Sukhdeep Dhillon

The discovery that fundamental lattice vibrations can carry angular momentum allows non‐magnetic materials to exhibit properties that until now have only been observable in magnetic materials. However, the role of these specific vibrations, commonly known as chiral phonons, in driving ultrafast spin dynamics has not yet been explored experimentally. Here, using broadband terahertz emission spectroscopy, we demonstrate that the non‐magnetic ferroelectric generates signals consistent with a ultrafast spin current after excitation with an ultrafast laser pulse, which we attribute to a chiral phonon–induced spin Seebeck effect. We suggest that the transient heating creates an non‐equilibrium phonon distribution, resulting in a net angular momentum and magnetization perpendicular to the thermal gradient, which is converted into a terahertz electromagnetic pulse via an inverse spin‐Hall effect in a thin metal overlayer. The emitted field reverses sign upon flipping the sample orientation or replacing Pt with W, consistent with spin‐current generation. Polarization‐resolved measurements reveal controllable terahertz ellipticity, indicating an additional polarization‐dependent ultrafast response. Our results establish chiral phonons as a route for generating ultrafast spin currents in non‐magnetic materials as well as providing a pathway toward terahertz sources with switchable handedness.

Quantum cascade lasers (QCLs) are unipolar semiconductor lasers first demonstrated in 1994. Since then, they have played a central role in advancing mid-infrared and terahertz photonics, becoming among the most reliable light sources in these regions of the electromagnetic spectrum. Their importance is further reinforced by their ability to generate self-starting optical frequency combs, whose investigation is motivated both by fundamental physics and by a wide range of applications, including molecular spectroscopy and free-space optical communications. This Roadmap provides a unified overview of current advances and emerging directions in QCL research. The chapters are organized into three main sections: device design and technology; frequency combs and pulse formation; and applications of QCLs. Each chapter reviews the relevant background, summarizes the current state of the art, and identifies key challenges and future directions within its specific research area.

With their tunable intraband absorption in the THz range and carrier dynamics on the order of tens of picoseconds, large HgTe nanocrystals (NCs) hold strong potential for THz optoelectronic devices, including modulators and detectors. Their THz intraband absorption has been attributed to multiple transitions of single carriers between quantized states. However, since most investigations have been limited to room temperature, many fundamental physical questions remain open. Here, we investigate the THz intraband absorption in both linear and nonlinear regimes across a broad temperature range. Our results highlight the essential role of thermally activated carriers in the linear regime, which has been so far neglected in existing models, and show that nonlinear intraband absorption in large HgTe NCs arises from an increase in chemical potential combined with changes in electronic temperature, both influenced by saturation effects associated with the progressive filling of electronic states. Furthermore, using mid-infrared pump–THz probe experiments, we probe the carrier dynamics and demonstrate that surface traps and ligand modes have a negligible effect on hot carrier recombination. The dominant recombination pathway is attributed to non-radiative interband processes mediated by optical phonon emission. This work provides new insight into the physical mechanisms underlying the THz intraband absorption of HgTe NCs in both linear and nonlinear regimes, refines their theoretical description, and highlights the potential of large HgTe NCs for THz optoelectronic devices such as modulators and detectors.

Van der Waals heterostructures and polymorphs have promised the realization of artificial materials with multiple physical phenomena such as giant optical nonlinearities, spin‐to‐charge interconversion in spintronics and topological carrier protection, through an infinitely diverse set of 2D quantum materials and their stacking order in a single layered device. However, their exploitation for the terahertz range has been limited with most investigations based around the optical domain, owing to the use of exfoliated material that inherently limits both the dimensions of the materials and the scalability for applications. Here, the combination of terahertz electronic and spin currents is demonstrated through the realization of large area complex crystalline heterostructures of topological insulators, transition metal dichalcogenides (TMDs) and ferromagnets. This is demonstrated through down‐conversion of optical beams into coherent terahertz currents, where the terahertz phase permits the decoupling of the physical phenomena, and bringing novel functionalities beyond those achievable in simple homostructures. In particular, the role of different TMD polymorphs (stacking orders ‐ 1T′, 2H, and 3R) is shown with the simple change of one atomic monolayer of the material stack entirely changing the terahertz responses – both electrical and magnetic ‐ of the artificial material. This allows to highlight ultrafast phenomena that combine both the electronic‐ and spin‐based processes in these structures. The importance of the crystal symmetry on the magnetic properties through proximity effects is further demonstrated, showing a nonlinearity of magnetic origin as a result of the 1T′ polymorph. As well as control of the terahertz currents through different polymorphs, this scalable integration of a set of highly diverse materials establishes a platform for next‐generation 2D heterostructures that integrate photonic, electronic, and spintronic properties into device architectures.

Coherent ultrafast detection has become an important method to study the temporal response of terahertz (THz) quantum cascade lasers (QCLs), bringing insights into the dynamics of modelocking and frequency comb operation in these complex structures. Coherent detection has been typically based on the use of nonlinear crystals and electro‐optic sampling, which are less sensitive to QCLs operating at high THz frequencies. This is because their response drops rapidly with frequency owing to phase matching conditions. Here, we develop coherent detection based on THz photoconductive antennas in vertical quarter‐wavelength cavities, where we can freely engineer the spectral response to enhance the THz detection at frequencies greater than 2 THz. We develop thick low temperature grown GaAs that is transferred onto polymer and metal coated substrates to create a cavity with an THz response that exceeds the response of non‐cavity detectors. This vertical THz cavity also permits the planar electrode geometry to be designed independently. Indeed, we show that the THz cavity can be combined with large surface area single contact electrodes to further enhance the spectral response. Although this proof‐of‐principle coherent detection is not fully optimised, it is used to coherently resolve the temporal response of a double‐metal THz QCL operating at 3 THz. This approach opens up perspectives to tune the response of THz photoconductive antennas and enhance their spectral response at a desired frequency.

Spintronic terahertz emitters, based on optically triggered spin-to-charge conversion processes, have recently emerged as a novel route toward compact and efficient THz sources. The next challenge for technologically relevant devices, however, remains to modulate the emission with low-energy consumption during operation. In order to achieve this, ferroelectric materials coupled to active spin–orbit layers such as two-dimensional transition metal dichalcogenides are potential candidates. In this work, we present the realization of a large area heterostructure of CoFeB/PtSe2/MoSe2 on a macroscopically bidomain LiNbO3 substrate. Using THz time-domain spectroscopy, we show that the ferroelectric polarization direction induces a sizable modulation of the THz emission. We rationalize these experimental results by using band structure and spin accumulation calculations that are consistent with an interfacial spin-to-charge conversion mediated by the inverse Rashba–Edelstein effect at the MoSe2/PtSe2 interface and being tuned by ferroelectricity in the adjacent LiNbO3 surface. This work demonstrates the relevance and technological potential of field effect spin–orbit architectures for novel THz technologies.

Interband cascade lasers (ICLs) are semiconductor lasers emitting in the mid-wave infrared (MWIR 3–6 μm) and can operate as frequency combs (FCs). These demonstrations are based on double section cavities that can reduce dispersion and/or are adapted for radio frequency operation. Here, we show that ICL FCs at long wavelengths, where the refractive index dispersion reduces, can be realized in a single long section cavity. We show FC generation for ICLs operating at λ ∼ 4.2 μm, demonstrating narrow electrical beatnotes over a large current range. We also reconstruct the ultrafast temporal response through a modified shifted wave interference Fourier transform spectroscopy setup with two fast MWIR detectors, which shows a frequency modulated response in free running operation. Further, we show that, through active mode-locking, the ICL can be forced to generate short pulses on the order of 3 ps. This temporal response is in agreement with Maxwell–Bloch simulations, highlighting that these devices possess long dynamics (∼100 ps) and potentially makes them appropriate for the generation of large peak powers in the MWIR.

Van der Waals heterostructures have promised the realisation of artificial materials with multiple physical phenomena such as giant optical nonlinearities, spin-to-charge interconversion in spintronics and topological carrier protection, in a single layered device through an infinitely diverse set of quantum materials. However, most efforts have only focused on exfoliated material that inherently limits both the dimensions of the materials and the scalability for applications. Here, we show the epitaxial growth of large area heterostructures of topological insulators (Bi2Se3), transition metal dichalcogenides (TMDs, WSe2) and ferromagnets (Co), resulting in the combination of functionalities including tuneable optical nonlinearities, spin-to-charge conversion and magnetic proximity effects. This is demonstrated through coherent phase resolved terahertz currents, bringing novel functionalities beyond those achievable in simple homostructures. In particular, we show the role of different TMD polymorphs, with the simple change of one atomic monolayer of the artificial material stack entirely changing its optical, electrical and magnetic properties. This epitaxial integration of diverse two-dimensional materials offers foundational steps towards diverse perspectives in quantum material engineering, where the material polymorph can be controlled at technological relevant scales for coupling applications in, for example, van der Waals nonlinear optics, optoelectronics, spintronics, multiferroics and coherent current control.

By leveraging the unique properties of epitaxially grown two-dimensional heterostructures, we demonstrate giant terahertz electronic and spin-currents that can be controlled and combined by the heterostructure stacking and the material polymorphs at the monolayer level.

Unlike traditional group‐6 transition metal dichalcogenides (TMDs), group‐10 TMDs such as PtSe₂ and PdTe₂ possess highly tuneable indirect bandgaps, transitioning from semiconducting in the near‐infrared to semimetal behavior with a number of monolayers (MLs). This opens up the possibility of TMD‐based mid‐infrared and terahertz optoelectronics. Despite this large potential, the optical properties of such materials have shown an extremely large disparity between that predicted and measured. For example, simulations show that a few MLs is required for the semiconductor–semimetal transition, whilst tens of MLs is found experimentally. This is a result of widely used optical extrapolation methods to determine bandgaps, such as the Tauc plot approach, that are not adapted here owing to i) nearby direct transitions, ii) the material dimensionality and iii) large changes in the non‐parabolic bandstructure with MLs. Here, uniquely combining optical ellipsometry to determine the complex permittivity, terahertz time resolved spectroscopy for the complex conductivity and in‐depth density functional theory (DFT) simulations, it is shown that the optical properties and bandstructure can be determined reliably and demonstrate clearly that the semiconductor‐semimetal transition occurs for PtSe₂ layers ≈5 MLs. The microscopic origins of the observed transitions and the crucial role of the Coulomb interaction for thin semiconducting layers, and that of interlayer van der Waals forces for multilayer semimetallic samples are also demonstrated. This work of combining complimentary experimental techniques and extensive simulations avoids the application of constrained extrapolation methods to determine the optical properties of group‐10 TMDs, and will be of importance for future mid‐infrared and terahertz applications.