This Letter reports the simultaneous excitation of two terahertz plasma waves in field-emission-driven microdischarges. As demonstrated by first-principle particle-in-cell simulations, we reveal that one wave results from intermittent field emission due to space-charge effects, while the other arises from a two-stream instability between cathode field-emission electrons and anode multipactor electrons, which can trap less energetic electrons, leading to circumnutation trajectories and increased collisionality. A dispersion relation is derived from fluid equations, showing good agreement with the simulated wave parameters. The findings of this Letter provide a foundation and potential pathway for designing novel terahertz sources at miniaturized scales.

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• Electrostatic waves & oscillations
• Field emission
• Plasma instabilities
• Plasma ionization
• Plasma-wall interactions
• Terahertz generation in plasmas
• Low-temperature plasma
• First-principles calculations in plasma physics
• Hydrodynamics
• Monte Carlo methods
• Particle-in-cell methods
• Test-particle methods