Abstract-Electrical tunability of terahertz nonlinearity in graphene

 Sergey Kovalev,  Hassan A. Hafez, Klaas-Jan Tielrooij, Jan-Christoph Deinert, Igor Ilyakov, Nilesh Awari,  David Alcaraz. Karuppasamy Soundarapandian, David Saleta, Semyon Germanskiy, Min Chen, Mohammed Bawatna, Bertram Green, Frank H. L. Koppens, Martin Mittendorff, Mischa Bonn, Michael Gensch, Dmitry Turchinovich, 

https://advances.sciencemag.org/content/7/15/eabf9809

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.

Abstract-Fast and Sensitive Terahertz Detection Using an Antenna-Integrated Graphene pn Junction

Sebastián Castilla, Sebastián Castilla, Bernat Terrés, Marta Autore, Leonardo Viti, Jian Li, Alexey Y. Nikitin, Ioannis,  Vangelidis,  Kenji Watanabe,  Takashi Taniguchi, Elefterios Lidorikis, Miriam S. Vitiello, Rainer Hillenbrand, Klaas-Jan Tielrooij, Frank Koppens.

https://pubs.acs.org/doi/10.1021/acs.nanolett.8b04171

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/ 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.

Abstract-Fast and Sensitive Terahertz Detection Using an Antenna-Integrated Graphene pn Junction

Sebastián Castilla, Bernat Terrés, Marta Autore, Leonardo Viti, Jian Li, Alexey Y. Nikitin, Ioannis Vangelidis, Kenji Watanabe, Takashi Taniguchi, Elefterios Lidorikis, Miriam S. Vitiello, Rainer Hillenbrand, Klaas-Jan Tielrooij, Frank H.L. Koppens

https://arxiv.org/abs/1905.01881

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-per-square-root-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.

Graphene Gives a Tremendous Boost to Future Terahertz Cameras

http://iconnect007.com/printfn/print.php?cdrID=116692

A study in Nano Letters reports on the development of a graphene-enabled detector for terahertz light that is faster and more sensitive than existing room-temperature technologies.

Detecting terahertz (THz) light is extremely useful for two main reasons. Firstly, THz technology is becoming a key element in applications regarding security (such as airport scanners), wireless data communication, and quality control, to mention just a few. However, current THz detectors have shown strong limitations in terms of simultaneously meeting the requirements for sensitivity, speed, spectral range, being able to operate at room temperature, etc.

Secondly, it is a very safe type of radiation due to its low-energy photons, with more than a hundred times less energy than that of photons in the visible light range.

Many graphene-based applications are expected to emerge from its use as material for detecting light. Graphene has the particularity of not having a bandgap, as compared to standard materials used for photodetection, such as silicon. The bandgap in silicon causes incident light with wavelengths longer than one micron to not be absorbed and thus not detected. In contrast, for graphene, even terahertz light with a wavelength of hundreds of microns can be absorbed and detected. Whereas THz detectors based on graphene have shown promising results so far, none of the detectors so far could beat commercially available detectors in terms of speed and sensitivity.

In a recent study, ICFO researchers Sebastián Castilla and Dr. Bernat Terrés, led by ICREA Prof. at ICFO Frank Koppens and former ICFO scientist Dr. Klaas-Jan Tielrooij (now Junior Group Leader at ICN2), in collaboration with scientists from CIC NanoGUNE, NEST (CNR), Nanjing University, Donostia International Physics Center, University of Ioannina and the National Institute for Material Sciences, have been able to overcome these challenges. They have developed a novel graphene-enabled photodetector that operates at room temperature, and is highly sensitive, very fast, has a wide dynamic range and covers a broad range of THz frequencies.

In their experiment, the scientists were able to optimize the photoresponse mechanism of a THz photodetector using the following approach. They integrated a dipole antenna into the detector to concentrate the incident THz light around the antenna gap region. By fabricating a very small (100 nm, about one thousand times smaller than the thickness of a hair) antenna gap, they were able to obtain a great intensity concentration of THz incident light in the photoactive region of the graphene channel. They observed that the light absorbed by the graphene creates hot carriers at a pn-junction in graphene; subsequently, the unequal Seebeck coefficients in the p- and n-regions produce a local voltage and a current through the device generating a very large photoresponse and, thus, leading to a very high sensitivity, high speed response detector, with a wide dynamic range and a broad spectral coverage.

The results of this study open a pathway towards the development a fully digital low-cost camera system. This could be as cheap as the camera inside the smartphone, since such a detector has proven to have a very low power consumption and is fully compatible with CMOS technology.

Abstract-Fast and sensitive terahertz detection using an antenna-integrated graphene pn-junction

Sebastian Castilla, Bernat Terres, Marta Autore, Leonardo Viti, Jian Li, Alexey Nikitin, Ioannis Vangelidis, Kenji Watanabe, Takashi Taniguchi, Elefterios Lidorikis, Miriam Serena Vitiello, Rainer Hillenbrand, Klaas-Jan Tielrooij, and Frank H.L. Koppens


https://pubs.acs.org/doi/10.1021/acs.nanolett.8b04171

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 photoactive material to overcome all of these limitations in one device. We introduce a novel detector for terahertz radiation that exploits the photo-thermoelectric 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 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 fulfils a combination that is currently missing in the state of the art. Importantly, based on the agreement we obtain between experiment, analytical model, and numerical simulations, we have reached a solid understanding of how the PTE eect gives rise to a THz-induced photoresponse, which is very valuable for further detector optimization.

Analyzing Intersubband Transitions of 2D Materials by Scattering Scanning Near-Field Optical Microscopy

Schematic illustration of charge carriers confined within a TMD flake comprising different thicknesses. Charge carriers in the ground state (blue) can be excited upon resonant light excitation to a higher state (pink). (Image credit: ICFO/Fabien Vialla)

Semiconducting heterostructures have been vital to the advancement of electronics and opto-electronics. Various applications in the terahertz and infrared frequency range make use of transitions, known as intersubband transitions, between quantized states in semiconductor quantum wells.

https://www.azonano.com/news.aspx?newsID=36319

Such intraband transitions demonstrate remarkably large oscillator strengths, close to unity. The discovery of these transitions in III-V semiconductor heterostructures had a large influence on the field of condensed matter physics and initiated the development of quantum well infrared photodetectors and also quantum cascade lasers.

In general, quantum wells of the highest quality are produced by employing molecular beam epitaxy (sequential growth of crystalline layers), which is a well-established method. Yet, it has two major drawbacks: It requires lattice-matching, thereby constraining the freedom in choosing the materials, and the thermal growth leads to atomic diffusion and increases interface roughness.

Since 2D materials naturally form a quantum well that has atomically sharp interfaces, they have the ability to overcome these drawbacks. They offer atomically sharp and defect-free interfaces, thereby allowing the formation of perfect quantum well, free of diffusive inhomogeneities. They eliminated the need for epitaxial growth on a matching substrate and hence can be easily isolated and coupled to other electronic systems such as optical or Si CMOS systems such as waveguides and cavities.

In their experiment, the group of scientists applied scattering scanning near-field optical microscopy (s-SNOM) as a novel strategy for performing spectral absorption measurements with a spatial resolution less than 20 nm. They exfoliated TMDs consisting of terraces of different layer thicknesses over lateral sizes of around a few microns. The researchers directly observed the intersubband resonances for these distinctive quantum well thicknesses inside a single device. They also tuned the charge carrier density in an electrostatic manner and illustrated intersubband absorption in both the conduction and valence bands. Detailed theoretical calculations were performed to complement and support these observations, which revealed non-local and many-body effects.Astonishingly, theoretical or experimental analysis of intersubband transitions in few-layer 2D materials has never been performed. As a result, in a research recently reported in the Nature Nanotechnology journal, ICFO researchers Peter Schmidt, Fabien Vialla, Mathieu Massicotte, Klaas-Jan Tielrooij, and Gabriele Navickaite, headed by ICREA Professor at ICFO Frank Koppens, in collaboration with the Institut Lumière Matière–CNRS, Technical University of Denmark, Max Planck Institute for the Structure and Dynamics of Matter, CIC nanoGUNE, and the National Graphene Institute, report on the first theoretical calculations and first experimental observation of intersubband transitions in quantum wells of few-layer semiconducting 2D materials (TMDs).

The outcomes of this research open the door for an unexplored area in this new category of materials and provide a preview of the physics and technology facilitated by intersubband transitions in 2D materials, such as infrared sources, detectors, and lasers with the capability for compact integration with Si CMOS.

Abstract-Terahertz Depolarization Effects in Colloidal TiO2 Films Reveal Particle Morphology

Soeren Alkaersig Jensen , Klaas-Jan Tielrooij , Euan Hendry , Mischa Bonn , Ivan Rychetský , and Hynek Soeren 

J.Phys.Chem. C, Just Accepted Manuscript

DOI: 10.1021/jp406897y

Publication Date (Web): December 10, 2013

Copyright © 2013 American Chemical Society
http://pubs.acs.org/doi/abs/10.1021/jp406897y

Films of colloidal TiO2 nanoparticles are widely used in photovoltaic and photocatalytic applications, and the nature of electrical conductivity in such materials is therefore of both fundamental and practical interest. The conductive properties of colloid TiO2 films depend strongly on their morphology and deviate greatly from the properties of the bulk material. We report ultrafast photoconductivity studies of films consisting of sintered TiO2 particles of very different sizes performed using time-resolved Terahertz spectroscopy. Remarkably, identical photoconductivity spectra are observed for films of particles with diameters of tens and hundreds of nm respectively. The independence of photoconductivity on particle size directly demonstrates that the terahertz photoconductive response of colloidal TiO2 films is not affected by carrier backscattering at particle boundaries as has previously been concluded, but rather by depolarization fields resulting from the spatial inhomogeneities in the dielectric function inherent to these types of films. Modelling of the influence of depolarization fields on the terahertz conductivity allows us to explain the measured data and gain insights into the morphology of the film. Specifically, we show that the observed photoconductivity spectra reflect percolated pathways in the colloidal TiO2 nanoparticles films, through which charge carrier diffusion can occur over macroscopic length scales.