New time lenses can magnify the time between individual photons, refining time-resolved photon-counting to the extent that ultrafast chemical and molecular processes can be measured.
3D Rendering of Photon Waves. Image Credit: agsandrew/Shutterstock.com
Novel “time-lenses” can magnify the difference in arrival times between individual photons in an ultrashort pulse of light.
The lens was developed by researchers in the United States using an optical set-up composed of “off-the-shelf” materials, demonstrating that the arrival times of individual photons in a femtosecond-length pulse could be stretched while not losing the quantum information the pulses carry.
The research, documented in a paper published in the journal Optica in 2021, could have important and positive connotations for time-correlated single-photon counting (TCSPC).
“Time-resolved photon-counting plays an indispensable role in precision metrology in both classical and quantum regimes. Therein, TCSPC has been the key enabling technology for applications such as fluorescence lifetime microscopy, time-gated Raman spectroscopy, photon-counting time-of-flight (ToF) 3D imaging light-in-flight imaging, and computational diffuse optical tomography,” the authors, including, Shu-Wei Huang and colleagues at the University of Colorado, Boulder explain.
One of the most common applications of TCSPC is in obtaining precise 3D images of a wide range of objects that vary in scale from molecules to mountains.
Picture This: How TCSPC works
The main principle of TCSPC is the detection of single photons and the measurement of their individual arrival times using another signal, commonly the light source, as a reference.
The method is statistical, which means that it requires a repetitive light source to accumulate enough photons to build an accurate model from the data.
TCSPC electronics are analogous to a fast stopwatch that has two inputs. The stopwatch is started by the “start” signal pulse, and ended by the “stop” signal pulse. Each start-stop sequence is a data point.
With a rapid repetition rate from a high repetition light source, millions of start-stop sequences can be measured in a very short time. The resulting data pattern represents fluorescence intensity over time.
Generally, one of the inputs to the TCSPC electronics, either start and stop, will be a pulse generated by a single photon. Single photons can be detected by photodetectors with intrinsically high gain.
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Using TCSPC requires ensuring that no more than one single photon event per light flash is detected because multi-photon events will affect the statistical picture and will give erroneous measurement results. This is a problem that is referred to as the “pulse pile-up problem.”
To make sure that only one photon per light flash is detected, the photon rate is kept low in comparison to the rate of the exciting lamp — usually 5% or lower.
When used to map an object or geological feature TCSPS measures the time taken for individual particles from a pulsed laser to travel to the detector after being bounced from an object.
The resolution of TCSPC has been improved over recent years through the addition of elements like photomultiplier tubes and superconducting nanowires. This has resulted in systems that can distinguish between photons as close together as 3 picoseconds, with a picosecond being a trillionth of a second.
The authors of this new study suggest that this resolution can be further improved by magnifying the time that separates the arrival of individual photons — the time between that start pulse and stop pulse in that start-stop sequence.
To do this they created a new time lens to pass pulse through.
Magnifying Time With a Time Lens
In their experiments, the researchers used a set-up that included an off-the-shelf single-photon detector, a 30 m spool of nonlinear optical fiber, and two pulsed lasers. Passing pulses from these lasers through their time lens, the researchers found the leading edge photons speeding up while trailing edge photons slowed down.
This meant that the gap between the start and stop signals was more clearly identifiable, but none of the information the signals carried was lost. The experimental set-up used by the team magnified the separation times of the photons by a factor of 130 with 97 percent of photons being successfully converted.
“In conclusion, we have demonstrated a TM-TCSPC that enables photon counting at the femtosecond regime. Using a temporal magnification ratio of 130, 550 fs SPTR has been achieved, enabling resolving 130-fs pulse width difference at a 22-fs accuracy,” the authors conclude.
By improving the time-lens implementation, the record length can be extended to more than 10 ns without sacrificing the sub-picosecond SPTR
Li. B., Bartos. J., Xie. Y., Huang S-W, Authors of the Study, 2021
The authors add that combined with computer vision TM-TCSPC-based, ToF 3D imaging will enable human face recognition even when the object is beyond the line of sight.
More research will be needed before a time lens like the one implemented by the team can be widely integrated into TCSPC. Yet, if it can be adopted, this development could herald the detailed imaging of molecular processes that proceed rapidly, such as chemical reactions and metabolic processes occurring at a cellular level.
References and Further Reading
Li. B., Bartos. J., Xie. Y., Huang S-W., [2021], ‘Time-magnified photon counting with 550-fs resolution,’ Optica, https://doi.org/10.1364/OPTICA.420816
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