Building efficient and ultrabroadband terahertz (THz) emitters remains a challenge in the field of THz photonics. Recently, spintronic THz sources emerged as a promising platform to fulfill the technological needs in terms of large THz bandwidth, high THz amplitude and versatility. In this talk, I will highlight the recent advances in studying laser-induced ultrafast spin and charge currents in magnetic heterostructures. Our results not only provide a basic understanding of ultrafast spintronic effects but might even allow one to further boost the perfomance of spintronic THz sources in the future.

Terahertz (THz) quantum cascade lasers (QCLs) based on double metal waveguides are compact sources of broadband THz radiation, which can also operate as frequency combs. We present a planarized double metal waveguide THz QCL platform, where the active region is embedded in a low-loss BCB polymer and covered by an extended top metallization. The latter enables placing bonding wires on the sides above the BCB-covered area, hindering the formation of any defects on the active region and enables the fabrication of waveguides with narrow widths below the bonding wire size. This can then be employed as a fundamental mode selection mechanism for comb operation without any side absorbers, and also features improved heat dissipation properties in continuous wave operation. The extended top metallization also enhances the RF properties of the device, as it encompasses a metallic cavity with the global ground plane. Experimentally, we present results on two different device geometries. First is a simple ridge waveguide with a width of 40 μm, narrow enough to act as a mode selection filter. Free-running frequency comb states with bandwidths above 600 GHz and single beatnotes up to -60 dBm are measured. With a strong external RF signal, close to the natural repetition frequency, we can broaden the emission to over 1.4 THz. The second type of device is a tapered waveguide, where the narrow sections act as a transversal mode filter, while the wider ones have lower waveguide losses and provide more gain. Due to a field-enhancement effect in the narrow sections, there is a significant enhancement in the four wave mixing, a third order nonlinear process responsible for comb formation. Free-running devices produce beatnotes close to -30 dBm, three orders of magnitude higher than for ridge devices. Improved comb performance is maintained also for high operating temperatures. A comb bandwidth above 200 GHz and a single beatnote above -60 dBm are measured at 115 K, very close to the maximum lasing temperature of 118 K. Beyond the improved laser and comb performance, the planarized waveguide platform also enables a relatively straightforward co-integration of active and passive elements.

Ultrastrong light-matter coupling has recently been achieved in several experimental platforms at different optical frequencies, leveraging on the collective enhancement of the interaction with the number of excitations and the simultaneous sub-wavelength electromagnetic field localization obtainable employing metallic resonators. Ever shrinking resonators have allowed to approach the regime of few electrons strong coupling, in which single-dipole properties can be modified by the vacuum field. In this work we will discuss two results that are relevant with respect to the limits of achievable light-matter coupling strength and to the measurement of such coupling in single, strongly subwavelength resonators. In the first experiment we demonstrate, theoretically and experimentally, the existence of a limit to the possibility of arbitrarily increasing electromagnetic confinement in polaritonic systems. Strongly sub-wavelength 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 in the regime of discrete-to-continuum strong coupling. In the second part of the work, we show that by combining an asymmetric immersion lens setup and complementary design of metasurfaces we are able to perform THz time domain spectroscopy of an individual, strongly subwavelength meta-atom. We unravel the linewidth dependence of planar metamaterials as a function of the meta-atom number indicating quenching of the Dicke superradiance. We investigate as well ultrastrongly coupled Landau polaritons at the single resonator level, measuring a normalized coupling ratio of 0.6 resulting from coupling of the fundamental mode of a single, deeply subwavelength LC resonatorto a few thousand electrons. Our findings pave the way towards the control of light-matter interaction in the ultrastrong coupling regime at the single electron/single resonator level. The proposed technique is way more general and can be useful to characterize the complex conductivity of micron-sized samples in the THz and sub-THz domain.

The field of photonic integration in system-on-a-chip devices (SoC) has become an enormous focus of research in recent years. Such highly integrated systems are not only of utmost importance for the development of highly integrated smart sensors, but also play an increasingly important role in communication technology, for example in the miniaturization of antenna systems in mobile devices. In many cases, photonic integration relies on the exploitation of the strong energy confinement of surface plasmon polaritons (SPPs) that allow the transport of energy from point to point within highly restricted (electromagnetic) space. While SPPs can be readily generated on flat metal surfaces at optical frequencies, it is much more difficult to obtain and observe SPPs in the terahertz frequency range. The reason for it is the lack of dispersion at terahertz frequencies, which inhibits a strong confinement of the SPPs. In order to increase the energy concentration of SPPs at lower frequencies, it is necessary to engineer the dispersion of the metal surfaces by geometric structuring. In the most general sense, such surfaces can be understood as metasurfaces. The propagating surface waves on such surfaces mimic the properties of SPPs and are called spoof surface plasmon polaritons (SSPPs). Here, we report the design, fabrication and experimental testing of dispersion-engineered metasurfaces that guide and manipulate SSPPs at will. Specifically, we investigate both the out-of-plane and in-plane confinement of the SSPPs during propagation along straight and curved routes on the metasurface. For this purpose, we tailored metasurface pathways of different width and shape. The metasurfaces were fabricated in the cleanroom. The electric field of the propagating SSPPs was measured in amplitude and phase by terahertz near-field spectroscopy, so that we obtained full information about the temporal dynamics of the SSPP propagation. We observed that the SSPPs maintained a strong out-of-plane and in-plane field confinement over the complete propagation distances of several millimeters. In addition, we designed patch antenna arrays that enable a frequency-selective coupling of SSPPs to free space. In combination with specifically implemented sensor properties of the metasurface structure, such combined metasurface/antenna systems can serve as smart, compact SoC devices in the terahertz frequency range.

Remarkably strong THz field emission from nonlinear metasurfaces excited by femtosecond lasers was recently reported. The field emitted from an ultrathin metasurface was shown to be comparable to that emitted from orders of magnitude thicker Zinc Telluride nonlinear crystal. Here we study this effect thoroughly by comparing the emission from metasurfaces fabricated on top of various substrates. We find that the presence of a thin ITO film, commonly used in the electron beam lithography process leads to two orders of magnitude stronger THz emission compared to the case of metasurface on glass. It also shows a different power law, signifying different dominant emission mechanisms. The enhancement is explained by the large optical nonlinearities of ITO at excitation wavelengths where the permittivity is near zero, in addition to further field confinement. We also show that coupling between the free electrons in the ITO and the surface plasmons on the gold nanoparticles leads to dynamic THz emission phenomena that were not reported to date. Specifically, we show that the generated THz pulse can be shortened in time, and thus broadened in frequency with twice the bandwidth compared to previous studies and to an uncoupled system. This phenomenon is attributed to hot electrons dynamics, which alter the optical response of ITO at sub-picosecond time scale. This modifies the coupling between the plasmonic metasurface and the free electrons in the ITO and drastically affect the nonlinear dynamics of the system. These findings shed new light on the fundamentals of THz emission from metasurfaces and open the door to design efficient and dynamic metasurface THz emitters.