Wide-bandgap Dielectrics Research Articles

Wavelength-Independent Performance of Femtosecond Laser Dielectric Ablation Spanning Over Three Octaves

Ultrafast laser breakdown of wide bandgap dielectrics is today a key for major technologies ranging from 3D material processing in optical materials to nanosurgery. However, a contradiction persists between the strongly nonlinear character of energy absorption and the robustness of processes to the changes of the bandgap/wavelength ratio depending on applications. While various materials and bandgaps have been studied, we concentrate here the investigations on the spectral domain with experiments performed with wavelength drivers varied from deep-ultraviolet (258 nm) to mid-infrared (3.5 $\mu$m). The measured fluence thresholds for single shot ablation in dielectrics using 200-fs pulses exhibit a plateau extending from the visible domain up to 3.5-$\mu$m wavelength. This is accompanied, after ablation crater analysis, by a remarkable invariance of the observed ablation precision and efficiency. Only at the shortest tested wavelength of 258 nm, a twofold decrease of the ablation threshold and significant changes of the machining depths are detected. This defines a lower spectral limit of the wavelength-independence of the ablation process. By comparison with simulations, avalanche ionization coefficients are extracted and compared with those predicted with the Drude model. This must be beneficial to improve predictive models and process engineering developments exploiting the new high-power ultrafast laser technologies emitting in various spectral domains.

Non-linear propagation effects of intense femtosecond pulses on below bandgap harmonics in solids

The non-linear propagation of the intense near-infrared (NIR) driving field in crystals poses a challenge and can offer an opportunity to control the spectral properties of harmonics in solids. Here, we have investigated the non-linear propagation effects in wide bandgap dielectrics such as Magnesium Oxide (MgO), Chromium (Cr) doped MgO (Cr: MgO), Sapphire (Sa) crystals, and fused silica (FS). Furthermore, we have generated second and third harmonics (TH) and measured the linear polarization dependence of harmonics in these thin solids to explore the non-linear response at a strong field. We observe spectral shifts and broadening of the driving field spectrum which is imprinted on the harmonics. We attribute these effects to the strong photoionization, generation of free-carrier density, and self-phase modulation effects. This work shows the sensitivity to control the spectral profile of harmonics by manipulating the driving field, showing the possibility of new tailored solid-state ultraviolet sources for optical diagnostics.

Features of coherent excitation of [formula omitted]Th

The paper studies the effects that arise at the coherent excitation of the anomalously low-lying isomeric level 3/2+(8.19±0.12 eV) in the 229Th nucleus driven by laser radiation in wide band gap dielectrics. It is shown that in such an excitation a) the appearance of additional decay channels with a width Γ of the 229mTh isomer due to electron states in the band gap leads to an effective “freezing” of the system in the ground state (the repopulation process at relatively small Γ and the Zeno effect at large Γ), making the process of nuclear excitation practically impossible, b) the probability of the α decay of a coherent superposition of the ground and isomeric states becomes equal to the half-sum of the α widths of both levels and is increased by about a factor of two in comparison with the probability of the α decay of 229Th, c) the intensity of the main α lines oscillates with the Rabi frequency and the total α spectrum shifts towards higher energies for long time measurements, d) the inverse population of nuclear levels required for the Thorium γ ray laser is achieved relatively quickly within the π pulse duration, e) the observation of the Zeno effect according to the Cook's scheme is possible for the system of three lower levels of the 229Th nucleus.

Polarization-dependent filamentation of femtosecond laser pulses in synthetic diamond

The filamentation process inside of the bulk of type IIa synthetic diamond with known crystallographic orientation has been studied as a function of the polarization state of the ultrashort laser pulses with a duration of 300 fs and a wavelength of 515 nm. The transmittance of the sample was measured using a photodiode, while the micro-image of the filament was recorded on the CMOS camera perpendicular to the propagation axis of the exciting laser radiation. The dependences of the transmittance and length of the filament on the polarization azimuth show a distinct modulation over the entire range of its change. Keywords: ultrashort laser pulses, synthetic diamond, photoluminescence, laser polarization, nonlinear absorption, wide-bandgap dielectrics.

Challenges of Overcoming Defects in Wide Bandgap Semiconductor Power Electronics

The role of crystal defects in wide bandgap semiconductors and dielectrics under extreme environments (high temperature, high electric and magnetic fields, intense radiation, and mechanical stresses) found in power electronics is reviewed. Understanding defects requires real-time in situ material characterization during material synthesis and when the material is subjected to extreme environmental stress. Wide bandgap semiconductor devices are reviewed from the point of view of the role of defects and their impact on performance. It is shown that the reduction of defects represents a fundamental breakthrough that will enable wide bandgap (WBG) semiconductors to reach full potential. The main emphasis of the present review is to understand defect dynamics in WBG semiconductor bulk and at interfaces during the material synthesis and when subjected to extreme environments. High-brightness X-rays from synchrotron sources and advanced electron microscopy techniques are used for atomic-level material probing to understand and optimize the genesis and movement of crystal defects during material synthesis and extreme environmental stress. Strongly linked multi-scale modeling provides a deeper understanding of defect formation and defect dynamics in extreme environments.

Attosecond synchronization of extreme ultraviolet high harmonics from crystals

The interaction of strong near-infrared (NIR) laser pulses with wide-bandgap dielectrics produces high harmonics in the extreme ultraviolet (XUV) wavelength range. These observations have opened up the possibility of attosecond metrology in solids, which would benefit from a precise measurement of the emission times of individual harmonics with respect to the NIR laser field. Here we show that, when high-harmonics are detected from the input surface of a magnesium oxide crystal, a bichromatic probing of the XUV emission shows a clear synchronization largely consistent with a semiclassical model of electron–hole recollisions in bulk solids. On the other hand, the bichromatic spectrogram of harmonics originating from the exit surface of the 200 μm-thick crystal is strongly modified, indicating the influence of laser field distortions during propagation. Our tracking of sub-cycle electron and hole re-collisions at XUV energies is relevant to the development of solid-state sources of attosecond pulses.

Ultrafast supercontinuum generation in bulk condensed media

Nonlinear propagation of intense femtosecond laser pulses in bulk transparent media leads to a specific propagation regime, termed femtosecond filamentation, which in turn produces dramatic spectral broadening, or superbroadening, termed supercontinuum generation. Femtosecond supercontinuum generation in transparent solids represents a compact, efficient and alignment-insensitive technique for generation of coherent broadband radiation at various parts of the optical spectrum, which finds numerous applications in diverse fields of modern ultrafast science. During recent years, this research field has reached a high level of maturity, both in understanding of the underlying physics and in achievement of exciting practical results. In this paper we overview the state-of-the-art femtosecond supercontinuum generation in various transparent solid-state media, ranging from wide-bandgap dielectrics to semiconductor materials and in various parts of the optical spectrum, from the ultraviolet to the mid-infrared spectral range. A particular emphasis is given to the most recent experimental developments: multioctave supercontinuum generation with pumping in the mid-infrared spectral range, spectral control, power and energy scaling of broadband radiation and the development of simple, flexible and robust pulse compression techniques, which deliver few optical cycle pulses and which could be readily implemented in a variety of modern ultrafast laser systems.

Laser amplification in excited dielectrics

Wide-bandgap dielectrics such as glasses or water are transparent at visible and infrared wavelengths. This changes when they are exposed to ultrashort and highly intense laser pulses. Different interaction mechanisms lead to the appearance of various transient nonlinear optical phenomena. Using these, the optical properties of dielectrics can be controlled from the transparent to the metal-like state. Here we expand this range by a yet unexplored mechanism in excited dielectrics: amplification. In a two-colour pump–probe experiment, we show that a 400 nm femtosecond laser pulse is coherently amplified inside an excited sapphire sample on a scale of a few micrometres. Simulations strongly support the proposed two-photon stimulated emission process, which is temporally and spatially controllable. Consequently, we expect applications in all fields that demand strongly localized amplification. Ultrashort high-intensity laser pulses change the properties of dielectrics in different ways. One unexpected outcome is light amplification in an excited dielectric, observed in a two-colour pump–probe experiment.

Conduction and valence band offsets of LaAl2O3 with (−201) β-Ga2O3

Wide bandgap dielectrics are needed as gate insulators and surface passivation layers on the emerging electronic oxide Ga2O3. X-ray photoelectron spectroscopy was used to determine the valence band offset at LaAl2O3 (LAO)/β-Ga2O3 heterointerfaces. LaAl2O3 was deposited by RF magnetron sputtering onto bulk Ga2O3 crystals. The bandgaps of the materials were determined by reflection electron energy loss spectroscopy to be 4.6 eV for Ga2O3 and 6.4 eV for LAO. The valence band offset was determined to be −0.21 ± 0.02 eV (staggered gap, type II alignment) for LAO on Ga2O3. This leads to a conduction band offset of 2.01 ± 0.60 eV for LaAO with Ga2O3. Thus, LAO provides excellent electron confinement but not hole confinement in LAO/Ga2O3 heterostructures.

Colloidal synthesis of Gd3+ doped ZrO2 based dielectrics and their structural and electrochemical property studies

Solution processed wide band gap dielectrics have nowadays started to receive renewed interest for practical application in semiconductor electronics. In this regard, undoped and gadolinium (Gd) doped zirconia (ZrO2) nanocrystallites were colloidally processed and their potential for dielectric applications has been demonstrated. X-ray diffraction measurements revealed the effective crystallization of nanostructures and the successful substitution of Gd ions into the cubic ZrO2 matrix. The particulate-like characteristics of undoped and Gd doped ZrO2 nanostructures were examined through the electron microscopes, which hardly revealed any difference among them. The optical band gap of ZrO2 nanostructures was determined to be around 4.64–4.80 eV from the absorbance measurements. The potential of Gd doped ZrO2 nanostructures for dielectric functions were evaluated through electrochemical impedance spectroscopic measurements. The improved capacitance values estimated from the Nyquist plots suggests the potential of the investigated materials for low power and low voltage electronic applications.

Constructive feedback for the growth of laser‐induced periodic surface structures

AbstractWe study the formation and growth of laser‐induced periodic surface structures (LIPSSs) with the finite‐difference time‐domain (FDTD) method. We use a recently proposed inter‐pulse feedback method to account for the evolution of the surface morphology between each laser pulse sent to the surface of the processed material. This method has been used with an ablation‐like mechanism, by removing material exposed to a light intensity higher than a given threshold.We propose an inverse mechanism, an expansion‐like mechanism, able to grow structures that the ablation‐like process cannot. This allows us to introduce the notions of constructive and destructive feedback and explains a strong contradiction between the standard Sipe‐Drude theory and the experimental observations, i.e. the formation on metals of structures usually linked to wide band gap dielectrics. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Extreme ultraviolet high-harmonic spectroscopy of solids.

Extreme ultraviolet (EUV) high-harmonic radiation emerging from laser-driven atoms, molecules or plasmas underlies powerful attosecond spectroscopy techniques and provides insight into fundamental structural and dynamic properties of matter. The advancement of these spectroscopy techniques to study strong-field electron dynamics in condensed matter calls for the generation and manipulation of EUV radiation in bulk solids, but this capability has remained beyond the reach of optical sciences. Recent experiments and theoretical predictions paved the way to strong-field physics in solids by demonstrating the generation and optical control of deep ultraviolet radiation in bulk semiconductors, driven by femtosecond mid-infrared fields or the coherent up-conversion of terahertz fields to multi-octave spectra in the mid-infrared and optical frequencies. Here we demonstrate that thin films of SiO2 exposed to intense, few-cycle to sub-cycle pulses give rise to wideband coherent EUV radiation extending in energy to about 40 electronvolts. Our study indicates the association of the emitted EUV radiation with intraband currents of multi-petahertz frequency, induced in the lowest conduction band of SiO2. To demonstrate the applicability of high-harmonic spectroscopy to solids, we exploit the EUV spectra to gain access to fine details of the energy dispersion profile of the conduction band that are as yet inaccessible by photoemission spectroscopy in wide-bandgap dielectrics. In addition, we use the EUV spectra to trace the attosecond control of the intraband electron motion induced by synthesized optical transients. Our work advances lightwave electronics in condensed matter into the realm of multi-petahertz frequencies and their attosecond control, and marks the advent of solid-state EUV photonics.

High Mobility Field-Effect Thin Film Transistor Using Room-Temperature High-<formula formulatype="inline"><tex Notation="TeX">$\kappa$</tex> </formula> Gate Dielectrics

This paper reports an InGaZnO thin-film transistor (TFT) that involves using fully room-temperature gate dielectrics on a flexible substrate. The wide bandgap dielectrics of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\hbox{HfO}}_{2}$</tex> </formula> and <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\hbox{Y}}_{2}{\hbox{O}}_{3}$</tex> </formula> exhibited favorable adhesion properties on a flexible substrate compared with conventional low- <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\kappa$</tex> </formula> <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\hbox{SiO}}_{2}$</tex> </formula> film. Based on the experimental results, the room-temperature <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\hbox{ IGZO/HfO}}_{2}$</tex></formula> TFTs demonstrated effective device integrity, and achieve a low drive voltage of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$&lt; {\hbox{2}}$</tex></formula> V, a low threshold voltage of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\hbox{0.46}} \pm {\hbox{006}}$</tex></formula> V, a low sub-threshold swing of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\hbox{110}} \pm {\hbox{6}}$</tex> </formula> mV/decade and an extremely high mobility of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">${\hbox{60.2}} \pm {\hbox{32}}$</tex></formula> cm <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^{2}/{\hbox{V}}\cdot{\hbox{s}}$</tex> </formula> . The excellent performance of this TFT indicated that it demonstrates considerable potential for active-matrix liquid crystal display applications requiring low power consumption and a high driving current.

Theoretical Studies on Multiphoton Absorption of Ultrashort Laser Pulses in Sapphire

We discuss in detail the multiphoton absorption theory for wide bandgap dielectrics that we have implemented in the framework of the time-dependent Nth order perturbation theory. In particular, we have carried out calculations on the N-photon absorption coefficient K N and interband transition rate W N for a perfect sapphire crystal (α-Al 2 O 3 ). Furthermore, we derive an expression on the laser-pulse induced seed electron density n 0 . Application of this theory on sapphire shows that n 0 , induced by the ultrashort laser pulse with pulse duration of 30 ps, wavelength of 1 μm, and peak intensity of 5 × 10 11 W/cm 2 at the focus point, cannot lead to multiphoton-based ablation. Indeed, our calculations show that the laser pulse in the N = 7 order absorption process generates a conduction electron density n 0 ≈ 6 × 10 13 cm -3 that is far too small compared with the required critical density n cr ≈ 10 19 - 10 21 cm -3 . This is in agreement with the extremely small value of the probability (P ind ≈ 2 × 10 -9 ) that we have estimated for a valence electron to be transferred to the conduction band under the influence of the abovementioned laser-pulse conditions. Therefore, we need to seek other mechanisms than multiphoton absorption, to explain ablation in sapphire. However, we show that by using larger photon energies and smaller order N values, it is possible to induce large enough multiphoton absorption excited electron densities, which will lead to ablation in sapphire, even for the abovementioned laser beam intensity. Finally, we discuss the Keldysh adiabatic parameter y and its interpretation in the case of our experimental pulse-laser setup.

Waferscale nanophotonic circuits made from diamond-on-insulator substrates

Wide bandgap dielectrics are attractive materials for the fabrication of photonic devices because they allow broadband optical operation and do not suffer from free-carrier absorption. Here we show that polycrystalline diamond thin films deposited by chemical vapor deposition provide a promising platform for the realization of large scale integrated photonic circuits. We present a full suite of photonic components required for the investigation of on-chip devices, including input grating couplers, millimeter long nanophotonic waveguides and microcavities. In microring resonators we measure loaded optical quality factors up to 11,000. Corresponding propagation loss of 5 dB/mm is also confirmed by measuring transmission through long waveguides.

Mechanisms of femtosecond laser ablation of dielectrics revealed by double pump–probe experiment

We study experimentally the electronic excitation mechanisms involved in the breakdown and ablation of wide band gap dielectrics. A femtosecond pump–probe interferometry technique, with 100 fs temporal resolution, allows measuring the modification of refractive index induced by ultra-short intense laser pulses. To get more information in the complex process of excitation and relaxation mechanisms involved during and after the interaction, we use a sequence of two excitation pulses: a first short pulse at 400 nm excites a controlled density of carriers, and a second one at 800 nm with variable pulse duration, from 50 fs to 10 ps, reaches an excited solid. In Al2O3, we show that the total density of carriers never exceeds the sum of the densities excited by the two pulses sent independently. This means that the second pulse deposits further energy in the material by heating the previously excited carriers, and that no electronic multiplication occurs. On the other hand, in SiO2, it is possible, under specific conditions, to observe an increase of carrier density due to impact ionization. All these results demonstrate that the avalanche process, which is often invoked in the laser breakdown literature, does not play a dominant role in optical breakdown induced by short pulses.

Modeling validity of femtosecond laser breakdown in wide bandgap dielectrics

In order to accurately calculate the femtosecond laser breakdown threshold of wide band-gap dielectrics using the photoionization and avalanche ionization theories, the applied range of these classical methods is demonstrated quantitatively by the comparison of the calculated results with the available experimental data. It is found that these standard theories to estimate laser ablation threshold are valid when the laser pulse duration is less than about 600fs and the laser wavelength is more than around 400nm. Besides, we demonstrate that the multi-photon ionization of wide band-gap dielectrics is the dominant breakdown mechanism when the laser wavelength is at the region of 0.4μm<λ<1.3μm. But the tunneling ionization mechanism performs a significant role when the laser wavelength is at the range of λ<1.3μm. Based on our calculation, we found that a valence band (VB) electron can absorb simultaneously the greatest number photons of ten in the multiphoton ionization process. Furthermore, it is revealed that the photoionization in dielectrics ionization process can provide seed electrons even at the pulse duration down to sub-10fs.

Nonradiative transition to the ground state and superelastic scattering of exited atoms upon impact on the surface of wide-bandgap dielectrics

The impact of excited cesium atoms on sapphire and glass surfaces have been experimentally studied. It is established that the probability of electron excitation quenching upon impact of an atom on the dielectric surface is close to unity. The velocity distribution of unexcited atoms upon scattering from the surface has been determined using the time-of-flight technique. The kinetic energies of most of these atoms are several tens of times greater than the energy of thermal motion of the excited atoms impinging on the surface. Conversion of the internal energy of atoms into their kinetic energy is explained in terms of nonradiative electron transitions with simultaneous excitation of lattice vibrations in the dielectric crystal. This mechanism of atomic excitation quenching near the dielectric surface explains the significant difference between the energies of atoms upon superelastic scattering and upon photodesorption from an adsorbed state.

Time resolved two‐colour VUV‐IR photoemission in wide band gap dielectrics

We have measured the relaxation of photo-excited carriers in different wide band gap dielectrics by using time resolved two-colour photoemission. Electrons are excited by an ultra-short pulse of VUV photons directly in the conduction band. They are then are re-excited by an IR probe pulse. The evolution of the photoemission spectra as a function of the pump-probe delay gives direct information on the relaxation of the population of excited electron. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Tunnelling in thin SiO 2

The conduction of electrons through ultra-thin insulators is accurately described by the Fowler-Nordheim mechanism. Electrons tunnel through the energy barrier at the cathode interface to the conduction band of the insulator and drift to the anode. The mechanism is the same as for field emission from cold cathodes in vacuum. In very thin oxides of 30-80 A, tunnelling is modulated by quantum-interference effects. Injected electrons form standing waves in the oxide conduction band interfering with their tunnelling probability. These quantum oscillations enable the determination of layer thicknesses to a high precision. Accompanying the tunnelling current, a ‘substrate-hole current’ is often observed by carrier separation techniques, but its origin is still in debate. We have now conducted experiments for several oxide layers between 30 and 70 A in which we show that the substrate-hole current coherently follows the quantum oscillations of the tunnel current. This new fact rules out the earlier explanations based on anode-hole tunnelling and valence-band electron tunnelling. Injection of electrons in wide band-gap dielectrics is by no means harmless. The energy loss of the electrons, especially at the anode interface, leads to degradation of the properties of the dielectric. This explains the generation of mesoscopic features, or ‘trees’, extending over several atomic distances. The formation of the ‘trees’ is modelled by computer simulation. The degradation of the dielectric leads to increased current leakage, negative or positive space-charge evolution and dielectric losses. Persisting charge transport renders strongly degraded insulators and finally causes dielectric breakdown.