The Science of Steering Electrons with Terahertz Radiation

By Atif SuhailReviewed by Louis CastelMay 20 2025

In chemistry and materials science, the ability to control where and how electrons are distributed within molecules is essential for steering chemical reactivity, tailoring electronic properties, and designing novel functional materials.¹

Image Credit: vchal/Shutterstock.com

Traditionally, electron control has been approached through chemical synthesis or static electric fields. However, recent advances in ultrafast laser technologies have introduced a dynamic frontier, using tailored terahertz (THz) light pulses to manipulate electrons in real-time.¹

These pulses offer a promising tool to guide electron motion and reorganize charge distributions in molecules without physically altering their structure. But how do scientists actually program electron behavior at the molecular level using light?

What Are Terahertz Light Pulses?

THz light pulses refer to bursts of electromagnetic radiation that fall within the 0.1 to 10 THz frequency range, sitting between microwave and infrared regions of the spectrum. This low-energy band corresponds to photon energies in the millielectronvolt (meV) range, making it particularly suited for probing and manipulating low-energy electronic, vibrational, and rotational states in molecules and materials.²

What makes THz radiation unique is its ability to interact with matter in a non-ionizing, yet highly dynamic manner. It can excite lattice vibrations, influence electron distributions, and even modulate charge or spin dynamics, all without breaking molecular bonds. This makes THz pulses ideal for ultrafast, non-destructive control of electron behaviour.^2-3

The ability of THz pulses to deliver sub-picosecond temporal resolution, combined with their broad spectral bandwidth and interaction with low-energy electronic processes, makes them a powerful tool for precisely manipulating electron distributions in molecules, and offers a pathway to control reactions, molecular orbitals, and charge dynamics at their most fundamental level.³

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Mechanism of Control

Controlling the distribution of electrons within molecules using THz pulses hinges on precise quantum interference techniques. By employing custom-shaped THz light pulses (often derived from two harmonically related femtosecond laser beams) scientists can induce and steer ultrafast electronic currents without permanently altering or breaking molecular bonds.⁴

This process exploits quantum interference between one-photon and two-photon absorption pathways. For instance, by tuning the phase difference (ϕ) between a fundamental frequency (ω) and its second harmonic (2ω), researchers can inject transient, directional currents within materials such as GaAs or monolayer graphene. The resulting current is proportional to the product of the two field amplitudes and the sine of their relative phase:

dJ/dt ∝ Eω²E₂ωsin(ϕ).

The ability to tailor the waveform, polarization, and spatial distribution of these laser pulses enables remarkable control over molecular orbitals, charge density, and even bond vibrational dynamics. This is achieved without initiating chemical reactions, allowing for reversible and non-destructive electronic manipulation.^4-5

Crucial to this level of control is the shaping and phase modulation of the THz pulses. Using spatial light modulators and combinations of circularly and linearly polarized beams, researchers can rotate the polarization axis of the emitted THz radiation and, consequently, direct the flow of charge within the molecular structure. This fine control allows manipulation of the electron's spatial distribution on femtosecond to picosecond timescales.⁴

For example, in graphene, the phase-controlled injection of currents using two-color laser fields (typically at 1480 nm and 740 nm) results in THz emission whose amplitude and polarity can be tuned dynamically. Remarkably, even a single atomic layer of graphene can emit measurable THz radiation, demonstrating the extreme sensitivity and potential of this approach for 2D materials.⁵

Experimental and Theoretical Advances

Recent advances in ultrafast spectroscopy and THz generation have transformed our ability to probe and manipulate electron dynamics at the molecular level. Core to this progress is the use of ultrashort laser pulses, generated by Ti:sapphire and Yb-doped lasers, whose energy can be scaled to the millijoule range via chirped pulse amplification (CPA), enabling strong-field THz interactions.⁶

Secondary sources, using effects like optical rectification and difference-frequency generation, extend this capability, with Ti:sapphire-pumped Optical Parametric Amplifiers (OPAs) producing pulses from the UV to THz range. These developments have made ultrafast spectroscopy widely accessible, with high temporal precision and broad spectral coverage.⁶

Theoretical and computational modeling has kept pace with experimental advancements, using quantum dynamics simulations and time-dependent density functional theory (TD-DFT) to predict how THz fields perturb molecular orbitals and drive charge redistribution. These models help design pulse sequences and anticipate reaction pathways that can be selectively activated or suppressed by THz excitation.⁶

One striking experimental demonstration of THz-induced control was the use of attosecond pump-probe spectroscopy to study the ultrafast ring-opening reaction of 1,3-cyclohexadiene. Here, time-resolved XUV absorption revealed the creation and decay of a transient charge-separated state, affirming predictions made by quantum chemical models. Similarly, in phenylalanine, attosecond pulses captured sub-femtosecond charge migration dynamics post-ionization, highlighting the potential of THz-like field interactions to initiate and monitor purely electronic processes before nuclear motion intervenes.^6-7

On the materials side, studies on thin GaAs layers have elucidated the contributions of surface and volume nonlinearities in THz generation. Experiments using metastructured GaAs demonstrated enhanced THz emission dominated by surface shift currents, challenging earlier assumptions that photocurrent effects were the sole contributors. These findings underline the importance of structural design and polarization control in optimizing THz generation and its coupling to electronic degrees of freedom.⁸

Moreover, high-field THz pulses are now being used in two-dimensional spectroscopy setups to explore coherent phonon dynamics and inter-subband transitions in quantum wells. These methods provide rich insight into energy relaxation pathways and many-body interactions.

Potential Applications

Chemical Synthesis

Intense THz fields enable selective activation or suppression of chemical bonds by manipulating electron density in a time-resolved manner. This opens pathways to steer reactions with unprecedented specificity, potentially revolutionizing photochemical synthesis and catalysis.⁹

Quantum Computing

THz-induced coherent control over electron states in molecules or quantum materials allows manipulation of qubit-like systems. This includes initializing, reading, or flipping states via field-induced transitions or tunneling, making THz fields promising for solid-state quantum logic and memory operations.⁹

Molecular Electronics

THz radiation can guide electron transport across molecules by modulating their internal charge distribution and energy levels. This could lead to devices where light controls current flow at the molecular scale, enabling ultra-compact photonic logic circuits.¹⁰

Ultrafast Spectroscopy

THz time-resolved techniques like optical-pump/THz-probe (OPTP) and THz-pump/optical-probe (TPOP) reveal transient states and carrier dynamics with sub-picosecond resolution. These methods are key to understanding energy transfer, phase transitions, and intermediate states in solids and nanostructures.⁶

Biochemistry

THz fields interact with low-frequency modes of biomolecules and can modulate electron transfer in proteins and enzymes. This paves the way for controlling biological function at the electronic level, with implications for drug development and bioengineering.⁹

Challenges and Future Outlook

Despite the progress, challenges remain. Generating precisely tuned THz pulses with sufficient intensity and stability is technically demanding. Additionally, understanding how complex, real-world molecules respond to these fields, especially in condensed-phase or biological environments, requires deeper theoretical and computational insights.¹⁰

Moving forward, integrating THz control into portable analytical tools or lab-on-a-chip systems could transform how researchers and engineers probe and manipulate molecular behavior in situ. As these technologies mature, the vision of on-demand electron distribution in functional molecules becomes increasingly tangible.

References and Further Readings

1. Marian, C. M., Understanding and Controlling Intersystem Crossing in Molecules. Annual Review of Physical Chemistry 2021, 72, 617-640 DOI: https://doi.org/10.1146/annurev-physchem-061020-053433.
2. Baxter, J. B.; Guglietta, G. W., Terahertz Spectroscopy. Analytical chemistry 2011, 83, 4342-4368.
3. Yang, C.-J.; Li, J.; Fiebig, M.; Pal, S., Terahertz Control of Many-Body Dynamics in Quantum Materials. Nature Reviews Materials 2023, 8, 518-532.
4. Choobini, A.; Ghaffari-Oskooei, S.; Shahmansouri, M.; Aghamir, F., Mechanisms of Thz Radiation in Laser-Plasma Interactions. arXiv preprint arXiv:2403.18499 2024.
5. Jana, K.; de Souza, A. B.; Mi, Y.; Gholam-Mirzaei, S.; Ko, D. H.; Tripathi, S. R.; Sederberg, S.; Gupta, J. A.; Corkum, P. B., Terahertz Generation Via All-Optical Quantum Control in Two-Dimensional and Three-Dimensional Materials. Physical Review B 2025, 111, L161405.
6. Maiuri, M.; Garavelli, M.; Cerullo, G., Ultrafast Spectroscopy: State of the Art and Open Challenges. Journal of the American Chemical Society 2019, 142, 3-15.
7. Attar, A. R.; Bhattacherjee, A.; Pemmaraju, C.; Schnorr, K.; Closser, K. D.; Prendergast, D.; Leone, S. R., Femtosecond X-Ray Spectroscopy of an Electrocyclic Ring-Opening Reaction. Science 2017, 356, 54-59.
8. Hale, L. L.; Jung, H.; Gennaro, S. D.; Briscoe, J.; Harris, C. T.; Luk, T. S.; Addamane, S. J.; Reno, J. L.; Brener, I.; Mitrofanov, O., Terahertz Pulse Generation from Gaas Metasurfaces. ACS Photonics 2022, 9, 1136-1142 DOI: 10.1021/acsphotonics.1c01908.
9. Hafez, H.; Chai, X.; Ibrahim, A.; Mondal, S.; Férachou, D.; Ropagnol, X.; Ozaki, T., Intense Terahertz Radiation and Their Applications. Journal of Optics 2016, 18, 093004.
10. Vilan, A.; Cahen, D., How Organic Molecules Can Control Electronic Devices. Trends in biotechnology 2002, 20, 22-29.

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