Praveen P, Iyengar Shruthi S, Bhattacharya Sarbari, Ananthamurthy Sharath
Appl Opt. 2016 Jan 20;55(3):585-94. doi: 10.1364/AO.55.000585.
We have studied in detail the situation wherein two microbeads are trapped axially in a single-beam Gaussian intensity profile optical trap. We find that the corner frequency extracted from a power spectral density analysis of intensity fluctuations recorded on a quadrant photodetector (QPD) is dependent on the detection scheme. Using forward- and backscattering detection schemes with single and two laser wavelengths along with computer simulations, we conclude that fluctuations detected in backscattering bear true position information of the bead encountered first in the beam propagation direction. Forward scattering, on the other hand, carries position information of both beads with substantial contribution from the bead encountered first along the beam propagation direction. Mie scattering analysis further reveals that the interference term from the scattering of the two beads contributes significantly to the signal, precluding the ability to resolve the positions of the individual beads in forward scattering. In QPD-based detection schemes, detection through backscattering, thereby, is imperative to track the true displacements of axially trapped microbeads for possible studies on light-mediated interbead interactions.
我们已经详细研究了两个微珠轴向被困在单光束高斯强度分布光阱中的情况。我们发现，从象限光电探测器（QPD）记录的强度波动的功率谱密度分析中提取的转折频率取决于检测方案。通过使用单波长和双波长的前向和背向散射检测方案以及计算机模拟，我们得出结论，在背向散射中检测到的波动承载着在光束传播方向上首先遇到的珠子的真实位置信息。另一方面，前向散射携带两个珠子的位置信息，其中来自沿光束传播方向首先遇到的珠子的贡献很大。米氏散射分析进一步表明，两个珠子散射的干涉项对信号有显著贡献，这使得在前向散射中无法分辨各个珠子的位置。因此，在基于QPD的检测方案中，通过背向散射进行检测对于跟踪轴向捕获的微珠的真实位移至关重要，以便可能对光介导的珠子间相互作用进行研究。
