F. Siegrist, A. Sommer, M. Schröder, Tobias Boolakee, K. Golyari, F. Krausz, M. Schultze
{"title":"Attosecond energy transfer dynamics in band-gap materials (Conference Presentation)","authors":"F. Siegrist, A. Sommer, M. Schröder, Tobias Boolakee, K. Golyari, F. Krausz, M. Schultze","doi":"10.1117/12.2316306","DOIUrl":"https://doi.org/10.1117/12.2316306","url":null,"abstract":"","PeriodicalId":287901,"journal":{"name":"Advances in Ultrafast Condensed Phase Physics","volume":"7 5","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2018-05-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"121015861","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
D. Franz, R. Nicolas, W. Boutu, Liping Shi, Q. Ripault, M. Kholodtsova, B. Iwan, Ugaitz Elu Etxano, M. Kovacev, J. Biegert, H. Merdji
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)
{"title":"Amplification of solid high harmonics in semiconductor nanostructures (Conference Presentation)","authors":"D. Franz, R. Nicolas, W. Boutu, Liping Shi, Q. Ripault, M. Kholodtsova, B. Iwan, Ugaitz Elu Etxano, M. Kovacev, J. Biegert, H. Merdji","doi":"10.1117/12.2306896","DOIUrl":"https://doi.org/10.1117/12.2306896","url":null,"abstract":"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]. \u0000\u0000[1] Vampa et al., Nat. Phys. 13, 659–662 (2017). \u0000[2] Franz et al., arXiv:1709.09153 [physics.optics] (2017)","PeriodicalId":287901,"journal":{"name":"Advances in Ultrafast Condensed Phase Physics","volume":"3 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2018-05-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"122685772","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"The optical conductivity of dielectrics after ultrafast multiphoton excitation (Conference Presentation)","authors":"V. Yakovlev, Michael S. Wismer","doi":"10.1117/12.2309294","DOIUrl":"https://doi.org/10.1117/12.2309294","url":null,"abstract":"","PeriodicalId":287901,"journal":{"name":"Advances in Ultrafast Condensed Phase Physics","volume":"36 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2018-05-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"115084796","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
H. Lakhotia, M. Zhan, Hee-Yong Kim, E. Goulielmakis
{"title":"Real space approach to high harmonic generation in solids (Conference Presentation)","authors":"H. Lakhotia, M. Zhan, Hee-Yong Kim, E. Goulielmakis","doi":"10.1117/12.2309921","DOIUrl":"https://doi.org/10.1117/12.2309921","url":null,"abstract":"","PeriodicalId":287901,"journal":{"name":"Advances in Ultrafast Condensed Phase Physics","volume":"34 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2018-05-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"128232973","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
J. Gaudin, I. Papagiannouli, V. Blanchet, D. Descamps, C. Fourment, S. Petit, J. Raty, N. Bernier, P. Noé, J. Dory
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. Noe 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 publis
{"title":"Ultrafast dynamics of phase change material probed by frequency domain interferometry (Conference Presentation)","authors":"J. Gaudin, I. Papagiannouli, V. Blanchet, D. Descamps, C. Fourment, S. Petit, J. Raty, N. Bernier, P. Noé, J. Dory","doi":"10.1117/12.2314987","DOIUrl":"https://doi.org/10.1117/12.2314987","url":null,"abstract":"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.\u0000\u0000Frequency 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.\u0000The 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.\u0000We 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.\u0000\u0000[1] P. Noe 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 publis","PeriodicalId":287901,"journal":{"name":"Advances in Ultrafast Condensed Phase Physics","volume":"1 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2018-05-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"130701546","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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.
{"title":"Contrasted dynamcis in the carrier relaxation in wide band gap oxides (Conference Presentation)","authors":"S. Guizard","doi":"10.1117/12.2307845","DOIUrl":"https://doi.org/10.1117/12.2307845","url":null,"abstract":"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. \u0000Two results concerning material which are extremely important for numerous application, namely silica (SiO2) and sapphire (Al2O3), and using different techniques, will be presented.\u0000First, 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. \u0000In 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.","PeriodicalId":287901,"journal":{"name":"Advances in Ultrafast Condensed Phase Physics","volume":"1 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2018-05-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"128734391","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
S. Kaassamani, R. Nicolas, D. Gauthier, D. Franz, W. Boutu, H. Merdji
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.
石墨烯是一种非凡的材料，单层碳原子以蜂窝状结构结合在一起，具有非凡的电子和光电子特性;如零带隙能、高电子迁移率和超高机械强度。石墨烯的电子特性会导致非线性光学过程，如高谐波的产生。在这里，我们研究了几种石墨烯结构中的高谐波产生。我们首次报道了在石英衬底上单层石墨烯中谐波产生的观察。我们测量了一个中红外飞秒激光器的九次谐波(233nm波长)，该激光器的波长为2.1µm，脉冲能量约为6 nJ，脉冲持续时间为85 fs，重复频率为18 MHz。我们的发现证实了最近的观察结果。然后，我们首次报道了在TEM网格上支持的独立石墨烯的谐波观测。独立石墨烯与基底上的石墨烯表现不同;这主要是由于其与衬底之间缺乏相互作用，从而改变了其带隙。我们将介绍高谐波产生与激光偏振、强度的关系的主要趋势，以及对损伤问题的研究Yoshikawa et al.， Science 356, 736 - 738 (2017)[2] Nicolas et al.提交。
{"title":"High harmonic generation in graphene (Conference Presentation)","authors":"S. Kaassamani, R. Nicolas, D. Gauthier, D. Franz, W. Boutu, H. Merdji","doi":"10.1117/12.2306789","DOIUrl":"https://doi.org/10.1117/12.2306789","url":null,"abstract":"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].\u0000[1] Yoshikawa et al., Science 356, 736_738 (2017)\u0000[2] Nicolas et al. submitted.","PeriodicalId":287901,"journal":{"name":"Advances in Ultrafast Condensed Phase Physics","volume":"77 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2018-05-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"114844284","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
S. Haacke, Li Liu, Edoardo Domenichini, P. Gros, X. Assfeld, A. Monari, Antonio Francés Monerris, M. Beley, Cristina Cebrián Ávila, Kévin Magra, M. Pastore
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
{"title":"Ultrafast excited state dynamics of NHC-Fe(II) complexes designed for light harvesting (Conference Presentation)","authors":"S. Haacke, Li Liu, Edoardo Domenichini, P. Gros, X. Assfeld, A. Monari, Antonio Francés Monerris, M. Beley, Cristina Cebrián Ávila, Kévin Magra, M. Pastore","doi":"10.1117/12.2307244","DOIUrl":"https://doi.org/10.1117/12.2307244","url":null,"abstract":"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. \u0000In 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.\u0000\u0000From 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. \u0000An 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. \u0000Despite 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","PeriodicalId":287901,"journal":{"name":"Advances in Ultrafast Condensed Phase Physics","volume":"39 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2018-05-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"126875216","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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.
{"title":"Ab-initio simulation for propagation of ultrashort laser pulse in solids (Conference Presentation)","authors":"M. Uemoto, K. Yabana","doi":"10.1117/12.2306727","DOIUrl":"https://doi.org/10.1117/12.2306727","url":null,"abstract":"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.\u0000\u0000We 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.\u0000\u0000We 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.\u0000\u0000As 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.","PeriodicalId":287901,"journal":{"name":"Advances in Ultrafast Condensed Phase Physics","volume":"9 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2018-05-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"133247689","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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