We present a systematic Raman study of unconventionally-stacked double-layer graphene, and find that the spectrum strongly depends on the relative rotation angle between layers. Rotation-dependent trends in the position, width and intensity of graphene 2D and G peaks are experimentally established and accounted for theoretically. Our theoretical analysis reveals that changes in electronic band structure due to the interlayer interaction, such as rotational-angle dependent Van Hove singularities, are responsible for the observed spectral features. Our combined experimental and theoretical study provides a deeper understanding of the electronic band structure of rotated double-layer graphene, and leads to a practical way to identify and analyze rotation angles of misoriented double-layer graphene.Recently there has been growing interest in double-layer graphene in which the two graphene layers are not conventionally stacked but relatively rotated by an arbitrary angle [1][2][3][4][5][6][7][8][9][10][11][12].Such graphene double layers are expected to display characteristics distinct from both monolayer 2 graphene as well as the extensively studied AB-stacked bilayer graphene [13][14][15][16]. Previous theoretical investigations suggest that electronic and optical properties of double layer graphene will strongly depend on this rotational angle [1][2][3][4][5]. Because the entire range of rotational angles is in principle experimentally accessible via artificial stacking, the properties of rotated double-layer graphene might be tuned to suit the application at hand, making this material a useful component in future nano-electronic devices. Limited low-energy electrical transport measurements have suggested that rotated graphene layers maintain the linear dispersion relation as in single-layer graphene [6,7].Furthermore, angle-resolved photoemission spectroscopy measurement has shown that rotated layers in multilayer epitaxial graphene exhibit weak interlayer interactions [8]. On the other hand, scanning tunnelling microscopy studies in the low rotation angle regime have demonstrated strong interlayer interactions such as carrier velocity renormalization and the occurrence of Van Hove singularities away from the Dirac point energy [9,10]. Despite these suggestive findings and the applications potential, there have unfortunately been no comprehensive experimental studies of the influence of rotation angle on the electronic properties of double-layer graphene.In this Letter we present a systematic experimental and theoretical study of rotated doublelayer graphene. We employ Raman spectroscopy, a powerful tool for investigating the electronic and vibrational properties of carbon-based materials [17][18][19][20], together with theoretical calculations of the electronic-structure-dependent Raman response. We experimentally sample a range of misorientation angles from 0 to 30 degrees in steps of ~1 degree, and we focus on the intensity, peak position, and peak width of the 2D and G Raman modes. Previous limited Raman stud...

We calculate linear and nonlinear optical susceptibilities arising from the excitonic states of monolayer MoS2 for in-plane light polarizations, using second-quantized bound and unbound exciton operators. Optical selection rules are critical for obtaining the susceptibilities. We derive the valley-chirality rule for the second-harmonic generation in monolayer MoS2, and find that the thirdharmonic process is efficient only for linearly polarized input light while the third-order two-photon process (optical Kerr effect) is efficient for circularly polarized light using a higher order exciton state. The absence of linear absorption due to the band gap and the unusually strong two-photon third-order nonlinearity make the monolayer MoS2 excitonic structure a promising resource for coherent nonlinear photonics.

We show that piezoelectric strain actuation of acoustomechanical interactions can produce large phase velocity changes in an existing quantum phononic platform: aluminum nitride on suspended silicon. Using finite element analysis, we demonstrate a piezo-acoustomechanical phase shifter waveguide capable of producing ±π phase shifts for GHz frequency phonons in 10s of μm with 10s of volts applied. Then, using the phase shifter as a building block, we demonstrate several phononic integrated circuit elements useful for quantum information processing. In particular, we show how to construct programmable multi-mode interferometers for linear phononic processing and a dynamically reconfigurable phononic memory that can switch between an ultra-long-lifetime state and a state strongly coupled to its bus waveguide. From the master equation for the full open quantum system of the reconfigurable phononic memory, we show that it is possible to perform read and write operations with over 90% quantum state transfer fidelity for an exponentially decaying pulse.

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