Direct Spatiotemporal Imaging of Charge Carrier Dynamics in Operative Semiconductor Devices

arXiv:2607.19157 · physics.optics, cond-mat.mes-hall, physics.app-ph · Submitted 2026-07-21 · Read on arXiv

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Introduction to the show: ident: Quantum Radio. Generated commentary on the latest quantum physics and condensed matter papers.

Kai: I'm Kai.

Mira: And I'm Mira, here with Lev.

Kai: Today's paper: "Direct Spatiotemporal Imaging of Charge Carrier Dynamics in Operative Semiconductor Devices".

Mira: The gist: This work introduces Carrier Dynamics by Pockels Imaging (CDPI), a sensitive, quantitative,

Kai: First, who's behind it and why it matters.

Title and authors: Kai: We talked about the title and authors of "Direct Spatiotemporal Imaging of Charge Carrier Dynamics in Operative Semiconductor Devices." It’s a very direct title, which really sets the expectation for what they are trying to achieve.

Mira: It promises a direct look at charge carrier dynamics, moving away from the indirect methods we usually rely on when studying these things.

Lev: So what is this supposed to mean in plain terms? What’s the core promise of this title?

Kai: The core promise is that they can get a direct access to the internal charge dynamics by probing local electric fields. They want to visualize photogenerated electron clouds in real space while simultaneously correlating their evolution with conduction and displacement currents.

Mira: So, when you take that apart, it means they are providing a unified picture of carrier transport by linking the optical effect to both measurable electrical quantities at the same time.

Lev: That sounds like they’re trying to establish a complete model of how carriers move from being excited optically all the way through to how they contribute to current flow.

Kai: Exactly, because they are probing those fundamental mechanisms—drift, diffusion, carrier–carrier interactions, trapping and detrapping—and seeing the spatiotemporal evolution of the carrier cloud morphology.

Mira: It’s not just seeing a picture; it’s understanding how those different physical processes interact in real time as you apply external conditions.

Lev: So this paper is trying to bridge that gap between the optical measurement and the electrical measurement for these devices.

The paper's summary: Kai: Now, let’s talk about what they actually found in this paper. They summarize their findings by saying that CDPI allows for a direct visualization of the optically generated electron cloud and its associated conduction and displacement current in the whole device.

Mira: They emphasize that this is a key point: this isn't just some localized measurement; they are mapping out how these currents are distributed throughout the entire volume.

Lev: So, for someone listening, what’s the practical implication of seeing it across the whole device rather than just a tiny spot?

Kai: Practically, it means you can study how those currents distribute in a way that gives you a picture of the overall transport behavior in that device.

Mira: It means they are linking that internal charge distribution directly to what we measure in the external circuit, which is distributed across the whole device.

Lev: That’s powerful because it allows for studying global transport phenomena instead of just local ones.

Kai: They show how this correlation works by writing down how the electron cloud density relates to spatial derivatives of the electric field at a given and initial time.

Mira: And then they express the total current as that sum of conduction and displacement terms in terms of that same electric field and its temporal derivative, which highlights their framework.

Lev: That formula is key because it shows how the measured quantities are mathematically linked together, right?

The paper's improvements: Kai: They discuss what they suggest to make this method better in terms of improvements. They point toward using materials beyond non-centrosymmetric semiconductors.

Mira: And they suggest pushing the time resolution and spatial dimensionality extension as areas where this technique can be enhanced.

Lev: So, for hardware engineers, does that mean we should just look at other materials or try to build faster systems?

Kai: Yes, because those are the suggested directions to push the method forward. The idea is to see if CDPI works in different crystal structures or if it can resolve things in more dimensions.

Mira: And they also suggest applications for any device involving the propagation of either electrically or optically generated excess carriers, which broadens the scope of what this technique can be used for.

Lev: So, from an engineering standpoint, are we talking about changing the physical materials we use to see if it works better?

Kai: That’s one path. And pushing spatial dimensionality is another way to extend the method’s capabilities.

Mira: It also suggests that this technique could be applied widely in any device where carriers are being generated either by electrical or optical means, which makes it very versatile.

Conclusion: Kai: So to wrap up on "Direct Spatiotemporal Imaging of Charge Carrier Dynamics in Operative Semiconductor Devices," the main implication is that this method gives full access to basic transport properties without the need for spatial averaging and the approximations implicit in any other experimental evaluation.

Lev: This is about getting that direct, quantitative visualization without having to rely on spatial averaging or those approximations you often have in other experimental evaluations.

Kai: They are connecting internal charges with the current flowing in an external circuit by modulating the refractive index, which they see as a breakthrough in optoelectronics.

Lev: Ultimately, this paper shows how to use Pockels Imaging to get direct insight into charge carrier dynamics through electric fields.

CNR IMM Institute for Microelectronics and Microsystems · Institute of Physics Charles University Institute of Nanotechnology CNR NANOTEC

physics.optics, cond-mat.mes-hall, physics.app-ph

Submitted: 2026-07-21

Updated: 2026-07-21

Comments: main+SI, 17+7 pages, 5+3 figures, 4 videos

Journal ref: Matter & Light 1, 100141, 2026

DOI: 10.1016/j.matlit.2026.100141

License: http://creativecommons.org/licenses/by-nc-nd/4.0/

Importance score: 89/100

The gist: The gist: This work introduces Carrier Dynamics by Pockels Imaging (CDPI), a sensitive, quantitative, and noninvasive method that enables direct access to internal charge dynamics by probing local

Key concepts

Carrier Dynamics: This refers to understanding how charge carriers (electrons and holes) evolve within a device over time. The method tracks their movement—including drift, diffusion, and interactions—by observing changes in the local electric field, which governs their motion.
Pockels Effect in CDPI: The Pockels effect is used as the core imaging mechanism. In this setup, a pump pulse excites carriers optically, and a probe pulse measures how the resulting local electric fields change spatially and temporally. This allows for noninvasive visualization of charge movement.
Electric Field-Current Relationship: The paper establishes a mathematical link between the measured electric field and the total current flowing in an external circuit. This relationship shows that the internal charge density is directly related to the spatial derivatives of the electric field, allowing researchers to correlate microscopic carrier behavior with macroscopic electrical measurements.

Terminology

Carrier Dynamics by Pockels Imaging — 7× in this paper
Coherence — 3× in this paper · explained in 2 episodes
Core — 3× in this paper · explained in 2 episodes
Papers — 3× in this paper · explained in 3 episodes
Generator — 1× in this paper · explained in 2 episodes

Summary

The gist: This work introduces Carrier Dynamics by Pockels Imaging (CDPI), a sensitive, quantitative, and noninvasive method that enables direct access to internal charge dynamics by probing local electric fields in operative semiconductor devices.

Understanding Charge Carrier Dynamics

Understanding charge carrier dynamics within electronic devices is crucial, as it governs both the internal evolution of signals and the externally measurable current<ref:2607.19157#pg2>. The technique provides direct insight into fundamental mechanisms—including drift, diffusion, carrier–carrier interactions, trapping and detrapping—as well as the spatiotemporal evolution of carrier cloud morphology<ref:2607.19157#pg3>. By probing the local electric field, CDPI visualizes photogenerated electron clouds in real space while simultaneously correlating their evolution with both conduction and displacement currents, thereby establishing a unified picture of carrier transport<ref:2607.19157#pg3>. This approach allows for the direct visualization of the optically generated electron cloud and of the associated conduction and displacement current, which is distributed in the whole device<ref:2607.19157#pg4>.

Methodology: Carrier Dynamics by Pockels Imaging (CDPI)

The novelty of this approach is represented by using the Pockels effect in a pump-probe electro-optical imaging configuration, which is called Carrier Dynamics by Pockels Imaging (CDPI) technique<ref:2607.19157#pg3>. The initial outcome of the CDPI experiments is a series of transmittance Pockels images T(x,y,t), each one corresponding to a progressive delay of the probing pulse with respect to the excitation pulse<ref:2607.19157#pg4>. The technique offers a different perspective to study free carrier dynamics, as it allows a direct visualization of the optically generated electron cloud and of the associated conduction and displacement current, this last being distributed in the whole device<ref:2607.19157#pg4>.

Analysis of Electric Field and Current Relationships

The core framework of CDPI is established by relating the electric field to the currents flowing in the external circuit<ref:2607.19157#pg4>. The electron cloud density can be written in terms of the electric field spatial derivatives at a given and initial time, in this form: δn(t, y) = εq [∂E(y,t) / ∂y − ∂E(y,t=0) / ∂y]<ref:2607.19157#pg4>. The total current J(t), sum of conduction and displacement terms, can be thus expressed as J(t) = qδn(y,t) + ε∂E(y,t)/∂t<ref:2607.19157#pg4>. This highlights the framework of our approach: CDPI measures the electric field, which encompasses the charge carriers inside the device and relates them to those measured in the external circuit<ref:2607.19157#pg4>.

Low Optical Perturbation Regime

In cases of low optical perturbation, where the electric field is only slightly perturbed by optical excitation impinging on the cathode side of the device, holes are quickly collected while the electron cloud drifts towards the anode<ref:2607.19157#pg5>. The clear motion of the electron cloud under different applied voltages can be directly imaged by applying the spatial and temporal derivative to relevant time-space charts of the electric field retrieved from EO experiments<ref:2607.19157#pg5>. The associated displacement current Jdispl(y) profiles are shown in Fig. 2d at time intervals of 5 ns<ref:2607.19157#pg5>. These profiles can be assumed to coincide almost exactly with the carriers’ cloud due to Eq. (3) because of the thin cloud's extension and has been thoroughly verified<ref:2607.19157#pg5>.

High Optical Irradiation Effects

Under larger levels of optical irradiation, the electric field undergoes a progressive bending, where the field reduces under the irradiated cathode while increases close to the anode upon increasing incident optical power<ref:2607.19157#pg9>. A significant quantity to evaluate is the collected charge, which can be obtained from the electric field step associated with the localized drifting electron cloud, according to ΔE = qε/Ns<ref:2607.19157#pg9>. The collected charge is retrieved instead by time integrating the approximated current Japprox(t)<ref:2607.19157#pg9>. The cloud of electrons reshapes from purely Gaussian to bi-Gaussian with more extended tail than front under high optical powers<ref:2607.19157#pg9>.

Conclusion and Broader Implications

CDPI provides full access to basic transport properties without the need for spatial averaging and the approximations implicit in any other experimental evaluation<ref:2607.19157#pg10>. The technique enables a direct and quantitative visualization of an optically generated electron cloud and of the associated conduction and displacement currents in the whole volume of operative devices<ref:2607.19157#pg10>. Furthermore, CDPI shows potentialities along several directions, including materials beyond non-centrosymmetric semiconductors, time resolution advancement, spatial dimensionality and resolution extension, and application to any device involving the propagation of either electrically or optically generated excess carriers<ref:2607.19157#pg13>. The correlation between internal charges with the current flowing in an external circuit achieved by modulating the refractive index represents a breakthrough in optoelectronics<ref:2607.19157#pg10>.

Numerical Validation

Numerical simulations confirm that the electron cloud retains a well-defined Gaussian spatial distribution at any instant and for the entire drift of the cloud when different applied voltages are applied<ref:2607.19157#pg11>. These simulations show that under high optical irradiation, an additional electron trapping level is necessary to account for pronounced tails in the moving clouds due to detrapping processes<ref:2607.19157#pg11>. The numerical modeling allows us to validate a fundamental statement in the CDPI technique, confirming that the displacement current can be used directly in localized-cloud conditions as a good match to the charge cloud shape and dynamics<ref:2607.19157#pg11>.

Experimental Setup Details

The radiation detectors used are based on CdTe:Cl semi-insulating crystals with nominal drift electrons and holes mobilities of μe= 10 50 cm2V-1s-1 and μh= 80 cm2V-1s-1, respectively<ref:2607.19157#pg4>. Two optical beams are impinging on the sample in a pump and probe configuration, coming from two independent pulsed lasers both of measured width around 1.0 ns<ref:2607.19157#pg4>. The probe beam is linearly polarized at 45° with respect to the vertical y-direction and experiences the electric field induced birefringence when crossing the sample<ref:2607.19157#pg4>. The raw P(x,y,t) images depend on the Ey(x,y,t) electric field distribution<ref:2607.19157#pg4>.

Key Findings on Carrier Dynamics

The study found that under different applied voltages and low optical intensity, the Gaussian shape of the electron cloud is well maintained during drift, with a width first increasing due to diffusion and then shrinking due to decreasing electric field profiles<ref:2607.19157#pg13>. At larger powers, Coulombic repulsion plays a dominant role, leading to a net increase in both front and tail widths of the cloud over time<ref:2607.19157#pg9>. The presence of trapping is indicated by pronounced tails in the moving clouds due to detrapping processes under high optical power<ref:2607.19157#pg9>. The all-optical approach correlates the dynamics of internal charges with the current flowing in an external circuit, which is a striking objective of this work<ref:2607.19157#pg10>.

Numerical Simulation Results

The timedependent drift-diffusion equations were solved by starting from an established model for the bulk of the CdTe and the contacts, here updated for transient conditions<ref:2607.19157#pg11>. Simulations confirm that under low optical irradiation, the electron cloud retains a well-defined Gaussian spatial distribution at any instant and for the entire drift of the cloud when different applied voltages are applied<ref:2607.19157#pg11>. The simulations also show that under high optical excitation levels, an additional electron trapping level is necessary to account for pronounced tails in the moving clouds due to detrapping processes<ref:2607.19157#pg9>.

Improvements for AI systems

Bold header: Direct visualization of internal charge dynamics

This allows AI systems to directly access fundamental mechanisms such as drift, diffusion, carrier–carrier interactions, trapping and detrapping, providing a unified picture of carrier transport by correlating the electric field with both conduction and displacement currents.

Bold header: Real-time correspondence between internal fields and external circuit behavior

The system can be improved to unveil the correspondence between the internal electric field reshaping and the charges supplied by the external circuit, i.e., the independently measured current, allowing for a direct link between the optical generated electron cloud and the associated conduction and displacement currents.

Bold header: Quantitative analysis of carrier transport properties

AI systems can move beyond approximations to directly access properties like mobility through non-approximated expression [Eq. (4)] which yields values very similar to those calculated assuming a constant electric field, offering more accurate estimation of local velocity at the local electric field.

Bold header: Spatiotemporal mapping under high optical perturbation

The system can be enhanced to analyze regimes where high optical excitation levels allow us to investigate perturbed space charge regimes, specifically observing pronounced tails in the moving clouds due to detrapping processes, which is critical for understanding device performance under intense irradiation.

Bold header: Detection of localized defects and trapping mechanisms

By incorporating numerical simulations that include specific trap distributions, the AI can identify phenomena such as a secondary peak in Fig. 3e very close to the cathode and confirm that trapping in CdTe detectors is enhanced by optical intensity and mostly associated with a defective layer possibly close to the surface.

Abstract

Understanding charge carrier dynamics within electronic devices is crucial, as it governs both the internal evolution of signals and the externally measurable current. Yet, direct imaging of these dynamics remains elusive: existing approaches are often indirect or inherently perturbative. Here, we introduce Carrier Dynamics by Pockels Imaging (CDPI), a sensitive, quantitative, and non-invasive method that enables direct access to internal charge dynamics. By probing the local electric field, CDPI visualizes photogenerated electron clouds in real space while simultaneously correlating their evolution with both conduction and displacement currents, thereby establishing a unified picture of carrier transport. The technique provides direct insight into fundamental mechanisms including drift, diffusion, carrier-carrier interactions, trapping and detrapping as well as the spatiotemporal evolution of carrier cloud morphology. Demonstrated in CdTe radiation detectors with micrometre-scale spatial resolution and nanosecond temporal resolution, CDPI is broadly applicable to systems exhibiting appreciable electro-optic responses. Beyond conventional semiconductors, the approach extends to organic, perovskite, and two-dimensional optoelectronic materials, offering a new paradigm for directly linking internal carrier dynamics to device-level functionality.