Understanding Terahertz Waves and Their Growing Importance

Terahertz waves, often abbreviated as THz, occupy the portion of the electromagnetic spectrum between microwaves and infrared light, with frequencies ranging from 0.1 to 10 THz. These waves hold immense promise for non-invasive imaging, high-speed data communication, and material characterization due to their ability to penetrate non-conducting materials like clothing, plastics, and paper while being absorbed by water and metals.

In Singapore, where advanced manufacturing and 6G telecommunications are national priorities, tunable THz sources could revolutionize sectors like semiconductors, biomedicine, and security screening. Traditional THz sources, however, suffer from limited tunability, high power consumption, and the need for bulky magnetic field modulation, hindering practical deployment.

Dirac Semimetals: The Quantum Playground for Next-Gen Electronics

Dirac semimetals are a class of topological quantum materials characterized by linear energy-momentum dispersion relations resembling those of massless Dirac fermions, first theorized in graphene but realized in 3D bulk forms like platinum ditelluride (PtTe₂). These materials exhibit unique electronic properties, including high carrier mobility and robustness against backscattering, making them ideal for ultrafast optoelectronics.

PtTe₂, a type-II Dirac semimetal, stands out due to its tilted Dirac cones and strong spin-orbit coupling, which amplify nonlinear optical responses. At Nanyang Technological University (NTU), researchers have leveraged these traits to push boundaries in spintronics—the manipulation of electron spin for information processing.

Quantum Geometry: The Hidden Key to Tunable Spintronic Emission

Quantum geometry refers to the geometric properties of Bloch wavefunctions in a material's band structure, quantified by the Berry curvature—a gauge-invariant measure analogous to magnetic fields in momentum space. In Dirac semimetals, this curvature governs anomalous transport phenomena like the spin Hall effect, where transverse spin currents are generated from charge currents without magnetic fields.

By electrically doping PtTe₂ via a ferroelectric substrate, NTU scientists shifted the Fermi level, altering the Berry curvature distribution. This modulation directly tunes the spin Hall conductivity, enabling precise control over THz emission from spin-to-charge conversion in an adjacent ferromagnetic layer.

The NTU Experiment: Engineering a Tunable THz Emitter

The breakthrough device integrates a thin PtTe₂ film with a ferromagnetic heterobilayer (CoFeB/Pt) on a ferroelectric lead magnesium niobate-lead titanate (PMN-PT) substrate. Under a fixed in-plane magnetic field, femtosecond laser pulses excite spin currents in PtTe₂, which are converted to THz radiation via inverse spin Hall effect.

• Electric voltage applied across PMN-PT induces piezoelectric strain, doping PtTe₂ and shifting its Fermi level by up to 100 meV.
• This doping repositions the Fermi surface relative to Dirac points, maximizing Berry curvature hotspots for enhanced spin Hall conductivity.
• Result: 21% peak-to-peak modulation of THz emission amplitude at 0.5 THz, with response time <1 ns.

Density functional theory simulations validated these shifts, confirming quantum geometry's pivotal role.

Spotlight on NTU's Research Powerhouse: Key Contributors

Lead author Ziqi Li, a PhD candidate in NTU's Division of Physics and Applied Physics, spearheaded the experimental design alongside Yingshu Yang and Yuanyuan Guo. Assoc Prof Elbert E.M. Chia and Prof Ranjan Singh from the School of Physical and Mathematical Sciences (SPMS) provided theoretical and THz spectroscopy expertise. Contributions from NTU's School of Materials Science and Engineering (MSE), including Lifei Xi and Chris Boothroyd, ensured high-quality PtTe₂ synthesis via molecular beam epitaxy.

Prof Singh's TeraX Lab, renowned for THz metamaterials and topological photonics, has pioneered on-chip THz routing and spintronic emitters, positioning NTU as Asia's THz hub.

NTU's Quantum Ecosystem Fuels Innovation

NTU's Quantum Science and Engineering Centre (QSEC), established in 2021, integrates quantum photonics, materials, and computing research. Facilities like the Facility for Analysis, Characterisation, Testing and Simulation (FACTS) enable atomic-scale fabrication and ultrafast spectroscopy essential for this work. Collaborations with NUS and A*STAR, evident in co-authors from NUS's Electrical Engineering, amplify Singapore's quantum talent pool.

This paper exemplifies NTU's alignment with Singapore's Research, Innovation and Enterprise 2025 plan, which allocates billions to quantum technologies.

Real-World Applications: From 6G to Biomedical Imaging

Tunable THz sources promise compact, efficient devices for:

• 6G Communications: Beam steering and spectrum agility for data rates >1 Tbps.
• Non-Destructive Testing: Defect detection in semiconductors without ionizing radiation.
• Medical Imaging: High-resolution skin cancer detection and drug delivery monitoring.
• Security: Stand-off detection of concealed explosives or biological agents.

The nonvolatile nature eliminates power-hungry magnets, reducing size and energy use by orders of magnitude.Read the full NTU paper on Nano Letters

Singapore's Quantum Leap: NQS and Industry Synergies

Launched in 2024, Singapore's National Quantum Strategy (NQS) invests S$300 million over five years in quantum RIE, elevating CQT@NUS as flagship while bolstering NTU's efforts. THz fits perfectly into quantum sensing and communication pillars, with potential commercialization via NTUitive and A*STAR's IME.

Singapore's semiconductor dominance (e.g., GlobalFoundries, Micron) positions THz for fab inspection, aligning with the S$37 billion RIE2030 plan prioritizing quantum.

Challenges Overcome and Future Horizons

Prior THz emitters required cryogenic cooling or pulsed magnets; NTU's room-temperature, continuous-wave compatible design addresses these. Future work may integrate with silicon photonics for on-chip THz transceivers or explore other quantum geometric phases like orbital magnetic moments.

Assoc Prof Chia notes, "Quantum geometry offers a new knob for ultrafast devices, beyond traditional doping." Prof Singh adds, "This paves CMOS-compatible THz for 6G." Scaling to arrays could yield phased THz antennas.

Impact on Singapore's Higher Education and Talent Pipeline

NTU's feat underscores Singapore's universities as quantum innovation engines, attracting global talent via scholarships like the President's Graduate Fellowship. Interdisciplinary training in SPMS and MSE equips graduates for quantum startups, with alumni founding THz firms.

As SG aims for quantum sovereignty, such research bolsters PhD programs and attracts funding, fostering a virtuous cycle of excellence. Explore NTU's quantum courses or QSEC initiatives for deeper involvement.

Photo by Markus Winkler on Unsplash

Conclusion: NTU Lighting the THz Path Forward

NTU's mastery of quantum geometry in Dirac semimetals marks a milestone in tunable THz technology, blending fundamental physics with practical engineering. For Singapore's universities, it reaffirms leadership in quantum materials, promising economic and societal dividends. As research evolves, expect spintronic THz devices transforming daily life—from smarter factories to healthier diagnostics.