Spatial-spectral mapping for long-duration broadband terahertz pulse generation in on-chip waveguide arrays

Abstract

Conventional approaches to terahertz (THz) pulse generation are restricted by the Fourier-transform limit, which hinders the creation of sources that combine long duration with broad bandwidth--a capability crucial for many spectroscopic and sensing applications. In this work, we overcome this challenge in the terahertz domain using an on-chip gradient waveguide array. The key is to spectrally disperse the pulse into spatially separated channels within a lithium niobate chip, effectively decoupling the design of temporal and spectral properties. We validate the source by distinguishing amino acid mixtures, demonstrating its tailored biosensing potential. This work establishes a novel mechanism for integrated THz generation, offering considerable promise for broadband spectroscopy and on-chip photonics.

Figures (4)

• Figure 1: Generation of long-duration broadband THz pulses. (a) High-quality broadband response using gradient metasurfaces. (b) Generation of long-duration, broad THz pulses using a specially designed on-chip gradient waveguide array. (c) The narrowband THz waves generated in different channels forms a long-pulse, broadband THz pulse. (d) THz generation process: the THz waves are generated within LN pillars, parts are coupled and confined to the low-loss air gap while propagating.
• Figure 2: The narrowband THz generation of a specific frequency. (a), The schematic of the proposed SWA. (b), The electric field distribution of the THz wave within the SWA. (c), The dispersion relations of the THz wave and the femtosecond laser pulse. The gray dotted line represents the pump laser pulses, which has a constant effective group index of 2.264. (d), The simulated time and space traces of THz E-field in the SWA. The inset depicts time traces of the THz E-field generated at two positions: $x$ = 0.05 mm and $x$ = 2.5 mm, showing that the oscillations increase as propagating forward. (e) Appearance of the fabricated SWA with designed geometric parameters. (f), Time traces of the THz E-field generated at the detection point in the fabricated SWA (top panel). The spectrum of the generated THz pulse in the fabricated SWA (bottom panel).
• Figure 3: The long-duration, broadband THz pulse generation in a gradient SWA. (a) Geometric parameters of the gradient SWA, and corresponding frequencies of the generated THz waves. (b) Appearance of the fabricated gradient SWA. (c) Time traces of the THz E-field generated in each channel of the gradient SWA. (d) Spectrum of all the THz pulses generated in the gradient SWA (experimental results). (e) Time traces of the THz E-field generated in each channel of the gradient SWA (simulation results). (f) Spectrum of all the THz pulses generated in the gradient SWA (simulation results).
• Figure 4: The on-chip scan-free identification of amino acids with the gradient SWA. (a), The calculated permittivity of His, Tyr and Glu. (b), Diagrams of detection of mixed amino acids. The mixture of amino acids, placed on the gradient SWA, is 5-$\mu$m thick and 2-mm-wide. The inset shows the structural formulas of His, Tyr and Glu. (c), The transmittance of detected samples. (d), Components detected in the samples according to the absorptance and the corresponding settings.