Bahl Research Group

Nonreciprocal integrated photonics

Modern integrated photonics technologies still lack optical isolators and circulators due to materials availability and optical loss issues. We are working towards integrated nonreciprocal devices that could be implemented in conventional foundry processes, without the need for specialized materials. Our isolators have established figure-of-merit records for contrast and insertion loss, and are designed for short wavelengths. [Learn more] [Nature Photonics News & Views article]

O.E. Orsel, D.B. Sohn, G. Bahl, "Lithium niobate isolators that outperform magneto-optics," Optics & Photonics News, Dec 2022. [Optica link]

D.B. Sohn*, O.E. Orsel*, G. Bahl, "Electrically driven linear optical isolation through phonon mediated Autler-Townes splitting," Nature Photonics 15, 822-827, doi:10.1038/s41566-021-00884-x, 2021. [Nature link]

D.B. Sohn, G. Bahl, "Direction reconfigurable non-reciprocal acousto-optic modulator on chip," APL Photonics 4, 126103, 2019. [AIP link]

D.B. Sohn, S. Kim, G. Bahl, "Time-reversal symmetry breaking with acoustic pumping of nanophotonic circuits," Nature Photonics, 12, pp.91-97, doi:10.1038/s41566-017-0075-2, 2018. [Nature link]

D.B. Sohn, S. Kim, G. Bahl, "Breaking optical symmetry with sound," Optics and Photonics News, vol.29, p.43, Dec 2018. [Optica link]

High-order topological insulators

An exciting aspect of topological insulators is the spatial and spectral protection of modes that are 1 dimension lower than the host material. Specifically, a 2D TI can protect 1D channels, while a 3D TI protects 2D surfaces. Our group is developing higher-order topological insulators that host modes over an extended range of dimensionalities, e.g. 3D materials that protect 1D line modes and 0D point modes, which opens up transformative new engineering possibilities.

J. Schulz*, J. Noh*, W.A. Benalcazar, G. Bahl, G. von Freymann, "Photonic quadrupole topological insulator using orbital-induced synthetic flux," Nature Communications 13, 6597, 2022. [Nature link]

S.S. Yamada, T. Li, M. Lin, C.W. Peterson, T.L. Hughes, G. Bahl, "Bound states at partial dislocation defects in multipole higher-order topological insulators," Nature Communications 13:2035, doi:10.1038/s41467-022-29785-5, 2022. [Nature link]

C. W. Peterson, T. Li, W. Jiang, T.L. Hughes, G. Bahl, "Trapped fractional charges at bulk defects in topological insulators," Nature 589, pp.376-380, 2021. [Nature link] [Learn more]

C. W. Peterson, T. Li, W. A. Benalcazar, T.L. Hughes, G. Bahl, "A fractional corner anomaly reveals higher-order topology," Science, vol. 368, iss. 6495, pp. 1114-1118, 2020. [Science link] [Learn more]

C.W. Peterson, W.A. Benalcazar, T.L. Hughes, G. Bahl, "Demonstration of a quantized microwave quadrupole insulator with topologically protected corner states," Nature, 555, pp.346-350, doi:10.1038/nature25777, 2018. [Nature link] [Learn more]

Related research on microwave systems

C. W. Peterson, W. A. Benalcazar, M. Lin, T. L. Hughes, G. Bahl, "Strong nonreciprocity in modulated resonator chains through synthetic electric and magnetic fields," Physical Review Letters, 123, 063901, 2019. [PRL link]

C.W. Peterson, S. Kim, J.T. Bernhard, G. Bahl, "Synthetic phonons enable nonreciprocal coupling to arbitrary resonator networks," Science Advances 4(6), eaat0232, doi:10.1126/sciadv.aat0232, 2018. [AAAS link]

Magneto-mechanical topological pumping

The robust transport of energy across 1 dimensional waveguides is susceptible to disorder and defects, leading to Anderson localization and undesirable scattering. We demonstrate that temporal topological pumping can produce 1D channels that are immune to disorder that appears in both space and time, allowing unidirectional energy transport even in the presence of severe disorder.

I. H. Grinberg, M. Lin, C. Harris, W. A. Benalcazar, C. W. Peterson, T. L. Hughes, G. Bahl, "Robust temporal pumping in a magneto-mechanical topological insulator," arXiv:1905.02778. [arXiv link]

Related research on magneto-mechanical systems

I.H. Grinberg, M. Lin, W.A. Benalcazar, T.L. Hughes, G. Bahl, "Observation of a trapped state at a dislocation in a weak magneto-mechanical topological insulator," arXiv:1909.01431. [arXiv link]

I.H. Grinberg, A. Mangu, C. W. Peterson, E. Wilken-Resman, J. T. Bernhard, G. Bahl, "Magnetostatic spring softening and stiffening in magneto-mechanical resonator systems," IEEE Transactions on Magnetics, doi:10.1109/TMAG.2019.2906864, 2019. [IEEE link]

Dynamic suppression of disorder-induced backscattering

Disorder often sets the fundamental limits of performance for photonic devices such as waveguides and resonators, by producing undesirable scattering. We have demonstrated that the breaking of parity symmetry using optomechanical interactions can significantly suppress disorder induced scattering in common dielectrics such as silica. [Learn mode: Rayleigh backscattering] [Learn more: Phonon backscattering]

S. Kim, J.M. Taylor, G. Bahl, "Dynamic suppression of Rayleigh light scattering in dielectric resonators," Optica 6(8), pp. 1016-1022, 2019. [Optica link]

S. Kim, X. Xu, J.M. Taylor, G. Bahl, "Dynamically induced robust phonon transport and chiral cooling in an optomechanical system," Nature Communications 8, 205, doi:10.1038/s41467-017-00247-7, 2017. [Nature link]

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,", 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]

Non-reciprocal Brillouin Scattering Induced Transparency

BSIT occurs due to the coupling between sound and light in resonator environments, 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, G. Bahl, "Imaging of acoustic pressure modes in opto-mechano-fluidic resonators with a single particle probe," Applied Physics Letters, 112, 071106, 2018. [AIP Link]

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]