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In the field of nanomechanics, parametric excitations are of interest since they can greatly enhance sensing capabilities and eliminate cross-talk. However, parametric excitations often rely on externally tuned springs, which limits their application to high quality factor resonators and usually does not allow excitation of multiple higher modes into parametric resonance. Here we demonstrate parametric amplification and resonance of suspended single-layer graphene membranes by an efficient opto-thermal drive that modulates the intrinsic spring constant. With a large amplitude of the optical drive, a record number of 14 mechanical modes can be brought into parametric resonance by modulating a single parameter: the pretension. In contrast to conventional mechanical resonators, it is shown that graphene membranes demonstrate an interesting combination of both strong nonlinear stiffness and nonlinear damping.
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This thesis is mainly devoted to the study of the quantum properties of optical parametric oscillators (OPOs), which are nowadays the sources of the highest-quality quantum-correlated light, apart from fundamental tools in the classical-optics realm, allowing for the conversion of laser light into virtually all regions of the optical spectrum. Regarding its content, the thesis might seem a bit unusual, because two thirds of it are devoted to a self-contained (though dense) introduction to quantum optics, including the quantum physics of harmonic oscillators, the quantization of the electromagnetic field in an open optical cavity and the detection of its output light, as well as the derivation of the basic model and known properties of OPOs. Hence, all the original results of the thesis are contained in the last third, were it is proven that all OPOs can be understood as multi-mode devices whose quantum properties can be explained in terms of three basic phenomena: bifurcation squeezing, spontaneous symmetry breaking, and pump clamping, which are introduced through simple, yet realistic examples.
Phase coherence of charge carriers leads to electron-wave interference in ballistic mesoscopic conductors. In graphene, such Fabry-Perot-like interference has been observed, but a detailed analysis has been complicated by the two-dimensional nature of conduction, which allows for complex interference patterns. In this work, we have achieved high-quality Fabry-Perot interference in a suspended graphene device, both in conductance and in shot noise, and analyzed their structure using Fourier transform techniques. The Fourier analysis reveals two sets of overlapping, coexisting interferences, with the ratios of the diamonds being equal to the width to length ratio of the device. We attribute these sets to a unique coexistence of longitudinal and transverse resonances, with the longitudinal resonances originating from the bunching of modes with low transverse momentum. Furthermore, our results give insight into the renormalization of the Fermi velocity in suspended graphene samples, caused by unscreened many-body interactions.
Oscillators, which produce continuous periodic signals from direct current power, are central to modern communications systems, with versatile applications such as timing references and frequency modulators. However, conventional oscillators typically consist of macroscopic mechanical resonators such as quartz crystals, which require excessive off-chip space. Here we report oscillators built on micron-size, atomically-thin graphene nanomechanical resonators, whose frequencies can be electrostatically tuned by as much as 14%. The self-sustaining mechanical motion of the oscillators is generated and transduced at room temperature by simple electrical circuitry. The prototype graphene voltage controlled oscillators exhibit frequency stability and modulation bandwidth sufficient for modulation of radio-frequency carrier signals. As a demonstration, we employ a graphene oscillator as the active element for frequency modulated signal generation, and achieve efficient audio signal transmission.
We assess the potential of two-terminal graphene-hBN-graphene resonant tunneling diodes as high-frequency oscillators, using self-consistent quantum transport and electrostatic simulations to determine the time-dependent response of the diodes in a resonant circuit. We quantify how the frequency and power of the current oscillations depend on the diode and circuit parameters including the doping of the graphene electrodes, device geometry, alignment of the graphene lattices, and the circuit impedances. Our results indicate that current oscillations with frequencies of up to several hundred GHz should be achievable.
We study the quantum properties of the polarization of the light produced in type II spontaneous parametric down-conversion in the framework of a multi-mode model valid in any gain regime. We show that the the microscopic polarization entanglement of photon pairs survives in the high gain regime (multi-photon regime), in the form of nonclassical correlation of all the Stokes operators describing polarization degrees of freedom.
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