Quasi-Phase-Matching Enabled by van der Waals Stacking of 3R-MoS2 for Enhanced Second Harmonic Generation and Spontaneous Parametric Down-Conversion

Core Concepts

This research demonstrates a novel method for achieving quasi-phase-matching (QPM) in nonlinear optical processes using van der Waals stacking of 3R-MoS2 layers, leading to enhanced second harmonic generation (SHG) and spontaneous parametric down-conversion (SPDC) for potential applications in various optical and quantum technologies.

Abstract

• Bibliographic Information: Tang, Y., Sripathy, K., Qin, H. et al. Quasi-Phase-Matching Enabled by van der Waals Stacking. (2024).
• Research Objective: To demonstrate a new method of achieving quasi-phase-matching (QPM) for nonlinear optical processes using van der Waals stacking of 3R-MoS2 layers, enhancing second harmonic generation (SHG) and spontaneous parametric down-conversion (SPDC).
• Methodology: The researchers exfoliated 3R-MoS2 flakes and stacked them with specific twist angles. They then used a femtosecond laser to generate SHG and measured the intensity as a function of twist angle and stacking order. SPDC was also measured using a continuous-wave laser, analyzing the photon pair correlations.
• Key Findings:
  • Stacking 3R-MoS2 layers with a 60° twist angle resulted in a significant enhancement of SHG intensity, exceeding the non-QPM limit.
  • The SHG enhancement was attributed to the constructive interference of SH waves due to the periodic modulation of nonlinear susceptibility achieved by the specific stacking order.
  • Enhanced SPDC was also observed in the stacked 3R-MoS2 structure, indicating an increased efficiency in generating entangled photon pairs.
• Main Conclusions:
  • Van der Waals stacking of 3R-MoS2 provides a practical and efficient way to achieve QPM for nonlinear optical processes.
  • This technique offers a promising platform for developing compact and tunable nonlinear optical devices for applications in frequency conversion, quantum communication, and other related fields.
• Significance: This research offers a new approach to QPM that overcomes limitations of traditional methods relying on periodically poled ferroelectric crystals. It paves the way for miniaturized, highly efficient nonlinear optical devices based on 2D materials.
• Limitations and Future Research: Future research could explore different 2D materials and stacking configurations to optimize QPM for specific wavelengths and applications. Further investigation into the long-term stability and scalability of these van der Waals stacked devices is also needed.

Stats

The SHG conversion efficiency (η) for 3R-MoS2 at a pump wavelength of 1550 nm was measured to be 4.64 × 10−6 under a power density of 30 GW/cm².
The coherence length (tc) for 3R-MoS2 for 1550 nm to 775 nm conversion was determined to be 572 ± 5 nm.
A three-layer stack of 3R-MoS2, with each layer having a thickness close to tc, demonstrated a 7.5-fold enhancement in SHG intensity compared to a single layer.
The damage threshold power density for the 3R-MoS2 samples at 1550 nm was found to be 34.82 GW/cm².
The SPDC measurements showed a peak-to-background ratio well above 2 at zero time delay (g(2)(0) ~ 4.5) for the stacked 3R-MoS2 structure, confirming the generation of correlated photon pairs.

Quotes

"This technique of using van der Waals stacking of 3R-MoS2 enables fine-tuning of the phase-matching condition and provides a larger interaction length to realize efficient and tunable nonlinear optical processes, with potential applications in areas such as frequency conversion, optical switching, and quantum communications."
"Our results highlight the potential of fabricating an incredibly small and lightweight van der Waals nonlinear crystal with QPM and a superior SHG to weight ratio, surpassing their bulky 3D counterparts."
"The adjustability of the twist angle in van der Waals stacking QPM holds significant promise in engineering SPDC-based quantum devices aimed at producing tailored quantum states like Bell and N00N states for applications in quantum optics and communication."

Key Insights Distilled From

Quasi-Phase-Matching Enabled by van der Waals Stacking

by Yilin Tang, ... at arxiv.org 11-04-2024

https://arxiv.org/pdf/2411.00303.pdf

Deeper Inquiries

How might this technique of van der Waals stacking for QPM be applied to other nonlinear optical phenomena beyond SHG and SPDC?

This technique of van der Waals stacking for quasi-phase-matching (QPM) holds immense potential for a wide array of nonlinear optical phenomena beyond second harmonic generation (SHG) and spontaneous parametric down-conversion (SPDC). Here are some promising avenues:

Higher-Order Harmonic Generation: The principle of stacking layers with controlled twist angles to achieve phase matching can be extended to generate higher-order harmonics, such as third-harmonic generation (THG), fourth-harmonic generation (FHG), and beyond. This opens possibilities for generating coherent light in new wavelength regimes, particularly in the ultraviolet and deep-ultraviolet, which are challenging to access with traditional methods.

Sum-Frequency Generation (SFG) and Difference-Frequency Generation (DFG): By carefully selecting the thicknesses and twist angles of the stacked layers, one can engineer the phase-matching conditions to enable efficient SFG and DFG processes. These techniques are valuable for applications such as optical up-conversion, down-conversion, and wavelength conversion in general.

Optical Parametric Oscillation (OPO): Van der Waals stacked structures could pave the way for compact and efficient OPO devices. By incorporating a suitable pump laser and a cavity design that supports the desired signal and idler wavelengths, one can achieve tunable light generation across a broad spectral range.

Nonlinear Optics in the Terahertz Regime: 2D materials like 3R-MoS2 have shown promise for terahertz (THz) generation and detection. QPM through van der Waals stacking could significantly enhance the efficiency of THz wave generation and manipulation, leading to advancements in THz imaging, spectroscopy, and communication systems.

Electro-Optic Modulation: Some 2D materials exhibit strong electro-optic effects, where their refractive index changes in response to an applied electric field. By integrating van der Waals stacked structures into waveguide configurations, one can create highly efficient electro-optic modulators for applications in optical communications and signal processing.
The key advantage of this technique lies in its versatility and the ability to tailor the phase-matching conditions to a specific nonlinear optical process by carefully selecting the materials, layer thicknesses, and twist angles.

Could imperfections in the stacking of the 3R-MoS2 layers, such as variations in twist angle or layer thickness, negatively impact the efficiency of QPM and the overall performance of the device?

Yes, imperfections in the stacking of the 3R-MoS2 layers can significantly impact the efficiency of QPM and the overall device performance. Here's how:

Twist Angle Variations: Precise control over the twist angle between adjacent layers is crucial for achieving the desired phase-matching condition. Even small deviations from the optimal twist angle can lead to a reduction in the overlap of the interacting waves, resulting in decreased SHG or SPDC efficiency. Larger variations can even introduce destructive interference, further diminishing the output signal.

Layer Thickness Fluctuations: The thickness of each 3R-MoS2 layer determines the phase shift experienced by the propagating waves. Variations in layer thickness can disrupt the intended periodic modulation of the nonlinear susceptibility, leading to a mismatch in the phase velocities of the fundamental and harmonic waves. This phase mismatch reduces the effective interaction length and lowers the conversion efficiency.

Lateral Misalignment: If the layers are not perfectly aligned laterally, the overlapping area where QPM occurs will be reduced. This effectively decreases the active region of the device, leading to lower output power and reduced efficiency.

Interface Quality: The presence of contaminants, defects, or air gaps at the interfaces between the stacked layers can introduce scattering losses and disrupt the flow of light, negatively impacting the overall device performance.
Addressing these challenges requires meticulous fabrication techniques. Advances in large-scale growth of high-quality 2D materials, combined with precise transfer and stacking methods, such as those employing robotic arm manipulation, are crucial for minimizing imperfections and realizing the full potential of van der Waals stacked QPM devices.

What are the potential long-term implications of using 2D materials like 3R-MoS2 in developing more efficient and compact quantum computing components?

The use of 2D materials like 3R-MoS2 in quantum computing holds transformative potential, promising more efficient, compact, and scalable quantum computing components. Here's a glimpse into the long-term implications:

Miniaturization of Quantum Devices: The atomically thin nature of 2D materials allows for the creation of extremely compact quantum devices. This is particularly advantageous for quantum computing, where reducing the size of components is essential for increasing qubit density and improving scalability.

Enhanced Photon-Qubit Interactions: 2D materials exhibit strong light-matter interactions, enabling efficient coupling between photons and quantum emitters, such as single-photon sources or quantum dots. This is crucial for developing robust and scalable quantum communication and information processing systems.

On-Chip Integration: The compatibility of 2D materials with existing semiconductor fabrication techniques facilitates their integration with conventional electronic and photonic devices. This paves the way for the development of hybrid quantum-classical systems, where quantum processors can be seamlessly integrated with classical control and readout electronics on a single chip.

Novel Quantum Computing Architectures: The unique properties of 2D materials, such as their tunable bandgaps and the ability to host exotic quantum states, open up possibilities for exploring novel quantum computing architectures. For instance, topological qubits, which are inherently protected from environmental noise, could potentially be realized in certain 2D materials.

Quantum Communication Networks: The efficient generation of entangled photon pairs in 2D materials, as demonstrated by the enhanced SPDC in 3R-MoS2, makes them promising candidates for building quantum communication networks. These networks would enable secure communication channels and distributed quantum computing.

Advancements in Quantum Sensing: The sensitivity of 2D materials to their surroundings makes them attractive for quantum sensing applications. By integrating them into quantum devices, one can create highly sensitive sensors for detecting magnetic fields, electric fields, and other physical quantities with unprecedented precision.
The realization of these long-term implications requires continued research and development efforts focused on addressing challenges such as improving material quality, controlling defects, and developing scalable fabrication techniques. Nevertheless, the unique properties and potential advantages of 2D materials like 3R-MoS2 position them as key players in shaping the future of quantum computing.