arXiv subjects

Harish Ravi

Publications and source records attributed to Harish Ravi.

5 recordsLinked to original sources

arXiv2017

Polarization dependent tuning of the Hanle effect in the ground state of Cs

We demonstrate that the Hanle effect can be tuned between magnetically induced absorption (MIA) and magnetically induced transmission (MIT) simply by changing the polarization of the input laser beam. The experiments are done using closed hyperfine transitions of the $ \rm D_2 $ line of ${\rm ^{133}Cs}$ ---$ F_g = 3 \rightarrow F_e = 2 $ and $ F_g =4 \rightarrow F_e = 5 $. The former shows a transformation from MIT to MIA, while the latter shows the opposite behavior. A qualitative explanation based on optical pumping and coherences among the magnetic sublevels of the ground state is borne out by a detailed density-matrix calculation. To increase the coherence time, the experiments are done in a Cs vapor cell with paraffin coating on the walls. The observed linewidth is extremely narrow ($\sim 0.1$ mG) compared to previous work in this area, making this a promising technique for all kinds of precision measurements.

physics.atom-ph↗

arXiv2017

Finding the number density of atomic vapor by studying its absorption profile

We demonstrate a technique for obtaining the density of atomic vapor, by doing a fit of the resonant absorption spectrum to a density-matrix model. In order to demonstrate the usefulness of the technique, we apply it to absorption in the ${\rm D_2}$ line of a Cs vapor cell at room temperature. The lineshape of the spectrum is asymmetric due to the role of open transitions. This asymmetry is explained in the model using transit-time relaxation as the atoms traverse the laser beam. We also obtain the latent heat of evaporation by studying the number density as a function of temperature close to room temperature.

physics.atom-ph↗

arXiv2016

Permanent EDM measurement in Cs using nonlinear magneto-optic rotation

We use the technique of chopped nonlinear magneto-optic rotation (NMOR) in a room temperature $^{133}$Cs vapor cell to measure the permanent electric dipole moment (EDM) in the atom. The cell has paraffin coating on the walls to increase the relaxation time. The signature of the EDM is a shift in the Larmor precession frequency which is correlated with the application of an E field. We analyze errors in the technique, and show that the main source of systematic error is the appearance of a longitudinal B field when the E field is applied. This error can be eliminated by doing measurements on the two ground hyperfine levels. Using an E field of 2.6 kV/cm, we place an upper limit on the electron EDM of $ 2.9 \times 10^{-22} $ e-cm ($95 \%$ confidence). This limit can be increased by 7 orders-of-magnitude---and brought below the current best experimental value---with easily implementable improvements to the technique.

physics.atom-ph↗

arXiv2015

Measuring the linewidth of a stabilized diode laser

We demonstrate a straight-forward technique to measure the linewidth of a grating-stabilized diode laser system---known as an external cavity diode laser (ECDL)---by beating the output of two independent ECDLs in a Michelson interferometer, and then taking the Fourier transform of the beat signal. The measured linewidth is the sum of the linewidths of the two laser systems. Assuming that the two are equal, we find that the linewidth of each ECDL measured over a time period of 2 \textmu s is about 0.3 MHz. This narrow linewidth shows the advantage of using such systems for high-resolution spectroscopy and other experiments in atomic physics.

physics.atom-ph↗

arXiv2013

Measurement of the electronic thermal conductance channels and heat capacity of graphene at low temperature

The ability to transport energy is a fundamental property of the two-dimensional Dirac fermions in graphene. Electronic thermal transport in this system is relatively unexplored and is expected to show unique fundamental properties and to play an important role in future applications of graphene, including opto-electronics, plasmonics, and ultra-sensitive bolometry. Here we present measurements of bipolar, electron-diffusion and electron-phonon thermal conductances, and infer the electronic specific heat, with a minimum value of 10 $k_{\rm{B}}$ ($10^{-22}$ JK$^{-1}$) per square micron. We test the validity of the Wiedemann-Franz law and find the Lorenz number equals $1.32\times(π^2/3)(k_{\rm{B}}/e)^2$. The electron-phonon thermal conductance has a temperature power law $T^2$ at high doping levels, and the coupling parameter is consistent with recent theory, indicating its enhancement by impurity scattering. We demonstrate control of the thermal conductance by electrical gating and by suppressing the diffusion channel using superconducting electrodes, which sets the stage for future graphene-based single microwave photon detection.

cond-mat.mes-hall↗