Staff directory
Klaas-Jan Tielrooij
Senior Group Leader
klaas.tielrooij(ELIMINAR)@icn2.cat
Ultrafast Dynamics in Nanoscale Systems
- ORCID: 0000-0002-0055-6231
Publications
2024
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Long lived photogenerated charge carriers in few-layer transition metal dichalcogenides obtained from liquid phase exfoliation
F Morabito; Kevin Synnatschke; Jake D. Mehew; Sebin Varghese; Charles J. Sayers; Giulia Folpini; Annamaria Petrozza; Giulio Cerullo; Klaas‐Jan Tielrooij; Jonathan N. Coleman; Valeria Nicolosi; Christoph Gadermaier Nanoscale Advances; 6 (4): 1074 - 1083. 2024. 10.1039/d3na00862b. IF: 4.600
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Microscopic understanding of the in-plane thermal transport properties of 2H transition metal dichalcogenides
Roberta Farris; Olle Hellman; Zeila Zanolli; David Saleta Reig; Sebin Varghese; Pablo Ordejón; Klaas‐Jan Tielrooij; Matthieu J. Verstraete Physical Review. B./Physical Review. b; 109 (12) 2024. 10.1103/physrevb.109.125422.
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Spatiotemporal Microscopy: Shining Light on Transport Phenomena
Vazquez, Guillermo D Brinatti; Morganti, Giulia Lo Gerfo; Block, Alexander; van Hulst, Niek F; Liebel, Matz; Tielrooij, Klaas-Jan Advanced Electronic Materials; 2024. 10.1002/aelm.202300584. IF: 5.300
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Spatiotemporal Microscopy: Shining Light on Transport Phenomena (Adv. Electron. Mater. 2/2024)
Vazquez, GDB; Morganti, GL; Block, A; van Hulst, NF; Liebel, M; Tielrooij, KJ Advanced Electronic Materials; 10 (2) 2024. 10.1002/aelm.202470009. IF: 5.300
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Terahertz-to-Visible Light Conversion in Graphene Metamaterials Controlled by Electrostatic Gate
Igor Ilyakov; A. N. Ponomaryov; David Saleta Reig; Connor Murphy; Jake D. Mehew; Thales V. A. G. de Oliveira; Gulloo Lal Prajapati; Ammar Arshad; Jan‐Christoph Deinert; Monica F. Craciun; Saverio Russo; Sergey Kovalev; Klaas‐Jan Tielrooij Journal Of The Institute Of Electronics, Information And Communication Engineers; : 1 - 2. 2024. 10.1109/irmmw-thz60956.2024.10697761.
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Ultrafast Charge and Exciton Diffusion in Monolayer Films of 9-Armchair Graphene Nanoribbons
Varghese, Sebin; Mehew, Jake Dudley; Wang, Hai I; Wuttke, Michael; Zhou, Yazhou; Muellen, Klaus; Narita, Akimitsu; Cummings, Aron W; Tielrooij, Klaas-Jan Advanced Materials; 2024. 10.1002/adma.202407796. IF: 27.400
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Ultrafast Umklapp-assisted electron-phonon cooling in magic-angle twisted bilayer graphene
Mehew, Jake Dudley; Merino, Rafael Luque; Ishizuka, Hiroaki; Block, Alexander; Merida, Jaime Diez; Carlon, Andres Diez; Watanabe, Kenji; Taniguchi, Takashi; Levitov, Leonid S; Efetov, Dmitri K; Tielrooij, Klaas-Jan Science Advances; 10 (6): eadj1361. 2024. 10.1126/sciadv.adj1361. IF: 11.700
2023
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A pre-time-zero spatiotemporal microscopy technique for the ultrasensitive determination of the thermal diffusivity of thin films
Varghese, S; Mehew, JD; Block, A; Reig, DS; Wozniak, P; Farris, R; Zanolli, Z; Ordejon, P; Verstraete, MJ; van Hulst, NF; Tielrooij, KJ Review Of Scientific Instruments; 94 (3): 34903. 2023. 10.1063/5.0102855. IF: 1.600
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Efficient terahertz harmonic generation in topological metamaterials
Kovalev, S; Tielrooij, K; Ilyakov, I; Deinert, JC; de Oliveira, T; Ponomaryov, O; Principi, A; Reig, D; Block, A; Varghese, S; Schreyeck, S; Brunner, K; Karczewski, G; Carbonell, C; Valenzuela, S; Molenkamp, L; Kiessling, T; Astakhov, GV International Conference On Infrared, Millimeter, And Terahertz Waves, Irmmw-Thz; 2023. 10.1109/IRMMW-THz57677.2023.10298895.
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Electron cooling in graphene enhanced by plasmon-hydron resonance
Yu, XQ; Principi, A; Tielrooij, KJ; Bonn, M; Kavokine, N Nature Nanotechnology; 18 (8): 898 - +. 2023. 10.1038/s41565-023-01421-3. IF: 38.300
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Enhanced Thermal Conductivity of Free-Standing Double-Walled Carbon Nanotube Networks
Mehew, JD; Timmermans, MY; Reig, DS; Sergeant, S; Sledzinska, M; Chávez-Angel, E; Gallagher, E; Torres, CMS; Huyghebaert, C; Tielrooij, KJ Acs Applied Materials & Interfaces; 15 (44): 51876 - 51884. 2023. 10.1021/acsami.3c09210. IF: 9.500
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Observation of Negative Effective Thermal Diffusion in Gold Films
Block, A; Yu, RW; Un, IW; Varghese, S; Liebel, M; van Hulst, NF; Fan, SH; Tielrooij, KJ; Sivan, Y Acs Photonics; 10 (4): 1150 - 1158. 2023. 10.1021/acsphotonics.2c01916. IF: 7.000
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Reversible Electrical Control of Interfacial Charge Flow across van der Waals Interfaces
Fu, S; Jia, XY; Hassan, AS; Zhang, H; Zheng, WH; Gao, L; Di Virgilio, L; Krasel, S; Beljonne, D; Tielrooij, KJ; Bonn, M; Wang, HI Nano Letters; 23 (5): 1850 - 1857. 2023. 10.1021/acs.nanolett.2c04795. IF: 10.800
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Terahertz nonlinear photonics based on the ultrafast thermodynamics of quantum materials
Tielrooij, KJ; Principi, A; Reig, DS; Varghese, S; Astakhov, G; Kovalev, S International Conference On Infrared, Millimeter, And Terahertz Waves, Irmmw-Thz; 2023. 10.1109/IRMMW-THz57677.2023.10298868.
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Ultrafast Tunable Terahertz-to-Visible Light Conversion through Thermal Radiation from Graphene Metamaterials
Ilyakov, I; Ponomaryov, A; Reig, DS; Murphy, C; Mehew, JD; de Oliveira, TVAG; Prajapati, GL; Arshad, A; Deinert, JC; Craciun, MF; Russo, S; Kovalev, S; Tielrooij, KJ Nano Letters; 23 (9): 3872 - 3878. 2023. 10.1021/acs.nanolett.3c00507. IF: 10.800
2022
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Electrically Tunable Nonequilibrium Optical Response of Graphene
Pogna E.A.A., Tomadin A., Balci O., Soavi G., Paradisanos I., Guizzardi M., Pedrinazzi P., Mignuzzi S., Tielrooij K.-J., Polini M., Ferrari A.C., Cerullo G. ACS Nano; 16 (3): 3613 - 3624. 2022. 10.1021/acsnano.1c04937. IF: 15.881
The ability to tune the optical response of a material via electrostatic gating is crucial for optoelectronic applications, such as electro-optic modulators, saturable absorbers, optical limiters, photodetectors, and transparent electrodes. The band structure of single layer graphene (SLG), with zero-gap, linearly dispersive conduction and valence bands, enables an easy control of the Fermi energy, EF, and of the threshold for interband optical absorption. Here, we report the tunability of the SLG nonequilibrium optical response in the near-infrared (1000-1700 nm/0.729-1.240 eV), exploring a range of EFfrom -650 to 250 meV by ionic liquid gating. As EFincreases from the Dirac point to the threshold for Pauli blocking of interband absorption, we observe a slow-down of the photobleaching relaxation dynamics, which we attribute to the quenching of optical phonon emission from photoexcited charge carriers. For EFexceeding the Pauli blocking threshold, photobleaching eventually turns into photoinduced absorption, because the hot electrons' excitation increases the SLG absorption. The ability to control both recovery time and sign of the nonequilibrium optical response by electrostatic gating makes SLG ideal for tunable saturable absorbers with controlled dynamics. © 2022 American Chemical Society. All rights reserved.
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Milliwatt terahertz harmonic generation from topological insulator metamaterials
Tielrooij, Klaas-Jan; Principi, Alessandro; Reig, David Saleta; Block, Alexander; Varghese, Sebin; Schreyeck, Steffen; Brunner, Karl; Karczewski, Grzegorz; Ilyakov, Igor; Ponomaryov, Oleksiy; de Oliveira, Thales V A G; Chen, Min; Deinert, Jan-Christoph; Carbonell, Carmen Gomez; Valenzuela, Sergio O; Molenkamp, Laurens W; Kiessling, Tobias; Astakhov, Georgy, V; Kovalev, Sergey Light-Science & Applications; 11 (1): 315. 2022. 10.1038/s41377-022-01008-y. IF: 20.257
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Ultrafast electronic heat dissipation through surface-to-bulk Coulomb coupling in quantum materials
Principi, Alessandro; Tielrooij, Klaas-Jan Physical Review b; 106 (11): 115422. 2022. 10.1103/physrevb.106.115422. IF: 3.908
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Ultrafast light-based logic with graphene
Tielrooij, KJ Nature Materials; 22 (8): 945 - 946. 2022. 10.1038/s41563-022-01367-2. IF: 47.656
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Unraveling Heat Transport and Dissipation in Suspended MoSe2 from Bulk to Monolayer
Saleta Reig D., Varghese S., Farris R., Block A., Mehew J.D., Hellman O., Woźniak P., Sledzinska M., El Sachat A., Chávez-Ángel E., Valenzuela S.O., van Hulst N.F., Ordejón P., Zanolli Z., Sotomayor Torres C.M., Verstraete M.J., Tielrooij K.-J. Advanced Materials; 34 (10, 2108352) 2022. 10.1002/adma.202108352. IF: 30.849
Understanding heat flow in layered transition metal dichalcogenide (TMD) crystals is crucial for applications exploiting these materials. Despite significant efforts, several basic thermal transport properties of TMDs are currently not well understood, in particular how transport is affected by material thickness and the material's environment. This combined experimental–theoretical study establishes a unifying physical picture of the intrinsic lattice thermal conductivity of the representative TMD MoSe2. Thermal conductivity measurements using Raman thermometry on a large set of clean, crystalline, suspended crystals with systematically varied thickness are combined with ab initio simulations with phonons at finite temperature. The results show that phonon dispersions and lifetimes change strongly with thickness, yet the thinnest TMD films exhibit an in-plane thermal conductivity that is only marginally smaller than that of bulk crystals. This is the result of compensating phonon contributions, in particular heat-carrying modes around ≈0.1 THz in (sub)nanometer thin films, with a surprisingly long mean free path of several micrometers. This behavior arises directly from the layered nature of the material. Furthermore, out-of-plane heat dissipation to air molecules is remarkably efficient, in particular for the thinnest crystals, increasing the apparent thermal conductivity of monolayer MoSe2 by an order of magnitude. These results are crucial for the design of (flexible) TMD-based (opto-)electronic applications. © 2022 The Authors. Advanced Materials published by Wiley-VCH GmbH
2021
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Decoupling the effects of defects on efficiency and stability through phosphonates in stable halide perovskite solar cells
Xie H., Wang Z., Chen Z., Pereyra C., Pols M., Gałkowski K., Anaya M., Fu S., Jia X., Tang P., Kubicki D.J., Agarwalla A., Kim H.-S., Prochowicz D., Borrisé X., Bonn M., Bao C., Sun X., Zakeeruddin S.M., Emsley L., Arbiol J., Gao F., Fu F., Wang H.I., Tielrooij K.-J., Stranks S.D., Tao S., Grätzel M., Hagfeldt A., Lira-Cantu M. Joule; 5 (5): 1246 - 1266. 2021. 10.1016/j.joule.2021.04.003. IF: 41.248
Understanding defects is of paramount importance for the development of stable halide perovskite solar cells (PSCs). However, isolating their distinctive effects on device efficiency and stability is currently a challenge. We report that adding the organic molecule 3-phosphonopropionic acid (H3pp) to the halide perovskite results in unchanged overall optoelectronic performance while having a tremendous effect on device stability. We obtained PSCs with ∼21% efficiency that retain ∼100% of the initial efficiency after 1,000 h at the maximum power point under simulated AM1.5G illumination. The strong interaction between the perovskite and the H3pp molecule through two types of hydrogen bonds (H…I and O…H) leads to shallow point defect passivation that has a significant effect on device stability but not on the non-radiative recombination and device efficiency. We expect that our work will have important implications for the current understanding and advancement of operational PSCs. © 2021 Elsevier Inc.
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Electrical tunability of terahertz nonlinearity in graphene
Kovalev S., Hafez H.A., Tielrooij K.-J., Deinert J.-C., Ilyakov I., Awari N., Alcaraz D., Soundarapandian K., Saleta D., Germanskiy S., Chen M., Bawatna M., Green B., Koppens F.H.L., Mittendorff M., Bonn M., Gensch M., Turchinovich D. Science Advances; 7 (15, eabf9809) 2021. 10.1126/SCIADV.ABF9809. IF: 14.143
Graphene is conceivably the most nonlinear optoelectronic material we know. Its nonlinear optical coefficients in the terahertz frequency range surpass those of other materials by many orders of magnitude. Here, we show that the terahertz nonlinearity of graphene, both for ultrashort single-cycle and quasi-monochromatic multicycle input terahertz signals, can be efficiently controlled using electrical gating, with gating voltages as low as a few volts. For example, optimal electrical gating enhances the power conversion efficiency in terahertz third-harmonic generation in graphene by about two orders of magnitude. Our experimental results are in quantitative agreement with a physical model of the graphene nonlinearity, describing the time-dependent thermodynamic balance maintained within the electronic population of graphene during interaction with ultrafast electric fields. Our results can serve as a basis for straightforward and accurate design of devices and applications for efficient electronic signal processing in graphene at ultrahigh frequencies. © 2021 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution License 4.0 (CC BY).
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Fabrication and characterization of large-area suspended MoSe2 crystals down to the monolayer
Varghese S., Reig D.S., Mehew J.D., Block A., El Sachat A., Chávez-Ángel E., Sledzinska M., Ballesteros B., Sotomayor Torres C.M., Tielrooij K.-J. JPhys Materials; 4 (4, 046001) 2021. 10.1088/2515-7639/ac2060. IF: 0.000
Many layered materials, such as graphene and transition metal dichalcogenides, can be exfoliated down to atomic or molecular monolayers. These materials exhibit exciting material properties that can be exploited for several promising device concepts. Thinner materials lead to an increased surface-to-volume ratio, with mono- and bi-layers being basically pure surfaces. Thin crystals containing more than two layers also often behave as an all-surface material, depending on the physical property of interest. As a result, flakes of layered materials are typically highly sensitive to their environment, which is undesirable for a broad range of studies and potential devices. Material systems based on suspended flakes overcome this issue, yet often require complex fabrication procedures. Here, we demonstrate the relatively straightforward fabrication of exfoliated MoSe2 flakes down to the monolayer, suspended over unprecedentedly large holes with a diameter of 15 µm. We describe our fabrication methods in detail, present characterization measurements of the fabricated structures, and, finally, exploit these suspended flakes for accurate optical absorption measurements. © 2021 The Author(s).
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Grating-Graphene Metamaterial as a Platform for Terahertz Nonlinear Photonics
Deinert J.-C., Alcaraz Iranzo D., Pérez R., Jia X., Hafez H.A., Ilyakov I., Awari N., Chen M., Bawatna M., Ponomaryov A.N., Germanskiy S., Bonn M., Koppens F.H.L., Turchinovich D., Gensch M., Kovalev S., Tielrooij K.-J. ACS Nano; 15 (1): 1145 - 1154. 2021. 10.1021/acsnano.0c08106. IF: 15.881
Nonlinear optics is an increasingly important field for scientific and technological applications, owing to its relevance and potential for optical and optoelectronic technologies. Currently, there is an active search for suitable nonlinear material systems with efficient conversion and a small material footprint. Ideally, the material system should allow for chip integration and room-temperature operation. Two-dimensional materials are highly interesting in this regard. Particularly promising is graphene, which has demonstrated an exceptionally large nonlinearity in the terahertz regime. Yet, the light-matter interaction length in two-dimensional materials is inherently minimal, thus limiting the overall nonlinear optical conversion efficiency. Here, we overcome this challenge using a metamaterial platform that combines graphene with a photonic grating structure providing field enhancement. We measure terahertz third-harmonic generation in this metamaterial and obtain an effective third-order nonlinear susceptibility with a magnitude as large as 3 × 10-8 m2/V2, or 21 esu, for a fundamental frequency of 0.7 THz. This nonlinearity is 50 times larger than what we obtain for graphene without grating. Such an enhancement corresponds to a third-harmonic signal with an intensity that is 3 orders of magnitude larger due to the grating. Moreover, we demonstrate a field conversion efficiency for the third harmonic of up to ∼1% using a moderate field strength of ∼30 kV/cm. Finally, we show that harmonics beyond the third are enhanced even more strongly, allowing us to observe signatures of up to the ninth harmonic. Grating-graphene metamaterials thus constitute an outstanding platform for commercially viable, CMOS-compatible, room-temperature, chip-integrated, THz nonlinear conversion applications. © 2021 American Chemical Society. All rights reserved.
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Hot carriers in graphene-fundamentals and applications
Massicotte M., Soavi G., Principi A., Tielrooij K.-J. Nanoscale; 13 (18): 8376 - 8411. 2021. 10.1039/d0nr09166a. IF: 7.790
Hot charge carriers in graphene exhibit fascinating physical phenomena, whose understanding has improved greatly over the past decade. They have distinctly different physical properties compared to, for example, hot carriers in conventional metals. This is predominantly the result of graphene's linear energy-momentum dispersion, its phonon properties, its all-interface character, and the tunability of its carrier density down to very small values, and from electron- to hole-doping. Since a few years, we have witnessed an increasing interest in technological applications enabled by hot carriers in graphene. Of particular interest are optical and optoelectronic applications, where hot carriers are used to detect (photodetection), convert (nonlinear photonics), or emit (luminescence) light. Graphene-enabled systems in these application areas could find widespread use and have a disruptive impact, for example in the field of data communication, high-frequency electronics, and industrial quality control. The aim of this review is to provide an overview of the most relevant physics and working principles that are relevant for applications exploiting hot carriers in graphene. © 2021 The Royal Society of Chemistry.
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Hot plasmons make graphene shine
Koppens F.H.L., Tielrooij K.-J. Nature Materials; 20 (6): 721 - 722. 2021. 10.1038/s41563-021-00952-1. IF: 43.841
[No abstract available]
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Hot-Carrier Cooling in High-Quality Graphene Is Intrinsically Limited by Optical Phonons
Pogna E.A.A., Jia X., Principi A., Block A., Banszerus L., Zhang J., Liu X., Sohier T., Forti S., Soundarapandian K., Terrés B., Mehew J.D., Trovatello C., Coletti C., Koppens F.H.L., Bonn M., Wang H.I., Van Hulst N., Verstraete M.J., Peng H., Liu Z., Stampfer C., Cerullo G., Tielrooij K.-J. ACS Nano; 15 (7): 11285 - 11295. 2021. 10.1021/acsnano.0c10864. IF: 15.881
Many promising optoelectronic devices, such as broadband photodetectors, nonlinear frequency converters, and building blocks for data communication systems, exploit photoexcited charge carriers in graphene. For these systems, it is essential to understand the relaxation dynamics after photoexcitation. These dynamics contain a sub-100 fs thermalization phase, which occurs through carrier-carrier scattering and leads to a carrier distribution with an elevated temperature. This is followed by a picosecond cooling phase, where different phonon systems play a role: graphene acoustic and optical phonons, and substrate phonons. Here, we address the cooling pathway of two technologically relevant systems, both consisting of high-quality graphene with a mobility >10000 cm2 V-1 s-1 and environments that do not efficiently take up electronic heat from graphene: WSe2-encapsulated graphene and suspended graphene. We study the cooling dynamics using ultrafast pump-probe spectroscopy at room temperature. Cooling via disorder-assisted acoustic phonon scattering and out-of-plane heat transfer to substrate phonons is relatively inefficient in these systems, suggesting a cooling time of tens of picoseconds. However, we observe much faster cooling, on a time scale of a few picoseconds. We attribute this to an intrinsic cooling mechanism, where carriers in the high-energy tail of the hot-carrier distribution emit optical phonons. This creates a permanent heat sink, as carriers efficiently rethermalize. We develop a macroscopic model that explains the observed dynamics, where cooling is eventually limited by optical-to-acoustic phonon coupling. These fundamental insights will guide the development of graphene-based optoelectronic devices. ©
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Long-lived charge separation following pump-wavelength–dependent ultrafast charge transfer in graphene/WS2 heterostructures
Fu S., du Fossé I., Jia X., Xu J., Yu X., Zhang H., Zheng W., Krasel S., Chen Z., Wang Z.M., Tielrooij K.-J., Bonn M., Houtepen A.J., Wang H.I. Science Advances; 7 (9, eabd9061) 2021. 10.1126/sciadv.abd9061. IF: 14.143
Van der Waals heterostructures consisting of graphene and transition metal dichalcogenides have shown great promise for optoelectronic applications. However, an in-depth understanding of the critical processes for device operation, namely, interfacial charge transfer (CT) and recombination, has so far remained elusive. Here, we investigate these processes in graphene-WS2 heterostructures by complementarily probing the ultrafast terahertz photoconductivity in graphene and the transient absorption dynamics in WS2 following photoexcitation. We observe that separated charges in the heterostructure following CT live extremely long: beyond 1 ns, in contrast to ~1 ps charge separation reported in previous studies. This leads to efficient photogating of graphene. Furthermore, for the CT process across graphene-WS2 interfaces, we find that it occurs via photo-thermionic emission for sub-A-exciton excitations and direct hole transfer from WS2 to the valence band of graphene for above-A-exciton excitations. These findings provide insights to further optimize the performance of optoelectronic devices, in particular photodetection. Copyright © 2021 The Authors, some rights reserved;
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Observation of giant and tunable thermal diffusivity of a Dirac fluid at room temperature
Block A., Principi A., Hesp N.C.H., Cummings A.W., Liebel M., Watanabe K., Taniguchi T., Roche S., Koppens F.H.L., van Hulst N.F., Tielrooij K.-J. Nature Nanotechnology; 16 (11): 1195 - 1200. 2021. 10.1038/s41565-021-00957-6. IF: 39.213
Conducting materials typically exhibit either diffusive or ballistic charge transport. When electron–electron interactions dominate, a hydrodynamic regime with viscous charge flow emerges1–13. More stringent conditions eventually yield a quantum-critical Dirac-fluid regime, where electronic heat can flow more efficiently than charge14–22. However, observing and controlling the flow of electronic heat in the hydrodynamic regime at room temperature has so far remained elusive. Here we observe heat transport in graphene in the diffusive and hydrodynamic regimes, and report a controllable transition to the Dirac-fluid regime at room temperature, using carrier temperature and carrier density as control knobs. We introduce the technique of spatiotemporal thermoelectric microscopy with femtosecond temporal and nanometre spatial resolution, which allows for tracking electronic heat spreading. In the diffusive regime, we find a thermal diffusivity of roughly 2,000 cm2 s−1, consistent with charge transport. Moreover, within the hydrodynamic time window before momentum relaxation, we observe heat spreading corresponding to a giant diffusivity up to 70,000 cm2 s−1, indicative of a Dirac fluid. Our results offer the possibility of further exploration of these interesting physical phenomena and their potential applications in nanoscale thermal management. © 2021, The Author(s).
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Terahertz signatures of ultrafast Dirac fermion relaxation at the surface of topological insulators
Kovalev S., Tielrooij K.-J., Deinert J.-C., Ilyakov I., Awari N., Chen M., Ponomaryov A., Bawatna M., de Oliveira T.V.A.G., Eng L.M., Kuznetsov K.A., Safronenkov D.A., Kitaeva G.K., Kuznetsov P.I., Hafez H.A., Turchinovich D., Gensch M. npj Quantum Materials; 6 (1, 84) 2021. 10.1038/s41535-021-00384-9. IF: 7.032
Topologically protected surface states present rich physics and promising spintronic, optoelectronic, and photonic applications that require a proper understanding of their ultrafast carrier dynamics. Here, we investigate these dynamics in topological insulators (TIs) of the bismuth and antimony chalcogenide family, where we isolate the response of Dirac fermions at the surface from the response of bulk carriers by combining photoexcitation with below-bandgap terahertz (THz) photons and TI samples with varying Fermi level, including one sample with the Fermi level located within the bandgap. We identify distinctly faster relaxation of charge carriers in the topologically protected Dirac surface states (few hundred femtoseconds), compared to bulk carriers (few picoseconds). In agreement with such fast cooling dynamics, we observe THz harmonic generation without any saturation effects for increasing incident fields, unlike graphene which exhibits strong saturation. This opens up promising avenues for increased THz nonlinear conversion efficiencies, and high-bandwidth optoelectronic and spintronic information and communication applications. © 2021, The Author(s).
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Thickness-Dependent Elastic Softening of Few-Layer Free-Standing MoSe2
Babacic V., Saleta Reig D., Varghese S., Vasileiadis T., Coy E., Tielrooij K.-J., Graczykowski B. Advanced Materials; 33 (23, 2008614) 2021. 10.1002/adma.202008614. IF: 30.849
Few-layer van der Waals (vdW) materials have been extensively investigated in terms of their exceptional electronic, optoelectronic, optical, and thermal properties. Simultaneously, a complete evaluation of their mechanical properties remains an undeniable challenge due to the small lateral sizes of samples and the limitations of experimental tools. In particular, there is no systematic experimental study providing unambiguous evidence on whether the reduction of vdW thickness down to few layers results in elastic softening or stiffening with respect to the bulk. In this work, micro-Brillouin light scattering is employed to investigate the anisotropic elastic properties of single-crystal free-standing 2H-MoSe2 as a function of thickness, down to three molecular layers. The so-called elastic size effect, that is, significant and systematic elastic softening of the material with decreasing numbers of layers is reported. In addition, this approach allows for a complete mechanical examination of few-layer membranes, that is, their elasticity, residual stress, and thickness, which can be easily extended to other vdW materials. The presented results shed new light on the ongoing debate on the elastic size-effect and are relevant for performance and durability of implementation of vdW materials as resonators, optoelectronic, and thermoelectric devices. © 2021 Wiley-VCH GmbH
2020
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Fast electrical modulation of strong near-field interactions between erbium emitters and graphene
Cano D., Ferrier A., Soundarapandian K., Reserbat-Plantey A., Scarafagio M., Tallaire A., Seyeux A., Marcus P., Riedmatten H., Goldner P., Koppens F.H.L., Tielrooij K.-J. Nature Communications; 11 (1, 4094) 2020. 10.1038/s41467-020-17899-7. IF: 12.121
Combining the quantum optical properties of single-photon emitters with the strong near-field interactions available in nanophotonic and plasmonic systems is a powerful way of creating quantum manipulation and metrological functionalities. The ability to actively and dynamically modulate emitter-environment interactions is of particular interest in this regard. While thermal, mechanical and optical modulation have been demonstrated, electrical modulation has remained an outstanding challenge. Here we realize fast, all-electrical modulation of the near-field interactions between a nanolayer of erbium emitters and graphene, by in-situ tuning the Fermi energy of graphene. We demonstrate strong interactions with a >1000-fold increased decay rate for ~25% of the emitters, and electrically modulate these interactions with frequencies up to 300 kHz – orders of magnitude faster than the emitter’s radiative decay (~100 Hz). This constitutes an enabling platform for integrated quantum technologies, opening routes to quantum entanglement generation by collective plasmon emission or photon emission with controlled waveform. © 2020, The Author(s).
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Plasmonic antenna coupling to hyperbolic phonon-polaritons for sensitive and fast mid-infrared photodetection with graphene
Castilla S., Vangelidis I., Pusapati V.-V., Goldstein J., Autore M., Slipchenko T., Rajendran K., Kim S., Watanabe K., Taniguchi T., Martín-Moreno L., Englund D., Tielrooij K.-J., Hillenbrand R., Lidorikis E., Koppens F.H.L. Nature Communications; 11 (1, 4872) 2020. 10.1038/s41467-020-18544-z. IF: 12.121
Integrating and manipulating the nano-optoelectronic properties of Van der Waals heterostructures can enable unprecedented platforms for photodetection and sensing. The main challenge of infrared photodetectors is to funnel the light into a small nanoscale active area and efficiently convert it into an electrical signal. Here, we overcome all of those challenges in one device, by efficient coupling of a plasmonic antenna to hyperbolic phonon-polaritons in hexagonal-BN to highly concentrate mid-infrared light into a graphene pn-junction. We balance the interplay of the absorption, electrical and thermal conductivity of graphene via the device geometry. This approach yields remarkable device performance featuring room temperature high sensitivity (NEP of 82 pW/Hz) and fast rise time of 17 nanoseconds (setup-limited), among others, hence achieving a combination currently not present in the state-of-the-art graphene and commercial mid-infrared detectors. We also develop a multiphysics model that shows very good quantitative agreement with our experimental results and reveals the different contributions to our photoresponse, thus paving the way for further improvement of these types of photodetectors even beyond mid-infrared range. © 2020, The Author(s).
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Terahertz Nonlinear Optics of Graphene: From Saturable Absorption to High-Harmonics Generation
Hafez H.A., Kovalev S., Tielrooij K.-J., Bonn M., Gensch M., Turchinovich D. Advanced Optical Materials; 8 (3, 1900771) 2020. 10.1002/adom.201900771. IF: 8.286
Graphene has long been predicted to show exceptional nonlinear optical properties, especially in the technologically important terahertz (THz) frequency range. Recent experiments have shown that this atomically thin material indeed exhibits possibly the largest nonlinear coefficients of any material known to date, paving the way for practical graphene-based applications in ultrafast (opto-)electronics operating at THz rates. Here the advances in the booming field of nonlinear THz optics of graphene are reported, and the state-of-the-art understanding of the nature of the nonlinear interaction of graphene with the THz fields based on the thermodynamic model of electron transport in graphene is described. A comparison between different mechanisms of nonlinear interaction of graphene with light fields in THz, infrared, and visible frequency ranges is also provided. Finally, the perspectives for the expected technological applications of graphene based on its extraordinary THz nonlinear properties are summarized. This report covers the evolution of the field of THz nonlinear optics of graphene from the very pioneering to the state-of-the-art works. It also serves as a concise overview of the current understanding of THz nonlinear optics of graphene and as a compact reference for researchers entering the field, as well as for the technology developers. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
2019
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Control of Terahertz Nonlinearity in Graphene by Gating
Hafez H.A., Tielrooij K.-J., Bonn M., Turchinovich D. International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz; 2019-September (8874288) 2019. 10.1109/IRMMW-THz.2019.8874288.
We study the dependence of the terahertz (THz) nonlinearity of graphene and its temporal dynamics on the free carrier concentration by THz-pump/THz-probe spectroscopy of a gated graphene at room temperature. The strong THz nonlinearity is attributed to electron heating by the driving THz field and shows a drastic dependence on the background electron concentration, demonstrating a wide-range tunability of the THz nonlinearity of graphene. © 2019 IEEE.
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Fast and Sensitive Terahertz Detection Using an Antenna-Integrated Graphene pn Junction
Castilla S., Terrés B., Autore M., Viti L., Li J., Nikitin A.Y., Vangelidis I., Watanabe K., Taniguchi T., Lidorikis E., Vitiello M.S., Hillenbrand R., Tielrooij K.-J., Koppens F.H.L. Nano Letters; 19 (5): 2765 - 2773. 2019. 10.1021/acs.nanolett.8b04171. IF: 12.279
Although the detection of light at terahertz (THz) frequencies is important for a large range of applications, current detectors typically have several disadvantages in terms of sensitivity, speed, operating temperature, and spectral range. Here, we use graphene as a photoactive material to overcome all of these limitations in one device. We introduce a novel detector for terahertz radiation that exploits the photothermoelectric (PTE) effect, based on a design that employs a dual-gated, dipolar antenna with a gap of ?100 nm. This narrow-gap antenna simultaneously creates a pn junction in a graphene channel located above the antenna and strongly concentrates the incoming radiation at this pn junction, where the photoresponse is created. We demonstrate that this novel detector has an excellent sensitivity, with a noise-equivalent power of 80 pW/Hz at room temperature, a response time below 30 ns (setup-limited), a high dynamic range (linear power dependence over more than 3 orders of magnitude) and broadband operation (measured range 1.8-4.2 THz, antenna-limited), which fulfills a combination that is currently missing in the state-of-the-art detectors. Importantly, on the basis of the agreement we obtained between experiment, analytical model, and numerical simulations, we have reached a solid understanding of how the PTE effect gives rise to a THz-induced photoresponse, which is very valuable for further detector optimization. © 2019 American Chemical Society.
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Graphene: The ultimate nonlinear material at terahertz frequencies
Hafez H.A., Kovalev S., Deinert J.-C., Jan-Tielrooij K., Bonn M., Gensch M., Turchinovich D. Optics InfoBase Conference Papers; Part F154-TSA 2019 2019. .
Graphene is an exceptional nonlinear material at terahertz (THz) frequencies, providing highly efficient THz high harmonics generation with nonlinear coefficients several orders of magnitude larger than those of other solids. © 2019 The Authors.
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Kinetic Ionic Permeation and Interfacial Doping of Supported Graphene
Jia X., Hu M., Soundarapandian K., Yu X., Liu Z., Chen Z., Narita A., Müllen K., Koppens F.H.L., Jiang J., Tielrooij K.-J., Bonn M., Wang H.I. Nano Letters; 19 (12): 9029 - 9036. 2019. 10.1021/acs.nanolett.9b04053. IF: 12.279
Due to its outstanding electrical properties and chemical stability, graphene finds widespread use in various electrochemical applications. Although the presence of electrolytes strongly affects its electrical conductivity, the underlying mechanism has remained elusive. Here, we employ terahertz spectroscopy as a contact-free means to investigate the impact of ubiquitous cations (Li+, Na+, K+, and Ca2+) in aqueous solution on the electronic properties of SiO2-supported graphene. We find that, without applying any external potential, cations can shift the Fermi energy of initially hole-doped graphene by ∼200 meV up to the Dirac point, thus counteracting the initial substrate-induced hole doping. Remarkably, the cation concentration and cation hydration complex size determine the kinetics and magnitude of this shift in the Fermi level. Combined with theoretical calculations, we show that the ion-induced Fermi level shift of graphene involves cationic permeation through graphene. The interfacial cations located between graphene and SiO2 electrostatically counteract the substrate-induced hole doping effect in graphene. These insights are crucial for graphene device processing and further developing graphene as an ion-sensing material. © 2019 American Chemical Society.
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Surface-Specific Spectroscopy of Water at a Potentiostatically Controlled Supported Graphene Monolayer
Dreier L.B., Liu Z., Narita A., Van Zadel M.-J., Müllen K., Tielrooij K.-J., Backus E.H.G., Bonn M. Journal of Physical Chemistry C; 123 (39): 24031 - 24038. 2019. 10.1021/acs.jpcc.9b05844. IF: 4.309
Knowledge of the structure of interfacial water molecules at electrified solid materials is the first step toward a better understanding of important processes at such surfaces, in, e.g., electrochemistry, atmospheric chemistry, and membrane biophysics. As graphene is an interesting material with multiple potential applications such as in transistors or sensors, we specifically investigate the graphene-water interface. We use sum-frequency generation spectroscopy to investigate the pH- and potential-dependence of the interfacial water structure in contact with a chemical vapor deposited (CVD) grown graphene surface. Our results show that the SFG signal from the interfacial water molecules at the graphene layer is dominated by the underlying substrate and that there are water molecules between the graphene and the (hydrophilic) supporting substrate. © 2019 American Chemical Society.
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Ultrathin Eu- and Er-Doped Y2O3 Films with Optimized Optical Properties for Quantum Technologies
Scarafagio M., Tallaire A., Tielrooij K.-J., Cano D., Grishin A., Chavanne M.-H., Koppens F.H.L., Ringuedé A., Cassir M., Serrano D., Goldner P., Ferrier A. Journal of Physical Chemistry C; 123 (21): 13354 - 13364. 2019. 10.1021/acs.jpcc.9b02597. IF: 4.309
Atomic layer deposited (ALD) Y2O3 thin films have been thoroughly investigated for optical or electronic applications. The coherent spectroscopy of lanthanide ions doped into this material has also recently attracted increasing interest in the field of quantum technologies for which they are considered promising candidates in quantum memories or as spin-photon interfaces. However, these most demanding applications require a deep control over the local positioning of the ions and their close environment in the crystalline matrix. This study focuses on the structural as well as optical properties of Eu3+ and Er3+ dopants in Y2O3 using photoluminescence (PL), luminescence decay times, and inhomogeneous line width (Γinh) measurements within this particular context. While as-grown ALD films do not provide an ideal host for the emitters, we demonstrate that by optimizing the deposition conditions and using appropriate annealing post treatments narrow inhomogeneous lines can be obtained for the 7F0↔5D0 transition of Eu3+ even for nanoscale films. Furthermore, about 1.5 ms lifetime has been measured for the infrared telecom transition of Er in ultrathin films (<10 nm), which is an order of magnitude higher than in nanoparticles of the same size. These results validate optimized rare-earth-doped ALD Y2O3 films as a suitable platform for photonics applications where few-nanometer-thick films with well-localized emitters are mandatory. This approach provides the first building blocks toward the development of more complex devices for quantum sensing or hybrid structures coupled with other systems such as two-dimensional materials. © 2019 American Chemical Society.