Harmonic quantum cascade laser terahertz frequency combs enabled by multilayer graphene top-cavity scatters

Area of Science:

• Quantum Optics
• Semiconductor Lasers
• Metamaterials

Background:

• Optical frequency combs are crucial metrological tools, concentrating power into modes spaced by multiples of the cavity free spectral range (FSR).
• Harmonic frequency combs (HFCs) offer increased optical power per mode, making them attractive for quantum correlation studies in quantum cascade lasers (QCLs).
• Generating stable, controlled-order HFCs in electrically pumped QCLs is challenging due to nonlinearity requirements.

Purpose of the Study:

• To demonstrate stable 2nd and 3rd order harmonic frequency comb emission in terahertz QCLs.
• To explore the use of graphene absorbers for controlling HFC generation in QCLs.
• To enable advanced quantum correlation studies using tailored HFCs.

Main Methods:

• Utilized terahertz quantum cascade lasers (QCLs) as the gain medium.
• Engineered distributed multilayer graphene absorbers.
• Patterned individual or coupled graphene absorbers on metallic waveguides to control harmonic generation order.

Main Results:

• Successfully demonstrated stable 2nd order HFC emission using a single graphene absorber.
• Achieved stable 3rd order HFC emission by employing a couple of equally spaced graphene absorbers.
• Showcased a method for controlling HFC order in QCLs via graphene integration.

Conclusions:

• Graphene absorbers provide an effective method for controlling and stabilizing harmonic frequency comb generation in terahertz QCLs.
• This technique facilitates the generation of specific HFC orders, paving the way for novel quantum correlation experiments.
• The integration of graphene offers a promising route for developing advanced metrological sources based on QCLs.