Towards On-Chip Self-Referenced Frequency-Comb Sources Based on Semiconductor Mode-Locked Lasers.

Miniaturization of frequency-comb sources could open a host of potential applications in spectroscopy, biomedical monitoring, astronomy, microwave signal generation, and distribution of precise time or frequency across networks. This review article places emphasis on an architecture with a semiconductor mode-locked laser at the heart of the system and subsequent supercontinuum generation and carrier-envelope offset detection and stabilization in nonlinear integrated optics

Scientific Opportunities with an X-ray Free-Electron Laser Oscillator

An X-ray free-electron laser oscillator (XFELO) is a new type of hard X-ray source that would produce fully coherent pulses with meV bandwidth and stable intensity. The XFELO complements existing sources based on self-amplified spontaneous emission (SASE) from high-gain X-ray free-electron lasers (XFEL) that produce ultra-short pulses with broad-band chaotic spectra. This report is based on discussions of scientific opportunities enabled by an XFELO during a workshop held at SLAC on June 29 - July 1, 2016Comment: 21 pages, 12 figure

Nonlinear optical functionalities of VO2- and GaN-based nanocomposites

This thesis presents fundamental research and concepts for active photonic elements operating in the telecom wavelength regime. The aim of the study is to determine the characteristics of the investigated nanostructures and to evaluate the implementation of the proposed materials in potential optical devices. In the first part of this thesis the optical properties as well as the photonic application of vanadium dioxide (VO2) nanocrystals (NCs) are studied. VO2 exhibits an easily accessible insulator-to-metal phase transition (IMT) near ambient temperatures. Upon excitation it undergoes an atomic rearrangement that is accompanied by a substantial modification of the complex dielectric function. When VO2 undergoes the IMT, the near-infrared transmission peaks of a moderate-finesse etalon containing a sub-wavelength layer of VO2 NCs are found to markedly shift in their spectral position and peak transmissivity. Both heat deposition and optical excitation permit to actively control the etalon’s functionality. Much less is known about the nonlinear optical properties of VO2 beyond the established IMT. To this end the nonlinear optical response of a thin film of VO2 NCs is investigated with open aperture z-scans involving femtosecond near-infrared pulses. A pronounced saturable absorption on the short-wave side of the resonance as well as a marked reverse saturable absorption in the telecom window are observed. The results hold promise for the use of VO2 nanocrystals as a saturable absorber, e.g., to mode-locked near-infrared lasers. In the second part a semiconductor heterostructure based on hexagonal ultranarrow GaN/AlN multi-quantum wells (MQWs) is investigated. The tailored inter-miniband (IMB) transition is characterized in terms of its linear as well as ultrafast nonlinear optical properties using the established pump-probe scheme. In line with theoretical predictions for LO-phonon scattering, a fast relaxation is found for resonant IMB excitation. In stark contrast, significantly larger relaxation times are observed for photon energies addressing the above barrier continuum. The last section reports on a new type of nonlinear metasurface taking advantage of these telecom-range IMB transitions. The heterostructure is functionalized with an array of plasmonic antennas featuring cross-polarized resonances at these near-infrared wavelengths and their second harmonic. This kind of nonlinear metasurface allows for substantial second harmonic generation at normal incidence which is completely absent for an antenna array without the heterostructure underneath

Effects of impurity band on multiphoton photocurrent from InGaN and GaN photodetectors

Multiphoton absorption of wide band-gap semiconductors has shown great prospects in many fundamental researches and practical applications. With intensity-modulated femtosecond lasers by acousto-optic frequency shifters, photocurrents and yellow luminescence induced by two-photon absorption of InGaN and GaN photodetectors are investigated experimentally. Photocurrent from InGaN detector shows nearly perfect quadratic dependence on excitation intensity, while that in GaN detector shows cubic and higher order dependence. Yellow luminescence from both detectors show sub-quadratic dependence on excitation intensity. Highly nonlinear photocurrent from GaN is ascribed to absorption of additional photons by long-lived electrons in traps and impurity bands. Our investigation indicates that InGaN can serve as a superior detector for multiphoton absorption, absent of linear and higher order process, while GaN, which suffers from absorption by trapped electrons and impurity bands, must be used with caution

Investigating the Liquid State of Carbon

Carbon materials have a many contemporary applications and new carbon allotropes are being discovered. However, while graphite and diamond are well understood, very little is known about the liquid state of carbon due to the high temperatures (above 5,000 K) and pressures (above 10 MPa) required for its formation. Initial studies used electrical heating to determine the melting point of graphite and the resistivity of liquid carbon. More recent studies used non-thermal laser melting to generate a metastable liquid that was studied with visible reflectivity and X-ray spectroscopies. Shock waves have also been used to transiently generate liquid carbon. Theoretical calculations of liquid carbon initially suggested the possibility of a liquid-liquid phase transition, but later ab initio quantum mechanical simulations showed only a continuous change in liquid coordination as its density increased. In this dissertation, extreme-UV (EUV) reflectivity and chirped coherent anti-Stokes Raman spectroscopy (c-CARS) were used to study non-thermally melted liquid carbon. Femtosecond laser pulses at 250 nm with a fluence of 0.45 J/cm2 (3.5 x 1012 W/cm2 intensity) were used to generate liquid carbon from an amorphous carbon substrate and the time evolution of EUV reflectivity was probed. EUV wavelengths from 20 to 42 nm were used with both s and p polarizations. The reflectivity decreased at all wavelengths probed as the material expanded and ablated. For wavelengths below 32 nm, the reflectivity decay time was less than ~2 ps. This time constant describes the lattice dynamics after melting, while above 32 nm, the reflectivity is also sensitive to the hot electron plasma generated by the melting pulse. From these results and equations for the behavior of a shock wave in a material, the electron temperature of the melted material was found to be 0.30 ± 0.6 eV. The reflectivity at two different polarizations was also used to calculate the complex refractive index of the material as it evolved over time. C-CARS spectra were obtained for highly ordered pyrolytic graphite (HOPG) and glassy carbon using CARS pump wavelengths of 400 nm and 800 nm. These spectra showed strong G peak resonance (1580 cm-1), corresponding to the relative vibrations of sp2 carbons in the material. The D peak (~1350 cm-1) resonance seen in Raman scattering of disordered graphite films was not observed in the CARS spectra. As this mode occurs when the excited electron scatters from a defect or phonon, it could be that the stimulated Stokes emission that occurs during the CARS process prevents such scattering. The sample was melted with an 800 nm, 90 fs laser pulse with fluences from 0.40 to 0.85 J/cm2 (intensities of 4.4 x 1012 to 9.4 x 1012 W/cm2). Delay times of less than 500 fs and as long as 100 ps all showed no broadening or shifting of the G peak, as would be expected for damaging and disordering of the material; only an intensity change is seen as the material ablates. Microscope images show permanent damage to the substrate and the fluences and times studied were comparable to those used in published reflectivity studies of liquid carbon. To advance the study of liquid carbon, a soft X-ray second harmonic generation (SHG) technique was developed and explored. X-ray absorption provides element-specific information on the electronic structure of a material that is sensitive to the environment around the element. Combining this with the interface specificity of SHG, provides a useful technique for studying solid-solid interfaces that are difficult to study otherwise. Our first soft X-ray SHG experiments on graphite films showed that the technique was indeed highly interface specific. The technique was also sensitive to resonance amplification when the input photons were at or above the carbon K-edge. A second experiment compared the boron/vacuum interface to a buried boron/carbon (Parylene-N) interface. The technique was sensitive to interface effects, showing larger SHG intensity at the boron K-edge for the boron/Parylene-N interface compared to the boron/vacuum interface. Ab initio quantum simulations were used to calculate the soft X-ray SHG spectra of these systems, verifying the interface sensitivity of the technique

Nonlinear Coherence Effects in Transient-Absorption Ion Spectroscopy with Stochastic Extreme-Ultraviolet Free-Electron Laser Pulses

We demonstrate time-resolved nonlinear extreme-ultraviolet absorption spectroscopy on multiply charged ions, here applied to the doubly charged neon ion, driven by a phase-locked sequence of two intense free-electron laser pulses. Absorption signatures of resonance lines due to 2$p$--3$d$ bound--bound transitions between the spin-orbit multiplets $^3$P$_{0,1,2}$ and $^3$D$_{1,2,3}$ of the transiently produced doubly charged Ne$^{2+}$ ion are revealed, with time-dependent spectral changes over a time-delay range of $(2.4\pm0.3)\,\text{fs}$. Furthermore, we observe 10-meV-scale spectral shifts of these resonances owing to the AC Stark effect. We use a time-dependent quantum model to explain the observations by an enhanced coupling of the ionic quantum states with the partially coherent free-electron-laser radiation when the phase-locked pump and probe pulses precisely overlap in time

Attosecond state-resolved carrier motion in quantum materials probed by soft x-ray XANES

Recent developments in attosecond technology led to table-top x-ray spectroscopy in the soft x-ray range, thus uniting the element- and state-specificity of core-level x-ray absorption spectroscopy with the time resolution to follow electronic dynamics in real-time. We describe recent work in attosecond technology and investigations into materials such as Si, SiO2, GaN, Al2O3, Ti, and TiO2, enabled by the convergence of these two capabilities. We showcase the state-of-the-art on isolated attosecond soft x-ray pulses for x-ray absorption near-edge spectroscopy to observe the 3d-state dynamics of the semi-metal TiS2 with attosecond resolution at the Ti L-edge (460 eV). We describe how the element- and state-specificity at the transition metal L-edge of the quantum material allows us to unambiguously identify how and where the optical field influences charge carriers. This precision elucidates that the Ti:3d conduction band states are efficiently photo-doped to a density of 1.9 x 1021 cm 3. The light-field induces coherent motion of intra-band carriers across 38% of the first Brillouin zone. Lastly, we describe the prospects with such unambiguous real-time observation of carrier dynamics in specific bonding or anti-bonding states and speculate that such capability will bring unprecedented opportunities toward an engineered approach for designer materials with pre-defined properties and efficiency. Examples are composites of semiconductors and insulators like Si, Ge, SiO2, GaN, BN, and quantum materials like graphene, transition metal dichalcogens, or high-Tc superconductors like NbN or LaBaCuO. Exiting are prospects to scrutinize canonical questions in multi-body physics, such as whether the electrons or lattice trigger phase transitions