Department of Physics and Astronomy at UC Riverside

Umar Mohideen
Professor of Physics
Ph.D. 1992, Columbia University

Experimental Condensed Matter Physics

E-mail: umar.mohideen@ucr.edu
Phone: (951) 827-5390
Fax: (951) 827-4529

Selected Publications

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. From the view point of applications, the role of the Casimir force in Micro Electromechanical Systems (MEMS) has been well recognized in the last few years. At device distance below 100 nanometers, the Casimir force becomes comparable or even exceeds the electric force which is normally used to actuate MEMS. Thus the Casimir effect is a serious limiting factor in the fabrication and operation of MEMS. This has motivated growing interest from the engineering community on ways to control and modify the Casimir force.

Starting in ‘98 we pioneered the use of the Atomic Force Microscope (AFM) for measurements of the normal Casimir force. Prior to that there were only a few attempts at its measurements. In 2002, we performing the first experimental demonstration of the lateral (horizontal) Casimir forces which acts parallel to the two surfaces used. The force around 10^-13N was demonstrated between two aligned surfaces, imprinted with nanoscale sinusoidal corrugations. This was the first experimental observation of an effect that was theoretically predicted in 1997. With our demonstration of the lateral Casimir force, quantum vacuum activated horizontal motion entirely originating from the quantum fluctuations of empty space might be possible in silicon chip micro-machines. We have recently done the first demonstration of the modulation of the Casimir force through a change in the dielectric properties of one boundary. Here, we measured the modification in the Casimir force between a silicon plate and gold sphere when the carrier density in the Silicon plate was altered by the incidence of a laser pulse. 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, New Scientist, Physics World, Research and Technology News, La Recherche and Vie, Discover etc.

2. Condensed Matter Experiments: 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 ).. We have used the NSOM to study domain walls in ferroelectric materials. 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 are using our expertise in sensitive force measurement techniques to study the interaction between single molecules or a single assembly of bound proteins. This is collaborative research done with Prof. V. Parpura (Cell Biology and Neurosciences) and Prof. Zandi (Physics). Physical phenomena exhibited by single molecules or an assembly of small number of particles might be very different from those exhibited by a ensemble mixture. One set of molecules we study are called the SNARE proteins which are present in the human brain. In the brain the signal transmission is electrical inside the neuron and chemical between neurons. The chemical transmitters are referred to as neurotransmitters. Over the last 15 years, it has become clear that 4 proteins are necessary for the attachment of vesicles containing the neurotransmitters to ends of neurons, a step that precedes the release of the neurotransmitters. Our research is focused on the mechanism of how the 4 proteins regulate the attachment of the vesicles. We have shown that only a pair of molecules are necessary for the attachment of the vesicle. Thus it is a highly non-equilibrium process, ideally adapted for single molecule investigations. We have also been able to identify the nature of the interaction between the proteins (coiled-coil vs ionic) and identify the location of the critical regions. Single molecule investigations are in their infancy and the interpretation of the results remain challenging. We hope in the coming years to play a decisive role in shaping the experimental landscape in this emerging field. We also believe that such single molecule research will be critical to isolating the key mechanisms in neuro signal transmission. With Prof. Zandi, we are studying the kinetics of viral capsid self-assembly.

Awards and Fellowships

Fellow of the American Physical Society (2004)

Fellow of the American Association for the Advancement of Science (2006)

Peer Reviewed Journal Publications

1. U. Mohideen, H.W.K. Tom, R.R. Freeman, J. Bokor and P.H. Bucksbaum, "Interaction of free electrons with an intense focused laser pulse in Gaussian and conical axicon geometries," Journal of Optical Society of America B, Vol. 9, pp. 2190-95 (1992).

2. M.H. Sher, U. Mohideen, H.W.K. Tom, O.R. Wood, G. Aumiller, and R.R. Freeman, "Soft X-ray Pulse-length Measurement by Pump-Probe Absorption Spectroscopy", Optics Letters, Vol. 18, pp. 646-48 (1993).

3. 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", Applied Physics Letters, Vol. 63, pp. 1310-12 (1993).

4. 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", Physical Review Letters, Vol. 71, pp. 509-512 (1993).

5. W.S. Hobson, U. Mohideen, S.J. Pearton, R.E. Slusher and F. Ren, "SiN x Passivated GaAs/AlGaAs Microdisk Lasers", Electronics Letters, Vol. 29, pp. 2199-00 (1994).

6. U. Mohideen, W.S. Hobson, S.J. Pearton, R.E. Slusher and F. Ren, "GaAs/AlGaAs Microdisk Lasers", Applied Physics Letters, Vol. 64, pp. 1911-13 (1994).

7. U. Mohideen, R.E. Slusher, F. Jahnke and Stephan Koch, "Semiconductor Microlaser Linewidths", Physical Review Letters, Vol. 73, pp.1785-88 (1994).

8. W.S. Hobson, F. Ren, U. Mohideen, and R.E. Slusher, “Silicon nitride encapsulation of sulfide passivated GaAs/AlGaAs microdisk lasers,” Journal of Vacuum Science and Technology, Vol.13, pp. 642-45 (1995).

9. U. Mohideen, R.E. Slusher, V. Mizrahi, T. Erdogan, M. Kuwata-Gonokami, P.J. Lemaire, J.E. Sipe, G.Martijn de Sterke, Neil G.R. Broderick, "Gap Solitons in Fiber Gratings," Optics Letters, Vol. 20, pp. 1674-76 (1995).

10. T.J. Yang, U. Mohideen, Mool C. Gupta, “ Near-field scanning optical microscopy of ferroelectric domain walls,” Applied Physics Letters, Vol. 71, pp. 1960-63 (1997).

11. U. Mohideen, H.U. Rahman, M.A. Smith, M. Rosenberg and D.A. Mendis, “Intergrain coupling in dusty plasma Coulomb crystals,” Physical Review Letters, Vol. 81, pp. 349-52 (1998).

12. T.J. Yang and U. Mohideen, “Nanoscale measurement of ferroelectric domain wall strain and energy by near-field scanning optical microscopy,” Physics Letters A, Vol. 250, pp. 205-210 (1998).

13. U. Mohideen and A. Roy, “Precision measurement of the Casimir force from 0.1 to 0.9 m m,” Physical Review Letters, Vol. 81, pp. 4549-52 (1998).

14. T.J. Yang, U. Mohideen, V. Gopalan and P. Swart, “Observation and mobility study of single 180 o domain walls using a Near-field Scanning Optical Microscope,” Ferroelectrics, Vol. 222, pp. 351-358 (1999).

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

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

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

18. 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).

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

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

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

22. A. Roy, C.Y. Lin and U. Mohideen, “Measurement of the Casimir Force using an Atomic Force Microscope,” Comments on Atomic and Molecular Physics, Issue D2, pp. 263-275, (2000).

23. G.L. Klimchitskaya, U. Mohideen, V.M. Mostepanenko, "Casimir and van der Waals forces between two plates or a sphere (lens) above a plate made of real metals,” Physical Review A, Vol. 61, pp.062107/1-12 (2000).

24. B.W. Harris, F. Chen, U. Mohideen, "Precision measurement of the Casimir force using gold surfaces", Physical Review A, Vol. 62, pp.052109/1-5 (2000).

25. H.U. Rahman, U. Mohideen, M.A. Smith, M. Rosenberg, D.A. Mendis, “Grain dynamics and inter-grain coupling in dusty plasma Coulomb crystals,” Physica Scripta, Vol.T89, pp.186-90 (2001).

26. A. Roy, U. Mohideen, “ A verification of quantum field theory - measurement of Casimir force ,” Pramana, Journal of Physics, Vol.56, (no.2-3), pp.239-43 (2001).

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

28. F. Chen, U. Mohideen, "Fiber optic interferometry for precision measurement of the voltage and frequency dependence of the displacement of piezoelectric tubes". Review of Scientific Instruments, Vol.72, p.3100-2 (2001).

29. M. Bordag, U. Mohideen, V.M. Mostepanenko, "New Developments in the Casimir Effect," Physics Reports, Vol. 353/1-3, pp.1-205 (2001).

30. F. Chen, U. Mohideen, G.l. Klimchitskaya and V.M. Mostepanenko, "Demonstration of the Lateral Casimir Force," Physical Review Letters, Vol. 88 (10), pp. 101801-4 (2002) .

31. F. Chen, U. Mohideen, G.l. Klimchitskaya and V.M. Mostepanenko, "Experimental and theoretical investigation of the lateral Casimir force between corrugated surfaces,” Physical Review A, Vol. 66 (3), pp. 0321131-15 (2002) .

32. F. Chen, U. Mohideen, G.l. Klimchitskaya and V.M. Mostepanenko, "New Features in the thermal Casimir Force at small separations," Physical Review Letters, Vol. 90 (16), pp. 160404 1-4 (2003) .

33. G.L. Klimchitskaya, U. Mohideen, “Constraints on Yukawa type hypothetical interactions from recent Casimir force measurements,” International Journal of Modern Physics A, Vol. 17 (29): pp. 4143-4152 (2002).

34. W. Liu, V. Montana , E.R. Chapman, U. Mohideen and V. Parpura, “Botulinum toxin type B micromechanosensor,” Proceedings of the National Academy of Sciences, Vol.100 (23), pp. 13621-13625 (2003).

35. F. Chen, U. Mohideen, G.l. Klimchitskaya and V.M. Mostepanenko, " Theory confronts experiment in the Casimir force measurements: quantification of errors and precision," Physical Review A, Vol. 69: pp. 022117 1-11 (2004).

36. E.V. Blagov, G.L. Klimchitskaya, U. Mohideen and V.M. Mostepanenko, “Control of the lateral Casimir force between corrugated surfaces,” Physical Review A, Vol. 69: pp. 044103 1-4, (2004).

37. Zou ZQ, Wei LY, Chen F, U. Mohideen, D. Bocian, "Solution STM images of porphyrins on HOPG reveal that subtle differences in molecular structure dramatically alter packing geometry," Journal of Porphorins and Phthalocyanines Vol. 9, pp. 387-392 (2005).

38. Chen F, Mohideen U , Milonni PW, " Limits on non-Newtonian gravity and hypothetical forces from measurements of the Casimir force," International Journal of Modern Physics A, Vol. 20, pp. 2222-2231 (2005).

39. Chen F, Mohideen U , Klimchitskaya GL, and V.M. Mostepanenko, “Investigation of the Casimir force between metal and semiconductor bodies," Physical Review A, Rapid Communication, Vol. 72, pp. 020101-1-4, (2005).

40. Chen F., U. Mohideen , G.L. Klimchitskaya, V.M. Mostepanenko, “Demonstration of the difference Casimir force for samples with different charge carrier densities,” Phys. Rev. Lett., Vol. 97, 170402 (2006).

41. Chen F., U. Mohideen , G.L. Klimchitskaya, V.M. Mostepanenko, “Investigation of the Casimir force between metal and semiconductor test bodies,” Physical Review A, Rapid Communication, 72 , 020101-1-4 (2005).

42. Chen F., U. Mohideen , G.L. Klimchitskaya, V.M. Mostepanenko, “ Experimental test for the conductivity properties from the Casimir force between metal and semiconductor, ” Physical Review A, 74 , (2006).

43. Chen F., U. Mohideen , “Recent experimental advances in precision Casimir force measurement with the Atomic Force Microscope, Journal of Physics A, 39 , 6233-6244 (2006).

44. Klimchitskaya G.L., F. Chen, R.S. Decca, E. Fishbach, U. Mohideen and V.M. Mostepanenko, , “Rigorous approach to the comparison between experiment and theory in Casimir force measurements,” Journal of Physics A, 39 , 6485-6493 (2006).

45. Liu W., V. Montana, J. Bai, E.R. Chapman, U. Mohideen, and V. Parpura, “Single molecule mechanical probing of the SNARE protein interactions,” Biophysical Journal, Vol. 91, p.1206-1212, (2006).

46. Chen F., U. Mohideen , G.L. Klimchitskaya, V.M. Mostepanenko, “Demonstration of the difference Casimir force for samples with different charge carrier densities,” Physical Review Letters, Vol. 97, 170402 (2006).

47.Chen F., G.L. Klimchitskaya, V.M. Mostepanenko, and U. Mohideen “Demonstration of the optical modulation of dispersion forces,” Optics Express, Vol. 15, 4823 (2007).

48. Chen F., G. L. Klimchitskaya, V. M. Mostepanenko, and U. Mohideen, "Control of the Casimir force by the modification of dielectric properties with light," Physical Review B, 035338 (2007).