Bahl Research Group
 




Raman laser cooling of solids

The ability to laser-cool any transparent solid, and specifically the laser refrigeration of indirect bandgap semiconductors, both remain unsolved challenges. Our work shows that photonic density of states engineering can resolve the fundamental requirements for achieving net cooling through Raman scattering in any solid. The enticing possibility of Raman cooling crystalline silicon with a single telecom wavelength pump is elucidated for the first time. [Learn more]

Y.-C. Chen, I. Ghosh, A. Schleife, P.S. Carney, G. Bahl, "Optimization of anisotropic photonic density of states for Raman cooling," arXiv.org:1705.00078, 2017. [arXiv preprint]

Y.-C. Chen, G. Bahl, "Raman Cooling of Solids through Photonic Density of States Engineering," Optica, 2(10), p.893-899, 2015. [Optica link] [arXiv preprint]



Brillouin Scattering Induced Transparency

BSIT occurs due to the coupling between sound and light in resonator environments, essentially resulting in inhibited light transmission that can be controlled by a secondary light source. Uniquely, this process permits nonreciprocal light transmission and optical isolation without any magnetic or magneto-optic materials. In addition, the group velocity of light can be increased or decreased as desired for 'slow' and 'fast' light applications. [Learn more]

J. Kim*, S. Kim*, G. Bahl [* equal contribution], "Complete linear optical isolation at the microscale with ultralow loss," Scientific Reports, 7:1647, 2017. [Nature link]

J. Kim, M. Kuzyk, K. Han, H. Wang, G. Bahl, "Non-reciprocal Brillouin scattering induced transparency," Nature Physics, 11, pp. 275-280, doi:10.1038/nphys3236, 2015. [Nature link] [Supplementary information]



OptoMechanoFluidic Sensors

[Click here for video summary (no audio) -- Optica 2016]

Optomechanofluidic systems permit us to develop highly sensitive acoustic sensors for fluids, particles, bioanalytes, chemical analytes, and gases, using only remote optical interfaces. These high-frequency devices hold promise for a variety of high-throughput sensing applications.

J. Suh, K. Han, C.W. Peterson, G. Bahl, "Real-time sensing of flowing nanoparticles with electro-opto-mechanics," APL Photonics 2, 010801, doi:10.1063/1.4972299, 2017. [AIP Open Access Link]

K. Han, J. Kim, G. Bahl, "High-Throughput Sensing of Freely Flowing Particles with OptoMechanoFluidics," Optica, vol.3, no.6, pp. 585-591, 2016. [Optica link] [Supplementary information] [Supplementary video]

K. Han, K. Zhu, G. Bahl, "Opto-Mechano-Fluidic Viscometer," Applied Physics Letters, 105, 014103, 2014. [AIP link] [arXiv link]

K. Han, J. Kim, G. Bahl "Aerostatically tunable optomechanical oscillators," Optics Express, Vol. 22, Issue 2, pp. 1267-1276, 2014. [OE link]

K. Zhu, K. Han, T. Carmon, X. Fan, G. Bahl, "Opto-Acoustic Biosensing with Optomechanofluidic Resonators," European Physical Journal Special Topics, 223, 1937-1947, 2014. [EPJ link]



Optomechanics on microfluidic systems

[Click here for video summary (no audio)]

This work forms the first experimental demonstration of optomechanically excited vibrational modes of a microdevice in the presence of water and viscous fluids. Through a series of experiments and theoretical analysis we demonstrate the optomechanical actuation mechanisms and mechanical mode shapes of these fluid-filled ultra-high-Q resonators.

G. Bahl, K. H. Kim, W. Lee, J. Liu, X. Fan, T. Carmon, "Brillouin cavity optomechanics with microfluidic devices," Nature Communications, 4:1994, doi:10.1038/ncomms2994, 2013. [Nature Link]

K. H. Kim*, G. Bahl*, W. Lee, J. Liu, M. Tomes, X. Fan, T. Carmon [* = equal contribution], "Cavity optomechanics on a microfluidic resonator with water and viscous liquids," Light: Science & Applications, 2, e110; doi:10.1038/lsa.2013.66, 2013. [Nature Link]

K. Han, K.H. Kim, J. Kim, W. Lee, J. Liu, X. Fan, T. Carmon, G. Bahl, "Fabrication and testing of microfluidic optomechanical oscillators," Journal of Visualized Experiments, vol. 87, e51497, doi:10.3791.51497, 2014. [JoVE link]

G. Bahl, X. Fan, T. Carmon, "Acoustic whispering-gallery modes in optomechanical shells," New Journal of Physics, Vol. 14, 115026, 2012. [NJP open access link] [doi link]



Brillouin Cooling

Leon Brillouin first reported on the scattering of light from sound in 1922. For the better part of a century since its discovery, it has been textbook knowledge that Brillouin scattering is primarily an acousto-optical amplification process. Our work attempts to change this notion, by providing the first experimental evidence of the cooling of a mechanical mode of a solid via Brillouin scattering.

S. Kim, G. Bahl, "Role of optical density of states in two-mode optomechanical cooling," Optics Express 25(2), pp.776-784, 2017. [OSA Link]

Y.-C. Chen, S. Kim, G. Bahl, "Brillouin Cooling in a Linear Waveguide," New Journal of Physics, 18, 115004, 2016. [NJP Open Access Link]

G. Bahl, M. Tomes, F. Marquardt, T. Carmon, "Observation of spontaneous Brillouin cooling," Nature Physics, Vol. 8, No. 3, pp. 203-207 (2012). doi:10.1038/nphys2206 [Nature Link] [Supplementary information]

M. Tomes, F. Marquardt, G. Bahl, T. Carmon, "Quantum mechanical theory of optomechanical Brillouin cooling," Phys. Rev. A 84, 063806, 2011. [PRA/APS Link]



Surface Acoustic Waves via forward SBS

Mechanical resonances excited by Brillouin interaction between sound and light in photonic microsystems have potential sensing applications, but have not previously been studied. Here the experimental excitation of mechanical resonances ranging from 49 to 1400 MHz is shown using forward Brillouin scattering. The mechanical modes that we generate are surface acoustic wave whispering gallery resonances of the microsphere device.

G. Bahl, J. Zehnpfennig, M. Tomes, T. Carmon, "Stimulated optomechanical excitation of surface acoustic waves in a microdevice," Nature Communications, 2:403, doi: 10.1038/ncomms1412 (2011).
[PDF Link] [Nature Link]




Stimulated Brillouin scattering allows for the experimental excitation of surface acoustic resonances in micro-devices, enabling vibration at rates in the range of 50 MHz to 12 GHz. The experimental availability of such mechanical whispering gallery modes in photonic-MEMS raises questions on their structure and spectral distribution. Here we calculate the form and frequency of such vibrational surface whispering gallery modes, revealing diverse types of surface vibrations including longitudinal, transverse, and Rayleigh-type deformations.

J. Zehnpfennig, G. Bahl, M. Tomes, T. Carmon, "Surface optomechanics: Calculating optically excited acoustical whispering gallery modes in microspheres," Optics Express, Vol. 19, pp.14240-8, 2011.
[OE Link] [arXiv Link]



We report on initial measurements of phase noise and continuous frequency tuning of surface acoustic wave optomechanical oscillators.

G. Bahl, J. Zehnpfennig, M. Tomes, and T. Carmon, "Characterization of Surface Acoustic Wave Optomechanical Oscillators," at the International Frequency Control Symposium (IFCS 2011), San Francisco, CA, May 2011.
[PDF Link]



Control of charging in resonant MEMS

We (@ Stanford) developed a fully-AC actuation scheme for electrostatic resonators, along with an oscillator architecture that circumvents the dielectric charging issue by eliminating any DC fields involved in the actuation of the device. This actuation technique is broadly applicable to resonant electrostatic MEMS such as gyroscopes, accelerometers, positioners, and micromirrors.

G. Bahl, J. Salvia, R. Melamud, B. Kim, R.T. Howe, and T. W. Kenny, "AC Polarization for Charge-Drift Elimination in Resonant Electrostatic MEMS and Oscillators," Journal of Microelectromechanical Systems, Vol. 20, No. 2, April 2011.
[PDF Link] [IEEE Link]



Dielectrics such as silicon nitride and silicon dioxide are common structural materials in microsystems. Our group (@ Stanford) has shown that oxide-coated silicon resonators have great potential as high-stability frequency references (quartz replacements in timing systems) due to their low temperature coefficient of frequency. However, since dielectrics are susceptible to charging, charge-induced drift of the resonance frequency in some of these resonators has been observed. My work has focused on characterizing and modeling this dielectric charge in the context of its electromechanical effects on resonant MEMS.

G. Bahl, R. Melamud, B. Kim, S. A. Chandorkar, J. Salvia, M. A. Hopcroft, D. Elata, R. G. Hennessy, R. N. Candler, R.T. Howe, and T. W. Kenny, "Model and observations of dielectric charge in thermally oxidized silicon resonators," Journal of Microelectromechanical Systems, Vol. 19, No. 1, Feb 2010.
[PDF Link] [IEEE Link]