Sanjukta Roy

Euler buckling-induced folding and rotation of red blood cells in an optical trap

We investigate the physics of an optically-driven micromotor of biological origin. A single, live red blood cell, when placed in an optical trap folds into a rod-like shape. If the trapping laser beam is circularly polarized, the folded RBC rotates. A model based on the concept of buckling instabilities captures the folding phenomenon; the rotation of the cell is simply understood using the Poincar`e sphere. Our model predicts that (i) at a critical intensity of the trapping beam the RBC shape undergoes large fluctuations and (ii) the torque is proportional to the intensity of the laser beam. These predictions have been tested experimentally. We suggest a possible mechanism for emergence of birefringent properties in the RBC in the folded state.We investigate the physics of an optically driven micromotor of biological origin. When a single, live red blood cell (RBC) is placed in an optical trap, the normal biconcave disc shape of the cell is observed to fold into a rod-like shape. If the trapping laser beam is circularly polarized, the folded RBC rotates. A model based on geometric considerations, using the concept of buckling instabilities, captures the folding phenomenon; the rotation of the cell is rationalized using the Poincaré sphere. Our model predicts that (i) at a critical power of the trapping laser beam the RBC shape undergoes large fluctuations, and (ii) the torque that is generated is proportional to the power of the laser beam. These predictions are verified experimentally. We suggest a possible mechanism for the emergence of birefringent properties in the RBC in the folded state.

A simple and inexpensive electronic wavelength-meter using a dual-output photodiode

In this Note we describe the design, construction, and calibration of an electronic wavelength-meter using a wavelength-sensitive dual-output photodiode. The device was constructed using electronic circuitry and then calibrated using various laser sources in the wavelength range 500–850 nm. This is an inexpensive device compared to the other similar devices available commercially. It is very useful for the determination of the wavelengths of diode lasers. It works well with multi-mode light, and there are no stringent alignment requirements. It is extremely compact and essentially a “pocket wavemeter.” The wavelength range for the operation of the wavelength-meter is 450 to 900 nm and it gives 1–3 nm accuracy, depending on the wavelength.

Evaporative Cooling of Atoms to Quantum Degeneracy in an Optical Dipole Trap

We discuss our experimental results on forced evaporative cooling of cold rubidium 87Rb atoms to quantum degeneracy in an Optical Dipole Trap. The atoms are first trapped and cooled in a magneto-optical trap (MOT) loaded from a continuous beam of cold atoms [1]. More than 1010 atoms are trapped in the MOT and then about 108 atoms are transferred to a Quasi-Electrostatic Trap (QUEST) formed by tightly focused CO2 laser (λ = 10.6μm) beams intersecting at their foci in an orthogonal configuration in the horizontal plane. Before loading the atoms into the dipole trap, the phase-space density of the atomic ensemble was increased making use of sub-doppler cooling at large detuning and the temporal dark MOT technique. In a MOT the phase-space density of the atomic ensemble is six orders of magnitude less than what is required to achieve quantum degeneracy. After transferring atoms into the dipole trap efficiently, phase-space density increases by a factor of 103. Further increase in phase-space density to quantum degeneracy is achieved by forced evaporative cooling of atoms in the dipole trap. The evaporative cooling process involves a gradual reduction of the trap depth by ramping down the trapping laser intensity over a second. The temperature of the cold atomic cloud was measured by time-of-flight (TOF) technique. The spatial distribution of the atoms is measured using absorption imaging. We report results of evaporative cooling in a single beam and in a crossed double-beam dipole traps. Due to the large initial phase space density, and large initial number of atoms trapped, the quantum phase transition occurs after about 600 ms of evaporative cooling in our optimized crossed dipole trap.

Direct evaporative cooling of 39 K atoms to Bose-Einstein condensation

We report the realization of a Bose-Einstein condensate of K-39 atoms without the aid of an additional atomic coolant. Our route to Bose-Einstein condensation comprises sub-Doppler laser cooling of large atomic clouds with more than 10(10) atoms and evaporative cooling in an optical dipole trap where the collisional cross section can be increased using magnetic Feshbach resonances. Large condensates with almost 10(6) atoms can be produced in less than 15 s. Our achievements eliminate the need for sympathetic cooling with Rb atoms, which was the usual route implemented until now due to the unfavorable collisional property of K-39. Our findings simplify the experimental setup for producing Bose-Einstein condensates of K-39 atoms with tunable interactions, which have a wide variety of promising applications, including atom interferometry to studies on the interplay of disorder and interactions in quantum gases.

Studies on cold atoms trapped in a Quasi-Electrostatic optical dipole trap

We discuss the results of measurements of the temperature and density distribution of cold Rubidium atoms trapped and cooled in an optical dipole trap formed by focussed CO2 laser beams at a wavelength of 10.6 μm from a cold, collimated and intense atomic beam of flux 2 × 1010 atoms/s produced using an elongated 2D+MOT. A large number of rubidium atoms (≥ 1010) were trapped in the MOT and the number density of atoms were further increased by making a temporal dark MOT to prevent density-limiting processes like photon rescattering by atoms at the trap centre. Subsequently, between 107 to 108 cold atoms at a temperature below 30 μK were transferred into a Quasi-Electrostatic trap (QUEST) formed by focussed CO2 laser beams at the MOT centre. Both single beam and crossed dual beam dipole traps were studied with a total output power of 50 W from the CO2 laser with focal spot sizes less than 100 microns. Various measurements were done on the cold atoms trapped in the dipole trap. The total atom number in the dipole trap and the spatial atom number density distribution in the trap was measured by absorption imaging technique. The temperature was determined from time-of-flight (TOF) data as well as from the absorption images after ballistic expansion of the atom cloud released from the dipole trap. The results from measurements are used to maximize the initial phase-space density prior to forced evaporative cooling to produce a Bose-Einstein Condensate.