We discuss recent progress in miniaturizing terahertz devices, facilitated by integrated photonic circuits. We show these provide ways to engineer dispersion, achieve field enhancement and realise complex functionalities on a single chip.

We present emission up to 3 THz from a phase-matched terahertz-optical photonic circuit, featuring a co-planar metallic cavity traversed by an optical rib waveguide and a dipolar antenna for efficient out-coupling of terahertz waves.

Photoconductive antennas (PCAs) have proven to be an excellent platform for efficient broadband terahertz (THz) generation and detection, especially in THz spectroscopy and sensing applications. However, since sub-picosecond optical pulses are required for pumping the PCAs, dispersion-induced pulse broadening hampers a full integration of such THz systems to optical fiber links and limits their compactness and portability [1]. In addition, modulation of PCAs is routinely done at 10-100 kHz, while MHz frequencies have not been well investigated. In this work, we demonstrate high-frequency modulation of THz pulses up to 75 MHz with a Mach-Zehnder modulator (MZM) by controlling the intensity of each single optical pump pulse of a 100 MHz femtosecond laser, while biasing the InGaAs PCA with V = 10 V DC. We compare the generated THz signals to direct PCA modulation at the same frequencies. To compensate the dispersion of fiber and free-space components, we implement a tunable Si prism pair and 1.2 m-long dispersion compensating fiber (DCF) in order to suppress the second-order dispersion of the complete system. As shown in the setup in Fig. 1 (a), one prism can be translated to fine-tune the pre-chirp induced into the 1550 nm pump pulse. In contrast to conventional prism setups and grating mirrors, which exploit angular dispersion, we utilize material dispersion since it has the opposite chirp sign and compensates the fiber dispersion at 1550 nm. The prisms exhibit nearly unitary transmission thanks to incidence at Brewster angle.

We demonstrate tunable dispersion compensation of femtosecond pulses at telecom wavelengths for broadband Terahertz generation in 7.5-meter fiber link using a Silicon prism pair. The prism pair enhances the amplitude of the THz E-field in InGaAs photoconductive antenna by up-to factor 5. As part of the fiber system, we employ a Mach-Zehnder modulator for pump intensity modulation at MHz frequencies and determine the dynamic range in electro-optic sampling. The performance is compared to direct modulation of the InGaAs antenna at the same frequencies. Our work establishes high compatibility between fiber and broadband THz systems.

Terahertz science and technology is possibly now at an inflection point where integrated photonic circuits become increasingly viable sources and detectors of such high-frequency radiation. Generation in both second order [1] and third order [2,3,4] nonlinear waveguides and architectures thereof has exploited either optical rectification or microcomb generation with subsequent optical-to-terahertz conversion at a uni-travelling carrier photodiode. These initial demonstrations showcase a possible route towards miniaturized terahertz chips that are seamlessly integrated with photonics.