Laser Cooling & Trapping of Atoms

1. Magneto-optical trap for cold Rb atoms:

   A SS chamber with eight-way (octagonal chamber) cross, attached with a six-way and four-way cross at suitable locations has been developed for a magneto-optical trap (MOT) which can be used for performing variety of experiments involving cold atoms. A turbo-molecular pump and a sputter-ion pump are used to pump the system down to a pressure of ~ 10^-9 torr. rubidium getter is connected in one of the ports with the help of vacuum feed-through to get the Rb atoms in the chamber for MOT. The chamber is routinely used for cooling and trapping of Rb atoms using three pairs of counter-propagating laser beams and a quadrupole magnetic field. The quadrupole magnetic field is applied by two coils connected in Anti-Helmholtz configuration to provide position dependent force to atoms toward the centre of the cross-section of the six cooling laser beams. It is possible to cool and trap ⁸⁵Rb and ⁸⁷Rb atoms using proper cooling and re-pumping laser beams in the MOT. The cloud of cooled and trapped atoms is observed by imaging the fluorescence at the beam intersection region using an IR-CCD camera.

   
   
   Figure 1: Experimental MOT setup

   Figure 1 shows our experimental setup for MOT. In the process, we have demonstrated two new techniques based on sub-Doppler polarization spectroscopy to obtain sharp dispersion-like spectra for Rb atoms and used them for firm locking of diode lasers. These dispersion-like spectra permit locking the laser frequency at the centre of an atomic transition without the need for laser frequency modulation and phase sensitive detection, as usually is the case. Moreover, the reference signal produced by these techniques is background free making the frequency stabilization less susceptible to ambient temperature variations. A tunable diode laser was frequency locked to the centre of a transition in Rb atom using the sharp dispersion-like spectrum with very good results; the estimated frequency fluctuations and drift were ~0.25 MHz and ~0.025 MHz, respectively, over an observation period of several hours.

2. Experiments with cold atoms:

   1. Study of impulsive force-induced dynamics of cold atom cloud:

      We have studied impulsive force-induced dynamics of cold ⁸⁵Rb atom cloud in a MOT. In this work, a pulsed (variable width) forcing laser beam was used to impart an impulsive force on the atom cloud during which the MOT was momentarily switched off to minimize the influence of forcing laser beam on atom cloud and was switched on again simultaneously with the switching off, of the forcing beam. The oscillatory motion of the atom cloud was observed by collecting fluorescence using a lens (L-1) onto a low-noise photo-detector (PD-1) as shown in Figure 2.

      
      
      Figure 2: Schematic of MOT setup for impulsive force-induced dynamics of cold atom cloud with time sequencing of the laser pulses used.

      The impulse imparted to the atom cloud by a variable pulse-width forcing beam was measured in terms of its initial drift velocity. The time-of-flight (TOF) method was used to estimate the drift velocity and velocity width of the atom cloud. The fluorescence from the atom cloud falling on a thin sheet of probe laser beam propagating below the trap was recorded by a combination of low noise level photo-detector (PD-2) and lens (L-2) to observe the TOF signal as a function of time (see Figure 3).

      
      
      Figure 3: Time-of-flight signals corresponding to different pulse width of forcing laser beam.

      The dynamics of the atom cloud was found to be governed by the initial drift velocity imparted to it. For smaller drift velocities, oscillations of the atom cloud were observed as soon as the trap laser beams were switched on to recapture the drifted atoms. With larger drift velocities, the atom cloud was found to traverse in the nonlinear region of trap with reduced trapping force and tendency to escape from the trap. These studies carried out over a wide range of drift velocities helped us to investigate the role of Doppler cooling particularly around the onset of the multiple scattering regime of MOT operation.

      
      
      Figure 4: Typical examples of damped oscillation signals for two different value of trap laser detuning and forcing beam pulse-width of 50 µs.

      Figure 4 shows the typical examples of experimentally observed oscillations in the fluorescence signals along with the theoretically calculated results for two different trap laser detuning of ∆ _L  =  -2 Γ and -3 Γ, where Γ = 2π x 5.9 MHz  is the natural line-width of the cooling transition. The single trapping laser beam intensity of 2.5 mW/cm² and magnetic field gradient of 12.5 G/cm were used. The spring constant (κ) and damping coefficient (α) of the trap were used to best fit the experimentally observed oscillations. The observed values of κ = (8.02 ± 0.11) x 10^-20 N/m [(3.34 ± 0.11) x 10^-20 N/m] and α = (6.19 ± 0.27) x 10^-23 Ns/m [(2.55 ± 0.27) x 10^-23 Ns/m] for ∆_L = -2 Γ [-3 Γ].

   2. Cooling and trapping of 85Rb atoms in the ground hyperfine level F=2 state:

      The ground state of ⁸⁵Rb atom consists of two hyperfine levels. Conventionally, the atoms are cooled and trapped in a magneto-optical trap (MOT) in the upper hyperfine ground state, F=3 by tuning a trapping laser near to 5 ²S_1/2 F=3 → 5 ²P_3/2 F’=4 resonant transition and re-pumping laser (for bringing atoms back into the cooling cycle) to 5 ²S_1/2 F = 2 → 5 ²P_3/2 F’=3 transition of ⁸⁵Rb atoms. We demonstrated cooling and trapping of ⁸⁵Rb atoms in a MOT in the lower hyperfine ground state, F=2. The trapping laser was tuned close to the cycling transition 5 ²S_1/2 F=2 → 5 ²P_3/2 F’=1, whereas the repumping laser frequency was tune to the 5 ²S_1/2 F=3 → 5 ²P_3/2 F’=3 transition. Figure 4 shows the relevant energy levels. Cold atoms in the lower state can be convenient for performing certain experiments. In our set-up ~ 6 x 10⁶,  atoms with a temperature of a few milli Kelvin were trapped. Figure 5 shows the fluorescence image of the laser cooled atom cloud.

      Figure 4: Relevant energy levels of 85Rb		Figure 5: The fluorescence image of 85Rb atoms

      Observation of transmission spectrum of a weak probe beam passing through the atom cloud confirmed that most of the atoms were in the lower level. Figure 6 shows the transmission spectra of a weak probe beam.

      
      Figure 6: Transmission spectra for the F=2 → F’=1, 2, 3 transitions and F=3 → F’=2, 3, 4 transitions for the 85Rb F=2 trap.

   3. Electromagnetically induced transparency in cold ⁸⁵Rb atoms:

      Electromagnetically induced transparency (EIT) in atomic systems is a quantum interference effect resulting in reduced absorption of a weak probe field in resonance with an atomic transition while propagating through a medium in presence of a strong coupling field. We have demonstrated a new experimental scheme to obtain the Λ-type EIT systems with cold ⁸⁵Rb atoms trapped in the lower hyperfine level, F-2 of the ground state.

      
      
      Figure 7: Λ-type system with cold atoms trapped in F =2 state.

      The cold atoms were obtained by directly cooling and trapping them into the lower hyperfine state as discussed above. Using mutually perpendicular linearly polarized coupling and probe beams, we studied two steady state Λ-type EIT systems when cold atoms were probed for transition into the hyperfine level F’=2 and F’=3 of the excited state 5 ²P_3/2 in the presence of coupling transitions F =3 → F’==2 and  F =3 → F’==3, respectively. In contrast with earlier performed experiments with cold atoms in this lower hyperfine ground state, our methodology enabled us to operate without periodic turning off of the trap. Figure 7 shows Λ-type system with cold atoms trapped in F=2 state.The experimental observations on the effects of uncoupled magnetic sublevel transitions and the coupling fields Rabi frequency on the EIT signals from these systems were found to be in good agreement with the theoretical calculations.

      
      
      Figure 8: The curves shown in (1a) and (2a) are the close-ups of the experimental EIT signals. The corresponding theoretically calculated curves are shown in (1b) and (2b), respectively.

   4. Effect of getter current on the performance of a Rb dark MOT:

      We have studied the effect of getter current on the loading characteristics of our Rb dark-MOT. The dark-MOT was obtained by suitably modifying the above described Rb MOT.  For this, we have used a red detuned cooling laser beam close to 5 ²S_1/2 (F=3) → 5 ²P_3/2(F’=4) transition of ⁸⁵Rb. A hollow re-pumping laser beam with 5 mW power and with frequency locked to the peak of 5 ²S_1/2 (F=2) → 5 ²P_3/2(F’=3) transition was used. The well collimated hollow re-pumping beam was generated by a two-lens optical setup with a dark circular spot placed near the focus of the lens system. The size of this hollow laser beam was changed by translating the dark spot near the focus of the lens system. The outer region of the trap consisted of cooling as well as re-pumping laser beams. However, due to the absence of the re-pumping beam in the internal region, the atoms were accumulated in the ‘‘dark” hyperfine state, F=2.

      
      
      Figure 9: Trap collection rate of the Rb dark MOT with getter current.

      The current through the Rb-getter (Ig) was varied to change the background vapour’s number density. The change in the associated value of the temperature was estimated from Doppler broadened spectra of a weak scanning probe laser beam passing through the background vapour. The optimum collection rate of the cold atoms in the dark state, F=2 was observed for the background temperature of ~ 400 K corresponding to a getter current of 4.0 A. Figure 9 shows the observed variation of the trap collection rate for dark-MOT with getter current.

List of Publications

Journal

• “Laser frequency stabilization and large detuning by Doppler-free dichroic lock technique: Application to Atom cooling”,   V. B. Tiwari, S . R.Mishra, H . S.Rawat, S .Singh, S .P .Ram, S .C .Mehendale,  Pramana - J. Physics, 65, 403, (2005).
  
  
• “Laser frequency stabilization using Doppler-free bi-polarization spectroscopy”,   V. B. Tiwari , S. Singh,  S. R. Mishra,  H. S. Rawat,  S. C. Mehendale,   Optics Communications, 263, 249 (2006).
  
  
•  “Laser frequency stabilization using a balanced bipolarimeter”,
  V. B. Tiwari , S. Singh,  S. R. Mishra,  H. S. Rawat,  S. C. Mehendale,
   Applied physics B, 83, 93 (2006).

• “ Measurements on impulsive force-induced dynamics of a cold 85Rb atom cloud in a magneto-optical trap”,  V. B. Tiwari , S. Singh,  H. S. Rawat,  M. P. Singh, 
   J. Phys. B: At. Mol. Opt. Phys., 41, 205301 (2008).

• “ Cooling and trapping of 85Rb atoms in the ground hyperfine F=2 state”,   V. B. Tiwari , S. Singh,  H. S. Rawat, S. C. Mehendale,  Phys. Rev. A, 78, 063421 (2008).
  
  
• “ High S/N ratio photo detector system for sensitive atomic physics experiments”,   S. Krishnan, S. Singh, A. Ray, V. B. Tiwari, H. S. Rawat,   J. Instrum. Soc. India, 38, 299-307 (2008).
  
  
• “Electromagnetically induced transparency in cold 85Rb atoms in the ground hyperfine F=2 state”, V. B. Tiwari , S. Singh,  H. S. Rawat, M. P. Singh, S. C. Mehendale, J. Phys. B: At. Mol. Opt. Phys., 43, 095503 (2010).
  
  
• “Efficient loading of a Rb dark-magneto optical trap by controlling current through a getter source” S. Singh, V. B. Tiwari ,  H. S. Rawat, J. Exp. Theor. Phys. (in press) (2010). 

Conferences

• “On temperature measurement of laser cooled atoms by time-of-flight method”   V. B. Tiwari, S. R. Mishra, H. S. Rawat, S. C. Mehendale,   Proc. National laser Symposium, Hyderabad, Dec. 15-27 (1999) p-179.
  
  
•  “Active frequency stabilization of diode laser using polarization spectroscopy”  V. B. Tiwari, S. C. Mehendale,   Proc. National laser Symposium, Delhi, Dec. 13-15 (2000) p-72.
  
  
• “Magneto-Optic trap for Rubidium atoms”,V. B. Tiwari, H. S. Rawat, S. R. Mishra, D. K. Arzare, S. Singh, S. C. Mehendale,  Proc. National laser Symposium, CAT, Indore, Dec. 19-21 (2001) p-411.
  
  
• “ A Comparative study of frequency stabilization techniques based on Zeeman and saturation absorption spectroscopy”,   V. B. Tiwari, S. R. Mishra, S. Singh, H .S. Rawat, S. C. Mehendale, Proc. National Laser Symposium, Thiruvananthpuram, Nov. 14-16 (2002) p-351.
  
  
•  “Characterization of a  Magneto-optic Trap for 87Rb atoms”,  V. B. Tiwari, S. R. Mishra, H. S. Rawat, S. Singh, S. P .Ram, S. C. Mehendale,  Proc. National laser Symposium, Mumbai, Jan. 10-13 (2005) p-382.
  
  
• “Power supply and associated instrumentation for tunable diode laser system for applications in high-resolution spectroscopy”,   J. Khanwalkar, R. Arya, S. Singh*, V. B. Tiwari, H. S. Rawat and T. P. S.  Nathan, Proc. National laser Symposium, Vellore, Dec. 7-10 (2005) p-433
  
  
• “Laser frequency stabilization using light-induced birefringence in atomic vapor” V. B. Tiwari, S. Singh, S. R. Mishra, H. S. Rawat, S. C. Mehendale,   International conference & HUMBOLDT KOLLEG, SCPCBGM, Nov 28-30(2005), BHU, Varanasi, p-20
  
  
• “Bi-polarization spectroscopy in atomic vapour: Application to laser frequency  stabilization”, V. B. Tiwari, S. Singh, S. R. Mishra, H. S. Rawat, S. C. Mehendale,  International conference-CDAMOP ,March 21-23 (2006), New Delhi, p-129.
  
  
• “Development of a low noise level photo-detector”   S. Singh, S. Krishnan, A. Ray, V. B Tiwari, H. S. Rawat, S. C. Mehendale,  Proc. National laser Symposium,  Indore, Dec. 5-8 (2006) p-416.
  
  
• “Cold atomic pulses from a magneto-optical trap”,  V. B. Tiwari, S. Singh, H. S. Rawat, M. P. Singh, S. C. Mehendale,  Proc. National laser Symposium, Vadodara, Dec. 17-20(2007) p-411.
  
  
• “Magneto-optical trap for  85Rb atoms in the ground hyperfine F=2 state”,   V. B. Tiwari, S. Singh, H. S. Rawat,  S. C. Mehendale,  Proc. National laser Symposium, Delhi, Jan. 7-10(2009) p-90.
  
  
• “Effect of hollow repumping beam on the relative population in ground hyperfine states of cold  85Rb atoms”,  S. Singh, V. B. Tiwari, H. S. Rawat,  S. C. Mehendale,   Proc. National laser Symposium, Delhi, Jan. 7-10(2009) p-92.
  
  
• “Performance study of a Rb dark magneto-optical trap using probe absorption spectroscopy”,   S. Singh, V. B. Tiwari, H. S. Rawat, Topical conf. on interaction of EM radiation with atoms, molecules and clusters”, RRCAT, Indore, March 3-6(2010)