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IEEE Photonics Technology Letters

Cascaded Optical Kerr Gate for Fluorescence Measurement With High Signal-to-Noise Ratio

Zhenqiang Huang, Zhen Kang, Wenjiang Tan, Jinhai Si, Xun Hou

Published Nov 23, 2023• 1 citation
Publisher

Abstract

Traditional optical Kerr gates (OKG) often suffer from leakage fluorescence background when sampling and measuring fluorescence spectra due to the limited extinction of the polarizers used in optical Kerr gates. In this letter, we propose a cascaded optical Kerr gate (COKG) for measuring fluorescence spectroscopy with a high signal-to-noise ratio by suppressing this leakage fluorescence background. Our proposed COKG consists of an optical Kerr gate with a fast-switching time at the front stage and an optical Kerr gate with a high transmission at the back stage. The experimental results show that the signal-to-noise ratio for the COKG can be improved by about two orders of magnitude compared to the single OKG.

DOI
10.1109/lpt.2023.3335236

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Traditional optical Kerr gates (OKG) often suffer from leakage fluorescence background when sampling and measuring fluorescence spectra due to the limited extinction of the polarizers used in optical Kerr gates. In this letter, we propose a cascaded optical Kerr gate (COKG) for measuring fluorescence spectroscopy with a high signal-to-noise ratio by suppressing this leakage fluorescence background. Our proposed COKG consists of an optical Kerr gate with a fast-switching time at the front stage and an optical Kerr gate with a high transmission at the back stage. The experimental results show that the signal-to-noise ratio for the COKG can be improved by about two orders of magnitude compared to the single OKG.
An optical Kerr gate (OKG) is an ultrafast optical switch based on the optical Kerr effect. The performance of a conventional OKG depends mainly on Kerr materials. Traditional Kerr materials do not demonstrate both large optical nonlinearity and an ultrafast response time. Therefore, the performance of a conventional OKG is limited by an inherent trade-off between high signal transmittance and fast switching time, which limits its application in many fields. We propose an improved femtosecond OKG with double gate pulses, based on the use of a birefringent crystal to realize an ultrashort switching time, even with a slow-response optical Kerr medium. We assessed the dependence of the double gate pulsed OKG (D-OKG)’s performance on the intensity ratio of the double gate pulses. A transmittance of 50% and a switching time of 142 fs were achieved. The D-OKG is convenient to construct, and its integrated performance is superior to that of a conventional OKG.
Signal-to-noise ratio of fluorescence detection from a single molecule is analysed by using time-gated techniques. It is found that the optimal signal-to-noise ratio can be obtained by choosing an appropriate gate time with a certain optical background. The dependences of molecular fluorescence lifetime and the optimal signal-to-noise ratio on the appropriate gate time are respectively discussed with two kinds of background sources, chaotic state with uniform distribution and coherent state with exponential distribution in time domain. For chaotic state background we find that a certain range for appropriate gate time can be obtained with a definite fluorescence lifetime, larger fluorescence lifetime would lower the value of optimal signal-to-noise ratios. For coherent state background we find that there is also a narrow range of appropriate gate time when lifetime of single molecule is less than that of background photons.
Abstract Based on the light-induced polarization ellipse rotation effect, we propose an optical Kerr gate (OKG) in which elliptically polarized probe light is used by introducing a pair of crossed quarter-wave plates into the conventional OKG arrangement, placed just before and after the Kerr medium. Transient birefringence induced by the pump beam in the Kerr medium leads to a rotation of the polarization ellipse of the probe beam, hence part of the probe beam would pass through the OKG. Theoretical calculations and experimental results both indicate that the OKG signal intensity would reach the optimum value independent of the pump polarization direction when the probe beam is circularly polarized. This modified OKG measurement is suitable to detect the third-order nonlinearities induced in anisotropic materials.
Classical time-resolved optical spectroscopy experiments are performed using sequences of ultrashort light pulses, with photon fluxes incident on the sample which are many orders of magnitude higher than real-world conditions corresponding to sunlight illumination. Spectroscopy and microscopy schemes that use quantum states of light have been widely described theoretically with fewer experimental demonstrations that typically require very long measurements that can extend for hours or more. Here, we show that time-resolved spectroscopy with quantum light can be performed without compromising measurement time or wavelength tunability, recording a fluorescence lifetime trace in biological samples in less than a second with acceptable signal-to-noise ratio. Starting from spontaneous parametric down-conversion driven by a continuous-wave laser, we exploit the temporal correlation between randomly generated signal/idler pairs to obtain temporal resolution, and their spectral correlation to select the excitation frequency. We also add spectral resolution in detection, using a 'photon-efficient' Fourier transform approach which employs a common-path interferometer. We demonstrate the potential of this approach by resolving, at the single-photon level, excitation energy transfer cascades from LH2 to LH1 in the photosynthetic membrane and disentangling the lifetimes of two dyes in a mixture. Our results provide a new approach to ultrafast optical spectroscopy, where experiments are performed under illumination intensity conditions comparable to real-world sunlight illumination.
A technique for reducing background and improving the signal-to-noise ratio in optical spectroscopy of weak atomic transitions is described. In this technique an incident laser is square-wave amplitude modulated and the resonance fluorescence from the atom is then detected out of phase with the excitation when the laser is off. Due to the finite lifetime of the atom’s excited state, appreciable signal is observed when the laser is off and instrumental scattering background is virtually eliminated. Applying this technique to an experiment on single laser-cooled In+ ions, we have observed a factor of 500 reduction in background and a factor of 5 increase in the signal-to-noise ratio.
We proposed an improved femtosecond optical Kerr gate with double gate pulses (DOKG) using a femtosecond laser, in which an ultrashort switching time can be realized even when a relatively slow response optical Kerr medium of carbon disulfide was used. The results showed that when the time delay and the light intensity ratio of the two pump pulses were adjusted to be about 140 fs and 1:0.9, the switching time and the maximum transmission efficiency of the DOKG could be optimized to be about 130 fs and 37%. The DOKG is convenient to be built and offers a good choice for applications based on ultrafast optical switch.