Microscopic circularly polarized fluorescence spectrometer

In the fields of materials science (chiral micro nano structure luminescence performance regulation), chemistry (micro aggregation chiral transfer), biomedicine (single-cell chiral probe imaging), and optoelectronic devices (micro nano scale CP-OLED array development), chiral luminescent samples (such as chiral perovskite nanocrystal arrays, single-cell targeted CPL probes, organic small molecule micro nano aggregates, micro CP-OLED luminescent units) often exhibit significant spatial heterogeneity and dynamic aggregation state evolution. Their core properties not only depend on molecular configuration, but are also closely bound to micro nano scale aggregation modes and interface interactions. Traditional circularly polarized fluorescence or circular dichroism characterization techniques can only obtain information from macroscopic or one-dimensional perspectives, making it difficult to decipher the chiral luminescence laws and mechanisms at the micro level. (Reference: Yeom et al., Advanced materials, 2020, 32 (1): e1903878)

Figure 1. Formation and characterization of chiral ultrafine particles

Any chiral characterization technique lacking spatial resolution is limited by the loss of micro area information, which can only reflect the average properties of the sample and cannot capture key micro nano scale chiral features. For example, ordinary fluorescence spectroscopy can measure the total luminescence intensity in the micro region, but cannot distinguish the difference between left/right circularly polarized light, and cannot obtain the micro region glum value, making it difficult to determine the chiral purity of individual nanocrystals; Although macroscopic circularly polarized fluorescence spectroscopy can measure the overall glum value, it cannot locate chiral luminescent "hotspots" (such as high glum regions in a certain micro area of a perovskite array), and cannot explain the influence of micro area heterogeneity on overall performance; ECD spectroscopy can characterize the ground state chirality of molecules, but cannot correlate the chirality changes of micro excited states (such as the relaxation of excited state conformation after the binding of intracellular probes to organelles), which are the core of micro CPL devices and biological targeted imaging; Transient fluorescence spectroscopy can measure the lifetime of excited states, but cannot locate the spatial coupling relationship between lifetime and polarization state, and cannot reveal the dynamic mechanism of chiral luminescence in the micro region. This requires the use of a micro circularly polarized fluorescence spectrometer, which can accurately lock the chiral signal in the micro/sub micron level spatial resolution, fill the gap in the characterization of chiral luminescence at the micro/nano scale, and maximize the complementarity with other technologies.

In the field of micro circularly polarized fluorescence detection, modern systems often integrate confocal optical paths, in-situ control modules, and multimodal synchronous acquisition (synchronously obtaining micro area CPL, total fluorescence, and bright field imaging data). The L-CPL/R-CPL signals of the micro area are separated by high-precision optoelectronic modulators (PEM), and focused on specific micro areas (such as single chiral nanocrystals and intracellular lysosomes) with high numerical aperture objective lenses to ensure the collection of chiral luminescence data in a unified spatial coordinate system, avoiding the loss of micro area information caused by "averaging" in traditional macroscopic detection and signal deviation caused by sample micro area displacement in stepwise detection. This integrated design can not only suppress artifacts such as linear birefringence and local fluorescence anisotropy in micro areas (such as uneven polarization interference in chiral thin film edge micro areas), but also analyze the coupling mechanism between micro area chirality, aggregation state, and interface interaction through dynamic monitoring (such as CPL changes during temperature induced micro area aggregation), providing a direct basis for the precise preparation of chiral micro nano materials (such as regulating nanocrystal size uniformity), micro CPL device micro area performance optimization (such as luminescent unit interface modification), and laying a methodological foundation for revealing the dynamic distribution law of chiral probes in cells and the influence of micro area heterogeneity on device performance.

The core value of a micro circularly polarized fluorescence spectrometer lies not only in the synergy of "micro resolution+circularly polarized detection", but also in the deep coupling and intelligent analysis of micro spatial information and chiral luminescence data. This fusion breaks through the limitations of "macroscopic averaging" and "data silos" in traditional chiral characterization. By establishing a cross scale correlation model of "molecular configuration micro aggregation state local CPL performance", it achieves the dual analysis of "spatial localization+performance quantification" of chiral luminescent systems. Especially in the context of rapid development of artificial intelligence and deep learning, the high-throughput micro area CPL data generated by this system (such as glue distribution maps and dynamic evolution curves of different micro areas) combined with deep learning algorithms can achieve high-throughput screening of chiral micro nano materials (quickly identifying high glue micro areas) and intelligent positioning of chiral probes within cells, breaking through the existing "macro trial and error" research paradigm and promoting chiral science research into a new stage of "spatial resolution+data-driven".

Application case:

1. In situ fluorescence+circular polarization+angular resolution+spatial conduction correlation characterization of edge luminescence characteristics of GaSe thin films

Combining circularly polarized luminescence (CPL) technology with spatial mapping can reveal in depth the distribution characteristics and variation patterns of the g-factor at different spatial positions in the material system. This measurement method, commonly known as CPL mapping or g-factor mapping, is an important development direction at the intersection of chiral optics and spatially resolved imaging technology. By combining microscopic imaging techniques with circular polarization detection modules, it is possible to achieve spatial distribution imaging of the g-factor size on the surface of samples at the micrometer or even nanometer scale, thereby establishing a "g (x, y)" two-dimensional spectrum. This spectrum reveals local chiral inhomogeneity in materials caused by crystal orientation, molecular stacking, chiral distortion, defect induction, or self-assembly processes, and reflects the spatial coupling relationship between structure chiral luminescence response in the system. For example, in chiral organic crystals, liquid crystal films, or supramolecular fibers, the g-values in different regions may significantly change due to differences in microscopic arrangement, forming chiral domains; Under external forces such as shear, electric field, and light field, this spatial distribution can be further regulated or reconstructed. CPL mapping is not only used in basic research to understand the structure-activity relationship of chiral materials, but also has important spatial information carrying and decoding potential in applications such as optical encryption, polarization display, anti-counterfeiting labels, etc. It is an important technical support for the spatial regulation research of chiral optical functional materials. (Reference: Hsu et al., Physical Review B, 2015, 91 (19): 195312.)

Figure 2. CPL mapping results of two-dimensional GaSe thin film. (a) Schematic diagram for polarization testing. (b) For CPL imaging of GaSe thin film in one of the ROIs, the central spot in the image is the excitation spot, and the generated fluorescence is collected along the plane to the edge. (c) The spatial distribution of CPL intensity within the sample plane obtained from (b). (d) Polarization direction at each luminescent position on the sample surface, as well as the system's automatic calculation of linear polarization degree

2. In situ fluorescence+circularly polarized+angle resolved correlation measurement guides VCSEL development

Combining circularly polarized luminescence (CPL) with angle resolved spectroscopy technology can achieve systematic analysis of the chiral properties of luminescence at different emission angles and photon momentum directions, thus revealing in depth the dependence of g-factor on luminescence orientation. This method is commonly referred to as Angle Resolved CPL (AR-CPL) measurement, and its core idea is to use the emission angle as a variable to construct a distribution map of g-values in the angular space (i.e., photon momentum space) by accurately measuring the left and right emission intensities in different emission directions. Compared with traditional point measurement CPL technology, AR-CPL not only provides emission chirality information, but also reflects the symmetry breaking of the excited states inside the material in space, the directionality of the emission dipole moment, and the relationship between chiral luminescence and structural anisotropy.

Figure 2. Excitation characteristics of InGaAs near-infrared VCSEL laser [3]

Furthermore, if an energy axis (wavelength or frequency) is introduced on the basis of angular resolution, a distribution map of the g-factor can be plotted in the two-dimensional space (k-E phase space) composed of photon momentum (k) and photon energy (E), namely the g (k, E) spectrum. This type of spectrum allows researchers to simultaneously track the coupling characteristics of luminescence directionality, dispersion behavior, and chiral strength, revealing the origin of chiral selectivity in different energy states. For example, in structures such as photonic crystals, metal organic frameworks (MOFs), chiral microcavities, exciton polaron systems, etc., the emission modes at different angles and energies may have different polarization characteristics. AR-CPL can be used to determine which excited states have strong chiral emission ability and which modes exhibit chiral flipping or resonance enhancement behavior.

In addition, this method can also be used to verify the predicted distribution of chiral emission states in the Brillouin zone in theoretical calculations, evaluate the synergistic effect of emission dipole moments in real space and momentum space, and observe frontier phenomena such as spin orbit coupling and chiral exciton behavior. The research on angle resolved CPL not only provides a multidimensional characterization method of space energy chirality for the luminescence mechanism of materials, but also lays the theoretical and technical foundation for designing chiral luminescent devices with directional selectivity, momentum spatial control ability, and high g-factor output, especially in the fields of circularly polarized lasers, chiral optical communication, and quantum information encoding, which have important development potential. (Reference: Hsu et al., Phys. Rev. B, 91, 195, 312 (2015))

3. Dynamic dependence of full Stokes parameters and g-factor on excited state evolution using circularly polarized fluorescence and time-resolved analysis

Combining circularly polarized luminescence (CPL) characterization with broadband transient time-resolved and full Stokes polarization analysis can achieve systematic analysis of chiral properties and polarization components of chiral luminescent materials at different time scales and energies (wavelengths), thus revealing the dynamic dependence of the luminescence asymmetry factor (lum) on the evolution of excited states. This method is called Time Resolved Full Stokes CPL (TR-FS-CPL), and its core idea is to use "time delay" as the key variable to precisely control the detection time window (from a minimum gate width of 2 ns to a maximum range of 2 ms) through an electronically gated enhanced CCD (ICCD), synchronously collect the intensity differences of left circularly polarized light (LCP) and right circularly polarized light (RCP) under different time delays, and separate the linear polarization components (S ₁: 0 °/90 ° linear polarization, S ₂:+45 °/-45 ° linear polarization), and finally construct the time energy two-dimensional space (t-E space) of glutamate. Dynamic distribution map (glue (t, E)). Compared with traditional steady-state CPL technology that can only obtain static glue values and traditional time-resolved CPL (TRCPL) that needs to balance sensitivity and bandwidth limitations, TR-FS-CPL can not only provide information on the intensity and polarization purity of chiral luminescence, but also reflect the temporal dynamics of excited state relaxation inside the material, the chiral transfer efficiency of different energy states (such as ligand states and high excited states of metal ions), and the boundary between linearly polarized induced CPL artifacts and true chiral signals.

Figure 3. Broadband steady-state and time-resolved full Stokes vector spectra of standard non chiral dyes in low viscosity and high viscosity aqueous solutions

Furthermore, if the broadband spectral coverage (400-900 nm) is retained on the basis of time resolution and full Stokes polarization, a holographic distribution map of glue (t, E, S ₙ) can be drawn in a three-dimensional space composed of "time delay (t) - photon energy (E) - polarization state (S ₁/S ₂/S ∝)", namely the glue (t, E, S ₙ) map (n=1,2,3). This type of spectrum allows researchers to simultaneously track the temporal evolution of luminescence, energy dispersion behavior, and coupling characteristics of polarization states (including chirality), revealing the origin and dynamic changes of chiral activity in different excited states. For example, in the chiral lanthanide complex Eu [(+) - fakam] ∝, TR-FS-CPL can distinguish the chiral differences between the nanosecond level ligand center (LC) state (13 ns lifetime, no CPL activity) and the ⁵ D ₁ → ⁷ F ₂ highly excited state (130 ns lifetime, | glum | ≈ 0.2), indicating that chiral transfer from ligands to metals requires excited state energy transfer; In chiral thermally activated delayed fluorescence (TADF) dyes (R/S-BPC), the chiral dynamics of transient luminescence (23 ns, glue ≈ ± 1.8 × 10 ⁻ ³) and delayed luminescence (18 μ s, glue stability consistent) can be synchronously quantified, demonstrating that the delayed fluorescence process does not disrupt molecular chirality; In the non chiral dye Rhodamine B, the time evolution of S ₁/S ₂ linearly polarized components can also be used to trace the CPL artifact (S3) induced by linear polarization and its correlation with molecular rotational dynamics (depolarize within 10 ns).

In addition, this method can also be used to verify the predicted excited state chiral relaxation paths in theoretical calculations, evaluate the transfer efficiency of chiral signals in different energy channels and time scales such as "ligand metal" and "transient delay", and observe frontier phenomena such as short-lived excited state chiral conservation and dynamic attenuation of linear polarization artifacts. The time-resolved full Stokes CPL study not only provides a multidimensional characterization method of "time energy polarization chirality" for the excited state mechanism of chiral luminescent materials, but also lays a theoretical and technical foundation for designing chiral luminescent devices with time window selectivity, energy channel regulation ability, and low artifact high glum output, especially in the fields of circularly polarized organic light-emitting diodes (CP-OLEDs), chiral biological imaging probes, and chiral photon sources in quantum information, which have important development potential. Through this technology, researchers can accurately screen highly stable chiral luminescent materials (such as the glum time stability of TADF dyes), optimize the excited state energy transfer pathway (such as the ligand sensitization efficiency of Eu ³+complexes), and provide standardized characterization solutions to solve the problem of artifact interference in traditional CPL measurements, promoting the transformation of chiral optoelectronic materials from basic research to practical applications.

(Reference: Reponen et al., Nature. July 2025; 643(8072):675-682.)