Geun Ho Ahn

All Publications

• Titanium:sapphire-on-insulator integrated lasers and amplifiers. Nature Yang, J., Van Gasse, K., Lukin, D. M., Guidry, M. A., Ahn, G. H., White, A. D., Vučković, J. 2024; 630 (8018): 853-859

  Abstract

  Titanium:sapphire (Ti:sapphire) lasers have been essential for advancing fundamental research and technological applications, including the development of the optical frequency comb1, two-photon microscopy2 and experimental quantum optics3,4. Ti:sapphire lasers are unmatched in bandwidth and tuning range, yet their use is restricted because of their large size, cost and need for high optical pump powers5. Here we demonstrate a monocrystalline titanium:sapphire-on-insulator (Ti:SaOI) photonics platform that enables dramatic miniaturization, cost reduction and scalability of Ti:sapphire technology. First, through the fabrication of low-loss whispering-gallery-mode resonators, we realize a Ti:sapphire laser operating with an ultralow, sub-milliwatt lasing threshold. Then, through orders-of-magnitude improvement in mode confinement in Ti:SaOI waveguides, we realize an integrated solid-state (that is, non-semiconductor) optical amplifier operating below 1 μm. We demonstrate unprecedented distortion-free amplification of picosecond pulses to peak powers reaching 1.0 kW. Finally, we demonstrate a tunable integrated Ti:sapphire laser, which can be pumped with low-cost, miniature, off-the-shelf green laser diodes. This opens the doors to new modalities of Ti:sapphire lasers, such as massively scalable Ti:sapphire laser-array systems for several applications. As a proof-of-concept demonstration, we use a Ti:SaOI laser array as the sole optical control for a cavity quantum electrodynamics experiment with artificial atoms in silicon carbide6. This work is a key step towards the democratization of Ti:sapphire technology through a three-orders-of-magnitude reduction in cost and footprint and introduces solid-state broadband amplification of sub-micron wavelength light.

  View details for DOI 10.1038/s41586-024-07457-2

  View details for PubMedID 38926612

  View details for PubMedCentralID 5734860

• An Inverse-Designed Nanophotonic Interface for Excitons in Atomically Thin Materials. Nano letters Gelly, R. J., White, A. D., Scuri, G., Liao, X., Ahn, G. H., Deng, B., Watanabe, K., Taniguchi, T., Vučković, J., Park, H. 2023

  Abstract

  Efficient nanophotonic devices are essential for applications in quantum networking, optical information processing, sensing, and nonlinear optics. Extensive research efforts have focused on integrating two-dimensional (2D) materials into photonic structures, but this integration is often limited by size and material quality. Here, we use hexagonal boron nitride (hBN), a benchmark choice for encapsulating atomically thin materials, as a waveguiding layer while simultaneously improving the optical quality of the embedded films. When combined with a photonic inverse design, it becomes a complete nanophotonic platform to interface with optically active 2D materials. Grating couplers and low-loss waveguides provide optical interfacing and routing, tunable cavities provide a large exciton-photon coupling to transition metal dichalcogenide (TMD) monolayers through Purcell enhancement, and metasurfaces enable the efficient detection of TMD dark excitons. This work paves the way for advanced 2D-material nanophotonic structures for classical and quantum nonlinear optics.

  View details for DOI 10.1021/acs.nanolett.3c02931

  View details for PubMedID 37695253

• Microwave Spin Control of a Tin-Vacancy Qubit in Diamond PHYSICAL REVIEW X Rosenthal, E. I., Anderson, C. P., Kleidermacher, H. C., Stein, A. J., Lee, H., Grzesik, J., Scuri, G., Rugar, A. E., Riedel, D., Aghaeimeibodi, S., Ahn, G., Van Gasse, K., Vuekovie, J. 2023; 13 (3)

  View details for DOI 10.1103/PhysRevX.13.031022

  View details for Web of Science ID 001122945200001

• Platform-agnostic waveguide integration of high-speed photodetectors with evaporated tellurium thin films OPTICA Ahn, G., White, A. D., Kim, H., Higashitarumizu, N., Mayor, F. M., Herrmann, J. F., Jiang, W., Multani, K. S., Safavi-Naeini, A. H., Javey, A., Vuckovic, J. 2023; 10 (3): 349-355

  View details for DOI 10.1364/OPTICA.475387

  View details for Web of Science ID 000983216600001

• Multi-dimensional data transmission using inverse-designed silicon photonics and microcombs. Nature communications Yang, K. Y., Shirpurkar, C., White, A. D., Zang, J., Chang, L., Ashtiani, F., Guidry, M. A., Lukin, D. M., Pericherla, S. V., Yang, J., Kwon, H., Lu, J., Ahn, G. H., Van Gasse, K., Jin, Y., Yu, S. P., Briles, T. C., Stone, J. R., Carlson, D. R., Song, H., Zou, K., Zhou, H., Pang, K., Hao, H., Trask, L., Li, M., Netherton, A., Rechtman, L., Stone, J. S., Skarda, J. L., Su, L., Vercruysse, D., MacLean, J. W., Aghaeimeibodi, S., Li, M. J., Miller, D. A., Marom, D. M., Willner, A. E., Bowers, J. E., Papp, S. B., Delfyett, P. J., Aflatouni, F., Vučković, J. 2022; 13 (1): 7862

  Abstract

  The use of optical interconnects has burgeoned as a promising technology that can address the limits of data transfer for future high-performance silicon chips. Recent pushes to enhance optical communication have focused on developing wavelength-division multiplexing technology, and new dimensions of data transfer will be paramount to fulfill the ever-growing need for speed. Here we demonstrate an integrated multi-dimensional communication scheme that combines wavelength- and mode- multiplexing on a silicon photonic circuit. Using foundry-compatible photonic inverse design and spectrally flattened microcombs, we demonstrate a 1.12-Tb/s natively error-free data transmission throughout a silicon nanophotonic waveguide. Furthermore, we implement inverse-designed surface-normal couplers to enable multimode optical transmission between separate silicon chips throughout a multimode-matched fibre. All the inverse-designed devices comply with the process design rules for standard silicon photonic foundries. Our approach is inherently scalable to a multiplicative enhancement over the state of the art silicon photonic transmitters.

  View details for DOI 10.1038/s41467-022-35446-4

  View details for PubMedID 36543782

  View details for PubMedCentralID PMC9772188

• Integrated passive nonlinear optical isolators NATURE PHOTONICS White, A. D., Ahn, G., Gasse, K., Yang, K., Chang, L., Bowers, J. E., Vuckovic, J. 2022

  View details for DOI 10.1038/s41566-022-01110-y

  View details for Web of Science ID 000893052000001

• Inverse Design of Optical Vortex Beam Emitters ACS PHOTONICS White, A. D., Su, L., Shahar, D. I., Yang, K., Ahn, G., Skarda, J. L., Ramachandran, S., Vuckovic, J. 2022

  View details for DOI 10.1021/acsphotonics.2c01007

  View details for Web of Science ID 000863653400001

• Photonic Inverse Design of On-Chip Microresonators ACS PHOTONICS Ahn, G., Yang, K., Trivedi, R., White, A. D., Su, L., Skarda, J., Vuckovic, J. 2022; 9 (6): 1875-1881

  View details for DOI 10.1021/acsphotonics.2c00020

  View details for Web of Science ID 000812511100001

• Inverse Spectral Design of Kerr Microcomb Pulses Lucas, E., Yu, S., Ahn, G., Yang, K., Vuckovic, J., Papp, S. B., Ilchenko, V. S., Armani, A. M., Sheldakova, J. V., Kudryashov, A. V., Paxton, A. H. SPIE-INT SOC OPTICAL ENGINEERING. 2021

  View details for DOI 10.1117/12.2576439

  View details for Web of Science ID 000695307500001

• Inverse-designed non-reciprocal pulse router for chip-based LiDAR NATURE PHOTONICS Yang, K., Skarda, J., Cotrufo, M., Dutt, A., Ahn, G., Sawaby, M., Vercruysse, D., Arbabian, A., Fan, S., Alu, A., Vuckovic, J. 2020

  View details for DOI 10.1038/s41566-020-0606-0

  View details for Web of Science ID 000521525600003

• Inverse-Designed Photonics for Semiconductor Foundries ACS PHOTONICS Piggott, A. Y., Ma, E. Y., Su, L., Ahn, G., Sapra, N. V., Vercruysse, D., Netherton, A. M., Khope, A. P., Bowers, J. E., Vuckovic, J. 2020; 7 (3): 569–75

  View details for DOI 10.1021/acsphotonics.9b01540

  View details for Web of Science ID 000526350900004

• 4H-silicon-carbide-on-insulator for integrated quantum and nonlinear photonics NATURE PHOTONICS Lukin, D. M., Dory, C., Guidry, M. A., Yang, K., Mishra, S. D., Trivedi, R., Radulaski, M., Sun, S., Vercruysse, D., Ahn, G., Vuckovic, J. 2020; 14: 330-334

  View details for DOI 10.1038/s41566-019-0556-6

• Toward inverse-designed optical interconnect Skarda, J., Yang, K., Ahn, G., Guidry, M. A., Vuckovic, J., IEEE IEEE. 2020

  View details for Web of Science ID 000612237500103

• Inverse design of microresonator dispersion for nonlinear optics Ahn, G., Yang, K., Skarda, J., Vuclovic, J., IEEE IEEE. 2020

  View details for Web of Science ID 000612090001279

• Synthetic WSe2 monolayers with high photoluminescence quantum yield. Science advances Kim, H., Ahn, G. H., Cho, J., Amani, M., Mastandrea, J. P., Groschner, C. K., Lien, D. H., Zhao, Y., Ager, J. W., Scott, M. C., Chrzan, D. C., Javey, A. 2019; 5 (1): eaau4728

  Abstract

  In recent years, there have been tremendous advancements in the growth of monolayer transition metal dichalcogenides (TMDCs) by chemical vapor deposition (CVD). However, obtaining high photoluminescence quantum yield (PL QY), which is the key figure of merit for optoelectronics, is still challenging in the grown monolayers. Specifically, the as-grown monolayers often exhibit lower PL QY than their mechanically exfoliated counterparts. In this work, we demonstrate synthetic tungsten diselenide (WSe2) monolayers with PL QY exceeding that of exfoliated crystals by over an order of magnitude. PL QY of ~60% is obtained in monolayer films grown by CVD, which is the highest reported value to date for WSe2 prepared by any technique. The high optoelectronic quality is enabled by the combination of optimizing growth conditions via tuning the halide promoter ratio, and introducing a simple substrate decoupling method via solvent evaporation, which also mechanically relaxes the grown films. The achievement of scalable WSe2 with high PL QY could potentially enable the emergence of technologically relevant devices at the atomically thin limit.

  View details for DOI 10.1126/sciadv.aau4728

  View details for PubMedID 30613771

  View details for PubMedCentralID PMC6314873

• Inverse designed Fano resonance in Silicon microresonators Yang, K., Skarda, J., Cotrufo, M., Ahn, G., Alu, A., Vuckovic, J., IEEE IEEE. 2019

  View details for Web of Science ID 000482226303042

• Large-area and bright pulsed electroluminescence in monolayer semiconductors NATURE COMMUNICATIONS Lien, D., Amani, M., Desai, S. B., Ahn, G., Han, K., He, J., Ager, J. W., Wu, M. C., Javey, A. 2018; 9: 1229

  Abstract

  Transition-metal dichalcogenide monolayers have naturally terminated surfaces and can exhibit a near-unity photoluminescence quantum yield in the presence of suitable defect passivation. To date, steady-state monolayer light-emitting devices suffer from Schottky contacts or require complex heterostructures. We demonstrate a transient-mode electroluminescent device based on transition-metal dichalcogenide monolayers (MoS2, WS2, MoSe2, and WSe2) to overcome these problems. Electroluminescence from this dopant-free two-terminal device is obtained by applying an AC voltage between the gate and the semiconductor. Notably, the electroluminescence intensity is weakly dependent on the Schottky barrier height or polarity of the contact. We fabricate a monolayer seven-segment display and achieve the first transparent and bright millimeter-scale light-emitting monolayer semiconductor device.

  View details for DOI 10.1038/s41467-018-03218-8

  View details for Web of Science ID 000428237700008

  View details for PubMedID 29581419

  View details for PubMedCentralID PMC5955902

• Polarization-resolved black phosphorus/molybdenum disulfide mid-wave infrared photodiodes with high detectivity at room temperature NATURE PHOTONICS Bullock, J., Amani, M., Cho, J., Chen, Y., Ahn, G., Adinolfi, V., Shrestha, V. R., Gao, Y., Crozier, K. B., Chueh, Y., Javey, A. 2018; 12 (601–607)
• Strain-engineered growth of two-dimensional materials NATURE COMMUNICATIONS Ahn, G., Amani, M., Rasool, H., Lien, D., Mastandrea, J. P., Ager, J. W., Dubey, M., Chrzan, D. C., Minor, A. M., Javey, A. 2017; 8: 608

  Abstract

  The application of strain to semiconductors allows for controlled modification of their band structure. This principle is employed for the manufacturing of devices ranging from high-performance transistors to solid-state lasers. Traditionally, strain is typically achieved via growth on lattice-mismatched substrates. For two-dimensional (2D) semiconductors, this is not feasible as they typically do not interact epitaxially with the substrate. Here, we demonstrate controlled strain engineering of 2D semiconductors during synthesis by utilizing the thermal coefficient of expansion mismatch between the substrate and semiconductor. Using WSe2 as a model system, we demonstrate stable built-in strains ranging from 1% tensile to 0.2% compressive on substrates with different thermal coefficient of expansion. Consequently, we observe a dramatic modulation of the band structure, manifested by a strain-driven indirect-to-direct bandgap transition and brightening of the dark exciton in bilayer and monolayer WSe2, respectively. The growth method developed here should enable flexibility in design of more sophisticated devices based on 2D materials.Strain engineering is an essential tool for modifying local electronic properties in silicon-based electronics. Here, Ahn et al. demonstrate control of biaxial strain in two-dimensional materials based on the growth substrate, enabling more complex low-dimensional electronics.

  View details for DOI 10.1038/s41467-017-00516-5

  View details for Web of Science ID 000411315800001

  View details for PubMedID 28931806

  View details for PubMedCentralID PMC5606995

• MoS2 transistors with 1-nanometer gate lengths SCIENCE Desai, S. B., Madhvapathy, S. R., Sachid, A. B., Llinas, J. P., Wang, Q., Ahn, G. H., Pitner, G., Kim, M. J., Bokor, J., Hu, C., Wong, H. P., Javey, A. 2016; 354 (6308): 99-102

  Abstract

  Scaling of silicon (Si) transistors is predicted to fail below 5-nanometer (nm) gate lengths because of severe short channel effects. As an alternative to Si, certain layered semiconductors are attractive for their atomically uniform thickness down to a monolayer, lower dielectric constants, larger band gaps, and heavier carrier effective mass. Here, we demonstrate molybdenum disulfide (MoS2) transistors with a 1-nm physical gate length using a single-walled carbon nanotube as the gate electrode. These ultrashort devices exhibit excellent switching characteristics with near ideal subthreshold swing of ~65 millivolts per decade and an On/Off current ratio of ~10(6) Simulations show an effective channel length of ~3.9 nm in the Off state and ~1 nm in the On state.

  View details for DOI 10.1126/science.aah4698

  View details for PubMedID 27846499