Light carries a chorus of harmonics, like a musical chorus that can be shifted to higher notes. For engineers building tiny optical circuits, tuning that chorus used to mean turning a dial or threading a temperature knob. But those methods are slow at the scale of modern photonics. A team at Queens College and The Graduate Center of the City University of New York asked if we could tune harmonics with light itself, and do it faster than a blink.
In a bold stroke, they used strong, non-resonant laser pulses to push the spectrum around, not by exciting electrons into long-lived states, but by nudging the energy levels of virtual excitons in atomically thin semiconductors. The outcome is a new way to control light with light in real time, opening the door to compact, tunable coherent light sources and ultrafast information processing.
Off-resonant fields reshape harmonic spectra
The core idea rests on two-dimensional transition metal dichalcogenides such as WS2 and MoS2. In these atomically thin crystals, electrons and holes can form bound states called excitons that play a starring role in light-matter interactions. The researchers overlapped a fundamental broadband pulse with a powerful gate pulse whose electric field amplitude was about 100 million volts per meter (10^8 V/m). The gate is off-resonant with the exciton resonance, but it couples strongly enough to mix with virtual excitons, creating photon-dressed virtual excitons that shift the energy landscape during the pulse.
In physics terms, the gate photons couple with the two-level exciton system, producing a blue shift of the exciton energies through the AC Stark effect. The fundamental pulse then probes these shifts as it creates second and third harmonics. The picture is a handshake between light and light: the gate field dresses the exciton, forming a hybrid light-matter state that repels energy levels and moves the harmonic’s peak in the spectrum. The shift is controlled by the gate intensity and the detuning between gate photons and the exciton resonance.
Crucially, this is not a simple perturbative tweak. The gate field is strong enough that the simple perturbation theory breaks down and the system enters a regime where the macroscopic polarization responds in a nonlinear, time-dependent way. The authors describe the interplay between a fast, coherent AC Stark response and a slower, incoherent response tied to nonlinear absorption and hot-carrier dynamics. The result is a spectrum that can be steered with speed limited by the gate pulse duration rather than by the lifetime of an excited electronic state.
Third-harmonic tuning tuned by photon-dressed excitons
In their experiments, the team used large-area monolayer WS2 and MoS2 grown by chemical vapor deposition on a silicon substrate. A broadband optical parametric amplifier provided the fundamental beam near 1770 nanometers for WS2 and a complementary wavelength for MoS2. The gate pulse came from a Ti:Sapphire laser tuned near 795 nanometers, with pulse durations around a few hundred femtoseconds. By overlapping the two beams in space and time and dialing the gate intensity, they watched the third harmonic spectrum move and reshape in real time. The setup is an elegant demonstration of non-resonant control in a solid-state playground.
What they found was striking: the third-harmonic peak could shift by as much as about 26 meV, and its intensity could drop to around 37 percent of the ungated signal at the strongest fields tested. The shift tracks gate intensity at low powers in a roughly linear fashion, then saturates as absorption inside the material becomes significant. A parallel effect is seen in the second harmonic, with similar tunability in WS2 and MoS2, though with smaller shifts of 10–20 meV. The sum-frequency signal, meanwhile, remains fixed in frequency, confirming that the tuning is a spectral reshaping driven by the dressed excitons rather than a simple overall boost in all frequencies.
Meticulous time-resolved measurements reveal the dynamics of the tuning. The shift has two faces: a fast, coherent component tied to the instantaneous gate field, and a slower, incoherent component arising from the gate pulse’s absorption and subsequent hot-carrier relaxation. The researchers fitted the data with a two-exponential model: a sub-picosecond coherent part and two picosecond decays. In plain language, turning up the gate makes the harmonic jump to a new color, then the system cools back down in two stages as energy flows away through the material.
A new toolkit for ultrafast, on-chip light control
Why should you care? Because this approach slices through a bottleneck that has long limited how fast we can tune nonlinear light sources in integrated photonics. Electrical gating can be fast in principle, but it loses energy to electronics and heats up. All-optical schemes are faster, yet they typically rely on resonant excitations that only work on the lifetimes of specific electronic states. The method described here sidesteps those constraints by working off resonance and leveraging the rapid, coherent AC Stark response of virtual excitons in atomically thin crystals. In practice, this means you could imagine compact, tunable light sources whose color and intensity can be swung on and off in a fraction of a pulse, enabling ultrafast reconfiguration of photonic circuits.
Moreover, the effect appears to be general across different two-dimensional semiconductors and across second- and third-harmonic generation. The authors show similar tuning in WS2 and MoS2, and for both second and third harmonic signals. That generality hints at a broader design principle: by engineering the coupling between light and excitons in 2D materials, you can sculpt optical spectra on ultrafast timescales. The practical take-home is a route to tunable coherent light sources that are slim, chip-scale, and capable of telegraphing information with light on terahertz timescales.
All of this rests on a single, clean trick: collide a strong, non-resonant gate beam with a craftily chosen fundamental pulse. The energy shifts you see are tiny in absolute terms, but they move light’s color in a controlled, reversible way. And because the setting is 2D materials, those controls happen in a platform that can sit on a photonic chip, whispering to waveguides, resonators, and detectors in a populated lab or a future factory. The team behind the work—led by Euclides Almeida at Queens College and The Graduate Center of the City University of New York, with collaborators George Trivizas and Matthew Feinstein—emphasizes that the concept can scale to other excitonic materials and even to other nonlinear processes such as high-harmonic generation in solids. It’s not a one-off trick; it’s a toolkit that could reshape how we think about tuning light with light.
