Breaking Spectral Boundaries: New Ultrafast Spectrometer Expands Detection Range for NIR Studies

Caution Sign for invisible near-infrared ytterbium laser ©Seetwo - stock.adobe.com

Researchers from Auburn University have disclosed a new NIR transient absorption spectrometer, pushing the boundaries of what is possible in ultrafast spectroscopy. The newly developed instrument can simultaneously detect across an impressive spectral range of 900–2350 nm, which significantly broadens the capabilities for studying electronic materials and ultrafast dynamics. Traditionally, such measurements required two separate configurations or experiments, but this new system simplifies the process, allowing efficient and cost-effective data collection in one go (measurement sequence). The work, conducted by Austin L. Dorris, Abdul Rashid Umar, and Christopher Grieco, is published in the journal Applied Spectroscopy (1).

See More Articles: Near-infrared (NIR) transient absorption (TA) spectroscopy

Expanding Detection in NIR Spectroscopy
Ultrafast transient absorption (TA) spectroscopy has long been a powerful tool for observing chemical and physical processes on the femtosecond to nanosecond timescale. These processes, including charge photogeneration and photochemistry, are critical for understanding the behaviors of various materials (1–3). However, traditional TA setups have been limited in their detection range, often stopping at around 1700 nm (1–3). This limitation meant that many low-bandgap materials and their full electronic transitions were not fully captured in previous studies.

The Auburn team’s work addresses this gap by developing an ultrabroadband spectrometer that significantly extends the detection range. The new system probes across 900–2350 nm in one experiment, eliminating the need for multiple setups. “Our approach simplifies NIR TA spectroscopy by integrating a single optical geometry to achieve this wide spectral range,” said Dorris (1).

Innovative Instrument Design
The newly designed instrument uses a femtosecond ytterbium laser with a pulse duration of 160 femtoseconds and a repetition rate of 10 kHz. This laser drives an optical parametric amplifier (OPA) to produce a pump pulse, which can be tuned from 315 to 2600 nm. Most notably, a second OPA generates a 1980 nm idler pulse, which is used to create the supercontinuum probe pulse needed for broadband detection (1).

At the heart of the system’s success is the use of an 8 mm yttrium aluminum garnet (YAG) crystal for supercontinuum generation. This crystal allows the generation of a stable, broad spectrum probe pulse, which is then shaped through a series of filters to ensure accurate detection across the entire 900–2350 nm range. The system’s high-speed, prism-based spectrometer detects the signal using custom-built indium gallium arsenide (InGaAs) cameras that are capable of capturing data at high repetition rates, up to 9 kHz (1).

Superior Signal Detection and Data Processing
Signal detection is achieved using a custom spectrometer equipped with two InGaAs cameras. The signal probe beam is overlapped with an actinic pulse to generate a differential absorption signal, while a reference probe beam passes through an unperturbed region of the sample for balanced detection. Data acquisition is managed by custom software, which calibrates the pixel-wavelength correlation and compensates for any artifacts in the data, such as background noise or group velocity dispersion (GVD) (1).

The spectrometer achieves a remarkably high signal-to-noise ratio (S/N) and sensitivity (10-5 AU), with further improvements possible by averaging more laser shots or scans. This level of sensitivity makes the system ideal for studying transient states like polarons and excitons in low-bandgap materials (1).

Applications and Future Improvements
The new spectrometer was tested on a polymer photovoltaic system, demonstrating its ability to capture the ultrafast dynamics of photogenerated charge carriers. Such capabilities are vital for advancing research in areas like organic photovoltaics and semiconductors, where understanding charge transfer processes is critical for improving device efficiency (1).

Looking ahead, the team envisions further enhancements, such as using different pump wavelengths or custom optical filters to improve spectral shaping and extend the continuum range. “We’ve demonstrated the power of this system in the NIR, but there’s potential to expand into the visible region as well, which would broaden its applicability even more,” explained Grieco (1).
This breakthrough in NIR TA spectroscopy by Auburn University’s research team represents a significant leap forward for the field. By expanding the detection range and simplifying the experimental setup, this new spectrometer offers a versatile, high-precision tool for exploring ultrafast processes in cutting-edge materials. The team’s work not only streamlines current research methods, but also sets the stage for future discoveries in photochemistry, photovoltaics, and beyond (1–3).

References

(1) Dorris A. L.; Umar A. R.; Grieco C. Ultrabroadband Near-Infrared Transient Absorption Spectrometer with Simultaneous 900–2350 nm Detection. Appl. Spectrosc. 2024, 78 (10),1043–1050. DOI: 10.1177/00037028241247072

(2) Schmidhammer U.; Jeunesse P.; Stresing G.; Mostafavi M. A Broadband Ultrafast Transient Absorption Spectrometer Covering the Range from Near-Infrared (NIR) down to Green. Appl. Spectrosc. 2014, 68 (10), 1137–1147. DOI: 10.1366/13-07214

(3) Berera, R.; van Grondelle, R.; Kennis, J. T. M. Ultrafast Transient Absorption Spectroscopy: Principles and Application to Photosynthetic Systems. Photosynth. Res. 2009, 101, 105–118. DOI: 10.1007/s11120-009-9454-y