Development of an On-Chip THz Spectrometer with Metamaterial Waveguides

The Terahertz spectral region is rich with spectral features of many physical systems. Common examples are the vibrational modes of organic compounds[1], rotational modes of molecular gas species, inter sub-band transitions in quantum confined structures, and crystal field splitting levels in rare earth doped glasses[2]. Continuous wave (CW) THz spectrometers have also been demonstrated and commercially exploited using photodiode sources LT-InGaAs photoconductive detectors[3] These systems typically offer superior spectral resolution when compared to the more commonly utilised THz time domain spectrometers (TDS). Typical Terahertz spectroscopy systems rely on interactions with a propagating wave guided by off-axis parabolic mirrors and a sample placed in either a collimated beam path or at a focal point of the mirrors. These systems are diffraction limited and interaction with samples that have a small cross section, for example small quantities of molecules is a challenge. Strongly subwavelength interactions have been demonstrated using a variety of techniques based around modified TDS systems such as aperture probe, scattering probe, and on-chip waveguide near field interactions. Moving towards confined near field interactions for CW spectrometers would bring increased spectral resolution to this area of research and better resolve transitions in small volume samples. Moreover, reducing the interaction volume through confinement of the terahertz fields on chip could enable nonlinear interactions with such samples in the strong-coupling regime. On chip spectrometers provide a useful tool for investigation of samples where the interaction cross section is much smaller than the wavelength of the probe radiation. Additionally, using metamaterials one can investigate enhancing the field of the THz radiation either through incorporation of resonator elements or through the waveguide itself. Here we demonstrate the first proof of concept of a monolithic Uni-travelling carrier photodiode (UTC-PD) based spectrometer integrated with a metamaterial waveguide to confine and guide the THz fields between the source and detector. The on-chip spectrometer is characterised in terms of bandwidth and dynamic range and compared against an equivalent free space spectrometer using antenna integrated UTC-PDs as the source and receiver.<br/>[1] M. B. Byrne <i>et al.</i>, “Terahertz vibrational absorption spectroscopy using microstrip-line waveguides,” <i>Appl. Phys. Lett.</i>, vol. 93, no. 18, p. 182904, Nov. 2008.<br/>[2] J. Seddon, R. Hermans, L. Ponnampalam, H. Shams, A. Seeds, and G. Aeppli, “Ultra-high-resolution software-defined photonic terahertz spectroscopy,” <i>Optica</i>, Sep. 2020.<br/>[3] D. Stanze, A. Deninger, A. Roggenbuck, S. Schindler, M. Schlak, and B. Sartorius, “Compact cw Terahertz Spectrometer Pumped at 1.5 μm Wavelength,” <i>J. Infrared Millim. Terahertz Waves</i>, vol. 32, no. 2, pp. 225–232, 2011.