My main research focus at UCR has been on (1) exploring the nature of the quantum vacuum by pioneering fundamental force measurements such as the normal and lateral Casimir force using an Atomic Force Microscope and (2) studying single molecule and nanostructures in condensed matter by scanning microscopy techniques such as the Atomic Force Microscope, Scanning Tunneling Microscope and the Near Field Scanning Optical Microscopes. I have also performed collaborative research activities in the areas of Plasma Physics and Biophysics. Below I provide a brief overview of our research activities.



1. Exploring the nature of the quantum vacuum - Measuring the Casimir Force: The physical nature of empty space as predicted by modern quantum field theories, the quantum vacuum, remains mysterious, replete with paradoxes. Even though by definition this space is empty of matter, it possesses infinite energy density as it is populated by quantum fluctuations (virtual particles). Inexplicably, such a large energy does not produce any observable gravitational effects (recent astrophysical research claims a link to the cosmological constant). The main distinction between the energy of quantum vacuum and other types of energy is that it is unobservable in the non-perturbed state. Also this energy cannot be registered and harnessed by physical devices to produce work. In all physical processes we measure only the difference of the energy under consideration and the energy of vacuum. So to observe vacuum we should disturb it and measure the produced effect. A unique way to do this is provided by the Casimir effect. H.B. G. Casimir predicted that two neutral parallel metal plates will change the properties of the quantum electromagnetic vacuum, and this would result in an attractive force between them. Another of the most startling aspects of the Casimir force is its shape dependence. It can be repulsive or attractive depending on the geometry of the boundary. The Casimir effect finds application in many branches of physics such as Quantum Field Theory, Gravitation and Cosmology, Condensed Matter Physics, Atomic Physics, and Mathematical Physics. Furthermore, the Casimir force can be used to set stringent limits on hypothetical forces and the existence of extra dimensions such as those predicted by modern unification theories. With proposed improvements our Casimir force measurements will rival those of accelerator based experiments for setting these limits at some distances. Due to the broad implications of the work, our experiments have been reported in the general media such as in The New York Times, The Times of London, Scientific American, Physikalische Blatter, Physical Review Focus etc.



1.1 Experimental and Theoretical Achievements with the Vertical Casimir Force: We have pioneered the use of the Atomic Force Microscope (AFM) for measurements of the Casimir force. The AFM with the potential to measure forces of 10-18N is ideally suited for precision force measurements. This precision was previously only used to do atomic imaging. We have adapted the AFM to make the first precision measurement of the Casimir force. The Casimir force between metal plates is <10-7N for realizable experimental configurations. Previous to our work, there were only few attempts at its measurement. Using the AFM we have made the most precise measurement of the Casimir force to date. Along with Prof. V.M. Mostepanenko and G.L. Klimchitskaya of St. Petersburg, Russia, we have successfully understood the corrections due to the finite conductivity of metals and surface roughness. The recent developments in the field of Casimir effects is summarized in a review article in Physics Reports co-authored by Prof. V.M. Mostpenanenko, Prof. M. Bordag and I.


1.2. Experimental Achievements with Shape Dependent Casimir Forces including the Lateral Casimir Force: The Casimir force is theoretically known to exhibit strong shape dependent properties. We have done the only experimental demonstration to date of the non-trivial boundary dependence of the Casimir force. This work has also been published in Physical Review Letters in '99. Here we showed that one can dramatically alter the normal Casimir force that attracts two parallel surfaces by introducing a very small periodic texture to the surfaces. We have now extended this work by experimentally demonstrating that one can even get lateral (horizontal) Casimir forces which act parallel to the two surfaces used. We demonstrated it between two surfaces imprinted with nanoscale sinusoidal corrugations. The lateral Casimir force measured was about a few hundred femto Newtons (10-13 N). With our demonstration of the lateral Casimir force, quantum vacuum activated lateral motion might be possible in silicon chip micro-machines.

In the near term we are continuing to investigate more shape dependent Casimir forces. Next we hope to improve the instrumental precision by 103. This should allow us to investigate (i) the temperature dependence (black body radiation effects) of the Casimir force and the effect of material properties on the Casimir force.


2. Condensed Matter Experiments: It is now well recognized that physical phenomena exhibited by single molecules or an assembly of small number of particles might be very different from those exhibited by their macroscopic counterparts. This area of physics which goes by the name of nanostructure physics or complexity physics, while ripe for experimentation, requires the development of new devices for their study. We have developed an optical microscope that provides resolution beyond the diffraction limit imposed by standard optics. To this end, we are now successfully operating a Near-field Scanning Optical Microscope (NSOM) with a spatial resolution of 80 nm (below the diffraction limit ). It is a proximal probe (the probe is maintained around 20 angstroms from the surface) adapted for precision optical spectroscopy and microscopy. We have used the NSOM to study domain walls in ferroelectric materials. This is the first nanoscale investigation of single domain walls ever undertaken. The pioneering nature of this work has been recognized by publications in such journals as Physical Review Letters, Physics Letters etc.


3. Bio-Physics: We now have a multidisciplinary collaborative research team involving Prof. V. Parpura, Department of Neuroscience, UCR; Prof. W. Kuhr, Department of Chemistry, UCR; and Prof. M. Ozkan, Department of Environmental Engineering, UCR. Here we are using our expertise in sensitive force measurement techniques to study the interaction force between single molecules involved in signal transmission in the human brain. Such single molecule research is critical to isolating the key mechanisms in neuro signal transmission.


Some Recent Publications


  1. Chen F., Mohideen, U., Klimchitskaya, G.L., Mostepanenko, V.M., "Demonstration of the lateral Casimir force," Physical Review Letters, vol.88, (no.10), APS, (2002). p.101801/1-4.

  2. Bordag M., Mohideen U., Mostepanenko V.M., "New Developments in the Casimir Effect," Physics Reports, vol.353, (no.1-3), Elsevier, (2001). p.1-205.

  3. Chen, F.; Mohideen, U. "Fiber optic interferometry for precision measurement of the voltage and frequency dependence of the displacement of piezoelectric tubes". Review of Scientific Instruments, vol.72, (no.7), AIP, (2001). p.3100-2

  4. Smith, M.A.; Goodrich, J.; Rahman, H.U.; Mohideen, U. "Measurement of grain charge in dusty plasma Coulomb crystals. IEEE Transactions on Plasma Science, vol.29, (no.2, pt.1), IEEE, (2001). p.216-20.

  5. Rahman, H.U.; Mohideen, U.; Smith, M.A.; Rosenberg, M.; Mendis, D.A. Grain dynamics and inter-grain coupling in dusty plasma Coulomb crystals. Physica Scripta Volume T, vol.T89, (2001). p.186-90.

  6. Harris, B.W.; Chen, F.; Mohideen, U. "Precision measurement of the Casimir force using gold surfaces", Physical Review A (Atomic, Molecular, and Optical Physics), vol.62, (2000). p.052109/1-5.

  7. Klimchitskaya, G.L.; Mohideen, U.; Mostepanenko, V.M. "Casimir and van der Waals forces between two plates or a sphere (lens) above a plate made of real metals. Physical Review A, (2000). p.062107/1-12.

  8. Yang, T.J.; Gopalan, V.; Swart, P.; Mohideen, U. "Experimental study of internal fields and movement of single ferroelectric domain walls. Journal of the Physics and Chemistry of Solids, vol.61, (2000). p.275-82.

  9. Tumer,T.; Bhattacharya,D.; Mohideen,U.; Rieben, R.; Souchkov, V.; Tom, H.; Zweerink, J.; "Solar Two Gamma-Ray Observatory," Astroparticle Physics, Vol. 11, pp. 271-3 (1999).

  10. Yang, T.J.; Gopalan,V.; Swart, P.; and Mohideen, U.; "Direct observation of pinning and bowing of a single ferroelectric domain wall," Physical Review Letters, Vol. 82, pp. 4106-8 (1999).

  11. Yang, T.J.; Gopalan, V.; Swart, P.; Mohideen, U. "Observation and Mobility Study of a Single 180 Domain Wall Using a Near-Field Scanning Optical Microscope," Ferroelectrics, vol.222, p.351-358 (1999).

  12. Roy, A.; and Mohideen, U.; "Demonstration of the non-trivial boundary dependence of the Casimir force," Physical Review Letters, Vol. 82, pp. 4380-83 (1999).

  13. G. L. Klimchitskaya, A. Roy, U. Mohideen, and V.M. Mostepanenko, "Complete roughness and finite conductivity corrections for the recent Casimir force measurement," Physical Review A, Vol. 60, pp. 3487-95 (1999).

  14. Mohideen U. and Roy A., Reply to "Comment on Precision measurement of the Casimir force from 0.1 to 0.9mm," Physical Review Letters, Vol. 83, pp. 3341 (1999).

  15. Roy, A., C.Y. Lin and U. Mohideen, "Improved precision measurement of the Casimir force," Physical Review D, Rapid Communication, Vol. 60, pp.111101-05 (1999).

  16. U. Mohideen and Anushree Roy, " A Precision Measurement of the Casimir force between 0.1 to 0.9 mm," Physical Review Letters, vol.81, (no.21), APS, (1998). p.4549-52.

  17. U. Mohideen, H.U. Rahman, M.A. Smith, M. Rosenberg and D.A. Mendis, "Inter-grain coupling in dusty plasmas," Accepted for publication in Physical Review Letters

  18. T.J. Yang and U. Mohideen, "Near-field Scanning Optical Microscopy of ferroelectric domain walls," App. Phys. Lett., 71, p. 1960 (1997).

  19. R.E. Slusher, A.F.J. Levi, U. Mohideen, S.L. McCall, S.J. Pearton and R.A. Logan, "Threshold Characteristics of Semiconductor Microdisk Lasers", App. Phys. Lett. 63, 1310-1312 (1993).

  20. U. Mohideen, M.H. Sher, H.W.K. Tom, G.D. Aumiller, O.R. Wood II, R.R. Freeman, J. Bokor and P.H. Bucksbaum, "High Intensity Above-Threshold Ionization of He and He Ion", OSA Proceedings on Short-Wavelength V: Physics with Intense Laser Pulses, P.B. Corkum, M.D. Perry, eds., (Opt.Soc.Am., Washington, DC 1993), 72-76.

  21. U. Mohideen, M.H. Sher, H.W.K. Tom, G.D. Aumiller, O.R. Wood II, R.R. Freeman, and J. Bokor, "High Intensity Above-Threshold Ionization of He", Phys. Rev. Lett. 71, 509-512 (1993).

  22. W.S. Hobson, U. Mohideen, S.J. Pearton, R.E. Slusher and F. Ren, "SiNx/Sulfide Passivated GaAs/AlGaAs Microdisk Lasers", Elec. Lett. 29, 2199-2200 (1994).

  23. U. Mohideen, W.S. Hobson, S.J. Pearton, R.E. Slusher and F. Ren, "GaAs/AlGaAs Microdisk Lasers", App. Phys. Lett. 64, p.13 (1994).