Abstract
We report a self-triggered asynchronous optical sampling terahertz spectroscopy system based on a single bidirectional mode-locked fiber laser and plasmonics-enhanced photoconductive nanoantennas. The fiber laser generates two optical mutually coherent pulse trains with a stable repetition rate difference, enabling time-domain terahertz spectroscopy without using any mechanical delay line, stabilization electronics, or external trigger. The resolved terahertz spectra over a 0.1–2 THz frequency range and a 30-second measurement time show more than a 70-dB dynamic range, revealing water absorption lines matching the HITRAN database, through a light-weight and compact spectroscopy setup.
Highlights
A promising alternative for introducing a variable time-delay between the pump and probe beams is optical sampling by cavity tuning (OSCAT)[14], which controls the time-delay by tuning the repetition rate of the femtosecond laser
It was proved to have similar utility in THz-TDS and terahertz asynchronous optical sampling systems (ASOPS-THz-TDS) based on electronic delay lines or multiple lasers were developed over the last few years[20,21,22,23]
When there is no average birefringence between the two pulse trains and the oscillator is wavelength multiplexed with a filtering mechanism, as in the recent demonstrations of ASOPS-THz-TDS and dual THz comb heterodyning[30,37], very small sampling steps may only be accomplished by cavity dispersion management since there is a practical limit to repetition rate
Summary
We report a self-triggered asynchronous optical sampling terahertz spectroscopy system based on a single bidirectional mode-locked fiber laser and plasmonics-enhanced photoconductive nanoantennas. Similar to OSCAT and ECOPS techniques, SLAPCOPS THz-TDS requires a phase control unit to precisely control the repetition rate difference between the pump and probe laser pulse trains and the jitter in the phase-control electronics can have detrimental effects on the spectral accuracy, dynamic range, and bandwidth of the spectroscopy system. Another promising alternative for high-speed THz-TDS is asynchronous optical sampling (ASOPS). The generated interferogram, which serves as the trigger signal, is sent to the PCI digitizer for data acquisition and time-base creation
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