All Highlights

An important challenge in the field of three-dimensional ultrafast laser processing is to achieve in the bulk structuring of silicon and narrow gap materials. Recent attempts by increasing the energy of infrared ultrashort pulses have simply failed. Here, we establish that it is because focusing with a maximum numerical aperture of about 1.5 with conventional schemes does not allow overcoming strong nonlinear and plasma effects before the focus. We circumvent this limitation by exploiting solid-immersion focusing, in analogy to techniques applied in advanced microscopy and lithography. By creating in a perfect spherical sample the conditions for an interaction with an extreme numerical aperture near 3, repeatable femtosecond optical breakdown and controllable refractive index modifications are achieved for the first time inside silicon. Our findings re-open the horizon towards the direct writing of 3D monolithic devices for silicon photonics. It also provides perspectives for new strong-field physics and warm-dense-matter plasma experiments.

More information can be found in: Chanal, M, Fedorov, VY, Chambonneau, M, Clady, R, Tzortzakis, S, Grojo, D. "Crossing the threshold of ultrafast laser writing in bulk silicon", NATURE COMMUNICATIONS 8, 773 (2017).

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We demonstrate both theoretically and experimentally that the harmonics from abruptly autofocusing ring-Airy beams present a surprising property: They preserve the phase distribution of the fundamental beam. Consequently, this "phase memory" imparts to the harmonics the abrupt autofocusing behavior, while, under certain conditions, their foci coincide in space with the one of the fundamental. Experiments agree well with our theoretical estimates and detailed numerical calculations. Our findings open the way for the use of such beams and their harmonics in strong field science.

More information can be found in: Anastasios D. Koulouklidis, Dimitris G. Papazoglou, Vladimir Yu. Fedorov, and Stelios Tzortzakis, "Phase Memory Preserving Harmonics from Abruptly Autofocusing Beams", Phys. Rev. Lett. 119, 223901 (2017).

[Editor’s Suggestion: Physical Review Letters]

Generation and application of energetic, broad-band terahertz pulses is an active and contemporary area of research. The main thrust is towards the development of efficient sources with minimum complexities – a true table-top setup. In this work, we demonstrate the generation of THz radiation via ultrashort pulse induced filamentation in liquids – a counterintuitive observation due to their large absorption coefficient in the THz regime. The generated THz energy is more than an order of magnitude higher than that obtained from the two-color filamentation of air (the most standard table-top technique). Such high THz energies would generate electric fields of the order of MV/cm, which opens the doors for various non-linear THz spectroscopic applications. The counterintuitive phenomenon has been explained via the solution of non-linear pulse propagation equation in the liquid medium.

More information can be found in: I. Dey, K. Jana, V. Y. Fedorov, A. D. Koulouklidis, A. Mondal, M. Shaikh, D. Sarkar, A. D. Lad, S. Tzortzakis, A. Couairon, and G. R. Kumar, "Highly efficient broadband terahertz generation from ultrashort laser filamentation in liquids", Nature Communications, 8, 1184 (2017).

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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).

The shape control of nanoparticles constitutes one of the main challenges in today's nanotechnology. The synthetic procedures are based on trial-and-error methods and are difficult to rationalize as many ingredients are typically used. For instance, concave nanoparticles exhibiting high-index facets can be obtained from Pt with different HCl treatments. These structures present exceptional capacities when are employed as catalysts in electrochemical processes, as they maximize the activity per mass unit of the expensive material. Here we show how atomistic simulations based on density functional theory that take into account the environment can predict the morphology for the nanostructures and how it is even possible to address the appearance of concave structures. To describe the control by etching, we have reformulated the Wulff construction through the use of a geometric model that leads to concave polyhedra, which have a larger surface-to-volume ratio compared to that for nanocubes. Such an increase makes these sorts of nanoparticles excellent candidates to improve electrocatalytic performance.

More information can be found in: Qiang Li, Marcos Rellán-Piñeiro, Neyvis Almora-Barrios, Miquel Garcia-Ratés, Ioannis N. Remediakis, Núria López, "Shape control in concave metal nanoparticles by etching", Nanoscale 9, 13089-13094 (2017).

We demonstrate that multiply coupled spinor polariton condensates can be optically tuned through a sequence of spin-ordered phases by changing the coupling strength between nearest neighbors. For closed four-condensate chains these phases span from ferromagnetic (FM) to antiferromagnetic (AFM), separated by an unexpected crossover phase. This crossover phase is composed of alternating FM-AFM bonds. For larger eight-condensate chains, we show the critical role of spatial inhomogeneities and demonstrate a scheme to overcome them and prepare any desired spin state. Our observations thus demonstrate a fully controllable nonequilibrium spin lattice.

More information can be found in: H. Ohadi, A. J. Ramsay, H. Sigurdsson, Y. del Valle-Inclan Redondo, S. I. Tsintzos, Z. Hatzopoulos, T. C. H. Liew, I. A. Shelykh, Y. G. Rubo, P. G. Savvidis, and J. J. Baumberg, "Spin Order and Phase Transitions in Chains of Polariton Condensates", Phys. Rev. Lett. 119, 067401 (2017).

Polariton lasers are coherent light sources based on the condensation of exciton-polaritons in semiconductor microcavities, which occurs either in the kinetic or thermodynamic (Bose-Einstein) regime. Besides their fundamental interest, polariton lasers have the potential of extremely low operating thresholds. Here, we demonstrate ultra-low threshold polariton lasing at room temperature, using an all-dielectric, GaN membrane-based microcavity, with a spontaneously-formed zero-dimensional trap. The microcavity is fabricated using an innovative method, which involves photo-electrochemical etching of an InGaN sacrificial layer and allows for the incorporation of optimally-grown GaN active quantum wells inside a cavity with atomically-smooth surfaces. The resulting structure presents near-theoretical Q-factors and pronounced strong-coupling effects, with a record-high Rabi splitting of 64 meV at room-temperature. Polariton lasing is observed at threshold carrier densities 2.5 orders of magnitude lower than the exciton saturation density. Above threshold, angle-resolved emission spectra reveal an ordered pattern in k-space, attributed to polariton condensation at discrete levels of a single confinement site. This confinement mechanism along with the high material and optical quality of the microcavity, accounts for the enhanced performance of our polariton laser, and pave the way for further developments in the area of robust room temperature polaritonic devices.

From: R. Jayaprakash, F. G. Kalaitzakis, G. Christmann, K. Tsagaraki, M. Hocevar, B. Gayral, E. Monroy & N. T. Pelekanos "Ultra-low threshold polariton lasing at room temperature in a GaN membrane microcavity with a zero-dimensional trap", SCIENTIFIC REPORTS 7, Article number: 5542 (2017).