Linear, time-invariant (LTI) systems are the staple of many electronic and photonic devices, as taught in undergraduate signals and systems courses. However, their behavior is constrained – they cannot generate new frequencies of light or exhibit nonreciprocal behavior. By breaking either linearity or time-invariance (through dynamic modulation), we can create integrated nonreciprocal devices such as isolators and circulators that are essential in communication systems and LiDAR.

By breaking the constraints of linearity and harnessing the high field enhancements in ultraconfined low-loss microresonators with high nonlinearities, we can generate new frequencies of light on chip from a single-frequency laser. In the extreme limit, hundreds to thousands of equally spaced frequencies can be generated in a low-noise, phase locked state – which is called a frequency comb. Frequency combs, in fact, were first generated at the turn of the century in mode-locked laser cavities, but on-chip microresonators have proven to be an appealing platform for miniaturizing these frequency combs. One of the most promising applications of frequency combs is in spectroscopy, whereby the multitude of frequencies of light are used to detect and sense molecules quantitatively through their spectral “fingerprints,” that is, absorption and dispersion at certain wavelength bands.

As an example application of such frequency combs to real-world technologies, we have demonstrated ultrafast spectroscopy using two combs generated on the same chip from a single laser. This specific technique – termed “dual-comb spectroscopy” – uses one comb as a reference local oscillator to coherently sample another comb, obviating bulky mechanically moving parts in conventional optical spectrometers and hence significantly improving speed, footprint and stability. The dispersion engineering capabilities of our record-high-Q Si₃N₄ resonators enabled us to generate very broadband on-chip dual combs. While these dual combs increased acquisition speed by 3-4 orders of magnitude, the achievable resolution was a rather coarse 450 GHz. Recently, we achieved a fine sub-MHz resolution without significantly compromising acquisition speed, by continuously scanning the laser and microheaters in tandem. With these advancements, on-chip dual-combs now compare favorably with state-of-the-art spectroscopy techniques in terms of footprint, resolution and speed. In more recent collaborative work, we have combined nonlinearity and nonreciprocity in silicon microresonators for chip-based LiDAR (light detection and ranging). Both frequency combs and nonreciprocal photonic chips could eventually contribute to full-duplex optical communications, or help autonomous vehicles become more economically viable by sensing their surroundings using tinier, mass-produced nonreciprocal LiDAR chips.

A. Dutt, C. Joshi, X. Ji, J. Cardenas, Y. Okawachi, K. Luke, A. L. Gaeta, M. Lipson, “On-chip dual comb source for spectroscopy,” Science Advances (2018). | “Single chip, single laser, dual comb“, Optics & Photonics News (2018).

Tutorials & Reviews

• I.A.D. Williamson, M. Minkov, A. Dutt, J. Wang, A. Y. Song, S. Fan, ”Integrated nonreciprocal photonic devices with dynamic modulation,” Proceedings of the IEEE 108, 1759 (2020).
• D. L. Sounas and A. Alù, “Non-reciprocal photonics based on time modulation,” Nature Photonics 11, 774 (2017).
• J. Leuthold, C. Koos, and W. Freude, “Nonlinear silicon photonics,” Nature Photonics 4, 535 (2010).
• A. L. Gaeta, M. Lipson, and T. J. Kippenberg, “Photonic-chip-based frequency combs,” Nature Photonics 13, 158 (2019).

Original research papers from our work

• K.Y. Yang*, J. Skarda*, M. Cotrufo*, A. Dutt, G.H. Ahn, M. Sawaby, D. Vercruysse, A. Arbabian, S. Fan, A. Alù, J. Vučković, “Inverse-designed non-reciprocal pulse router for chip-based LiDAR,” Nature Photonics 14, 369 (2020). [nonlinear, nonreciprocal]
• S. Buddhiraju, Y. Shi, A. Song, C. Wojcik, M. Minkov, I.A.D. Williamson, A. Dutt, S. Fan, “Absence of unidirectionally propagating surface plasmon-polaritons in nonreciprocal plasmonics,” Nature Communications 11, 674 (2020). [nonreciprocal]
• C. Joshi, A. Farsi, A. Dutt, B.Y. Kim, X. Ji, Y. Zhao, A. Bishop, M. Lipson, A.L. Gaeta, “Frequency domain quantum interference with entangled photons from an integrated microresonator,” Phys. Rev. Lett. 124, 143601 (2020). [nonlinear, quantum]
• T. Lin, A. Dutt, C. Joshi, X. Ji, C.T. Phare, Y. Okawachi, A.L. Gaeta, M. Lipson, “Broadband ultrahigh-resolution chip-scale scanning soliton dual-comb spectroscopy,” arXiv:2001.00869 (2020). [nonlinear, frequency combs, spectroscopy]
• L. Yuan, A. Dutt, M. Qin, S. Fan, X. Chen, “Creating locally interacting Hamiltonians in the synthetic frequency dimension for photons,” Photon. Research 8, B8 (2020). [nonlinear, quantum, time-modulated]
• A. Dutt, M. Minkov, Q. Lin, L. Yuan, D. A. B. Miller, S. Fan, “Experimental demonstration of dynamical input isolation in nonadiabatically modulated photonic cavities,” ACS Photonics, 6, 162 (2019). [time-modulated]
• A. Dutt, C. Joshi, X. Ji, J. Cardenas, Y. Okawachi, K. Luke, A. L. Gaeta, M. Lipson, “On-chip dual comb source for spectroscopy,” Science Advances 4, e1701858 (2018). [nonlinear, frequency combs, spectroscopy]
• X. Ji, F. A. Barbosa, S. P. Roberts, A. Dutt, J. Cardenas, Y. Okawachi, A. Bryant, A. L. Gaeta, M. Lipson, “Ultra-low-loss on-chip resonators with sub-milliwatt parametric oscillation threshold,” Optica 4, 619 (2017). [nonlinear, frequency combs]
• G. Patwardhan, X. Gao, A. Sagiv, A. Dutt, J. Ginsberg, A. Ditkowski, G. Fibich, A. L. Gaeta, “Loss of polarization of elliptically polarized collapsing beams,” Phys. Rev. A 99, 033824 (2019). [nonlinear]
• A. Dutt, S. Miller, K. Luke, J. Cardenas, A. L. Gaeta, P. Nussenzveig, M. Lipson, “Tunable squeezing using coupled ring resonators on a silicon nitride chip,” Opt. Lett. 41, 223 (2016). [nonlinear, quantum]
• S. A. Miller, Y. Okawachi, S. Ramelow, K. Luke, A. Dutt, A. Farsi, A. L. Gaeta, M. Lipson, “Tunable frequency combs based on dual microring resonators,” Opt. Express 23, 21527 (2015). [nonlinear, frequency combs]
• A. Dutt, K. Luke, S. Manipatruni, A. L. Gaeta, P. Nussenzveig, M. Lipson, “On-chip Optical Squeezing,” Phys. Rev. Applied 3, 044005 (2015). [nonlinear, quantum]