Recent intense electrical and optical studies of graphene have pushed the material to the forefront of optoelectronic research. Of particular interest is the few terahertz (THz) frequency regime where efficient light sources and highly sensitive detectors are very challenging to make. Here we present THz sources and detectors made with graphene field effect transistors (GFETs) enhanced by a double-patch antenna and an on-chip silicon lens. We report the first experimental observation of 1-3 THz radiation from graphene, as well as four orders of magnitude performance improvements in a GFET thermoelectric detector operating at ~2 THz. The quantitative analysis of the emitting power and its unusual charge density dependence indicate significant non-thermal contribution from the GFET. The polarization resolved detection measurements with different illumination geometries allow for detailed and quantitative analysis of various factors that contribute to the overall detector performance. Our experimental results represent a significant advance towards practically useful graphene THz devices. Subject terms: Physical sciences, Materials science, Condensed matter3 Manuscript textThe gapless electronic structure of graphene 1 is a unique property that has drawn significant attention from both basic sciences and practical applications 2 . In particular, it enables broadband interaction of photons with the two dimensional (2D) atomic layer from the far infrared up to the ultraviolet 3 . This has led to various optoelectronic devices operating with photons in the visible 4-7 , near infra-red [8][9][10][11] , mid infra-red [12][13][14][15][16] and far infrared [17][18][19][20][21][22][23][24] . Applications of graphene field effect transistors (GFET) in the few terahertz (THz) frequency range are particularly appealing since it's one of the least developed regimes lying in the gap between efficient manipulation with electronics and photonics [25][26][27] . Here we perform combined THz emission-detection measurements using devices made with monolayer graphene. Our results represent the first study of THz emission from graphene, as well as significant improvements in GFET thermoelectric THz detectors.A common bottleneck in graphene photonic and optoelectronic devices is the limited light-matter interaction, because of the 2D crystal's sub-nanometer thickness.This has led to the 'greybody' radiation 28,29 range that is notoriously difficult to work with. For the emitter, we observe a radiated power that is significantly larger than the anticipated thermal radiation, suggesting additional radiation channels at our disposal for devising efficient graphene THz sources.For the detector, we achieve four orders of magnitude sensitivity improvements, which, in conjunction with its high speed 19,20 , makes the GFET a strong competitor to other contemporary THz sensors.The antenna is designed to have a size of 45×31 µm 2 as shown in Fig.1 indicates an optimal operation frequency of 2.1THz. The electric field distribution at ...

Stokes and anti-Stokes Raman scattering are performed on atomic layers of hexagonal molybdenum ditelluride (MoTe2), a prototypical transition metal dichalcogenide (TMDC) semiconductor. The data reveal all six types of zone center optical phonons, along with their corresponding Davydov splittings, which have been challenging to see in other TMDCs. We discover that the anti-Stokes Raman intensity of the low energy layer-breathing mode becomes more intense than the Stokes peak under certain experimental conditions, and find the effect to be tunable by excitation frequency and number of atomic layers. These observations are interpreted as a result of resonance effects arising from the C excitons in the vicinity of the Brillouin zone center in the photon-electron-phonon interaction process.

We report the experimental observation of radiative recombination from Rydberg excitons in a two-dimensional semiconductor, monolayer WSe2, encapsulated in hexagonal boron nitride.Excitonic emission up to the 4s excited state is directly observed in photoluminescence spectroscopy in an out-of-plane magnetic field up to 31 Tesla. We confirm the progressively larger exciton size for higher energy excited states through diamagnetic shift measurements. This also enables us to estimate the 1s exciton binding energy to be about 170 meV, which is significantly smaller than most previous reports. The Zeeman shift of the 1s to 3s states, from both luminescence and absorption measurements, exhibits a monotonic increase of -factor, reflecting nontrivial magnetic-dipole-moment differences between ground and excited exciton states. This systematic evolution of magnetic dipole moments is theoretically explained from the spreading of the Rydberg states in momentum space.

We report experimental observation of 2s exciton radiative emission from monolayer tungsten diselenide, enabled by hexagonal boron nitride protected high-quality samples. The 2s luminescence is highly robust and persists up to 150K, offering a new quantum entity for manipulating the valley degree of freedom. Remarkably, the 2s exciton displays superior valley polarization and coherence than 1s under similar experimental conditions. This observation provides evidence that the Coulomb-exchange-interactiondriven valley-depolarization process, the Maialle-Silva-Sham mechanism, plays an important role in valley excitons of monolayer transition metal dichalcogenides. 71.35.Cc 3The coupled spin-valley physics [1] in monolayer (1L) transition metal dichalcogenide (TMDC) semiconductors has inspired great strides towards realizing valleytronic devices harnessing these two-dimensional (2D) materials [2][3][4][5]. The two energetically degenerate 1L-TMDC valleys with opposite angular momentum can be selectively populated with circularly polarized optical excitation, and the valley polarization can be detected both optically [2][3][4] and electrically [5]. Further, coherent superposition of valley excitons can be generated with linearly polarized light [6] or a sequence of laser pulses with opposite circular polarization [7], which allows for rotation of the valley pseudospin with magnetic Zeeman effect or optical Stark effect [8,9]. Such coherent manipulations of valley pseudospin are at the heart of future quantum valleytronic devices, and requires thorough understanding and efficient control of various valley depolarization and decoherence processes.In general, intervalley scattering can occur due to both extrinsic mechanisms such as disorder scattering, and intrinsic mechanisms such as the Coulomb exchange interaction [10]; the competition between these different valley relaxation channels is a topic under active debate [7,[11][12][13]. So far many of the valleytronic studies focus on the 1s exciton, the ground state of Coulomb-bound electron-hole pairs, which is readily accessible in 2D TMDC monolayers [2][3][4][5][6][7][8][9]14]. Excitons also have higher energy states that form the hierarchical Rydberg-like series [15][16][17], similar to a hydrogen atom. It is desirable to access the valley pseudospin of these higher quantum number exciton states, which in previous studies have been employed to demonstrate the exceptionally large exciton binding energy [15][16][17][18][19] and to probe exciton internal quantum transitions [20]. Yet it is relatively challenging to generate radiative emission from these states, as can be understood from Kasha's rule [21]: photon emission quantum yield is appreciable only for the lowest energy excited state, which for the charge neutral exciton, is the 1s state. In this Letter, we report that with efficient removal of disorder and phonon scattering channels, the 2s exciton luminescence from monolayer tungsten diselenide (1L-WSe2) becomes accessible for valleytronic investigat...

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