Dynamics of micrometer-sized dielectric objects can be controlled by optical tweezers with scanning light, however, the trapped objects fail to track the scan when drag exceeds the trapping by too quick movement. On the other hand, optical vortices (OVs), which have a property of carrying angular momenta, can directly control torque on objects rather than their position. Laguerre-Gaussian (LG) beams are the most familiar examples of OV and have been studied extensively so far. Revolution of the objects trapped by the LG beams provides typical models of nonequilibrium statistical system, but stable mid-water trapping by the LG beams becomes essential to evaluate physical properties of the system without extrinsic hydrodynamic effects,. Nevertheless, off-axis revolutions of small objects trapped in mid-water by the LG beams have not yet been established with secure evidences. Here we report stable off-axis trapping of dielectric spheres in mid-water using high-quality LG beams generated by a holographic complex-amplitude modulation method. Direct evidence of the three-dimensional off-axis LG trapping was established via estimating the trapping position by measuring the change of revolution radii upon pressing the spheres onto glass walls. Resultantly, the axial trapping position was determined as about half the wavelength behind the beam waist position. This result provides a direct scientific evidence for possibility of off-axis three-dimensional trapping with a single LG beam, moreover, suggests applications as powerful tools for studying energy-conversion mechanisms and nonequilibrium nature in biological molecules under torque.
In this paper, we report fabrication and evaluation of an organic light emitting diode (OLED) with organic multiple
quantum wells (OMQWs). We fabricated the OMQWs using organic molecular beam deposition, consisting of TPD
(N,N'-bis(3-methilphenyl)-N.N-diphenyl benzidine) and DCM (4-(Dicyanomethilane)-2-methil-6-(p-dimethilamino-styryl)-
4H-pyran). We measured photoluminescence (PL) and electroluminescence (EL) properties to characterize the
OMQWs. In both measurements the peaking wavelengths clearly become shorter as the thickness of the QWs becomes
narrower. This result obviously indicates that subband states are achieved in the quantum wells. In addition, we did
numerical analysis to assign the effective mass of electrons in the OMQWs. Consequently, the effective mass of
electrons can be estimated as 0.05 m0.
We report holographic generation of higher-order Laguerre-Gaussian (LG) beams using a liquid crystal on silicon
spatial light modulator (LCOS-SLM) device. In our experimental set-up, a flat-top light beam was projected
on the LCOS-SLM to generate LG beams of various mode indices without changes of the optical system. Additionally,
the size of the holographic phase pattern was optimized for each beam to maximize the mode purity
of the obtained beam. Holographic generation of LG beams is easily influenced by a distortion of the optical
system and deviation of the phase setting from an ideal one. Nevertheless, we obtained high-quality LG beams
with an additional phase pattern on the LCOS-SLM for canceling the distortion of the optical system and with
calibration of the phase control voltage for precise expression of the phase patterns. Numerical analyses are
also performed for two-dimensional beam profiles to verify the quality of the obtained beams. Through fitting
the obtained profiles to theoretical ones, we calculate the correlation coefficients R between the observed and
fitted profiles to find that R > 0.95 for all beams and that the correlation coefficients behave similarly to the
theoretically estimated mode purities, facts indicating that the quality of the obtained LG beams is close to the
theoretical limit in our experiments.