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Picosecond Transient Absorption Spectroscopy System

PicoTAS– Picosecond to Second Transient Absorption Spectrometer

About Unisoku/TII Group As a member of Tokyo Instruments, Inc. (TII Group), UNISOKU has been developing, manufacturing and custom designing the world’s cutting-edge research instruments since 1974. It was chosen as one of Japan’s “Global Niche Top 100 Companies” by the Japanese Ministry of Economy, Trade and Industry in March 2014. It is striving to be the leading company in Japan as “No.1 in Nano-Technology measurement and Photonics”. PicoTAS Transient Absorption Spectroscopy Systems picoTAS PicoTAS is one of UNISOKU’s newest innovative product capable of measuring Picosecond to Second Transient Absorption Spectrometer. It uses the combination of Asynchronous Pump Light and Probe Light technique and the Randomly Interleaved Pulse Train (RIPT) method. This newly patented technology allows seamless bridge of the picosecond to nanosecond time range, eliminates the notorious 1 ns – 20 ns time gap, avoids the pitfalls and limitations of optical delays in traditional pump and probe methods and nanosecond flash photolysis. Unisoku has been developing this product in cooperation with Nihon University, Osaka University and Meijo University in the framework of the Japan Science and Technology Agency’s “Development of Systems and Technologies for Advanced Measurement and Analysis (JST-SENTAN)” program.

Outstanding Features

  1. Unisoku’s patented RIPT technique eliminates deficiencies of conventional TAS systems.
  2.  Asynchronous Operation
  3. Complete coverage of gap time region (1 ns–20 ns)
  4. Broad wavelength range from VIS to NIR
  5. Transient absorption measurement from 100 picosecond to second
  6. Completely eliminates photoluminescence contamination, which has plagued conventional TAS methods.
  7. Can be easily upgraded to include TCSPC lifetime capability
  8.  Compact size: can be placed on lab bench, optical bench is not required (ns model),
  9. Convenient to operate: No complex optical system for alignment, easy to operate, user-friendly software.

What is TAS ( Transient Absorption Spectroscopy)?

Time-resolved spectroscopy is the study of dynamic processes in materials or chemical compounds by means of spectroscopic techniques. Both TAS and Fluorescence lifetime measurement are time resolved methods, but the latter is limited to a fluorescent process at the first stage of the reaction (excited singlet state). TAS, on the other hand, can explore and analyze multi-step complex processes in a wide time region (short-lived radical species, charge transfer states, etc.), to predict emissive, non emissive states and dark states.

Conventional TAS

Conventional TAS usually refers to the pump-probe spectroscopic technique for probing and characterizing the electronic and structural properties of short-lived excited states (transient states) of photochemically or photophysically relevant molecules. In a typical conventional experimental set up, the sample is first excited by a pump pulse (pump light), then struck by a delayed probe pulse (probe light), and the impact of the probe pulse on the sample is observed by measurement of time-resolved absorption change of the probe light, analyzed against wavelength or time to study the dynamics of the excited state.

Absorbance (after pump)-Absorbance (before pump)= Δ Absorbance

Δ Absorbance records any change in the absorption spectrum as a function of time and wavelength. TAS curve along wavelength provides information of various intermediate species involved in chemical reaction at different wavelengths, and TAS against time, measured at a given wavelength, can provide information such as inter-system crossing, intermediate unstable electronic states, trap states, surface states etc.

PicoTAS from Unisoku

PicoTAS uses the Asynchronous Pump Light and Probe Light technique combined with the Randomly Interleaved Pulse Train (RIPT) method [1], which was featured by Science [2] and Nature Photonics [3]. This is Unisoku’s newly patented technology [4], allows for seamless bridge of the picosecond and nanosecond time ranges, eliminating the notorious 1 ns – 20 ns time gap in conventional TAS instruments.

[1] Nakagawa, T.; Okamoto, K.; Hanada. H.; Katoh, R. Opt. Lett.2016, 41, 1498-1501.
[2] Yeston, J. Science 352, 670 (2016)
[3] Donati, G. Nature Photonics 10, 285 (2016)
[4] US Patent 9,709,497, 2017

What is Randomly Interleaved Pulse Train (RIPT) Method

The RIPT technique involves detecting the waveforms of uncorrelated pump pulses and probe pulse and calculating the time delay between the two. In the RIPT method, signal waveforms of both pump light and probe light are recorded by high-speed detectors on each pump light irradiation. The delay time of a probe pulse after the pump pulse is calculated from these waveforms. Each light intensity of probe light pulse that is transmitted through a sample is recorded by using a detector with amplifier, and the intensity is plotted against the delay time. When pump light irradiation is repeated, the delay time differs every time because pump light and probe light are asynchronous. In this way, the plot generates a continuous curve after many repeats of pump light irradiation. By executing delta optical density calculation, transient absorption curve is reconstructed. This elegant direct method avoids the pitfalls and limitations of optical delays in traditional pump and probe methods and nanosecond flash photolysis.

The main features of RIPT

  • In RIPT method, the ratio between pump /probe light pulses is 1:Many, while this ratio is 1:1 for Pump-Probe method and 1: ∞ for CW method
  • In RIPTI, Pump and Probe are asynchronous
  • In RIPT, Time difference is calculated passively
  • By repetition of pumping cycles with randomly-interleaved-pulse-train probing, TA signal with high time resolution and wide time range is reconstructed by high-speed data processingOptics Letters, 41, 1498 (2016) ; Highlighted in Nat. Photon., 10 (2016), 285; Science, 352 (2016), 669 

What are the Differences between picoTAS and Conventional Techniques?

Eliminate “Gap time Region” (1-20ns):

There are mainly two conventional transient absorption (TA) techniques, Pump & Probe method and Nanosecond Flash Photolysis method. Both have difficulty in measuring the “Gap Time Region” (1‐20 nanoseconds) in which many important phenomena exist.

picoTAS uses Randomly Interleaved Pulse Train (RIPT method). It can measure wide time range of time including the time region from 1 to 20 nanoseconds.

Eliminating the influence of fluorescence:

picoTAS has the capability to eliminate the influence of fluorescence, therefore, not only non-fluorescent but also fluorescent intermediates can be detected and identified correctly. This is accomplished by picking up the signal intensity just before the rise of each probe pulse signal and repeating this procedure for a series of probe pulse train data set, and creating a background curve, which includes a photoluminescence decay signal generated by the pump pulse. Subtracting the background curve from individual probe pulse train data resulting in a transient absorption curve that is free of any photoluminescent contamination