A major frontier in strong field laser physics and nonlinear optics is the interaction of powerful terahertz (THz) pulses with matter. A plethora of scientific challenges and applications are presently under study, like table-top electron acceleration, THz-enhanced attosecond pulse generation and strong electric and magnetic THz field interactions with matter. However, despite the rapid development of THz science during the last two decades, the majority of available table-top THz sources remain rather weak limiting the interactions of THz radiation with matter mostly in the realm of linear optics. In this work, using intense ultrashort mid-infrared laser pulses to drive laser beam filamentation in ambient air, we demonstrate generation of sub-millijoule single-cycle THz pulses with unprecedented THz conversion efficiency (>2%), exceeding by far any previously reported experimental values for plasma-based THz sources. Moreover, due to the large bandwidth of the generated THz radiation (∼20 THz), the peak THz electric and magnetic fields exceed the 100 MV/cm and 33 tesla, respectively. Based on the reported experimental findings and theoretical estimates, it is projected that soon multi-millijoule THz pulses with peak electric and magnetic fields in the gigavolt per centimeter and kilotesla level, respectively, will become available. Quasi-static ultrashort electric and magnetic bursts at these intensities will enable extreme nonlinear and relativistic science.
More information can be found in: A. D. Koulouklidis, C. Gollner, V. Shumakova, V. Y. Fedorov, A. Pugžlys, A. Baltuška, and S. Tzortzakis, "Observation of extremely efficient terahertz generation from mid-infrared two-color laser filaments." Nature Communications, 11, 292 (2020).
Exciton polaritons in high quality semiconductor microcavities can travel long macroscopic distances (> 100 µm) due to their ultralight effective mass. The polaritons are repelled from optically pumped exciton reservoirs where they are formed; however, their spatial dynamics is not as expected for pointlike particles. Instead we show polaritons emitted into waveguides travel orthogonally to the repulsive potential gradient and can only be explained if they are emitted as macroscopic delocalized quantum particles, even before they form Bose condensates.
More information can be found in: Peter Cristofolini, Z. Hatzopoulos, Pavlos G. Savvidis, and Jeremy J. Baumberg, "Generation of Quantized Polaritons below the Condensation Threshold." Phys. Rev. Lett., 121(6), 067401 (2018).
Photocatalytic water splitting for hydrogen production is an emerging and promising strategy for converting solar energy into chemical fuels. To that end, the development of robust and highly active semiconductor materials is of eminent importance in this field. Here, we demonstrate high-surface-area mesoporous networks comprising interconnected β-Ni(OH)2 modified CdS nanocrystals (NCs) as highly active and stable photocatalysts for hydrogen generation. Compared to single-component CdS assemblies, Ni-modified materials present a strong enhancement of photocatalytic performance for hydrogen evolution under visible light irradiation (λ ≥ 420 nm). By controlling the formation of β-Ni(OH)2 species, the mesoporous β-Ni(OH)2/CdS heterojunction networks at a 10 wt % Ni content reached an outstanding photocatalytic H2-evolution rate of 1.4 mmol h–1 at 20 °C (or ∼35 mmol g–1 h–1 mass activity), associated with an apparent quantum yield (QY) of 72% at 420 nm in a 5 M NaOH aqueous solution containing 10% v/v ethanol as sacrificial reagent. Mechanistic study with UV–vis/near-infrared, photoluminescence, and electrochemical impedance spectroscopy and photocatalytic performance evaluation reveals that the improved photocatalytic performance arises from the strong electronic coupling and charge-transferred states at the p–n β-Ni(OH)2/CdS heterojunctions. These β-Ni(OH)2 modified CdS mesoporous assemblies have important implications for renewable hydrogen generation technologies.
More information can be found in: Ioannis Vamvasakis, Ioannis T. Papadas, Theocharis Tzanoudakis, Charalampos Drivas, Stelios A. Choulis, Stella Kennou, and Gerasimos S. Armatas, "Visible-Light Photocatalytic H2 Production Activity of β-Ni(OH)2-Modified CdS Mesoporous Nanoheterojunction Networks." ACS Catal., 8, 8726-8738 (2018).
A primary limitation of the intensively researched polaritonic systems compared to their atomic counterparts for the study of strongly correlated phenomena and many-body physics is their relatively weak two-particle interactions compared to disorder. Here, we show how new opportunities to enhance such on-site interactions and nonlinearities arise by tuning the exciton-polariton dipole moment in electrically biased semiconductor microcavities incorporating wide quantum wells. The applied field results in a twofold enhancement of exciton-exciton interactions as well as more efficiently driving relaxation towards low energy polariton states, thus, reducing condensation threshold.
More information can be found in: S. I. Tsintzos, A. Tzimis, G. Stavrinidis, A. Trifonov, Z. Hatzopoulos, J. J. Baumberg, H. Ohadi, and P. G. Savvidis, "Electrical Tuning of Nonlinearities in Exciton-Polariton Condensates." Phys. Rev. Lett., 121(3), 037401 (2018).
In this study, we devise a facile polymer-assisted sol-gel chemical method to prepare highly porous, crystalline implanted SrTiO3 (STO) nanoparticles and demonstrate their performance for photocatalytic hydrogen generation from water. X-ray scattering, electron microscopy, and nitrogen physisorption data corroborate that the as-made catalysts comprise 100-nm-sized nanocuboid particles containing a highly internal porous structure (BET surface area ∼176 m2 g−1) with uniform mesopores (ca. 5.8 nm in diameter). Interestingly, a partial substitution of N and C for O is attained in STO lattice with this synthetic protocol, according to the elemental analysis, and infrared (IR) and X-ray photoelectron spectroscopy (XPS) studies. Compared to STO:C,N, the STO:C,N mesoporous decorated with Pt nanoparticles (ca. 3 nm) present unique attributes that allow for an impressive improvement of up to 74-fold in photocatalytic H2-production activity. By combining UV–vis/NIR optical absorption, photoluminescence, Raman and electrochemical impedance spectroscopy, we show that this improved performance arises from the unique nanostructure, which provides massive surface active sites, and the proper alignment of defect states and conduction band-edge position of the STO:C,N semiconductor with respect to the interband transitions of metal, which permit efficient plasmon-induced interfacial electron transfer between the Pt–STO:C,N junction.
More information can be found in: I. Tamiolakis, D. Liu, F.-X. Xiao, J. Xie, I.T. Papadas, T. Salim, B. Liu, Q. Zhang, S.A. Choulis & G.S. Armatas, "Mesoporous Implantable Pt/SrTiO3:C,N Nanocuboids Delivering Enhanced Photocatalytic H2-Production Activity via Plasmon-Induced Interfacial Electron Transfer." Appl. Catal. B: Environ., 236, 338–347 (2018).
Synthesis of high-performance and cyclic stable photocatalysts has been remaining a significant challenge. In this work, we report the synthesis of high-surface-area mesoporous networks of CoO NPs through a polymer-templating self-assembly method and demonstrate their potential application in the reductive detoxification of aqueous Cr(VI) solutions under UV and visible light irradiation. Electron microscopy images and N2 adsorption measurements corroborate the presence of a porous network of interconnected CoO NPs (ca. 18 nm in size) with large internal surface area (up to 134 m2 g−1) and narrow pore-size distribution (ca. 4.4–4.8 nm in diameter). Conjunction of optical absorption and electrochemical impendence spectroscopy results indicates that the band edge positions of constituent CoO NPs meet the electric potential requirements for reducing Cr(VI) and splitting water to oxygen. We show that mesoporous assemblies of hexagonal CoO NPs effectively overcome the kinetic barriers for the oxidation reaction, manifesting a remarkably photocatalytic Cr(VI) reduction activity at acidic pH with an apparent quantum yield (AQY) of 1.61% and 0.17% at wavelengths of 375 and 440 nm, respectively. We demonstrate that, apart from oxygen evolution reaction, photoconversion of harmful Cr(VI) into non-toxic Cr (III) involves also a hydroxyl radical-mediated oxidation process by intercepting oxidation products with on-line mass spectrometry and fluorescence spectroscopy in control catalytic experiments.
More information can be found in: G. Velegraki, J. Miao, Ch. Drivas, B. Liu, S. Kennou, G.S. Armatas, "Fabrication of 3D mesoporous networks of assembled CoO nanoparticles for efficient photocatalytic reduction of aqueous Cr(VI)", Appl. Catal. B: Environ. 221, 635-644 (2018).