Portable, broadband, and sensitive terahertz time-domain spectrometer

Abstract

Terahertz time-domain spectroscopy (THz-TDS) is a method used in research and industry for non-invasive characterization of products and materials. Many THz-TDS systems rely on parametric conversion in semiconductor crystals to generate and detect phase-locked THz pulses, providing reliable access to frequencies below 3 THz. Accessing higher frequencies, however, often requires a sophisticated near-infrared (NIR) source delivering sub- 100 fs pulses to access the required spectral bandwidth and thin nonlinear crystals (few hundred micrometers thick) to minimize phase mismatch during both the THz generation and detection processes. As a result, broadband THz- TDS configurations rely on laser systems which are often bulky and costly, resulting in inefficient THz generation and detection processes due to a limited nonlinear interaction length in the crystals. To overcome these limitations, we introduce three modules to a THz-TDS system employing a compact and cost-effective pulsed laser. First, a fiberbased component is used to broaden the output laser spectrum and compress the pulse duration. This module provides the NIR frequency content needed for broadband THz generation through optical rectification and a pulse duration short enough to efficiently resolve high THz frequencies during electro-optic sampling. The other two modules utilize a thick nonlinear crystal with a periodically patterned surface to optimize the efficiencies of the broadband THz generation and detection processes. In this configuration, a long nonlinear interaction length is guaranteed while noncollinear phase matching provides access to a broad spectral range. The combination of these modules extends the THz spectrum from 3 THz to beyond 6 THz with a peak dynamic range >50 dB at 3.5 THz.