OLIS Introduces FFT-Based Solution to Eliminate Artifacts in CPL Measurements
A 2023 paper by Sasha Feldmann reminds everyone that circularly polarized luminescence (CPL)
is very easily contaminated by linearly polarized luminescence (LPL) and that any contamination
results in both LPL and CPL being used in calculating g LUM .
Early publications focused the concern on the equipment itself being a cause of LPL
contamination. But Feldmann’s paper strongly emphasizes the contribution of LPL by the
sample itself.
One very pointed statement is “Acquiring reliable CPL spectra of highly oriented samples with
large optical anisotropy is very challenging, as these commonly give rise to strong linear
polarization artifacts.” And “…the most common artifacts and pitfalls are discussed, especially
for the study of thin films, for example, based on molecules, polymers, or halide perovskites, as
opposed to dilute solutions of emitters.”
Feldmann points out that many samples have strong LPL. Thus, no matter how careful an
instrument manufacturer is in reducing LPL contribution, contamination by sample-induced LPL
remains a major concern. (See citations to follow.)
OLIS CPL spectrometers are designed with components which do not introduce LPL.
However, Feldmann’s concern about samples with inherent LPL signals (e.g., samples outside of
lanthanides) led us to a renewed focus on this topic. How to improve our technology?
Our solution took nearly two years to implement, but it is now available on all new OLIS CPLs
and it can be added to all original OLIS CPL spectrometers.
We have pioneered a data collection and processing method using Fast Fourier Transform
(FFT). This method takes the raw signals captured by the photon counting detector and
mathematically separates the three phase forms: zero polarization, quarter-wave polarization
(CPL), and half-wave polarization (LPL).
This elegant and exact method ensures artifact-free gₗᵤₘ values and thus reliable
interpretation of chiral luminescence.
All existing OLIS CPL systems can be upgraded to incorporate this FFT technology through a
straightforward electronics and software update.
In offering this solution to our clients, OLIS reaffirms its commitment to scientific integrity,
reproducibility, and continuous innovation in luminescence spectroscopy.
For details or upgrade information, contact sales@olisweb.com and 706-353-6547.
Not sure how important LPL is? Let’s go back to Riehl’s earliest work on this topic, Chemical
Reviews, 1986: Circularly Polarized Luminescence Spectroscopy:
III. B. Artifacts in CPL Measurements
In most application of CPL, the circularly polarized components of the emitted light represent
only a small fraction of the total emitted light intensity (usually <10 -2 ). CPL spectrometers are
generally capable of determining g EM [g LUM ] with a sensitivity of approximately 1 part of 10 -4 to
10 -6 , depending on the total light intensity. One of the problems in CPL measurements is the
fact that it is not possible to reliably determined CPL results if the emitted light is also partially
linearly polarized.
Linear polarization in emission results from a distribution of emitting molecules that is not
isotropic. As described above, even for molecular systems which have random orientational
distributions in the ground state, the distribution of emitting molecules may not be random due
to photoselection by the incident excitation beam.
The problem associated with the measurement of CPL in the presence of linearly polarized
luminescence was first recognized by Steinberg and Gafni (1972), who demonstrated that even
if the emitting sample was achiral, an apparent CPL signal was detected if the emission was
linearly polarized … Dekkers et al., have presented some experimental results that support their
conclusion and demonstrated experimentally that the artifact, if present, and a true CPL signal
are additive …
It should be noted that the artifact one detects when linear polarization is present is difficult to
distinguish from true circularly polarized emission. Very small realignments of the optical
components can yield apparent signals of opposite signs and variable magnitude. It is extremely
important when CPL experiments are performed to check for linear polarization in the emission
by looking for a signal at 2f m . If it is present, an f m signal does not then necessarily indicate the
presence of CPL.
Sensitive Instrument for the Study of Circular Polarization of Luminescence | Review of
