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 an optically packaged thin film lithium niobate device. We show that this packaged device is capable of cryogenic operation and is resistant to extreme thermal shock.

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.

Recent research revealed that in resonators with deep subwavelength gaps coupled to two-dimensional electron gases, propagating plasmons lead to energy leakage, hindering polaritonic resonance. This study introduces plasmonic reflectors to create an artificial energy stopband, confining terahertz-range plasmons and recovering polaritonic resonances. Using this approach demonstrates a normalized coupling ratio of 0.36, enabling the observation of polaritonic resonances not seen without plasmonic reflectors.

It was recently demonstrated that, in deep subwavelength gap resonators coupled to two-dimensional electron gases, propagating plasmons can lead to energy leakage and prevent the formation of polaritonic resonances. This process, akin to Landau damping, limits the achievable field confinement and thus the value of light-matter coupling strength. In this work, we show how plasmonic reflectors can be used to create an artificial energy stopband in the plasmon dispersion, confining them and enabling the recovery of the polaritonic resonances. Using this approach we demonstrate a normalized light-matter coupling ratio of ^Ω_ω^R₀ = 0.36 employing a single doped quantum well with a resonator’s gap size of 250 nm equivalent to λ/3000 in vacuum, a geometry in which the polaritonic resonances would not be observable in the absence of the plasmonic reflectors.

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.

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.

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.

We will present experiments on ultrastrongly coupled, strongly subwavelength resonators in arrays down to single element. We will discuss the limitations of the planar resonators approach when employing extremely subwavelength gaps and we will present the new developments towards single-object, few electrons ultrastrongly coupled systems.

In this work, by combining an asymmetric Silicon immersion lens configuration and a complementary resonator design, far-field transmission measurement of a single subwavelength split-ring resonator, ultra-strongly coupled to inter Landau level transitions in a single quantum well, is resolved. The highest coupling is 60%, achieved for only 2000 coupled electrons for a single resonator on an InSb quantum well. This technique can be useful to characterize the complex conductivity of micron-sized samples, such as exfoliated graphene or transition metal dichalcogenide flakes, in the THz domain.

In this work, we theoretically and experimentally show that the confinement of an electromagnetic field below critical length-scales can excite high momentum matter resonances and can ultimately limit the light-matter coupling enhancement in an ultrastrong coupling regime.

In this work, we theoretically and experimentally show that the confinement of an electromagnetic field below critical length-scales can excite high momentum matter resonances and can ultimately limit the light-matter coupling enhancement in an ultrastrong coupling regime.

Subwavelength electromagnetic field localization has been central to photonic research in the last decade, allowing us to enhance sensing capabilities as well as increase the coupling between photons and material excitations. The strong and ultrastrong light–matter coupling regime in the terahertz range using split-ring resonators coupled to magnetoplasmons has been widely investigated, achieving successive world records for the largest light–matter coupling ever achieved. Ever shrinking resonators have allowed us to approach the regime of few-electron strong coupling, in which single-dipole properties can be modified by the vacuum field. Here, we demonstrate, theoretically and experimentally, the existence of a limit to the possibility of arbitrarily increasing electromagnetic confinement in polaritonic systems. Strongly subwavelength fields can excite a continuum of high-momenta propagative magnetoplasmons. This leads to peculiar nonlocal polaritonic effects, as certain polaritonic features disappear and the system enters the regime of discrete-to-continuum strong coupling.

We report a physical limit for reducing the modal volume of a cavity and ultimately increasing the light-matter coupling strength. Extremely confined photonic mode below a critical length-scale can introduce a large in-plane wave vector and excite a continuum of high momentum matter resonances. This excitations act as loss channels and reduce the field confinement by smearing the distribution of surface charges, thus consequently limiting the achievable field enhancement.

We probe ultrastrong light-matter coupling between metallic terahertz metasurfaces and Landau-level transitions in high-mobility two-dimensional electron and hole gases. We utilize heavy-hole cyclotron resonances in strained Ge and electron cyclotron resonances in InSb quantum wells, both within highly nonparabolic bands, and compare our results to well-known parabolic AlGaAs/GaAs quantum well systems. Tuning the coupling strength of the system by two methods, lithographically and by optical pumping, we observe a behavior clearly deviating from the standard Hopfield model previously verified in cavity quantum electrodynamics: An opening of a lower polaritonic gap.

We present our investigation on ultrastrong coupling of the Landau level transitions in a two dimensional electron gas to complementary split ring resonator arrays, as a terahertz metasurface. To achieve a higher coupling rate, the capacitive gap of the resonator is reduced systematically. This increment in the coupling rate is expected due to the enhanced vacuum field fluctuations in a smaller volume mode. However, our results indicate that the gap size reduction introduces a broadening in the upper polariton branch below a certain gap width.