Electrical Control of Second-Harmonic Generation in a WSe2 Monolayer Transistor (1504.05247v1)

Abstract: Nonlinear optical frequency conversion, in which optical fields interact with a nonlinear medium to produce new field frequencies, is ubiquitous in modern photonic systems. However, the nonlinear electric susceptibilities that give rise to such phenomena are often challenging to tune in a given material, and so far, dynamical control of optical nonlinearities remains confined to research labs as a spectroscopic tool. Here, we report a mechanism to electrically control second-order optical nonlinearities in monolayer WSe2, an atomically thin semiconductor. We show that the intensity of second-harmonic generation at the A-exciton resonance is tunable by over an order of magnitude at low temperature and nearly a factor of 4 at room temperature through electrostatic doping in a field-effect transistor. Such tunability arises from the strong exciton charging effects in monolayer semiconductors, which allow for exceptional control over the oscillator strengths at the exciton and trion resonances. The exciton-enhanced second-harmonic generation is counter-circularly polarized to the excitation laser, arising from the combination of the two-photon and one-photon valley selection rules that have opposite helicity in the monolayer. Our study paves the way towards a new platform for chip-scale, electrically tunable nonlinear optical devices based on two-dimensional semiconductors.

• The paper reveals that SHG intensity in a WSe₂ monolayer transistor can be tuned over fourfold at room temperature and an order of magnitude at low temperatures.
• It employs resonant excitonic charging effects to precisely control oscillator strengths, achieving up to 15 times enhancement at the A exciton resonance.
• The experimental setup, featuring optical parametric amplification, micro-PL, and polarization-resolved measurements, uncovers key insights into valley selection rules and device integration.

Electrical Control of Second-Harmonic Generation in a WSe₂ Monolayer Transistor

In this paper, the authors present a sophisticated approach to manipulating the nonlinear optical behavior of atomically thin semiconductors, specifically focusing on WSe₂ monolayers. The paper integrates semiconductor physics and nonlinear optics, elucidating the electrical control of second-order optical nonlinearities. It highlights the significance of this control mechanism for applications in chip-scale photonic devices.

The intensity of second-harmonic generation (SHG) in a monolayer WSe₂ transistor has been demonstrated to be electrically tunable. By employing a field-effect transistor (FET), the research showcases that the SHG intensity can be varied by over a factor of four at room temperature. At low temperatures, the modulation extends over an order of magnitude. This tunability emerges from the resonant responses of neutral and charged excitons, influenced by electrostatically controlled charging effects, presenting a novel mechanism distinct from traditional electric-field-induced SHG.

The SHG manipulation leverages the strong excitonic charging effects characteristic of TMDs, which enable precise control over oscillator strengths at exciton and trion resonances. Experimentally, this manifests as resonant SHG being subject to significant enhancement when on-resonance with the A exciton, achieving over 15 times the intensity compared to off-resonance values. Impressively, the effective second-order susceptibility is approximately ~60 pm/V, analogous to elevated-energy excitations in similar semiconductors.

Key experimental procedures include the use of an optical parametric amplifier for SHG and PL measurements, confirming the quadratic power dependence of SHG under various conditions. Notably, the micro-PL setup and polarization-resolved techniques underscore the mechanistic insights into valley selection rules, revealing counter-circular polarization relationships between excitation and SHG in these monolayers.

The implications of this work are manifold. Practically, this research opens pathways for developing electrically tunable nonlinear optical devices integrated with current semiconductor technology, potentially revolutionizing on-chip optical communications. Additionally, from a theoretical perspective, it provides deeper understanding into the correlation between crystal symmetry, valley physics, and optical selection rules, which could inform future material design and device engineering in 2D semiconductor systems.

Looking ahead, advancements in device engineering, including optimized gating strategies and waveguide integration, promise enhanced tunability and efficiency. Further, these findings set the stage for bandgap engineering via alloying and heterostructure formations, potentially expanding the wavelength range accessible for second-harmonic processes. Such CMOS-compatible architectures suggest promising avenues for applications extending to optical signal processing and integrated photonic circuits.