An imaging system developed by researchers from the Institut national de la recherche scientifique (INRS) captures the photoluminescence lifetimes of rare-earth doped nanoparticles in the micro- to millisecond range. The high-precision shortwave infrared (SWIR) imaging technique paves the way for application in biomedical and information security where accuracy and dependability are essential.

Rare-earth doped nanoparticles (RENPs) possess unusual light-emitting properties that researchers find useful, like the ability to emit in the UV, visible, and SWIR ranges. The photoluminescence lifetime of RENPs has the advantage of being minimally affected by external conditions. As a result, measuring it through imaging provides data from which accurate and highly reliable information can be derived, such as energy transfer processes and photoluminescence efficiency.

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(From left) Professor Jinyang Liang, researcher Miao Liu, and professor Fiorenzo Vetrone in front of the SWIR-PLIMASC device. Courtesy of INRS. 

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So far, SWIR imaging techniques have not kept pace with advances in the design and synthesis of RENPs with optical properties. “Until now, existing optical systems have offered limited possibilities due to inefficient photon detection, limited imaging speed, and low sensitivity,” professor Jinyang Liang said.

Typically, the photoluminescence lifetime of RENPs is measured by counting time-correlated single photons. This method requires a large number of excitations to be repeated at the same location, because the detector can only process a certain number of photons for each excitation. The long photoluminescence lifetimes of RENPs in the IR spectrum limit the excitation’s repetition rate. As a result, significant pixel dwelling time is needed to build the photoluminescence intensity decay curve.

To improve SWIR photoluminescence lifetime acquisition, the researchers created SWIR photoluminescence lifetime imaging microscopy using an all-optical streak camera (SWIR-PLIMASC), a photoluminescence lifetime mapping platform for RENPs in the SWIR spectral range.

SWIR-PLIMASC blends scanning optics with a high-sensitivity indium gallium arsenide (InGaAs) CCD camera. The use of scanning optics enables the camera to achieve a 1D imaging speed of up to 138.9 kHz in the spectral range of 900 to 1700 nm, which allows the photoluminescence lifetime of RENPs to be quantified in a single shot. SWIR-PLIMASC also can map the 2D SWIR photoluminescence lifetime distribution, by 1D scanning of the sample.

To evaluate the system and assess its possible use in anti-counterfeiting and thermometry applications, the researchers prepared a series of erbium ion (Er³⁺)- and holmium ion (Ho³⁺)-doped RENPs with distinct SWIR photoluminescence decay lifetimes.

The team used Er³⁺-doped RENPs to demonstrate SWIR-PLIMASC for multiplexed anti-counterfeiting applications. The system revealed complex topology in photoluminescence lifetime-based taggants, demonstrating its potential for use in high-security anti-counterfeiting.

Leveraging Ho³⁺-doped RENPs as temperature indicators, the researchers applied the device to SWIR photoluminescence lifetime-based thermometry. SWIR-PLIMASC accurately sensed temperature in a biological phantom, demonstrating superior detection ability compared to conventional thermal imaging.
 
According to the researchers, SWIR-PLIMASC is the first high-sensitivity, high-speed SWIR optical imaging system.

“It has several advantages,” researcher Miao Liu said. “For instance, it responds to a wide spectral range, from 900 nm to 1700 nm, allowing photoluminescence to be detected at different wavelengths and/or spectral bands.”

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Miao Liu, a Ph.D. student in energy and materials science at INRS, is first author on the paper describing a high-precision shortwave infrared imaging technique to improve the efficiency of photoluminescence lifetime imaging. Courtesy of INRS. 

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The system can directly capture photoluminescence lifetimes in the IR spectrum, from microseconds to milliseconds, in one snapshot, with a 1D imaging speed that can be tuned from 10.3 kHz to 138.9 kHz. By allocating the temporal information of photoluminescence to different spatial positions, SWIR-PLIMASC ensures that the entire process of 1D photoluminescence intensity decay can be recorded in a single snapshot, without repeated excitation. “You save time, but still get high sensitivity,” Liu said.

The system could find broad use in materials characterization, information science, security, and biomedicine. In the biomedical field, it could be used to fight cancer.

“As our system applies to the temperature-based photoluminescence lifetime imaging of rare-earth ions, we believe that the data obtained could, for example, help to detect cancer cells even earlier and more accurately. The metabolism of those cells raises the temperature of the surrounding tissues,” professor Fiorenzo Vetrone said.

SWIR-PLIMASC could also be valuable for storing information at enhanced security levels to prevent falsification of documents and data. When used in photoluminescence lifetime studies, the system could contribute to a better understanding of the luminescence process and, thus, to the design of new RENPs with improved optical properties.

The research was published in Advanced Science (www.doi.org/10.1002/advs.202305284).

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