Lasers Induce and Assess Esoteric Behavior in Electronic Materials

It seems as if almost every day there’s a credible report or academic paper detailing yet another “trick” enabled by lasers (note that I mean “trick” in the positive, beneficial sense of the word). This is ironic because when Theodore H. Maiman demonstrated the first ruby (optical) laser in 1960 to the press (as it was called in those days, and now called the “media”), the somewhat jaded journalists quipped that the laser was “a solution looking for problems to solve.”

Well, we know now how that turned out. Lasers and their many manifestations are now standard tools. Both vital and flexible, they continue to enable countless products and developments in science, medicine, industry, and consumer products ranging from somewhat obvious to leading-edge, esoteric advances.

Two recent examples show how lasers are allowing a re-think of what’s possible with regard to basic materials used by the electronics industry.

Seeing Magnetism in “Non-Magnetic” Metals

Scientists have cracked a century-old physics mystery by detecting magnetic signals in non-magnetic metals using only light and a revamped laser technique. Previously undetectable, these faint magnetic “whispers” are now measurable, revealing hidden patterns of electron behavior.

For over a century, scientists have known about the ordinary Hall effect, where the Lorentz force deflects the electrons and the transverse Hall voltage arises. In plain words, the electric currents “bend” in a magnetic field. In magnetic materials like iron, this effect is strong and well understood as the anomalous Hall effect (AHE)—an anomalously large Hall voltage saturates with the applied magnetic field. However, in ordinary, non-magnetic metals like copper or gold, the effect is much weaker.

In theory, a corresponding phenomenon called the magneto-optic Kerr effect (MOKE) should help scientists visualize how electrons behave when light and magnetic fields interact. But at visible wavelengths, this optical Hall effect (OHE) has been too subtle to detect. The OHE has been primarily measured at terahertz and infrared frequencies where the effective electronic displacement is larger.

The sensitivity to the Kerr signal can be increased by modulating the external magnetic field. However, when electromagnets are utilized, this can be done only at impractical low rates and amplitudes.

Now, a multi-university team led by Hebrew University (Israel), joined by researchers at the Weizmann Institute of Science (Israel), Pennsylvania State University (USA), and University of Manchester (UK), has solved this dilemma. They upgraded the MOKE using a 440-nm, 40-mW laser to measure how magnetism alters light’s reflection.

They combined a 440-nm blue laser with large-amplitude modulation of the external magnetic field to dramatically boost the technique’s sensitivity (Fig. 1). The result: They were able to pick up magnetic “echoes” in non-magnetic metals like copper, gold, aluminum, tantalum, and platinum — a feat previously considered near-impossible.

The team found that what appeared to be random “noise” in their signal wasn’t random at all. Instead, it followed a clear pattern tied to a quantum property called spin-orbit coupling. That property links how electrons move to how they spin, a key behavior in modern physics. The technique offers a non-invasive, highly sensitive tool for exploring magnetism in nominally non-magnetic metals, but without the need for massive magnets or cryogenic conditions. It was also able to determine the Gilbert damping parameters.

There’s some scientific closure here: Edwin Hall (of the eponymous effect) attempted to measure this property using a beam of light, but he was unsuccessful. He summarized his efforts in the closing sentence of one of his papers in 1881: “I think that, if the action of silver had been one-tenth as strong as that of iron, the effect would have been detected. No such effect was observed.”

What’s the practical use here? As with so many deep-physics experiments, the answer is simple: We don’t know, at least not yet. It could — the key word being “could” — have implications for the design of magnetic memory, spintronic devices, and even quantum systems. Full details are in their intense and sophisticated paper “A sensitive MOKE and optical Hall effect technique at visible wavelengths: insights into the Gilbert damping” published in Nature Communications.

Controlling Thin Semiconductors via Terahertz Light

Physicists at Bielefeld University and the Leibniz Institute for Solid State and Materials Research (both in Germany) developed a method to control atomically thin semiconductors using ultra-short light pulses. The project could pave the way for components that are controlled at unprecedented speeds directly by terahertz light.

(Note that these researchers refer to their terahertz waves as “light,” while many engineers would see them as RF energy. Of course, terahertz waves and what we consider the optical energy have some spectrum overlap. Further, as both are electromagnetic waves following Maxwell’s equations, using the term “light” isn’t wrong, just unusual).

The scientists were able to experimentally demonstrate that the optical and electronic properties of the material could be selectively altered using light pulses. This technique allows for real-time control of the electronic structure on timescales of less than a picosecond.

Conventional methods to induce such fields use gating techniques based on electronic circuitry, which is restricted to microwave response rates and face challenges in achieving device-compatible ultra-fast, sub-picosecond control.

The team used an ultra-fast field effect in atomically thin molybdenum disulfide (MoS₂) embedded within a hybrid 3D-2D terahertz nanoantenna. This nanoantenna transforms an incoming terahertz electric field into a vertical ultra-fast gating field in MoS₂, simultaneously enhancing it to the megavolt/centimeter (MV/cm) level (Fig. 2). It’s not a field-effect transistor (FET) — at least, not yet.

The team achieved this control by designing nanoscale antennas that transform terahertz light into vertical electric fields within atomically thin materials such as MoS₂. As explained by project leader and physics professor Dr. Dmitry Turchinovich from Bielefeld University, “Our approach uses the terahertz light itself to generate the control signal within the semiconductor material — allowing an industry-compatible, light-driven, ultra-fast optoelectronic technology that was not possible until now.”

The antenna consists of two gold electrodes, top and bottom, vertically separated by an Al₂O₃ dielectric spacer layer. The electrodes are horizontally displaced such that they only overlap in the middle section of the antenna, which has lateral dimensions of 10 × 10 μm. The antenna has a bowtie dipole shape, enabling efficient coupling of the broadband free-space terahertz fields to its electrodes and strong local field enhancement in the sub-wavelength antenna. The entire antenna structure is deposited on a glass substrate.

How do lasers enter into this story? Time-resolved optical spectroscopy of characteristic exciton resonances in MoS₂ were used with a THz pump-optical probe (TPOP). The pumped-THz field was generated using optical rectification of a 2-mJ, 800-nm laser pulse with 100-fs duration in a lithium-niobate crystal.

This produced a broadband single-cycle pulse with a frequency range of 0.2 to 2.5 THz and a center frequency of 0.4 THz propagating into free space. The terahertz beam was then focused and directed at the antenna at normal incidence. Using this admittedly complicated arrangement, they were able to confirm that these terahertz waves flipped the optical and electronic properties of the ultra-thin material at those rates, making the scheme suitable for control.

Details are in their lengthy paper with a short, clear title, “Terahertz field effect in a two-dimensional semiconductor,” also published in Nature Communications.