Researchers have developed a new terahertz (THz) spectroscopic system that incorporates 3D-printed neural networks to manipulate and analyze THz light. The system utilizes a titanium-sapphire laser to generate ultrashort optical pulses, which are then used to produce and detect THz radiation. One part of the laser beam activates a plasmonic photoconductive nano-antenna array, the THz emitter, generating pulses of THz light. These THz pulses are then directed and focused onto a detector, another high-sensitivity plasmonic nano-antenna array, using off-axis parabolic mirrors. The remaining part of the optical laser beam is used to trigger the THz detector after passing through a delay line, allowing for time-domain measurements.
The detected signal, representing the interaction of THz and optical fields, is amplified and processed to obtain a time-domain signal. This system boasts a high signal-to-noise ratio, exceeding 90 decibels, and can operate across a broad bandwidth of up to 5 THz. Measurements are taken rapidly, with each trace captured in 5 seconds and averaged over 10 pulses within a 400 picosecond time window.
A key feature of this development is the integration of computationally designed diffractive neural networks. Fabricated using high-resolution 3D printing, these networks are positioned within the THz beam path, between the emitter and detector. These networks, characterized by a 1×1 centimeter input aperture, are designed to perform specific optical functions on the THz beam, such as spectral filtering. The design process involves complex simulations based on the Rayleigh-Sommerfeld equation of diffraction, allowing for precise control over the amplitude and phase modulation of the THz waves as they propagate through the 3D-printed layers.
The neural networks are digitally designed and optimized using machine learning techniques, considering the material properties and fabrication constraints of the 3D printing process. A training process is employed to refine the network parameters for desired spectral responses, balancing factors like signal power and spectral resolution. The resulting 3D structures, constructed from materials with specific refractive properties in the THz range, act as sophisticated optical elements capable of performing complex THz signal processing tasks. This advancement paves the way for new possibilities in THz imaging, sensing, and communication technologies.
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