The speed limit of optoelectronics

The pursuit of ever-faster electronics has led researchers to explore the potential of light-field-driven charge motion in semiconductors. The ultimate goal is to achieve optical clock rates (0.3-1 PHz), far exceeding current capabilities. Strong-field excitation using ultrashort laser pulses has been explored as a means to create and manipulate electron wavepackets with sub-femtosecond precision. However, this method often populates multiple conduction bands, leading to a reduced effective bandwidth and hindering high-fidelity signal processing. The challenge lies in understanding and overcoming the limitations imposed by the underlying physics of electron dynamics within the solid. This research addresses this challenge by investigating a novel approach for generating and controlling single-band Bloch wavepackets in a wide-bandgap dielectric.

Previous research has demonstrated the use of strong-field excitation to generate electron wavepackets within sub-femtosecond time intervals. Studies on various materials have shown the potential for ultrafast manipulation of electronic properties. However, the challenges of multi-band population and reduced bandwidth have limited the speed and fidelity of signal processing. This work builds upon this existing literature by focusing on single-band excitation to address these limitations and to explore the fundamental limits of high-speed optoelectronic signal processing.

The researchers employed a novel technique called linear petahertz photoconductive sampling (LPPS). This method utilizes vacuum-ultraviolet (VUV) light pulses, generated via high-harmonic generation, to populate the first conduction band of lithium fluoride (LiF) selectively via single-photon excitation. The subsequent motion of the Bloch wavepacket is controlled by a few-cycle optical "gate" pulse. The resulting current from gate-field-induced charge separation is then measured. To precisely determine the duration of the injection time window, a deconvolution method was used. The gate field vector potential was determined through an in-situ attosecond streaking measurement for benchmarking purposes. The LPPS signal was analyzed to extract information about the electron dynamics and to determine the limits of ultrafast wavepacket steering, by measuring deviations of the LPPS signal from a linearly scaled low-intensity reference recording at varying gate field intensities. Theoretical modeling using time-dependent optical Bloch equations, incorporating the band structure and dipole-coupling matrix elements from density-functional theory, was used to complement the experimental findings. The study included detailed description of the experimental setup, VUV pulse generation and characterization, current detection techniques, sample geometry, the considerations regarding filter material and other specifics of the experimental setup.

The key findings include the successful generation of a single-band Bloch wavepacket in LiF within approximately 1 fs using single-photon excitation. LPPS is shown to be a high-fidelity technique for petahertz-scale optical field retrieval. The experimental results, supported by theoretical simulations, reveal that population transfer to higher conduction bands and the associated group velocity inversion are the primary mechanisms limiting the speed of electric current control in solids. This limit manifests as a deviation in the LPPS signal at high gate field intensities, occurring approximately one to two femtoseconds after carrier injection. This deviation is directly linked to non-adiabatic Landau-Zener transitions between conduction bands. The simulations confirmed that switching off the couplings between conduction bands eliminates the time-advanced deviation. The study experimentally demonstrates conductivity switching in a dielectric on a one-femtosecond timescale, corresponding to a near-petahertz bandwidth.

The results of this study demonstrate a fundamental limit for classical field-driven signal processing in solids arising from interband transitions. The limitation on the speed at which electric currents can be controlled is not simply a materials property but is an intrinsic consequence of the band structure and interband couplings in the solid. The achieved near-petahertz bandwidth signifies significant progress in ultrafast optoelectronics. The observed effects, linked to population transfer and group velocity inversion, are broadly applicable to other solid-state materials, indicating that similar limitations may arise in other systems. The high-fidelity of the LPPS technique opens new avenues for studying ultrafast electron dynamics in solids.

This research demonstrates a novel approach for generating and controlling single-band Bloch wavepackets, enabling optical field sampling with near-petahertz bandwidth. The identification of population transfer to higher conduction bands and associated group velocity inversion as a limiting factor for high-speed electric current control suggests a fundamental constraint for classical signal processing in solids. This work establishes a basis for future optoelectronic devices potentially operating at petahertz frequencies. Future studies could focus on material engineering strategies to mitigate the effects of interband transitions and further enhance the speed and fidelity of optoelectronic devices.

The simulations in this study focused on one-dimensional cuts of the LiF Brillouin zone due to computational constraints. This simplification may affect the quantitative agreement between experimental observations and theoretical predictions. The current noise level, limited by the VUV source power and electrical detection noise, is a factor affecting the signal-to-noise ratio and the precision of the measurements. Improvements in the VUV source could significantly improve experimental accuracy.