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

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.

💡 Research Summary

This paper presents a novel on‑chip method for generating terahertz (THz) pulses that simultaneously possess long temporal duration and broad spectral bandwidth, overcoming the conventional Fourier‑transform limit that ties pulse length to spectral width. The authors achieve this by spatial‑spectral mapping: the broadband spectrum of an ultrashort femtosecond pump is dispersed across multiple, diffraction‑free waveguide channels fabricated in a lithium‑niobate (LN) chip. Each channel acts as a narrowband THz source whose central frequency is set by the effective permittivity of the waveguide, which is controlled through the geometry of lithium‑niobate pillars and air slots.

The basic building block is a slot waveguide array (SWA). Finite‑difference‑time‑domain (FDTD) simulations show that by choosing pillar width l, thickness h, and slot width s, the phase‑velocity of the THz mode can be matched to the group velocity of the pump at a desired frequency. For example, an SWA with l = 133 µm, h = 50 µm, s = 8 µm yields phase‑velocity matching at 0.36 THz, producing a narrowband (≈23 GHz) THz pulse. Electro‑optic sampling of a single‑channel device confirms a ~60 ps long THz waveform centered at 0.36 THz.

To obtain broadband coverage, the authors cascade several SWAs with gradually varying pillar widths on the same chip, forming a “gradient SWA.” Six parallel channels are fabricated with pillar widths ranging from 135 µm down to 90 µm, generating six discrete frequencies (0.353, 0.370, 0.384, 0.406, 0.416, 0.433 THz). Each channel emits a ~50 ps narrowband pulse; when these pulses are summed in time they produce a composite THz waveform that lasts ~60 ps and spans the 0.35–0.44 THz band. Measured spectra match the design targets, and full‑wave simulations reproduce the temporal and spectral behavior, confirming that the spatial‑spectral mapping works as intended and that diffraction‑induced interference—problematic in free‑space metasurfaces—is eliminated by the on‑chip confinement.

The authors exploit this multi‑frequency capability for scan‑free biosensing. They model the permittivity of three amino acids (histidine, tyrosine, glutamic acid) whose characteristic absorption peaks lie at 0.773, 0.963, and 1.201 THz. A three‑channel gradient SWA is designed with pillar widths of 82.4 µm, 53.4 µm, and 38.2 µm to emit THz tones exactly at those frequencies. A 5 µm‑thick, 2 mm‑wide mixture of the amino acids is placed on the chip; the transmitted THz power at each tone is recorded. By comparing the attenuation at the three frequencies, the system correctly identifies pure compounds, binary mixtures, and the ternary mixture, demonstrating parallel, instantaneous spectral readout without mechanical scanning.

In summary, the work introduces (1) a spatial‑spectral mapping strategy that decouples temporal and spectral design, (2) a gradient LN waveguide array that generates long‑duration, broadband THz pulses on a monolithic chip, and (3) a proof‑of‑concept scan‑free spectroscopic sensor for biomolecular identification. The approach opens pathways for integrated THz systems in high‑speed communications, on‑chip quantum information processing, and multi‑modal spectroscopy. Current limitations include the modest frequency range (0.35–0.44 THz) and pulse length (~60 ps); extending the bandwidth and duration will require further geometric optimization and low‑loss transmission structures.