Highly efficient broadband THz upconversion with Dirac materials

The use of the THz frequency domain in future network generations offers an unparalleled level of capacity, which can enhance innovative applications in wireless communication, analytics, and imaging. Communication technologies rely on frequency mixing, enabling signals to be converted from one frequency to another and transmitted from a sender to a receiver. Technically, this process is implemented using nonlinear components such as diodes or transistors. However, the highest operation frequency of this approach is limited to sub-THz bands. Here, we demonstrate the upconversion of a weak sub-THz signal from a photoconductive antenna to multiple THz bands. The key element is a high-mobility HgTe-based heterostructure with electronic band inversion, leading to one of the strongest third-order nonlinearities among all materials in the THz range. Due to the Dirac character of electron dispersion, the highly intense sub-THz radiation is efficiently mixed with the antenna signal, resulting in a THz response at linear combinations of their frequencies. The field conversion efficiency above 2$%$ is provided by a bare tensile-strained HgTe layer with a thickness below 100 nm at room temperature under ambient conditions. Devices based on Dirac materials allow for high degree of integration, with field-enhancing metamaterial structures, making them very promising for THz communication with unprecedented data transfer rate.

💡 Research Summary

The paper presents a breakthrough approach to terahertz (THz) frequency up‑conversion that overcomes the sub‑THz limitations of conventional nonlinear components such as diodes and transistors. The authors employ a high‑mobility, tensile‑strained HgTe heterostructure whose electronic band inversion creates a three‑dimensional Dirac semimetal at room temperature. This Dirac character yields an exceptionally large third‑order nonlinear susceptibility (χ^(3)), one of the strongest reported for THz frequencies.

In the experimental configuration, a narrow‑band 0.7 THz pulse from the accelerator‑based TELBE source serves as a strong pump, while a photoconductive antenna (PCA) driven by a synchronized Ti:sapphire amplifier generates a weak broadband signal spanning 0.1–0.5 THz. Both beams are co‑propagated through the HgTe layer (70 nm thick, grown on a CdTe substrate with CdHgTe buffer and cap layers). Four‑wave mixing (FWM) in the Dirac material produces two up‑converted THz bands at frequencies f_low = 2f_T − f_a and f_high = 2f_T + f_a, where f_T is the pump frequency and f_a the PCA signal frequency. Electro‑optical sampling (EOS) records the time‑domain waveforms, and Fourier analysis reveals clear peaks in the 0.9–1.3 THz (low) and 1.5–1.9 THz (high) ranges.

The measured field conversion efficiency κ = E_low/E_a exceeds 2 % and reaches up to 2.5 % for certain PCA frequencies, corresponding to a conversion loss of less than 20 dB. This performance is achieved with a bare HgTe film thinner than 100 nm, operating at ambient temperature and pressure—far superior to prior demonstrations that required cryogenic conditions or micrometer‑scale bulk samples. The extracted χ^(3) ≈ 6.4 × 10⁻¹⁰ m² V⁻² matches the record values reported for HgTe quantum‑well structures and graphene, confirming the material’s strong nonlinearity.

A detailed theoretical analysis based on the acceleration model shows that χ^(3) scales as η τ³ v_F k_F, where η≈1 reflects the non‑parabolic dispersion, τ is the carrier scattering time, v_F the Fermi velocity, and k_F the Fermi wavevector. Pump‑probe measurements at the FELBE facility yield τ ≈ 0.2 ps, while fitting of the conversion efficiency suggests an effective τ ≈ 0.5 ps—both longer than in other Dirac systems, thereby enhancing the nonlinear response.

Polarization studies reveal that the up‑conversion is maximal when the pump and signal fields are parallel (α = 0° or 180°) and reduced but still finite for orthogonal polarizations (α = 90°). This deviates from the simple |cos α| dependence predicted by a naïve semiclassical model, indicating that more sophisticated mechanisms—such as momentum‑space displacement during scattering or ultrafast carrier heating—contribute.

The dependence of the up‑converted field on the pump strength follows a power law E_low ∝ E_T^β with β = 1.6 ± 0.1, close to the theoretical β = 2 derived from Eq. (2). Slight sub‑quadratic behavior is attributed to saturation effects at the highest pump fields (E_T ≈ 86 kV cm⁻¹).

Overall, the work demonstrates that a thin, strained HgTe Dirac semimetal can serve as an ultra‑compact, room‑temperature THz frequency mixer with conversion efficiencies surpassing 2 %. Its compatibility with planar integration and field‑enhancing metamaterials makes it a promising platform for future high‑speed THz communication links, imaging systems, and spectroscopy applications. The authors suggest that further improvements could be achieved by incorporating resonant metamaterial antennas to boost local fields or by stacking multiple HgTe layers to increase the effective interaction length while preserving the Dirac dispersion.