The effect of alcohol solution on single human red blood Cells (RBCs) was investigated using nearinfrared laser tweezers Raman spectroscopy (LTRS). In our system, a low-power diode laser at 785 nm was applied for the trapping of a living cell and the excitation of its Raman spectrum. Such a design could simultaneously reduce the photo-damage to the cell and suppress the interference from the fluorescence on the Raman signal. The denaturation process of single RBCs in 20% alcohol solution was investigated by detecting the time evolution of the Raman spectra at the single-cell level. The vitality of RBCs was characterized by the Raman band at 752 cm −1 , which corresponds to the porphyrin breathing mode. We found that the intensity of this band decreased by 34.1% over a period of 25 min after the administration of alcohol. In a further study of the dependence of denaturation on alcohol concentration, we discovered that the decrease in the intensity of the 752 cm −1 band became more rapid and more prominent as the alcohol concentration increased. The present LTRS technique may have several potential applications in cell biology and medicine, including probing dynamic cellular processes at the single cell level and diagnosing cell disorders in real time.

We present the theoretical analysis and the numerical modeling of optical levitation and trapping of the stuck particles with a pulsed optical tweezers. In our model, a pulsed laser was used to generate a large gradient force within a short duration that overcame the adhesive interaction between the stuck particles and the surface; and then a low power continuous-wave(cw) laser was used to capture the levitated particle. We describe the gradient force generated by the pulsed optical tweezers and model the binding interaction between the stuck beads and glass surface by the dominative van der Waals force with a randomly distributed binding strength. We numerically calculate the single pulse levitation efficiency for polystyrene beads as the function of the pulse energy, the axial displacement from the surface to the pulsed laser focus and the pulse duration. The result of our numerical modeling is qualitatively consistent with the experimental result.

We propose and demonstrate a novel detection scheme of clock signals and obtain an ultrahigh resonance contrast up to 90%, which leads to the remarkable improvement of the precision of the signal-to-noise ratio. The frequency stability in terms of Allan deviation of the proposed detection scheme is improved by an order of magnitude under equivalent conditions. © 2012 Optical Society of America OCIS codes: 020.3690, 210.3810, 290.2558. Atomic clocks play a key role in current society life, especially in timekeeping, synchronization, and communication [1]. Active efforts have been made to improve performance of vapor-cell atomic clocks, particularly regarding contrast and signal-to-noise ratio (SNR). These two parameters play an important role in the achievement of good frequency stability. In the present day, people use optical pumping and the time domain Ramsey double-pulse approach [2,3] to interrogate an atomic ensemble contained in a closed cell. In that case the Allan deviation for characterizing frequency stability is [3,4] στwhere Q is the atomic line quality factor, SNR is the signal-to-noise ratio, and τ is the averaging time. Here T c is the cycle time of the atomic clock. In state-of-the-art devices, frequency stability is limited by shot noise, which results, for example, from residual light reaching the detector in optically pumped devices [5]. Over the past years, several techniques such as coherent population trapping [6] and N resonance [7] have been proposed to eliminate systematic effects that deteriorate clock performance and improve their short-term frequency stability. To date, contrasts up to 30% using absorption detection have been reported [5,8]. The frequency stability of the order of 10 −15 has been reported in a pulsed optically pumped (POP) vapor-cell clock [8]. However, the further improvement is limited by the difficulty of increasing signal contrast and atomic line quality factor.To address these problems, we develop a novel detection technique to make better performance for a vaporcell atomic clock, which is characterized by using dispersive detection, with the advantages of higher sensitivity and SNR over the absorptive counterpart [9,10]. In our modified scheme, probe light background is blocked by an analyzer in the measurement, and the shot noise is strongly suppressed. The forward-scattering signals, which result from magneto-optical rotation, has carried the Ramsey fringe information. We obtain the Ramsey fringe with an ultrahigh contrast up to 90%, under which the Allan deviation is improved by about an order of magnitude over the absorptive detection. On the other hand, the Allan deviation is defined by the atomic line quality factor. The magneto-optical rotation angle can exceed π∕2 when the microwave excitation increases and the forward-scattering signal diminishes quickly near the microwave resonance line. In this condition, a pseudoresonance with a narrower linewidth appears in the central zone of Ramsey fringe. The narrower linewidth leads to a higher atomic line q...