Time-Resolved Photoluminescence (TRPL)

Time-resolved photoluminescence (TRPL) is a powerful technique that measures the photoluminescence decay (emission lifetime) of materials after pulsed excitation, thereby probing their optical and electronic properties. By tracking how excited states relax back to the ground state, TRPL distinguishes between radiative and non-radiative pathways, yielding lifetimes that report on charge-carrier dynamics, trap/defect activity, and other loss mechanisms. Compared with steady-state photoluminescence (PL), which focuses on spectral intensity, TRPL resolves the temporal decay of the luminescence signal, enabling quantitative extraction of recombination rates, multi-exponential or distributed lifetimes, and diffusion- or transfer-limited behavior. These parameters are broadly useful for screening and optimizing photonic materials, such as semiconductors, perovskites, polymers, and quantum-emitter platforms, and for interpreting contrast in bioimaging and other applications.

Time-resolved photoluminescence (TRPL) is a technique used to investigate how materials emit light after pulsed optical excitation and the temporal decay of that emission following excitation. TRPL differs from steady-state photoluminescence by resolving the characteristic time it takes for the luminescence signal to decay, instead of solely the spectral information (intensity as a function of wavelength). By analyzing the temporal decay of the luminescence signal, TRPL can reveal lifetimes, from which properties such as recombination rates of excited carriers, relaxation pathways, material defects, and carrier diffusion dynamics can be inferred ^{1 2}. These properties are widely studied in various fields, including materials science, semiconductor photonics, solar cell research, and biological applications.

TRPL plays a crucial role in various scientific and industrial applications due to its ability to resolve ultrafast carrier recombination dynamics with high temporal precision.

Nanosecond–picosecond time resolution enables precise diagnosis of material and device behavior, guiding design changes (e.g., passivation, composition, doping) that improve performance across quantum technology, bioimaging, displays, and photovoltaics.

Common TRPL uses include the following:

• Microscopy and Spectroscopy: Imaging methods can extend beyond traditional intensity-based analysis by measuring the relaxation of the excited state of the sample under study, known as the photoluminescence “lifetime”. TRPL enables techniques such as Fluorescence Lifetime Imaging Microscopy (FLIM) and Phosphorescence Lifetime Imaging Microscopy (PLIM), which are useful for mapping molecular environments in tissues, differentiating cancerous from healthy cells, separating bound and free fluorophores, quantifying local pH, and more.
• Semiconductors and Photonic Devices: TRPL is key to evaluating defect densities, non-radiative recombination, and carrier trapping, all of which impact the efficiency of light-emitting devices. Longer photoluminescence lifetimes are generally indicative of low defect density and efficient radiative recombination and, therefore, often characteristic of high-quality semiconductors. In contrast, shorter lifetimes often point to the presence of defects, impurities, or traps that introduce fast, non-radiative decay pathways, which quench the luminescence ³.
• Organic and Perovskite Photovoltaics: Improving power-conversion efficiency is crucial to optimizing solar-cell materials. TRPL provides information on charge-carrier dynamics and recombination mechanisms by distinguishing bulk from surface recombination, pinpointing trap-assisted losses, and validating passivation steps that lengthen carrier lifetimes ⁴.

Time-resolved photoluminescence (TRPL) setups pair a synchronized excitation source with efficient photon collection and low-jitter timing electronics, so photon arrival times relative to each laser pulse can be histogrammed into a decay curve and fitted for lifetimes and pathways.

Figure 1 sketches a typical arrangement:

• Excitation and triggering: A pulsed laser (e.g., picosecond or femtosecond laser) at a specific wavelength drives the excitation at a defined repetition rate; a trigger/marker from the laser or from a signal generator provides the timing reference for subsequent time stamping.
• Sample and collection optics: The excitation beam is shaped and focused onto the sample in a microscope or simple free-space setup. The emitted light is then collected, filtered to remove the pump, and sent, via free space or a fiber, to the detector. Setups can be confocal or widefield, with optional add-ons like a spectrometer or basic environmental control.
• Photon detection: After excitation, the sample’s emitted photons are collected and directed toward a time-resolved detector, such as a photomultiplier tube (PMT), single-photon avalanche diode (SPAD), or superconducting nanowire single-photon detector (SNSPD).
• Data acquisition and analysis: The electrical signal is then captured by a high-resolution timing electronics (e.g., a time-to-digital converter, TDC), which records the laser trigger (“start”) and each detected photon (“stop”) as precise timestamps. A histogram of photon arrivals relative to the trigger yields the PL single-exponential or multi-exponential decay curve; fitting returns lifetimes and amplitudes. Since each photon corresponds to an energetic recombination event, analyzing the distribution of photon arrival times provides insight into how long and through which channels excited states persist before relaxing back to the ground state. A single-channel lifetime follows $\frac{1}{k_r + k_{nr}}$, where $k_{r}$ and $k_{nr}$ are the radiative and non-radiative rate constants, respectively. Deviations from a single-exponential photoluminescence decay often arise from competing relaxation pathways, such as non-radiative recombination, quenching, or intersystem crossing. In semiconductors, carrier trapping, surface states, and exciton–exciton interactions can also contribute. These processes typically lead to multi-exponential or stretched-exponential, reflecting the complexity of the underlying dynamics.

Common challenges in TRPL due to Conventional Timing Electronics

In time-resolved photoluminescence (TRPL), the instrument response is effectively the convolution of each stage, so their timing variances add (approximately) in quadrature. The dominant contributor sets the limit, making it essential to minimize added jitter, dead time, and data-path bottlenecks. Therefore, requirements on the timing electronics are stringent.

Typical challenges include:

• Timing Jitter: TRPL experiments require precise timing resolution, often in the range of a few to tens of picoseconds, to accurately characterize fast decays. Even when leveraging femtosecond-class lasers, the jitter contribution from other electrical components in the setup, such as detectors and timing electronics, can blur the actual arrival times of photons, broadening the instrument response function (IRF) and degrading the sharpness of decay curves. This makes it harder to resolve fast recombination processes or distinguish overlapping decay channels. As material systems become more complex, the need to resolve multi-exponential decay becomes increasingly dependent on timing fidelity. Therefore, a precise acquisition system with low jitter is crucial for advanced TRPL studies.
• Acquisition Speed and Data Throughput: TRPL experiments rely on detecting single photons, so adequate statistics are essential, especially for weak signals or rapid dynamics. If the system’s electronics are too slow to process this large volume of data, some photon events (timestamps) will be discarded, which reduces statistical accuracy and biases the apparent lifetimes. Therefore, the system’s data processing speed must keep up with the rate of photon detection.
• Synchronization Across Experimental Components: Advanced TRPL experiments often require synchronization between the laser source, photon detector, and external signal triggers. Signal generators employed to orchestrate TRPL experiments require a stable frequency range to span the relevant repetition-rate range (kHz for long-lived phosphorescence to tens of MHz for fluorescence). Pattern/trigger sources should offer tunable pulse parameters and integrate cleanly with the timing reference used for start/stop stamping.
• Multichannel and Synchronized Acquisition and Analysis: Coordinating all synchronized signals in a TRPL experiment and time-stamping them relative to the excitation pulse is a non-trivial task. Without accurate synchronization and scalable multi-channel acquisition, experiments become limited in scope or prone to misalignment. While post-hoc realignment can fix small skews, it introduces complexity and may not recover true timing when channel skew, drift, or missing timestamps are present. Stable referencing with known inter-channel skew is essential for reliable and comparable decay analysis across detectors.

Swabian Instruments’ Time Taggers are data acquisition and analysis systems for precise time-correlated experiments. They are engineered to meet the demanding requirements of modern Time-Resolved PhotoLuminescence (TRPL) experiments by providing a unique combination of powerful hardware and software specifications.

From a hardware perspective, Swabian Instruments’ Time Taggers provide a set of features for advanced TRPL acquisition, including:

• Picosecond Timing Jitter is essential for high-resolution experiments, especially when working with fast decay dynamics. Swabian Instruments’ Time Tagger X achieves timing jitter as low as 1.5 ps, enabling researchers to resolve rapid decay processes and distinguish overlapping lifetime components with precision.
• Short Dead Time for Fast Acquisition: Data collection efficiency is maximized with dead times as short as 1.5 ns, which is significantly shorter than the typical dead time of commercial detectors on the market. The ability to capture input signals at high frequency helps achieve a full characterization of the decay-enhancing signal-to-noise ratio (SNR).
• Multi-Channel Capabilities: The precise timestamping of input signals across multiple channels with picosecond resolution enables the integration of external synchronization signals for flexible experiments that may include external pixel, line, or frame triggers (for example, in a scanning microscope setup). All inputs are equivalent (no fixed start/stop roles), simplifying wiring and reconfiguration.

Swabian Instruments’ software platform allows for real-time data streaming of raw timestamps and on-the-fly processing. This approach simplifies TRPL data acquisition and analysis, including software features such as:

• Powerful yet intuitive software engine: Swabian Instruments offers a comprehensive Software Development Kit (SDK) with an extensive API in common programming languages (Python, MATLAB, LabVIEW, C#, and C++) for seamless automation and integration with existing experimental setups. The intuitive GUI of Time Tagger Lab enables the setup of a lifetime decay experiment in just a few clicks.
• Flexible adjustments of trigger levels, delay times, conditional filtering, and customizable gating are available to maximize experiment efficiency. An adjustable synthetic dead time helps reject retriggers/afterpulsing, improving measurement SNR. Conditional filtering enables users to discard events on a selected input based on events in another input channel, which is particularly helpful when leveraging excitation sources at high repetition rates, where not all laser events are followed by a detection click.
• Expanded features: Swabian Instruments software Virtual Channels enable computations of existing physical channels or other virtual channels. For example, DelayedChannel() can serve to make a copy of a signal with a specified pulse shift, and can serve to combine with the Conditional filter feature for efficient data handling. In addition, one can filter out certain regions of a data stream by building start and stop gates with the GatedChannel(). Another important Virtual Channel for users with PhotoMultiplier Tubes (PMTs), where the detector signal may have inconsistent pulse widths and amplitude, is the ConstantFractionDiscriminator(), implemented at a software level for precise pulse characterization without requiring an external piece of hardware equipment that may contribute to added jitter and sources for error.
• Advanced clock synchronization options: In laser-synchronous TRPL, the excitation laser serves as the clock. The ReferenceClock functionality of the Time Tagger software implements a software PLL that rescales time tags on the fly to this reference, averaging down timing noise and keeping the analysis phase-aligned, even when a Conditional filter is enabled on the laser channel.

On the signal generation side, Swabian Instruments’ Pulse Streamer is a signal generator with digital and analog output capabilities that facilitates synchronization and laser triggering capabilities:

• From a hardware perspective, the Pulse Streamer 8/2 offers 8 digital outputs with a sampling rate up to 1 GHz, which is sufficient to satisfy experimental requirements. The rise and fall times < 300 ps combined with jitter < 50 ps RMS are suitable features for well-defined laser triggering. In addition, 2 analog outputs are available for experimental control of additional equipment.
• The intuitive and versatile implementation of pulse sequences via GUI or API (Python, MATLAB, LabVIEW) provides a unique approach for pattern generation.

To summarize, advanced timing electronics, which offer multi-channel inputs, low timing jitter, minimal dead time, and real-time streaming of raw timestamps, facilitate powerful data acquisition. Flexible, software-based analysis streamlines the analysis of lifetime decay profiles to extract recombination rates, defect densities, and other key material parameters. Swabian Instruments Time Taggers’ multi-channel, low-jitter, short dead time acquisition, and efficient data handling result in high-resolution, high-fidelity lifetime measurements across a wide range of materials. This approach preserves fast decays at high repetition rates and accommodates multi-detector flexible configurations where simultaneous channels must remain phase aligned. The combination of Swabian Instruments’ Pulse Streamer and Time Taggers for signal generation and data acquisition provides a versatile yet powerful platform for advanced TRPL studies and the next generation of experiments.