Mini review of ultrafast fluorescence polarization spectroscopy [invited]

Femtosecond fluorescence conical optical parametric amplification spectroscopy

Parametric superfluorescence (PSF), which originated from the optical amplification of vacuum quantum noise, is the primary noise source of femtosecond fluorescence non-collinear optical parametric amplification spectroscopy (FNOPAS). It severely affects the detection limit of FNOPAS to collect the femtosecond time-resolved spectra of extremely weak fluorescence. Here, we report the development of femtosecond fluorescence conical optical parametric amplification spectroscopy (FCOPAS), aimed at effectively suppressing the noise fluctuation from the PSF background. In contrast to traditional FNOPAS configurations utilizing lateral fluorescence collection and dot-like parametric amplification, FCOPAS employs an innovative conical fluorescence collection and ring-like amplification setup. This design enables effective cancellation of noise fluctuation across the entire PSF ring, resulting in an approximate order of magnitude reduction in PSF noise compared to prior FNOPAS outcomes. This advancement enables the resolution of transient fluorescence spectra of 4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM) dye molecules in ethanol, even at an optically dilute concentration of 10-6 mol/l, with significantly enhanced signal-to-noise ratios. This improvement will be significant for extremely weak fluorescence detection on the femtosecond time scale.

Relaxation dynamics of alkyl derivatives of fluorescein MitoFluo and C8-Fl in solutions with liposomes

We present results of experimental and theoretical studies of excited state dynamics in two alkyl derivatives of fluorescein, MitoFluo and C₈-Fl in solutions with liposomes. The liposomes DOPC and soybeanPC + 20% Cardiolipin (Azo-Cl), modelling cellular and inner mitochondrial membranes, respectively, were used in experiments. Both types of liposomes were shown to reduce significantly the fluorescence quantum yield as compared to that of pure fluorescein derivatives in solutions, while DOPC liposomes also caused a noticeable (ca 10 nm) red shift of fluorescence maximum. The study of fluorescence polarization decay has been carried out where important fluorescence parameters: polarization anisotropy, fluorescence lifetimes, and rotational diffusion times have been determined. It was shown that the isotropic fluorescence decay of C₈-Fl in liposome containing solutions was single-exponential and the anisotropic decay was double-exponential for both types of lyposomes. In the case of MitoFluo both isotropic and anisotropic fluorescence decays were fitted satisfactory only with double-exponential functions. The interpretation of the experimental data obtained was supported by ab initio calculations of the structure and excitation properties of MitoFluo and C₈-Fl in aqueous solution. The analysis of anisotropic fluorescence decay allowed for isolation of the contributions of fluorescein derivatives free in solution from those embedded in liposomes. Also, the experimental data suggest that MitoFluo interacts with liposomes more effectively than C₈-Fl. Basing on the experimental and theoretical results obtained we conclude that free C₈-Fl and MitoFluo molecules in solution were mostly in their dimer forms.

Time-division-multiplexed observation bandwidth for ultrafast parametric spectro-temporal analyzer

Parametric spectro-temporal analyzer (PASTA) has been demonstrated as a powerful tool for ultrafast spectrum measurement with superior frame rate and resolution. Compared with other time-stretch-based counterparts, the temporal focusing mechanism enlarges the initial condition and enables the observation of arbitrary waveform, especially the emission spectrum. However, due to the limited conversion bandwidth of the parametric mixing-based time-lens, the observation bandwidth of PASTA is constrained within the C (conventional) band, which hinders its practical applications. To overcome this constraint, both stokes and anti-stokes conversions of the parametric mixing process are leveraged, and the concept of time division multiplexing (TDM) is introduced to ensure their separability. Therefore, the TDM-based PASTA system successfully demultiplexes the C band and L (long) band spectra in two adjacent temporal frames. It is capable of reconstructing the wavelength-to-time sequence for arbitrary waveform over a record 58-nm observation bandwidth, which can be further improved by optimizing the filters and amplifiers. Meanwhile, both of these two bands achieve 20-pm resolution, 10-MHz frame rate, and -30-dBm sensitivity. Moreover, this TDM concept can also be applied to other parametric mixing-based temporal imaging systems to enlarge the working wavelength band, such as temporal magnification.

Emulated laser-acupuncture system

A novel laser-acupuncture system was developed that can be used to implement the manipulation methods of traditional acupuncture, such as lifting and thrusting. A 780 nm laser diode with a maximum power of 90 mW was used as the light source. The focus point of the laser beam was adjustable by changing the position of the lens, facilitating the implementation of the lifting and thrusting methods of traditional Chinese medicine and achieving various stimulation depths at the acupuncture point. The images for the light spots from the outlet of the emulated laser acupuncture were captured at various distances and their sizes were calculated. The result showed that the diameter of the focused light spot (i.e., at the focus point) was 0.11 mm, which is close to the diameter of commonly used needles (with diameters of approximately 0.22 mm). The area of the light spot 1 cm from the focus point was approximately 50 times larger, indicating that the unit power might be 1/50 of the power of the focus point. To study the effect of emulated laser acupuncture on human meridians, after stimulating the Shenmen point (HT7) of five subjects and obtaining their Ryodoraku values of the heart meridian and the small-intestine meridian, a paired t test showed that the laser stimulation incorporating lifting and thrusting was significantly higher than the laser stimulation without lifting and thrusting (p<0.05). The result is consistent with traditional acupuncture in that acupuncture incorporating lift and thrust is more effective than that without lift and thrust.