Low-Power Lasers Now Control Material Vibrations for Faster Electronics

Scientists have developed a new nonlinear spectroscopic technique enabling monitoring and manipulation of coherent phonons in few-layer 2H-MoTe2. Shaoxiang Sheng and colleagues at the Max Planck Institute for Solid State Research, in a collaboration between institutions including the Chinese Academy of Sciences and the Indian Institute of Science Education and Research, have developed this method which operates with sharply reduced laser power, approximately 10kW/cm2. The technique allows real-time observation of how these phonons modulate the material’s Kerr nonlinearity, offering a background-free way to detect subtle optical responses and actively control the flow of energy within the material. It provides a key step towards advances in areas such as energy conversion and quantum computation.

Low-power coherent phonon control reveals subtle nonlinear optical effects in 2H-MoTe2

Real-time monitoring and control of coherent phonons in few-layer 2H-MoTe2 was achieved at a laser power of approximately 10kW/cm2. This represents a reduction of over three orders of magnitude compared to previously required intensities exceeding 10GW/cm2. The dramatic decrease enables observation of subtle nonlinear optical responses previously obscured by strong background noise, a persistent challenge in studying these materials. Conventional methods demanded such high power that delicate phonon dynamics were lost within the signal, but this new technique provides a background-free signal. This allows for precise manipulation of the material’s Kerr nonlinearity and opens avenues for controlling energy flow. The significance of this reduction in power lies in its ability to probe materials without inducing unwanted heating or damaging delicate quantum states, crucial for preserving the integrity of the observed phenomena. Furthermore, the lower power requirement broadens the accessibility of this technique, allowing for its implementation in a wider range of research laboratories.

Coherent oscillations of the out-of-plane A1g phonon mode of 2H-MoTe2 were detected, lasting approximately 5.0ps, consistent with prior transient reflection measurements. These coherent phonons represent collective atomic vibrations that persist for a short duration after initial excitation. Further spectral analysis revealed a short-lived phonon mode at around 480cm-1, attributed to the A1b mode of the quartz substrate. A weaker peak at 202cm-1 corresponding to the A1r mode of quartz also confirmed the technique’s sensitivity. The observation of substrate-related phonon modes highlights the importance of considering the influence of the supporting material on the measured signal. The observed cosine-like oscillation for both the A1g and A1b modes suggests a displacive excitation mechanism, where ultrafast laser pulses induce atomic displacement and coherent oscillations. Time-dependent density-functional theory calculations support this interpretation by reproducing key features in the pump-probe spectra. However, the absence of the Raman-active E2g mode of 2H-MoTe2 indicates that this method currently prioritises detection of fully symmetric A mode, limiting its ability to thoroughly map all phonon dynamics within the material. This selectivity may be due to the specific polarisation of the laser pulses used or the inherent sensitivity of the XPM technique to certain vibrational symmetries. Future work could explore alternative experimental configurations to access the E2g mode and provide a more complete picture of the material’s phonon landscape.

Cross-phase modulation reveals coherent phonon dynamics in few-layer materials

The technique centres on a phase-sensitive nonlinear spectroscopic method, exploiting the interaction between light waves. It specifically uses cross-phase modulation (XPM), where one light pulse alters the phase of another. Ultrafort pulses initially excite coherent phonons, atomic-level vibrations within the material, and these phonons periodically modulate the material’s Kerr nonlinearity. This modulation subtly alters a delayed ‘probe’ pulse, creating detectable spectral changes. The Kerr nonlinearity describes the change in refractive index of a material in response to an applied electric field, and its modulation by phonons provides a sensitive probe of their dynamics. XPM offers a significant advantage over traditional methods by directly measuring the phase shift induced by the phonons, rather than relying on intensity changes, leading to enhanced sensitivity and reduced background noise.

Few-layer samples of 2H-MoTe2, specifically three to five layers thick, were targeted with ultrashort laser pulses lasting approximately ten femtoseconds. Laser powers were maintained at around ten kilowatts per square centimetre, with pulse energies of ten picajoures, offering superior phase sensitivity for examining quantum materials. The low power employed minimises background noise, a significant improvement over conventional nonlinear methods requiring over ten gigawatts per square centimetre. The use of few-layer materials is crucial as it enhances the interaction between light and the material’s vibrational modes, due to the increased surface-to-volume ratio and reduced screening effects. The ultrashort pulse duration ensures that the initial excitation occurs on a timescale faster than the phonon oscillation period, allowing for the observation of coherent dynamics. The precise control of pulse energy and duration is essential for optimising the excitation process and maximising the signal-to-noise ratio.

Low-power laser control of atomic vibrations in a layered semiconductor

Controlling light-matter interactions at the atomic level is vital for developing faster electronics and more efficient energy technologies. Researchers have now demonstrated a way to monitor and manipulate these interactions, specifically coherent phonons, within the layered material 2H-MoTe2, using remarkably little laser power. While replicating this precise control in other materials presents a challenge, this success establishes an important principle for future research. The layered structure of 2H-MoTe2, with its strong interlayer coupling, plays a significant role in the observed phonon dynamics and may contribute to the efficiency of the XPM process. Understanding how these interactions vary across different layered materials is an important area for future investigation.

It opens avenues for designing more efficient electronic devices and energy harvesting technologies. This new technique provides a pathway to actively govern material responses using light, offering unprecedented precision in controlling coherent phonons within layered materials. By operating at approximately 10kW/cm2, limitations caused by strong background noise were overcome, enabling real-time observation of subtle nonlinear optical responses. Exploiting cross-phase modulation allowed selective amplification or suppression of specific phonon modes; this precise control was demonstrated through a dual-pump pulse scheme, allowing manipulation of the material’s vibrational state. The ability to selectively control phonon modes could lead to the development of novel phononic devices, where sound waves are used to process information or transport energy. Furthermore, this technique could be used to tailor the optical properties of materials, enhancing their performance in applications such as solar cells and light-emitting diodes. The potential for integrating this control mechanism into functional devices represents a significant step towards realising the full potential of coherent phonon manipulation in advanced technologies.

Researchers successfully monitored and controlled coherent phonons within a few-layer sample of 2H-MoTe2 using a novel phase-sensitive nonlinear spectroscopic technique operating at low laser power, around 10kW/cm2. This matters because controlling these atomic vibrations is crucial for developing faster electronics and more efficient energy technologies. The method enabled selective amplification or attenuation of phonon modes via cross-phase modulation, offering unprecedented precision in material control. Future work will likely focus on replicating this control in other layered materials and exploring the integration of these findings into functional phononic devices or improved optoelectronic components.