin films grown by molecular beam epitaxy. The azimuthal dependence of the emission indicates the presence of a magnetic nonlinearity and a nonmagnetic surface nonlinearity.

Terahertz emission via ultrashort pulse excitation of magnetic metal films

We observe terahertz emission via optical rectification of a 1.5 eV, 50 fs pulse in single crystal iron thin films. The azimuthal dependence of the emission indicates the presence of a magnetic and surface nonlinearity.

Design for a kinematic, variable flux microcapillary array molecular beam doser

The design of a microcapillary array for dosing gases in ultrahigh vacuum (UHV) is reported. The current design improves upon previous designs in ease of fabrication and assembly, and combines the attributes of minimum dead volume, minimum gas load on the vacuum system, collimated molecular flow, the ability to optimize dosing geometry and withdraw the doser when not in use, and ultimate UHV compatibility. These design characteristics result in an optimum source–target spacing enabling uniform, quantitative exposure of samples up to 1in. in diameter.

Ablation cleaning techniques for high-power short-pulse laser produced heavy ion targets

It has become apparent in the last few years that the light ion surface contamination on short-pulse laser targets is a major impediment to the acceleration of heavier target ions. Mitigation strategies have been tested in experiments at the Los Alamos Trident Laser facility using one arm of the Trident laser at 150 ps to ablatively clean a large area of heated targets in a single short that are subsequently irradiated by the Trident 30 TW short-pulse arm to accelerate the bulk target ions to high energies. This process was used on targets consisting of 15 microns of vanadium. The 150 ps pulse rids the rear of the target of its omnipresent surface contamination layer, consisting mainly of water vapor and hydrocarbons, and allows the Trident 30 TW short-pulse arm to illuminate the target and accelerate ions via the Target Normal Sheath Acceleration (TNSA) mechanism. Because this mechanism relies on a laser generated electrostatic sheath, the ions with the lightest charge to mass ratio (i.e. protons) would be accelerated preferentially at the expense of heavier ions. However with the contamination layer removed, and hence the bulk of the available protons, the TNSA mechanism is able to accelerate the bulk material ions to high energies. Our experimental results are discussed and compared to the LASNEX rad-hydro code to validate and improve our predictive capabilities for future acceleration experiments.

Fe(001) thin films for x-ray diffraction and terahertz emission studies

Our interests in growing thin films of iron (Fe) are twofold. First, Fe(001) films are ideal targets for an ultrafast x-ray diffraction instrument designed to understand complex behavior, such as melting or solid-solid phase transitions, in shock-loaded materials. Second, we have discovered that thin iron films generate picosecond, broadband terahertz frequencies after intense femtosecond pulse excitation by a Ti:sapphire laser. Excluding film thickness, the criteria for samples used in both experiments are identical due to the similarities of the experiments. Iron films are grown in ultrahigh vacuum (UHV) conditions on (001)-oriented magnesium oxide (MgO) substrates. We have investigated the effects of substrate preparation on the film quality and have found that films grown in UHV on UV/ozone-cleaned and annealed MgO(001) have a better crystal quality than films grown on as-received MgO(001). However, both substrate preparation methods produce continuous, (001)-oriented thin films of iron.

Terahertz spectroscopy of ultrafast demagnetization in ferromagnetic iron

We use ultrafast terahertz transmission and emission spectroscopy to study ultrafast demagnetization in ferromagnetic iron. We show that demagnetization proceeds with a 1.7 ps/spl plusmn/0.3 ps lifetime in ferromagnetic films at room temperature.

Ultrafast demagnetization in ferromagnetic iron

We report unambiguous observation of an ultrafast component to demagnetization in epitaxially-grown ferromagnetic iron films after absorption of an intense femtosecond pulse. Demagnetization occurs in 2.5±0.5 ps, comparable to the measured electronic relaxation time. Much recent controversy exists regarding the proper interpretation of optical measurements of ultrafast magnetization changes studied by timeresolved, magneto-optical Kerr (TR-MOKE) techniques. These results are believed to be a combination of both sample magnetization changes and the induced, time-dependent Fresnel reflection modifications that simultaneously occur after femtosecond pulse excitation. It has been shown in Kerr ellipticity and field rotation experiments that there exists subpicosecond modifications to the magnetization in nickel which is significantly faster than conventional spin-lattice relaxation [1]. However, due to the presence of a pump-induced, time dependent reflectivity occurring on the same time scale as the ultrafast magnetization changes, the measured TR-MOKE results are difficult to interpret [2, 3]. In contrast to these all-optical techniques, which probe transitions between states of different energy, ultrafast terahertz emission spectroscopy provides an alternative to TR-MOKE for studies of demagnetization following intense femtosecond excitation. In order to unambiguously determine the demagnetization time in ferromagnetic iron, we have studied the terahertz emission and the induced changes to the optical conductivity at terahertz frequencies of a ferromagnetic iron single crystal after excitation by an ultrashort optical pulse. For the photoinduced THz emission, it is the induced time-varying magnetic field which acts as the source of the emission. While emission studies of material properties have thus far been limited to determination of electronic properties of materials, recent work has extended this tool to studies of ultrafast demagnetization [4, 5]. In order to simultaneously determine the electronic dynamics in the experiment, we have performed room temperature optical pump, terahertz probe studies of the relaxation of the conductivity (electronic subsystem), at several pump fluences. We observe a strong dependence of the initial fast relaxation time on the pump fluence (sample heating), in contrast with the absence of bandwidth dependence of the emitted pulse at these fluences [5]. These results are not consistent with the simple “effective spin temperature” approximation previously used to model ultrafast demagnetization [4]. Our results suggest the presence of a demagnetization mechanism on the same time scale as the electron-phonon thermalization time in iron. In the experiment, we pump a 12 nm thick single crystal sample with the output of an amplified Ti:S laser (pulse length = 100 fs) operating at a 1 kHz repetition rate with a pulse energy of 2 mJ/pulse. The iron films studied are grown by molecular beam epitaxy by a method which is known to produce bulk base-centered cubic (bcc) single crystal Fe [6, 7]. To measure the associated induced conductivity dynamics, we employ a standard optical pump, terahertz probe experiment, where we use a portion of output of the amplified Ti:sapphire laser to generate THz, via optical rectification in a 1 mm thick (110) ZnTe single crystal, which we detect via standard electro-optic sampling techniques. The remainder of the amplifier output is available to pump the sample. Due to the absence of resonant transitions in this material at a 800 nm, the primary effect of the pump pulse, P(t), is an ultrafast heating of the sample. From the known absorption coefficient of iron, we calculate that the absorption length at 800 nm is ~15 nm. As a result, approximately 10% of our incident optical power is absorbed by the sample, with the remainder either being lost to front surface reflection or transmitted through this film [1]. This pump pulse excites electrons in the metal and raises this subsystem temperature, TE, by an amount dependent on the absorbed fluence and the temperature-dependent electronic specific heat, CE=γTE, of iron. As a result of this ultrafast heating, the samples optical conductivity, σ, is perturbed, changing the transport scattering time, JFH2-4

Terahertz Emission Spectroscopy of Ultrafast Demagnetization in Iron

We have observed ultrafast demagnetization of ferromagnetic metals after femtosecond pump pulse excitation by both transmission and emission spectroscopy. We observe demangtization occuring in iron with a 2 ps time constant.

Terahertz emission via ultrashort pulse excitation of magnetic metal surfaces

Terahertz emission from a single crystal iron film is studied. The terahertz emission results from optical rectification of an ultrashort optical pulse due to the magnetic and surface contributions to the second order nonlinearity

An Ultrafast X‐Ray Diffraction Apparatus for the Study of Shock Waves

The use of X‐ray diffraction for the study of shock physics has been pursued for decades. Conceptually, changes in the diffraction line, including broadening and shifts, provide details about the nature of compression, plasticity, phase, and kinetics of the phase transition for the material being shock‐loaded. In practice, X‐ray source brightness, sample preparation, and turn‐around times have limited the applicability to a few crystalline systems. We report our development of an ultrafast X‐ray diffraction instrument suitable for studying rapid phase changes, including both solid‐solid and solid‐melt, in shock‐loaded materials. Due to the relatively small sample sizes needed and to the ability to conduct thousands of shock physics experiments with these small samples, we can build up the statistics required to study elastic‐plastic transitions, the kinetics of phase changes, as well as the mechanistic details of phase changes in nearly all materials, including high‐Z samples. An overview of the technique...