Trap split with Laguerre-Gaussian beams

Abstract

The optical trapping techniques have been extensively used in physics, biophysics, micro-chemistry, and micromechanics to allow trapping and manipulation of materials ranging from particles, cells, biological substances, and polymers to DNA and RNA molecules. In this Letter, we present a convenient and effective way to generate a novel phenomenon of trapping, named trap split, in a conventional four-level double- atomic system driven by four femtosecond Laguerre-Gaussian laser pulses. We find that trap split can be always achieved when atoms are trapped by such laser pulses, as compared to Gaussian ones. This work would greatly facilitate the trapping and manipulating the particles and generation of trap split. It may also suggest the possibility of extension into new research fields, such as micro-machining and biophysics. Since Ashkin's seminal work on trapping a particle through the use of radiation forces exerted by a Gaussian laser beam [1], the technique of optical trapping has been developed rapidly, becoming an integral tool in variety of different fields to manipulate neutral atoms, nanoparticles, cells, DNA and RNA molecules as well as microsized dielectric particles [2][3][4][5][6][7][8][9][10]. However, as these techniques are based on the optical gradient force which is dependent on the size of the particle [11], smaller particles are much more difficult to trap [12,13], Enhancement of the gradient force by using the femtosecond (fs) pulses could provide an efficient method to address the issue. During recent years, due to the tremendous technological advancement in the generation of fs lasers, there has been a resurgence of interest in this field [14,15] and the optical trapping technologies have been further developed [16][17][18][19][20]. Tamai et al. reported the feasibility of achieving stable laser trapping of water-soluble CdTe quantum dots by using a high repetition-rate picosecond Nd:YLF laser with two orders of magnitude lower than that used in continuous wave laser trapping [17]. Not only does fs laser lead to efficient trapping [16,17], but also it results in discoveries of novel physical phenomena including the deposition of nanostructured CdS films [18] and controlling the direction of the scattered nanoparticles [20].Recently, Okamoto's group observed the stable trapping of 60-nm gold nanoparticles when they are trapped by the fs laser pulses. They have discovered a new trapping phenomenon due to the nonlinear optical effect in which the stable trap site splits into two positions, as the incident peak-laser power approaches a threshold level, named trap split [9]. More recently, Chakraborty and Sarma have shown how to control optical trap potential (OTP) in an N-type four level atomic system by tuning the beam waist of the chirped fs Gaussian pulses and the detuning frequency [21]. They have shown that the potential splits with increasing the Rabi frequency, a behaviour analogous to that in trapping of nonparticles with fs pulses observed in Okamoto's study. In a recent paper by two ...

Atom localization using Laguerre–Gaussian beams

“…|1 is coupled to the excited levels |3 and |4 by control and probe fields E 31 and E 41 with carrier frequencies ω 31 and ω 41 , respectively. Two coupling fields E 32 (x, y) and E 42 (x, y) with carrier frequencies ω 32 and ω 42 couple the excited states to another lower state |2 . The spontaneous decay rates on the dipole-allowed transitions are denoted by γ 13 , γ 23 , γ 14 , and γ 24 .…”

Laguerre-Gaussian modes generated vector beam via nonlinear magneto-optical rotation

“…Moreover, the phase singularity of the LG modes on the beam axis dictates zero intensity at the beam center 46 , leading the intensity pattern of the LG modes to take the form of a doughnut or even concentric rings. It has been demonstrated that the characteristics of the optical phenomena can be influenced by the LG beams 47 – 49 . Mahmoudi et al showed that the use of LG beams narrowed the linewidth of the optical spectrum of the multi-photon resonance phenomena 50 .…”

Spatially dependent atom-photon entanglement

“…It has been found that the linewidth of the optical spectrum due to the multi-photon transition becomes narrower in the presence of a LG beam 15 . They have also studied trap split in an atomic system interacting with femtosecond LG laser pulses 16 . More recently, we have demonstrated that the LG beam can decrease the full width at the half maximum of the output probe intensity in the electromagnetically induced focusing 17 .…”