Amplification of solid high harmonics in semiconductor nanostructures (Conference Presentation)

High-harmonic generation (HHG) is a powerful tool to generate coherent attosecond light pulses in the extreme ultraviolet. However, the low conversion efficiency of HHG at the single atom level poses a significant practical limitation for many applications. Enhancing the efficiency of the process defines one of the primary challenges in the application of HHG as an advanced XUV source. In this work, we demonstrate a new mechanism, which in contrast to current methods, enhances the HHG conversion efficiency purely on a single particle level. We show that using a bichromatic driving field, sub-optical-cycle control and enhancement of the tunnelling ionization rate can be achieved, leading to enhancements in HHG efficiency by up to two orders of magnitude. Our method advances the perspectives of HHG spectroscopy, where isolating the single particle response is an essential component, and offers a simple route toward scalable, robust XUV sources.

We demonstrate up to two orders of magnitude enhancements in high harmonic generation efficiency via sub-cycle control and scaling of the ionization rate in a two colour laser field.

Experimental study on the energy transmissivity of high -power nanosecond laser pulse focusing in air versus input laser energy has been carried out. There are three stages: when the input laser energy is relatively low，all of the laser energy can pass through the focusing point; when the input laser energy increases gradually，energy transmissivity decreases steeply from nearly 100% to lower values; when the input laser energy keeps on increasing，the transmissivity decreases slowly further. By defining the critical electron density and the corresponding critical time point，successful theoretical analysis has been carried out for the above experimental phenomena. When the free-electron density is less than the critical value，multiphoton ionization plays the dominating role; when the free-electron density is higher than the critical value, the inverse bremsstrahlung process dominates. The critical time point is the time when laser pulse reaches critical electron density in the process of laser pulse passing through the air; its position on the laser pulse is determined by the input laser pulse energy，meanwhile this position determines the energy transmissivity of the laser pulse. So we can calculate the critical electron density by measuring the transmissivity versus input energy and comprehend the characteristics of transmissivity for laser pulse focusing in the air.

The radio-frequency (RF) emissions in a range from 30 MHz to 800 MHz from the plasma, which is produced by the nanosecond laser (532 nm, 8 ns) induced breakdown of atmospheric air, are presented. A spectrum analyzer which can scan over a spectral range of 9 kHz-26.5 GHz is used to record the RF-range radiation intensities of the emission from the plasma. RF electromagnetic radiations from the laser induced breakdown of atmospheric air are obtained for different input laser energies. A half-wave plate and a Glan prism are used to vary the input laser energy. Experimental results show that the intensities of RF radiation in a range of 30-200 MHz increase with the increase of laser energy, but the intensities of RF radiation in a 360-600 MHz frequency range decrease. To study the effect of input laser polarization on the RF radiation, we adopt the input lasers with vertical and horizontal polarization respectively. When the polarizations of the input laser and the antenna are the same, the RF radiation intensity is relatively high, and the frequency lines are relatively abundant. The changing relationship between the total power of RF radiation and the energy of the input laser is calculated and analyzed. It is observed that the total power of RF radiation first increases and then decreases with the increase of input laser energy. The influences of the plasma electron density on the plasma frequency and the plasma attenuation coefficient are investigated to explain the relationship between the total power of the RF radiation and the laser energy. The RF radiation is caused by the following processes. The generated electrons and ions are accelerated away from the core by their thermal pressures. This leads to charge separation and forming the electric dipole moments. These oscillating electric dipoles radiate electromagnetic waves in the RF range. Furthermore, the interactions of electrons with atomic and molecular clusters within the plasma play a major role in RF radiation, and the low frequency electromagnetic radiation takes place from the plasma that is far from fully ionized state. Further study of the characteristics of RF electromagnetic radiation is of great significance for understanding the physical mechanism of the interaction between laser and matter.

We present the results on the electromagnetic (em) radiation emitted in the 70 MHz - 1 GHz frequency range from the laser induced breakdown of atmospheric air. Laser pulses (7 ns) from second harmonic of an Nd:YAG laser (532 nm) were used to breakdown atmospheric air to form plasma. During the plasma evolution and expansion, dipole moment is induced in the homonuclear molecules of nitrogen and oxygen (the main components of atmospheric air), which naturally have no permanent electron dipole moments. The RF spectra originating from the longitudinal oscillation of these induced dipoles was detected using the RH-799 broadband Diamond antenna. A spectrum analyzer (Agilent PSA E444A, 3 Hz to 50 GHz) was used to monitor and record the RF spectrum from plasma. By tuning the length of the antenna, lines corresponding to the different resonant frequency were observed at different laser energies. The total emitted RF energy was found to be increasing with the input laser energy up to certain input laser energy, beyond which emission properties were modified drastically. This was observed due to the presence of multiple breakdown sources due to the self-focusing of the ns laser pulses, modifying the collisions between the plasma electrons and eventually modifying the induced dipole moment in the detection range. The emitted radiation showed a specific polarization property associated with the input em radiation.

Silicon nanowires (Si-NWs) have been extensively studied for their numerous applications in nano-electronics. The most common method for their synthesis is the vapor–liquid–solid growth, using gold as catalyst. After the growth, the metal remains on the Si-NW tip, representing an important issue, because Au creates deep traps in the Si band gap that deteriorate the device performance. The methods proposed so far to remove Au offer low efficiency, strongly oxidize the Si-NW sidewalls, or produce structural damage. A physical and chemical characterization of the as-grown Si-NWs is presented. A thin shell covering the Au tip and acting as a barrier is found. The chemical composition of this layer is investigated through high resolution transmission electron microscopy (TEM) coupled with chemical analysis; its formation mechanism is discussed in terms of atomic interdiffusion phenomena, driven by the heating/cooling processes taking place inside the eutectic-Si-NW system. Based on the knowledge acquired, a new efficient etching procedure is developed. The characterization after the chemical etching is also performed to monitor the removal process and the Si-NWs morphological characteristics, demonstrating the efficiency of the proposed method and the absence of modifications in the nanostructure.

With the advent of multi-petawatt lasers, the relativistic transparency regime of laser-plasma interactions becomes readily accessible for near-solid density targets. Initially opaque targets that undergo relativistic self-induced transparency (RSIT) have already shown to result in promising particle acceleration and radiation generation mechanisms, as well as relativistic optical and photonics phenomena that modify the spatial, temporal, spectral and polarization properties of the laser pulse itself. At the maximum laser intensities currently available, this opaque-to-RSIT transition regime can be achieved through ultrafast ionization, heating and expansion of initially ultrathin foil targets. Here, we review findings from our programme of work exploring this regime experimentally and numerically, including changes to the laser energy absorption, mechanisms for laser-driven particle acceleration and the generation of a relativistic plasma aperture. New physics induced by this aperture, such as the production of intense light with higher order spatial modes and higher harmonics, and spatially-structured and temporally-varying polarization states, is summarized. Prospects for exploring the physics of the RSIT regime with higher intensity and high repetition rate lasers, including expected new phenomena such as high-field effects and the application of new techniques such as machine learning, are also discussed; outlining directions for the future development of this promising laser-plasma interaction regime.

The dependence of external focusing geometries and polarization in generation of supercontinuum by femtosecond laser pulse in air

We are investigating the use of a low-divergence (100, μrad) KrF laser to trigger 2.8-MV gas switches for lage pulsed-power systems. Using less than 0.1 J of laser energy, we have demonstrated subnanosecond-jitter triggering (1σ) of these switches, which are insulated with pure SF6, at voltages between 75-90 percent of the self-breakdown voltage of the switch. The time delay between the laser pulse and the switch closure is relatively insensitive to variations in voltage, with delay changing by as little as 1.5 ns between 80-90 percent of the self-breakdown voltage. We also present parametric studies of laser triggering using lenses with different focal lengths at various input laser energies.

The problem of the amplification of high harmonics generated during the above-threshold ionization of atoms in a high-power laser wave field is examined for the first time. An estimate of the gain coefficient as a function of the parameters of the atom beam and the pump wave is given.

Recently we proposed a paradigm shift in light confinement strategy showing how relaxed total internal reflection and photonic skin-depth engineering can lead to sub-diffraction waveguides without metal (S. Jahani and Z. Jacob, "Transparent sub-diffraction optics: nanoscale light confinement without metal," Optica 1, 96-100 (2014)). Here, we show that such extreme-skin-depth (e-skid) waveguides can counter-intuitively confine light better than the best-case all-dielectric design of high index silicon waveguides surrounded by vacuum. We also analytically establish that figures of merit related to light confinement in dielectric waveguides are fundamentally tied to the skin depth of waves in the cladding, a quantity surprisingly overlooked in dielectric photonics. We contrast the propagation characteristics of the fundamental mode of e-skid waveguides and conventional waveguides to show that the decay constant in the cladding is dramatically larger in e-skid waveguides, which is the origin of sub-diffraction confinement. We also propose an approach to verify the reduced photonic skin depth in experiment using the decrease in the Goos-Hanschen phase shift. Finally, we provide a generalization of our work using concepts of transformation optics where the photonic-skin depth engineering can be interpreted as a transformation on the momentum of evanescent waves.

Properties of various core-shell silicon nanowires are investigated by extensive first-principles calculations on the geometric optimization as well as electronic band structures of the nanowires by using pseudopotential plane-wave method based on the density-functional theory. We show that different geometrical structures of silicon nanowires with various core compositions, formed by stacking of atomic polygons with pentagonal or hexagonal cross sections perpendicular to the wire axis, can be stabilized by doping with various types of semiconductor (Si, Ge), nonmetal (C), simple metal (Al), and transition metal (TM), $3d$ (Ti, Cr, Fe, Co, Ni, Cu), $4d$ (Nb, Mo, Pd, Ag), and $5d$ (Ta, W, Pt, Au), core atoms. Dopant atoms are fastened to a linear chain perpendicular to the planes of Si-shell atoms and are located through the center of planes. According to the stability and energetics analysis of core-shell Si nanowires, the eclipsed pentagonal and hexagonal structures are energetically more stable than the staggered ones. Electronic band structure calculations show that the pentagonal and hexagonal Si-shell nanowires doped with various different types of core atoms exhibit metallic behavior. Magnetic ground state is checked by means of spin-polarized calculations for all of the wire structures. The eclipsed hexagonal structure of Si-shell nanowire doped with Fe atom at the core has highest local magnetic moment among the magnetic wire structures. Electronic properties based on band structures of Si-shell nanowires with different dopant elements are discussed to provide guidance to experimental efforts for silicon-based spintronic devices and other nanoelectronic applications.

Silicon nanowires (SNWs) have recently attracted a lot of interest due to its explosive potential for nanoelectronic applications. To materialize these applications, the key role of thermal oxidation behavior of SNWs must be thoroughly understood. In the first half of the paper we review the results on SNW oxidation published up to date. It is commonly observed that SNW oxidation shows a retarded oxidation rate and stronger temperature dependence compared to those of planar Si; in addition, the SNW oxide thickness is also a strong function of SNW diameter. In the second half, we report our results on the SNW oxidation using an in-situ steam generation (ISSG) approach. ISSG oxidation is applied to oxidize the SNWs generated by a top-down lithography-etch approach, as an important technique to reduce the SNW diameter below 10nm. It is found that SNW oxidation with ISSG also exhibits a retared oxidation compared to that of planar Si using the same ISSG process, and a less dependance on SNW sizes than furnace SNW oxidation.

Molybdenum disulfide (MoS2) has gained significant attention as a promising catalyst for hydrogen evolution reaction (HER). The catalytic performance of MoS2 can be enhanced by either altering its structure or regulating external conditions. In this study, a novel approach combining the introduction of sulfur vacancy (VS) and biaxial tensile strain to create more active sites and modulate the band structure of monolayer MoS2 is proposed. To achieve the desired strain level, nano-cones (NCs) array substrates facilely fabricated by dip-pen nanolithography (DPN) are employed. The magnitude of the applied tensile strain can be finely tuned via adjusting the height of the NCs. Furthermore, on-chip electrochemical devices are constructed based on artificial structures, enabling precise optimization of HER performance of MoS2 through the synergistic effect of VS and strain. Combined with the d-band theory, it reveals that the HER properties of VS-MoS2 are highly dependent on the degree of tensile strain. This study presents a promising avenue for the design and preparation of high-performance 2D catalysts for energy conversion and storage applications.

High harmonic generation (HHG) enables the production of coherent light in the extreme-ultraviolet (XUV) spectral region. However, the radiation conversion efficiency from infrared to the XUV is low. We observed enhancement of HHG in gas mixtures of H2 and Ne compared to pure gases H2 or Ne. Our experiments show that at an intensity of ∼1.5×1014 W/cm2 and in mixtures of gases with significantly different ionization potentials (IPs), the initial HHG takes place in the component with lower IP (H2), inducing excited states and facilitating ionization and HHG in the component with higher IP (Ne). We show that the combination of the two gas components results in an enhancement or suppression of the output of high harmonics as compared to pure gases, which is also confirmed by the employed phenomenological model.