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Fluorescence lifetime measurement
The principles of photoluminescence lifetime
Photoluminescence (PL) lifetime is the time it takes for a molecule, such as a fluorescent dye, to return to its ground state by emitting photons following photoexcitation. In the simplest terms, it is defined as the time it takes for the PL intensity to decrease to approximately 37% of its initial value immediately after excitation (Figure 1). The PL lifetime varies from picoseconds to milliseconds, depending on the type of emitting material. For example, emitting materials used in organic electroluminescence devices require high luminescence efficiency and short fluorescence lifetimes to achieve high-speed response.
Figure 1 : Definition of fluorescence lifetime
What is a TRPL measurement
Time-resolved photoluminescence (TRPL) measurement is a method of measuring the temporal change in the intensity of photoluminescence, such as fluorescence and phosphorescence, emitted from a sample after photo excitation with a very short pulse of light. Interesting examples of TRPL measurements are fluorescence and phosphorescence lifetimes in organic and inorganic compounds, as well as semiconductors that undergo interband recombination.
TRFL / Fluorescence lifetime measurement methods and principle
There are several methods for measuring TRPL, two of the well-known techniques include time-correlated single photon count (TCSPC) and the streak method.
Time-correlated single photon counting (TCSPC) method
TCSPC is a fluorescence lifetime measurement method that uses a Time-to-Amplitude Converter (TAC) as its fundament. A fluorescence lifetime measurement system using the TCSPC method generally consists of an excitation pulse light source, sample holder, detector, and data analysis system. Photomultiplier tubes are used primarily as detectors. The TAC is like a high-speed stopwatch that starts when the excitation light is emitted and stops when the fluorescence photon from the sample is detected. The time difference between the start and the stop points is then output as a voltage. The voltage output from the TAC is then converted into time information via a Multi-Channel Analyzer (MCA) (Figure 2a). After accumulating several start-stop cycles the obtained data is plotted as a histogram of counts versus time (Figure 2b) resulting in the fluorescence decay curve where a fluorescence lifetime could be determined (Figure 3).
In recent years, Time to Digital Converter (TDC) is often used instead of TAC. The main reason a TDC is used nowadays is to downsize the measurement devices, where a TDC combines the time-to-amplitude conversion with the amplitude-to-digital conversion, thereby reducing the component's size and increasing its efficiency.
Hamamatsu Photonics' compact fluorescence lifetime measurement system, the Quantaurus-TauⓇ, deploys TDC technology.
Figure 2(a): Principle of TCSPC method
Figure 2(b): Principle of TCSPC method
Figure 3: Example of output fluorescence lifetime measurement data
In this video, we introduce how to operate Quantaurus-Tau.
Streak camera method
A streak camera is a device that captures light-related phenomena occurring within an extremely short time. A streak camera is mainly composed of a photocathode, an accelerating electrode, a sweeping electrode, a microchannel plate (MCP), and a phosphor screen. Photons that enter the streak camera are first converted to electrons at the photocathode, and then accelerated by passing through the accelerating electrode. When the accelerated electrons pass through the sweeping electrode, a variable voltage in time is applied to the electrodes to deflect the electrons up and down. After which, the deflected electrons are multiplied several orders of magnitude via the MCP gain and are converted back into light upon hitting on the phosphor screen. The fluorescence of the phosphor screen produces a streak image that is captured via a read-out camera, with the horizontal axis representing space and the vertical axis representing time (Figure 5). The brightness of the streak image is proportional to the intensity of each photon.
Furthermore, by placing a spectrometer in front of the streak camera, it becomes possible to obtain a streak image with the horizontal axis representing wavelength and the vertical axis representing time. This is described as Time-Resolved Emission Spectroscopy (TRES) that combines time-resolved measurements with spectral analysis, allowing for the study of both the temporal and spectral characteristics of photoluminescence.
The Hamamatsu Photonics universal streak camera has an extremely high time resolution of less than 800 femtoseconds (fs).
Figure 4: Streak camera operating principle diagram
Figure 5: Example of streak image
In this video, we introduce examples of measurements and principles using a streak camera.
Fluorescence lifetime measurement of luminescent materials
Thermally activated delayed fluorescence from blue organic LED materials
Third generation OLED materials, well known for their thermally activated delayed fluorescence (TADF) are highly investigated to further improve their high efficiency and cost saving potential. In order to achieve the desired high efficiency, the molecular design is an important factor to minimize the thermally accessible gap between the lowest singlet excited state (S1) and the lowest triplet excited state (T1), defined as ΔEst. This approach facilitates harvesting the excitons of both S1 and T1 states, thereby increasing the efficiency of the OLED material.
This paper uses the Quantarus Tau fluorescence lifetime spectrometer and a streak camera system to study the small ΔEst with TADF between 1-3 µs. The modularity of these systems also allowed characterization of the short fluorescence component of OLED films, measured in nanoseconds.
Data courtesy of Prof. Chihaya Adachi, Hajime Nakanotani Center for Organic Photonics and Electronics Research, Kyushu Univ.
Q. Zhang, B. Li, S. Huang, H. Nomura, H. Tanaka and C. Adachi, nature photonics. 8, 326 (2014)
System used : Quantaurus-Tau
Streak camera system with spectro-temporal capabilities for studying two-dimentional pervoskites
2D halide perovskites emerged as a highly interesting material class due to their combination of strong light-matter coupling, chemical versatility, and suitability for a wide range of optoelectronic applications. Moreover, they exhibit strong interactions among electronic, optical, and vibrational excitations, making them interesting for both basic research and future technologies.Despite their unique potential, a major challenge is the difficulty in controlling electrical doping in these materials. In this example, Transient Microscopy was used to study gate-tunable devices based on 2D perovskites, combining time-resolved measurements with spectrally- and spatially-resolved analysis for monitoring reversible electrical injection of free charge carriers. The latter was achieved for both n- and p-type scenarios with an all-optical readout, demonstrating an electrically tunable optical response. The high sensitivity and temporal resolution of the Hamamatsu Photonics streak camera uniquely allowed for direct monitoring of recombination dynamics and spatial propagation of charged exciton complexes in 2D hybrid perovskites. This offered exciting opportunities to employ electrically tunable, optical excitations in hybrid inorganic-organic semiconductors for nanoscale optoelectronic devices.
Temperature- and spatially-dependent photoluminescence
Temperature-dependent streak camera images capture the photoluminescence (PL) within the trion emission range, highlighting how spectral broadening increases over time at higher temperatures. The white dashed lines serve as visual references for 20 K and 50 K.
Time- and spectrally-resolved PL emission
a. Streak camera image of the PL emission in the n-doped regime at 5 K.
b. PL spectra for different time-intervals after the excitation, extracted from (a).
Data courtesy of Prof. Alexey Chernikov, his group, and their collaborators (Institute of Applied Physics and Würzburg-Dresden Cluster of Excellence, TU Dresden).
Jonas D. Ziegler, Yeongsu Cho, Sophia Terres, Matan Menahem, Takashi Taniguchi, Kenji Watanabe, Omer Yaffe, Timothy C. Berkelbach, Alexey Chernikov, Advanced Materials 2023, 35, 2210221
Featured products and systems
We have two major types of fluorescence lifetime measurement systems in our lineup, depending on the time resolution required.
The compact fluorescence lifetime measurement system measures fluorescence lifetime from sub-nanoseconds to milliseconds. It is easy to operate and provides highly accurate fluorescence lifetime measurements.
Fluorescence lifetime measurement system with high sensitivity and high temporal resolution of 800 fs by two-dimensional photon counting method. Fluorescence lifetime of multiple wavelengths is measured simultaneously.
Case study
To evaluate the organic optoelectronic materials and devices being developed by the Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, various measurement methods such as photoluminescence(PL) quantum yield measurement and fluorescence lifetime measurement are required. For these evaluations, our Quantaurus series and streak cameras have been introduced.
We interviewed Prof. Chihaya Adachi, Director of the Center, and Assoc. Prof. Hajime Nakanotani about establishing absolute photoluminescence quantum yield measurement methods, the impact of our Quantaurus series on their research, and their prospects for future research.
Other measurement methods
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