We present a model to describe thermophysical and optical properties of two-temperature systems consisted of heated electrons and cold ions in a solid lattice that occur during ultrafast heating experiments. Our model is based on ab initio simulations within the framework of density functional theory. The optical properties are obtained by evaluating the Kubo-Greenwood formula. By applying the material parameters of our ab initio model to a two-temperature model we are able to describe the temperature relaxation process of femtosecond-laser-heated gold and its optical properties within the same theoretical framework. Recent time-resolved measurements of optical properties of ultrafast heated gold revealed the dynamics of the interaction between femtosecond laser pulses and solid state matter. Different scenarios obtained from simulations of our study are compared with experimental data [Chen, Holst, Kirkwood, Sametoglu, Reid, Tsui, Recoules, and Ng, Phys. Rev. Lett. 110, 135001 (2013)].

The resolution enhancement of laser-induced forward transfer (LIFT) is investigated through the pre-patterning of Cr on the donor substrate. 85 nm dots are first patterned on a microscope slide, and an 800 nm wavelength and 130 fs pulse laser with a beam waist of ∼9 μm is used to transfer the Cr dots to an acceptor substrate. The threshold fluence is found to be ∼0.15 the threshold fluence of a similar continuous film, which is thought to be due to the fact that no force is needed to tear away Cr from the film itself, unlike in a continuous film experiment. Since the volume of the material limits the transfer feature sizes instead of the laser parameters, as in a continuous film system, minimum transferable feature diameters are significantly lower compared to the continuous film case. Also, the transferred feature diameters are not dependent on the laser parameters, so the diameters are consistent across a wide range of fluences. The force per unit area generated by the laser at threshold fluence is estimated to be ∼3 GPa, which is consistent with previous results in the literature. The simplified model that our pre-patterned Cr LIFT experiment represents would make it an ideal case for benchmarking molecular dynamics simulations of femtosecond laser ablation.

The effect of donor film thickness and laser beam fluence on the size of laser-induced forward transfer (LIFT) spots is studied to achieve sub-100 nm features. A 130 fs, 800 nm laser is focused on ultrathin Cr films, and the transfer and ablation thresholds of these films at various thicknesses are determined. The minimum transfer spot size decreases with decreasing donor film thickness and incident laser fluence. Minimum LIFT spots of 70-450 nm diameter are obtained from films of 20-80 nm thickness, respectively. The 70 nm diameter transfer spots obtained from sputtered continuous films are the smallest to date.

AC conductivity has long been of interest in the study of electronic structure and transport properties of Warm Dense Matter. Using a chirped pulse probe technique, we have obtained single-shot measurements of the temporal evolution of AC conductivity during electron energy relaxation in non-equilibrium warm dense gold produced by femtosecond-laser heating with energy density up to 4.1MJ/kg (8×1010J/m3). The results uncover important changes that have been masked in earlier studies. Equally significant, they provide the first benchmark for testing an ab-initio model that is used to calculate electron heat capacity, electron-ion coupling and AC conductivity in a single, first principles framework. While measurements of the real part of AC conductivity corroborate our theoretical temperature-dependent electron heat capacity, they point to an electron-ion coupling factor of ~2.2×1016W/m3 K, substantially below that predicted by theory. In addition, measurements of the imaginary part of AC conductivity reveal the need to improve theoretical treatment of intraband contribution at very low photon energy.

An abrupt change in energy transport has been observed in femtosecond laser heated gold when the absorbed laser flux exceeds ~7×10(12) W/cm(2). Below this value, the absorbed flux is carried by ballistic motion of nonthermal electrons produced in interband excitation. Above this value energy transport appears to include ballistic transport by nonthermal electrons and heat diffusion by thermalized hot electrons. The ballistic component is limited to a flux of ~7×10(12) W/cm(2). This offers a unique benchmark for comparison with theory on nonequilibrium electron transport.

This chapter contains sections titled: Direct Femtosecond Laser Nanostructuring and Nanopatterning on Metals Laser-Induced Forward Transfer: an Approach to Direct Write of Patterns in Film Form Laser-Induced Forward Transfer: Transfer of Micro-Nanomaterials on Substrate Laser-Induced Forward Transfer for the Fabrication of Devices Acknowledgments References

We report a systematic investigation of the electronic structure of chemisorbed alkali atoms (Li-Cs) on a Ag(111) surface by two-photon photoemission spectroscopy. Angle-resolved two-photon photoemission spectra are obtained for 0-0.1 monolayer coverage of alkali atoms. The interfacial electronic structure as a function of periodic properties and the coverage of alkali atoms is observed and interpreted assuming ionic adsorbate/substrate interaction. The energy of the alkali atom σ-resonance at the limit of zero coverage is primarily determined by the image charge interaction, whereas at finite alkali atom coverages, it follows the formation of a dipolar surface field. The coverage- and angle-dependent two-photon photoemission spectra provide information on the photoinduced charge-transfer excitation of adsorbates on metal surfaces. This work complements the previous work on alkali/Cu(111) chemisorption [Phys. Rev. B 2008, 78, 085419].

The adsorption of Cs and Ba on Cu(111) is investigated by means of one- and two-photon photoemission experiments and theoretically by first-principles calculations. The spectral properties of these systems, induced by both surface and adatom states, are studied at submonolayer coverage through angle-resolved measurements. A coverage-dependent analysis is also exploited in the assignment of the observed electronic states. The comparison with ab initio calculations allows identification of all the spectral features induced by Cs and Ba chemisorption. The theoretical analysis concerns the limiting single adatom case, treated in an embedding approach with a one-dimensional potential for the surface. The agreement between the calculated density of states and the experimental spectra confirms that the model substrate retains all the relevant physics entering in the adsorption process. The differences between the electronic structures of Cs and Ba on the Cu(111) surface can be attributed to the group-dependent screening of the core potentials as manifested by the ionic radii and ionization potentials (alkali vs alkaline earth).

We present a systematic time-resolved two-photon photoemission study of the electronic and dynamical properties of Li through Cs adsorbed on Cu(111) and Ag(111) surfaces. A fundamental problem in surface science is how to describe the electronic structure of a chemisorption interface based on the intrinsic properties of the interacting materials. Because of their simple s-electron structure, elements of the alkali atom group comprise paradigmatic adsorbates in many theories of chemisorption, whereas the complementary experimental studies are sparse and incomplete. Through a combination of spectroscopic and femtosecond time-resolved surface measurements, we are able to probe systematically the binding energies, symmetries, and electron and nuclear relaxation dynamics of the initially unoccupied alkali atom resonances. As a prelude, we study the two-photon photoemission process occurring at the bare Ag(111) surface. We develop a quantitative model for two-photon photoemission process, where the nonresonant and k-dependent two-photon absorption between the lower and upper sp-bands is modeled by the optical Bloch equations, and the angle-dependent intensities are described by the Fresnel equations. Our two-photon photoemission spectra of Li through Cs chemisorbed Cu(111) and Ag(111) surfaces reveal two resonances with the m=0 and m=+-1 symmetry ('m' is the projection of the orbital angular momentum ‘l’ onto the surface plane). For the m=0 resonance, which is derived from the hybridization of the ns and npz orbitals of alkali atoms, we find a binding energy of 1.84-1.99 eV below the vacuum level, which is independent of the alkali atom period, and tunes with coverage in a universal manner. At 0.3-0.7 eV higher energy, we discover and identify the m=+-1 resonance by its characteristic angular intensity distribution, which derives from the antisymmetry of the npx and npy orbitals. We implement a quantitative model for the alkali atom chemisorption based on the dominant Coulomb interactions invoked by Langmuir and Gurney. Moreover, the time-resolved photoemission measurements on Cs/Ag(111) surface reveal an unprecedented nonexponential electronic population decay, which is indicative of the bond rupture; we follow the femtochemistry, i.e., the dissociative wave packet motion, on nearly the picosecond time scale.

Chemisorption of atoms and molecules controls many interfacial phenomena such as charge transport and catalysis. The question of how the intrinsic properties of the interacting materials define the electronic structure of their interface remains one of the most important, yet intractable problems in surface physics. Through two-photon photoemission spectroscopy we determine a common binding energy of $\ensuremath{\sim}1.8--2.0\text{ }\text{eV}$ with respect to the vacuum for the unoccupied resonance of the $ns$ valence electron of alkali atoms (Li-Cs) chemisorbed at low coverage (less than 0.1 monolayer) on noble metal [Cu(111) and Ag(111)] surfaces. We present a theoretical model based on the semiempirical potentials of the adsorbates and the substrates, their principal mode of interaction through the Coulomb interaction, and the ab initio adsorption structures. Our analysis reveals that atomic size and ionization potential independent interfacial electronic structure is a consequence of the Coulomb interaction among the $ns$ electron, the alkali-atom ionic core, and the induced image charge in the substrate. We expect the same interactions to define the effective electronic potentials for a broad range of molecule/metal interfaces.

We have performed a joint experimental and theoretical study of the unoccupied electronic structure of alkali adsorbates on the (111) surfaces of Cu and Ag. Combining angle- and time-resolved two-photon photoemission spectroscopy with wave packet propagation calculations we show that, along with the well known sigma resonance oriented along the surface normal, there exist long-lived alkali-localized resonances oriented parallel to the surface (pi symmetry). These new resonances are stabilized by the projected band gap of the substrate and emerge primarily from the mixing of the p and d Rydberg orbitals of the free alkali atom modified by the interaction with the surface.

We have measured angle-dependent photoemission spectra for one-photon and two-photon excitation from Ag(111). The observed dispersion of the sp-band transition of Ag(111) can be reproduced using a nearly-free-electron model for the initial and final states involved. The observed dispersion agrees with the known band structure. We illustrate how the strong refraction of low-energy electrons becomes a limiting factor to obtain quantitative band-structure information. Conversely, low-energy electrons of a well-defined direct optical interband transition can provide a sensitive probe of the inner potential. We observe asymmetric two-photon photoelectron intensity distributions with respect to detection along the surface normal. These intensity distributions can be well described by a phenomenological model which employs the Fresnel equations to calculate the electric field components of the incident radiation inside the sample. Very good agreement is found using tabulated optical constants and a momentum matrix element, which is oriented along the surface normal. In contrast, the observed intensity distribution for one-photon photoemission from Ag(111) does not fit the simple Fresnel model. We interpret this as the influence of surface photoemission. By comparison to Cu(001), we show that the expected intensity distributions of the Fresnel model for one-photon photoemission and two-photon photoemission are valid for an orientation of the momentum matrix element along the surface normal if the influence of additional effects like surface photoemission can be neglected.

We record movies surface plasmon dynamics in nanostructured Ag films by time-resolved photoemission electron microscopy with resolution of 330 attosecond/frame. By combining the interferometric time-resolved two-photon photoemission spectroscopy (ITR-2PP)[1-3] with photoemission electron microscopy (PEEM)[4] we develop a new technique for imaging surface plasmon dynamics in nanostructured metal films. Two-photon photoemission process mediated by the excitation of surface plasmons leads to the emission of photoelectrons from metal surfaces with a spatial distribution that is given by the inhomogeneous electric field (polarization) excited in the sample raised to the 4 power. The resulting emission distribution is imaged by PEEM electron optics with 50 nm spatial resolution. By recording images excited by identical 10 fs, 3.1 eV pulses with a delay that is defined within ±25 as, we record the emission that is an outcome of the interference between the pump and probe polarizations [2]. Thus obtained moves provide information on the dynamics of the coherent polarization excited in the sample. We explore surface plasmon dynamics in smooth Ag films with nanometer scale discontinuities, which allow the coupling of free electromagnetic fields into propagating surface plasmon modes, as well as films with random nanoscale roughness [5], which exhibit localized surface plasmon modes. Through interferometric measurements we can determine the time scales for the electronic dephasing in individual plasmon modes on <10 fs time scale. In addition, we demonstrate the quantum control of plasmon fields. By selecting the specific phase of the excitation fields [3], we control the specific plasmon field distributions excited in the metal sample with 50 nm spatial resolution and 50 as temporal precision. The combination of ultrafast optical methods and electron microscopic techniques embodied in interferometric time-resolved photoemission electron microscopy (ITR-PEEM) opens the way to studies of coherent electron dynamics in nanostructured materials on femtosecond temporal and nanometer spatial scales. 1. H. Petek and S. Ogawa, Prog. Surf. Sci. 56, 239 (1997). 2. S. Ogawa, et al., Phys. Rev. Lett. 78, 1339 (1997). 3. H. Petek, et al., Phys. Rev. Lett. 79, 4649 (1997). 4. H. H. Rotermund, Surf. Sci. Rep. 29, 267 (1997). 5. M. Merschdorf, et al., Appl. Phys. A 71, 547 (2000). QTuD2-3-INV

Theoretical and experimental studies on the two-photon photoemission excited by femtosecond laser pulses from the sp-band of Ag(111) in normal direction are reported. At low temperatures, the two-photon photoemission spectrum is found to consist mainly of nonresonant direct transitions from the lower to the upper sp-band across the L-projected band gap involving the upper sp-band as virtual intermediate states. This direct photoemission signal is calculated from the Ag band structure using optical Bloch equations and dipole moments derived from the nearly free electron approximation. For finite temperatures, a minor additional temperature-dependent, continuous contribution to the two-photon photoemission spectrum is experimentally determined. This background due to the secondary electrons is theoretically modeled combining the Debye model and Fermi Liquid Theory to account for electron relaxation effects during the two-photon photoemission process.