Proceedings Volume 10673

Advances in Ultrafast Condensed Phase Physics

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Proceedings Volume 10673

Advances in Ultrafast Condensed Phase Physics

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Volume Details

Date Published: 2 August 2018
Contents: 7 Sessions, 5 Papers, 14 Presentations
Conference: SPIE Photonics Europe 2018
Volume Number: 10673

Table of Contents

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Table of Contents

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  • Front Matter: Volume 10673
  • Novel Tools for Ultrafast Spectroscopy
  • Ultrafast Excitation Dynamics I
  • Ultrafast Excitation Dynamics II
  • Electron Dynamics in Nanosystems
  • High Harmonic Generation, Attosecond Physics in Solids
  • Ultrafast Excitation Dynamics III
Front Matter: Volume 10673
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Front Matter: Volume 10673
This PDF file contains the front matter associated with SPIE Proceedings Volume 10673, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.
Novel Tools for Ultrafast Spectroscopy
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Ultrafast dynamics of phase change material probed by frequency domain interferometry (Conference Presentation)
Jérôme Gaudin, Irene Papagiannouli, Valérie Blanchet, et al.
Chalcogenide Phase-Change Materials (PCMs), mainly GeSbTe-based alloys, have already been widely used for optical data storage in DVD-RAM or CD-RW. Thanks to their unique reversible and very fast amorphous to crystalline phase transition which is characterized by an uncommon huge change in optical and electrical properties, PCMs are now extensively studied aiming at developing innovative emerging non-volatile memories such as phase change random access memory (PCRAM) or storage class memories (SCM) in order to replace current dominant Flash memory technology [1]. The interaction of PCMs with a fs light pulse has attracted significant attention due to fundamental interest since the possible non-thermal amorphous↔crystal phase transition could be used as a process to drive the phase change above the thermal “speed limits” [2]. Our experiments address the investigation of ultra-fast phenomena of fundamentals laser-material interaction. Frequency domain interferometry (FDI) [3] is a pump-probe experiment that gives access to the variation of the refractive index of a material. A pump pulse (25 fs, 800 nm, 1kHz) is used to trigger a phase transition. The probe beam is made of two pulses (120 fs, 532 nm) delayed by 9 ps in our case which are focused on the pump/sample interaction point. The first probe pulse impinges the surface of the sample before the pump pulse, and is thus reflected on the unperturbed material, while the second one that arrives after the pump pulse, is reflected on the pump-heated material. Both pulses are then sent in a spectrometer where they interfere in the frequency domain. The intensity variation and phase shifts in the interference pattern (right image in the fig. 1) can be used to retrieve variations of the optical constant of the heated material. The interference pattern is simultaneously measured for the S ans P polarization independently. The samples are amorphous GeSbTe-based thin film deposited by magnetron sputtering in a 200 mm industrial deposition tool at in the LETI clean-rooms. A 10 nm thick SiN capping layer of hwas been coated deposited on top of the GST films in order to prevent surface oxidation. We will present the results obtained on prototypical PCMs thin films, i.e. Ge2Sb2Te5 and GeTe. Experiments have been conducted in the fluence range (from 17 to 31 mJ/cm2 ) allowing us to trigger the amorphous to crystal phase transition. Dynamics on the sub-ps time scale shows a very rapid switch mainly attributed to the real part of the refractive index. The polarisation resolved FDI permits to foster information on the behaviour of the surface. A clear phase shift is attributed to a contraction, in the nm range, and the sub-ps time scale. The results presented will be discussed and compared to on-going ab-initio simulations. [1] P. Noé et al., “Phase Change Materials for Non-Volatile Memory devices: From Technological Challenges to Materials Science Issues”, Topical Review in Semicond. Sci. Technol., to be published (2017). [2] D. Loke et al. “Breaking the Speed Limits of Phase-Change Memory” Science 336, 1566 (2012) [3] J.P. Geindre et al., “Frequency-domain interferometer for measuring the phase and amplitude of a femtosecond pulse probing a laser-produced plasma” Optics Letters 19, 1997 (1994).
Ultrafast Excitation Dynamics I
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Optically excited structural transition in atomic wires on surfaces at the quantum limit: a femtosecond ultrafast surface electron diffraction study
Michael Horn von Hoegen
Ultrafast electron diffraction is employed for study of structural dynamics at surfaces in the time domain. Experiments are performed in a pump probe setup with fs-laser excitation and subsequent probing through diffraction of a fs electron pulse at a temporal resolution of 350 fs. The system of interest is one atomic layer of indium atoms on a Si(111) surface: Through self-assembly In atomic wires and form which exhibits a Peierls-like, insulator to metal phase transition which can be driven non-thermally through a femtosecond-laser pulse. Through the transient intensity of the diffraction spots we observe the lifting of the Peierls transition and melting of a charge density wave in only 700 fs, heating of the surface in 6 ps, and formation of a metastable and supercooled phase which exists for nanoseconds.
Ultrafast charge cooling and carrier multiplication in semiconductor nanocrystals and superlattices
Aditya Kulkarni, Laurens D. A. Siebbeles
We studied charge carrier photogeneration, cooling, carrier multiplication (CM) and charge mobility and decay in: a) isolated PbSe nanocrystals in solution, b) films of PbSe nanocrystals coupled by organic ligands, and c) 2D percolative networks of epitaxially connected PbSe nanocrystals. The studies were performed using ultrafast pump-probe spectroscopy with optical or terahertz/microwave conductivity detection. The effects of electronic coupling between the nanocrystals on charge mobility were characterized by frequency-resolved microwave and terahertz photoconductivity measurements. Reducing the size of ligand molecules between nanocrystals in a film strongly increases the charge mobility. Direct connection of nanocrystals in a percolative network yielded a sum of electron and hole mobilities as high as 270±10 cm2V-1s-1. We found that a high mobility is essential for multiple electron-hole pairs formed via CM to escape from recombination. The coupling between the nanocrystals was found to strongly affect the competition between cooling of hot charges by phonon emission and CM. In percolative networks of connected nanocrystals CM is much more efficient than in films with ligands between the nanocrystals. In the e networks CM occurs in a step-like fashion with threshold near the minimum photon energy of twice the band gap.
Effect of size confinement in C exciton dynamics of few-layered MoS2 nano-sheets
Manobina Karmakar, Sayantan Bhattacharya, Subhrajit Mukherjee, et al.
We study transient absorption response of few-layered MoS2 nano-flakes in dispersion, mainly focusing on its high energy exciton (commonly known as C exciton). We use a simple sono-chemical exfoliation technique to obtain confined nano-crystals of MoS2 of average diameter 2 nm, inter-dispersed in the flakes and study the effect of quantum confinement on this layered semiconductor. We emphasize on the interplay between exciton bleaching and excited state absorption upon a blue-detuned pumping. The relaxation times for the exciton are found and for the nano-crystals the radiative relaxation process is found to be slower as compared to that of the nano-flakes.
Ultrafast Excitation Dynamics II
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Contrasted dynamcis in the carrier relaxation in wide band gap oxides (Conference Presentation)
Understanding the dynamics of excited carriers in wide band gap materials is a requirement to describe a broad range of physical mechanisms such as scintillator response, radiation induced damage of crystals, or laser-induced breakdown in optical materials and coatings. The difficulty arises from the competition between all the different relaxation channels: electron-phonon collisions, impact ionization, exciton and transient or permanent defects formation. Ultrashort laser pulses are ideal tool to investigate transparent materials since they allow to induce a large excitation density, and provide a temporal resolution high enough to track in real time the carrier relaxation. Two results concerning material which are extremely important for numerous application, namely silica (SiO2) and sapphire (Al2O3), and using different techniques, will be presented. First, in Al2O3, we have measured in a broad temporal range – from 30fs to 8 ns - the absorption induced by photo-excited carriers using time revolved absorption spectroscopy. By changing the intensity of the pump pulse, and thus the initial excitation density, we could measure the induced absorption on more than two orders of magnitude and demonstrate that the carrier relaxation dynamics exhibit a complex decay, and strongly depends on the initial density of excited carriers. We have developed a two steps model based on rate equations and taking into account the laser damping, which allows to fully reproduce the decay and the amplitude of the measured absorption. We demonstrate that in sapphire the electrons are mobile and can recombine with any hole. With this experiment and our modelling we can explain for instance the complex decay of luminescence observed when sapphire is irradiated with heavy ions or VUV photons. In SiO2, an important problem related to optical breakdown is the impact ionization which can lead to avalanche: electron excited by an intense laser can gain high kinetic energy in the conduction band and collide with valence electron (impact ionization) thus multiplying the excited carrier density. By using a sequence of double pump pulse we could control independently the two key parameters: plasma density and temperature. Under appropriate conditions, using time resolved interferometry as a probe, we could directly observe for the first time an electronic avalanche induced by a laser pulse. Again a complete modeling, using multiple rate equation and taking into account the laser propagation,; allow to completely describe the experimental results.
The optical conductivity of dielectrics after ultrafast multiphoton excitation (Conference Presentation)
An intense laser pulse can transiently turn a dielectric into a conducting medium by exciting electron-hole pairs. If the pulse is as short as a few optical cycles, and the excitation process is highly nonlinear, then injected charge carriers form a highly non-equilibrium state. We investigate the optical conductivity of such transient states. Understanding the electrical properties of these states is important to utilize a fast change of optical conductivity for ultrafast metrology. We find that the average effective mass of laser-excited electrons and holes significantly increases with the peak intensity of the laser pulse. Also, we find that the optical conductivity induced by an intense pulse is sensitive to its carrier-envelope phase. We performed simulations using our recently developed numerical model where the time-dependent Schrödinger equation (TDSE) or semiconductor Bloch equations (SBE) are solved in three spatial dimensions using the basis of Kohn-Sham orbitals [1]. As the input, our code uses band energies and momentum matrix elements obtained from DFT codes (Abinit, Wien2k, GPAW). The equations of motion are solved in the velocity gauge within the independent-particle approximation. Evaluating the polarization response, we benefit from our method for correcting artifacts typical to velocity-gauge simulations [2]. As an example, the average effective mass of charge carriers excited by a 750-nm 4-fs laser pulse in diamond increases by a factor of 1.7 as the amplitude of the laser pulse increases from 0.5 V/Å to 1.4 V/Å. For SiO2, this is a factor of 3.9. This result is important for interpreting pump-probe measurements designed to study the temporal dynamics of strong-field charge-carrier injection, where a probe pulse accelerates electrons and holes generated by an intense few-cycle pump pulse. Also, the significant increase of the effective mass for intense pulses must be taken into account when the Drude model is applied to describe excitation-induced changes of optical properties of dielectrics and semiconductors. The effects that we study two main origins: First, a stronger field populates a larger number of bands. Second, multiphoton and tunneling transitions driven by a strong field populate a large part of the Brillouin zone, especially if the amplitude of the vector potential is comparable to or exceeds the size of the Brillouin zone. REFERENCES [1] Wismer, M.S., M.I. Stockman, V.S. Yakovlev. Ultrafast optical Faraday effect in transparent solids. arXiv:1612.08433 [cond-mat.other], 2016. [2] V. S. Yakovlev, M. Wismer. Adiabatic corrections for velocity-gauge simulations of electron dynamics in periodic potentials. Comp. Phys. Comm. 217, 82 (2017).
Attosecond energy transfer dynamics in band-gap materials (Conference Presentation)
Florian Siegrist, Annkatrin Sommer, Malte Schröder, et al.
If the energy of a photon exceeds the band-gap of a material, an electron can be promoted from the valence into the conduction band with a probability described by the linear absorption coefficient. For photons with energies below the bandgap, the medium is transparent. However, even large band-gap materials that are transparent for visible light in the linear regime can become absorptive in the presence of strong electric fields either through multi-photon, or via tunneling transitions. To study the intricate dynamics of these excitation pathways we developed Attosecond Polarization Sampling (APS). This method resolves the light-matter energy transfer dynamics with attosecond temporal resolution and allows real-time recording of the evolution of both the linear and nonlinear polarization wave driven by the external electric field inside a material. In APS, the laser electric field E(t) is recorded after passage through the sample in two different settings: First, we attenuate the laser pulses before sending them through the material (here: 10 μm fused silica (SiO2)), ensuring only linear effects occur. In the second step, we use intensities close to the damage threshold of SiO2 so that both linear and non-linear interaction occurs. Comparison of the two electric fields yields slight differences in the instantaneous temporal phase of the pulse carrying information about the time evolution of the induced nonlinear polarization wave. The observed phase shifts record the field amplitude dependency of the nonlinear refractive index due to the Kerr-effect including its response time and can be directly related to the degree of light-field induced conduction band population. Consequently, these measurements, as shown in Fig. 1, allow to determine the amount of energy exchanged between light-field and sample with sub-optical-cycle time resolution. In the case of strong-field illumination of SiO2 we find that the nonlinear energy transfer has a strong reversible component occurring only during short time intervals around the field crests of the strong laser field and corresponding to the non-permanent transfer of electrons into conduction band-states. Build-up and immediate decay of this transient conduction band population are the result of a bi-directional energy transfer between field and matter and conform to the assumption of energy absorption from the field during the first half of the laser pulse and re-emission of energy into the same field within the duration of the few-cycle laser pulses. Most interestingly, we find settings where the transient energy exchange yields a significant modification of the optical and electronic material properties for femtosecond time intervals but no lasting energy transfer (i.e. residual conduction band population) is observable. This ultrafast, dissipation free switching of material properties might turn useful for a future all-optical, ultrafast and loss-free signal metrology.
Electron Dynamics in Nanosystems
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High harmonic generation in graphene (Conference Presentation)
Shatha Kaassamani, Rana Nicolas, David Gauthier, et al.
Graphene is a remarkable material, a monolayer of carbon atoms bonded together in a honeycomb structure that exhibits extraordinary electronic and optoelectronic properties; such as a zero band gap energy, high electron mobility and ultrahigh mechanical strength. The electronic properties of graphene can lead to nonlinear optical processes such as high harmonic generation. Here, we investigate high harmonic generation in several graphene configurations. We first report on the observation of harmonic generation in monolayer graphene on a quartz substrate. We measured up to the ninth harmonic (233 nm wavelength) from graphene of a mid-infrared femtosecond laser, whose wavelength is 2.1 µm, pulse energy around 6 nJ, pulse duration 85 fs, and repetition rate 18 MHz. Our findings confirm recent observations [1]. We then report for the first time on the observation of harmonics from free-standing graphene supported on TEM grids. Free-standing graphene, in contrast to graphene on a substrate behaves differently; mainly due to the lack of its interaction with the substrate which alters its band gap. We will present major trends of high harmonic generation dependence with laser polarization, intensity and a study on damages issues [2]. [1] Yoshikawa et al., Science 356, 736_738 (2017) [2] Nicolas et al. submitted.
Ultrafast carrier dynamics in AlGaN/GaN superlattices by time-dependent reflectivity measurements (Conference Presentation)
Felix Mahler, Klaus Reimann, Jens W. Tomm, et al.
Gallium-nitride-based structures have become more and more important in recent years. Especially InGaN/GaN multi-quantum well (MQW) structures are used for optoelectronic devices such as light emitting diodes and diode lasers in the blue and green spectral region and for detectors and power amplifiers. AlGaN/GaN-based structures have the potential to extend optoelectronics towards the ultraviolet spectral region. Thus, carrier dynamics in MQW structures and superlattices containing aluminum are of strong interest. The ultrafast processes of nonequilibrium carriers in such semiconductor superlattices are not yet fully understood. Therefore, we have investigated the carrier dynamics in Al0.18Ga0.82N/GaN superlattice samples by pump-probe measurements. The samples consist of 60 periods with 2 nm barriers and 3 nm quantum wells. A SiN coating prevents degradation effects during excitation [1]. In addition, GaN bulk material was measured. For the measurements, we used an Yb-based oscillator amplifier system (repetition rate 1 MHz) pumping an optical parametric amplifier, allowing second-harmonic wavelengths between 325 nm and 460 nm with a pulse length of 40 fs. Time-dependent pump-probe measurements at room temperature were performed in reflection because absorption in the GaN template between the superlattice and the substrate prevents transmission measurements. After interaction with the sample, the probe beam was spectrally resolved to determine transient spectra. In the measurement, carriers are excited by the pump laser pulse above the superlattice band gap energy. Two processes are involved in the ensuing intra-band relaxation: the first leads to the thermalization of carriers by carrier-carrier scattering, the second is the cooling of carriers by phonon scattering [3]. Due to the polar properties of GaN-based superlattices, one expects a much stronger coupling between electrons and optical phonons compared to GaAs-based systems. This should result in a much faster cooling process. The photo-excited carriers lead to band gap renormalization and therefore to an increase of the refractive index at energies below the band gap and to a decrease of the refractive index at energies above the band gap. These changes manifest themselves in the transient reflectivity measured in our pump-probe experiments. Exciting 240 meV above the superlattice band gap, we see a decrease in reflectivity of up to 4 percent at an excitation density of 580 µJ/cm2 per pulse, decaying with a time constant of 1.7 ps. Furthermore, carrier cooling rates in superlattices and in bulk materials are compared [3]. References [1] C. Netzel, J. Jenschke, F. Brunner, A. Knauer, and M. Weyers; J. Appl. Phys. 120, 095307 (2016) [2] Jagdeep Shah; Ultrafast Spectroscopy of Semiconductors and Semiconductor Nanostructures, Springer, Berlin (1996) [3] Y. Rosenwaks, M. C. Hanna, D. H. Levi, D. M. Szmyd, R. K. Ahrenkiel, and A. J. Nozik; Phys. Rev. B 48, 14675 (1993)
High Harmonic Generation, Attosecond Physics in Solids
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Amplification of solid high harmonics in semiconductor nanostructures (Conference Presentation)
Dominik Franz, Rana Nicolas, Willem Boutu, et al.
Nanoscale amplification of non-linear processes in solid-state devices opens novel applications in nano-electronics, nano-medicine or high energy conversion for example. Coupling few nano-joules laser energy at a nanometer scale for strong field physics is demonstrated. We report enhancement of high harmonic generation in nano-structured semiconductors using nanoscale amplification of a mid-infrared laser in the sample rather than using large laser amplifier systems. Field amplification is achieved through light confinement in nano-structured semiconductor 3D waveguides. The high harmonic nano-converter consists of an array of zinc-oxide nanocones. They exhibit a large amplification volume, 6 orders of magnitude larger than previously reported [1] and avoid melting observed in metallic plasmonic structures. The amplification of high harmonics is observed by coupling only 5-10 nano-joules of a 3.2 µm high repetition-rate OPCPA laser at the entrance of each nanocone. Harmonic amplification (factor 30) depends on the laser energy input, wavelength and nanocone geometry [2]. [1] Vampa et al., Nat. Phys. 13, 659–662 (2017). [2] Franz et al., arXiv:1709.09153 [physics.optics] (2017)
Real space approach to high harmonic generation in solids (Conference Presentation)
Harshit Lakhotia, Minjie Zhan, Hee-Yong Kim, et al.
The microscopic electronic arrangement and charge densities defines the physical, optical and chemical properties of the material. The scientific advancement in understanding the arrangement of matter has led to better understanding of both chemical as well as physical aspects of materials. For the bulk periodic or crystalline materials X-ray or electron beams can be used to decipher the crystalline arrangement and thereby map the electron density. However, even with these techniques at hand, the direct visualization of the valence electron densities inside solids remains an insurmountable challenge. Currently the understanding of high harmonic generation(HHG) in solids is mostly limited to dynamic of electron hole pair on the conduction and valence bands of solids however to gain insight into valence charge densities this process needs to explained with real space motion of electrons. Recent efforts in 2D materials have made significant steps in this direction but a sufficiently complete real-space picture is still lacking. We present a real space approach to high harmonic generation in solids in strong field regime. We show that in strong field regime the motion of electron in presence of laser field is mostly governed by it and periodic potential acts as a perturbation to the potential. Solving for equation of motion in presence of these two fields we show the dependence of emitted high harmonic radiation on laser field and potential. All the experimental observations such as cut-off dependence, wavelength dependence, orientation dependence and dependence of emitted intensity on input electric field reconcile with the theoretical prediction, hence bolstering our model. We also predict that such model can be used to reconstruct the valence electron densities for solids.
Ultrafast Excitation Dynamics III
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Attosecond electron pulse trains and applications to time-resolved diffraction and microscopy
Yuya Morimoto, Peter Baum
Attosecond spectroscopy with laser-generated photons can in principle resolve electronic processes in real time, but a movie-like space–time imaging is impeded by the wavelength, which is ~100 times longer than atomic distances. Here we advance attosecond science to sub-atomic spatial resolution by using sub-relativistic electron beams instead of the high-harmonic photons. A beam of 70-keV electrons at 4.5-pm de Broglie wavelength is temporally modulated by the electric field of laser cycles into a train of attosecond pulses with the help of a dielectric modulation element. The pulses in the train have 820-as duration and maintain the degree of coherence of the original electron beam. We demonstrate the feasibility of analytic attosecond–Angstrom imaging by recording time-resolved Bragg diffraction from a singlecrystalline silicon. Real-space electron microscopy with the attosecond electron pulses visualizes the propagation of optical waves at a dielectric membrane with sub-wavelength and sub-optical-cycle resolution. This unification of attosecond science with electron diffraction/microscopy will enable the direct visualization of fundamental and complex light-matter interaction in space and time.
Ultrafast excited state dynamics of NHC-Fe(II) complexes designed for light harvesting (Conference Presentation)
Stefan Haacke, Li Liu, Edoardo Domenichini, et al.
The development of renewable energy sources is one of the biggest challenges in the 21st century. Within this context, great efforts are spent to develop new materials for cheaper and sustainable solar energy conversion schemes. Commercial dye-sensitized solar cells (DSSCs) are based on Ru(II) transition metal complexes as photo-sensitizers. But, ruthenium is rare and expensive, hence iron, abundant and cheap, is a good candidate to replace it. However, Fe(II) complexes are notorious for their ultrafast excited state spin crossover (SCO) into low-energy quintuplet states (5T2), cutting short on their use for light-harvesting applications relying on photo-sensitization. Very recently, it was shown that SCO can be avoided in Fe(II) complexes featuring N-heterocyclic carbene (NHC) ligands [1], and excited state lifetimes up to 26 ps were reported [2], making these complexes promising photo-sensitizers in DSSCs or photo-catalytic applications. In this work, the effect of structural parameters and variations of the proto-typical octahedric Fe(II)-NHC complexes and up to ten different variants thereof were investigated by femtosecond transient absorption and picosecond fluorescence spectroscopy at room temperature in order to understand which structural and electronic factors contribute to increasing the excited state metal-to-ligand charge transfer state (3MLCT) lifetime. From an energetic perspective, the aim of the chemical design is to increase the ligand field splitting so as to have the 5T2 state higher in energy than 3MLCT. The use of the strong -donating character of the carbene ligands led to a breakthrough in this respect. The experiments show that at minimum three carbene bonds are required to prevent SCO. Their hybridization with the metal-centered orbitals is optimal when the octahedral symmetry of the six coordinating Fe(II) bonds is respected. Bidentate ligands preserving the octahedral geometry are thus expected to induce a larger ligand field per carbene bond than tridentate ones, with smaller bite angles. We show indeed that three carbene bonds in bidentate ligands lead to the same 3MLCT lifetime as four carbene bonds in tridentate moieties. An increased conjugation across the organic ligands is also beneficial since it lowers the 3MLCT energy. We made use of this effect in several complexes with increasing electron accepting character of the ligands, leading for the record lifetime complex (26 ps) to the theoretical prediction of the 3MLCT state being lower in energy than 5T2 [3]. However, since the 5T2 requires a significant bond lengthening [4], a possible effect of the ligand substitutions on the structural rigidity of Fe-C bonds cannot be excluded. Despite the successful development of these complexes displaying sufficiently long excited state lifetimes, DSSCs turn out to have very low power conversion efficiency (<0.5 %) [3]. While charge recombination was identified as a potential drawback of the present chemical design [5], our latest experiments seem to indicate that the grafting and electronic coupling mechanisms to TiO2 surfaces is less effective than for comparable Ru complexes. The project is funded by the French ANR programme (ANR-16-CE07-0013-02). References: [1] Liu, Y.; Harlang T.; Canton, S.; Chabera, S.; Suarez Alcantara, K., Fleckhaus, Göransson, E.; Corani, A.; Lomoth, R.; Sundström, V.; Wärnmark, K. Chem. Comm. 2013, 6412-6414. [2] Duchanois, T.; Etienne, T.; Cebrián, C.; Liu, L.; Monari, A.; Beley, M.; Assfeld, X.; Haacke, S.; Gros, P. C. Eur. J. Inorg. Chem., 2015, 2469-2477 [3] Liu, L.; Duchanois, T.; Etienne, T.; Monari, A.; Beley, M.; Assfeld, X.; Haacke, S.; Gros, P. C., Phy. Chem. Chem. Phys. 2016, 18, 12550-12556. [4] Fredin, L.A.; Pápai, M.; Rozsályi, E.; Vankó, G.; Wärnmark, K.; Sundström, V.; Persson, P. J. Phys. Chem. Lett. 2014, 5, 2066−2071.
Ab-initio simulation for propagation of ultrashort laser pulse in solids (Conference Presentation)
Nowadays ab-initio calculations are recognized as an essential and indispensable tool in materials science. Although density functional theory has been widely used, it is a theory for electronic ground states. To describe electronic excitations and dynamics, time-dependent density functional theory (TDDFT) has been developed. Solving the time-dependent Kohn-Sham equation, the basic equation of the TDDFT, in real time, it has been possible to explore ultrafast electron dynamics induced by ultrashort laser pulses with typical resolutions of 0.02 nm in space and 1 as in time. We are developing a novel ab-initio simulation method to describe a propagation of ultrashort laser pulses in a bulk medium based on the TDDFT. A key innovation in our simulation method is the multiscale combination of simulations in two different scales, electromagnetic field analysis for the propagation of pulsed light and the TDDFT calculation for the electron dynamics in atomic scale induced by the pulsed light. Our method allows us to describe interactions between an ultrashort laser pulse and bulk materials without any empirical parameters, in particular the energy transfer from the pulsed light to electrons in the medium. The energy transfer is significant in practical usages of the pulsed light, for example, to understand the initial stage of non-thermal laser processing. Our method provides a useful platform of numerical experiments that faithfully describe optical experiments such as pump-probe measurements. We believe that the simulation method will contribute much to progresses in wide fields of optical sciences. We apply the method to interactions between an intense and ultrashort pulsed light and nanoscale semiconducting materials: silicon nanofilms and silicon 3D nanostructures. Under the irradiation of the intense pulsed light, our calculations indicate that the optical properties of the silicon changes from insulator to metal, owing to the multi-photon carrier excitations. For a propagation of a pulsed light in silicon nanofilms, we solve a coupled problem of 1D-Maxwell equations for the electromagnetic fields of the pulsed light and 3D electron dynamics described by the time-dependent Kohn-Sham equation. Penetrating the silicon nanofilms, the waveform of the pulsed light is found to be modulated during the propagation in the film: suppression in the high intensity amplitude, distortion in the tail part, and so on. A collaboration with an experimental research group is ongoing on this subject. As 3D silicon nanostructures, we consider two cases: a nanospheres of about 500 nm diameter in which a focusing of pulsed light takes place, and a bowtie-shaped nanogap composed of square nanoblocks of about 400 nm side in which a near field enhancement is expected. For the strong intensity beam, the spatial distribution of the energy transfer is modulated by the carrier excitation induced by the focused light, and it decreases the lifetime of the light confinement.
Optoelectronic measurements of light fields by sub-cycle carrier injection in dielectrics (Conference Presentation)
Dmitry Zimin
We present a new technique to measure electric fields over a 1 PHz bandwidth spanning the infrared to ultraviolet with sub-femtosecond temporal resolution via the sub-cycle control of injected charge carriers in dielectric media. The fidelity of the reconstructed electric fields are benchmarked against attosecond streaking and electro-optic sampling and provide detailed information about the temporal evolution of the charge carrier density in materials exposed to strong laser fields. The resulting optoelectronic technique allows for many methods of attosecond physics to be applied to a compact, table-top measurement, without necessitating the generation of attosecond XUV pulses. In the measurement device, a pair of metal electrodes separated by a small gap are deposited on the surface of a dielectric. The size of the gap is adjusted to the focal spot of the laser. CEP stable few cycle laser pulses with central wavelength around 780nm are focused in the gap between electrodes to promote carriers from the valence band into the conduction band. Injected carriers are then spatially separated by the laser field being detected, creating an electric dipole. The screening effect from the electrodes generates a measurable current in an external circuit. We show that when one pulse is incident on the sample, the detected current signal reveals information about the compression of the optical pulse and it's CEP. When two cross-polarized optical pulses with precise time-delay are incident on the dielectric, so that first (strong) pulse injects the carriers, while second (weak) pulse drives them towards electrodes, the detected current records a waveform as their relative delay is modified. The recorded time-delay signal can be used to reconstruct the waveform of the drive arm with the detection bandwidth covering more than 1 PHz (mid-infrared to ultraviolet). An experimental comparison with conventional methods such as the electro-optical sampling and attosecond streaking was performed, which both verify its fidelity and provide new insight into carrier dynamics on the few femtosecond timescale. By using the new technique to measure nonlinear polarization and light-matter energy transfer in solids we demonstrate that the method can be used for attosecond physics experiments which were previously possible only with attosecond beamlines. In addition we show the presence and control of non-linear effects in the dielectric samples by changing sample orientation and energies of both (injection and drive) pulses. The new method allows the performance of attosecond measurements with the following advantages with respect to conventional methods: operation in ambient conditions (no vacuum attosecond beamline is required), compact and simple experimental setup, large detection bandwidth, large dynamic range of the detection, high signal to noise ratio, and an all-solid-state detection apparatus.