Our research focuses on the development of novel photonic devices and their applications. The following are specific research topics that we have recently worked on. Each area is strongly correlated, and we always look forward to extending our expertise into other areas of research to impact the world.

Kerr Frequency Combs and Nonlinear/Quantum Photonics

Recent advances in high-Q microresonators can boost many nonlinear/quantum processes, and can generate Kerr frequency combs, whose spectral lines are discrete and equidistant [1,2]. While the mode-locked laser-based comb generation requires a bulky and complex setup, the microresonator-based Kerr frequency combs can be very compact and have revolutionized many areas of modern optics including telecommunications, time-frequency metrology, and sensing applications.

Our research focuses on the development of novel microresonators and other related photonic devices that can overcome the various challenges in nonlinear processes and frequency combs, for example, resonator dispersions, film thickness, fabrication challenges, phase-matching bandwidth, etc.

[1] S. Kim, et al, "Dispersion Engineering and Frequency Comb Generation in Thin Silicon Nitride Concentric Microresonators," Nature Communications, 8, 372 (2017).
[2] Y. Xuan, … S. Kim, et al"High-Q Silicon Nitride Micro-Resonators Exhibiting Low Power Frequency Comb Initiation," Optica, 3, 1171-1180 (2016).
[3] S. Kim and M. Qi, "Broadband Second-Harmonic Phase-Matching in Dispersion Engineered Slot Waveguides," Optics Express, 24, 773-786 (2016).

Hybrid Photonic Devices (Metamaterials/Plasmonics)

Realizing the Photonic Integrated Circuits (PICs) is one of the ultimate goals in photonics. Today’s advanced nanotechnology makes their realization feasible; however, due to the fundamental limit in light confinement, their relatively large device sizes remain as a challenge. On the other hand, over the last decade, there has been significant scientific progress in subwavelength light confinement using the metamaterial and/or plasmonic nanostructures.

Here, we are working on further scaling down the photonic devices, using the silicon/metamaterial and silicon/plasmonic hybrid nanostructures. Especially, our recent approach, which is based on all-dielectric metamaterials, can achieve an extremely high photonic integration by reducing the device crosstalk significantly [1]. While the previous light confinements have been demonstrated on a plasmonic platform, which suffers from a high metallic loss, this method is based on a monolithic silicon-on-insulator (SOI), which is low-loss, low-cost, and readily grown in a CMOS foundry. This work should have a broader impact in the photonic society, introducing a new scheme for the high-density PICs.

[1] S. JahaniS. Kim, et al. " Photonic Skin-Depth Engineering on a Silicon Chip using All-Dielectric Metamaterials," arXiv:1701.03093. (equal contribution)
[2] S. Kim and M. Qi, "Mode-Evolution Based Polarization Rotation and Coupling Between Silicon and Hybrid Plasmonic Waveguides," Scientific Reports, 5, 18378 (2015).
[3] S. Kim and M. Qi, "Polarization Rotation and Coupling Between Silicon Waveguide and Hybrid Plasmonic Waveguide," Optics Express, 23, 9968-9978 (2015).
[4] S. Kim and M. Qi, "Copper Nanorod Array Assisted Silicon Waveguide Polarization Beam Splitter," Optics Express, 22, 9508-9516 (2014).

Passive/Active Photonic Devices

Silicon photonics, or more generally, integrated on-chip photonics, use high-index lossless dielectrics to confine and guide the light at telecommunication wavelengths. The ultimate goal of on-chip photonics is to realize the commercially available photonic circuits, and the compatibility with the well-established CMOS manufacturing system makes this promising. Currently, many industrial companies such as IBM, Intel, and HP as well as the US, China, and European governments are actively working on this research area.

Here, we are working on the development of various passive/active photonic devices. Especially, the Nanophotonics Center at Texas Tech University is dedicated to the development of high quality III-V wide bandgap semiconductors (GaN, AlN, hBN, and etc.), which can be used as an active media in various photonic devices such as Lasers, LEDs, and detectors. Our research goal is to develop novel active photonic devices that can enhance light-matter interactions and boost devices’ energy efficiency.

[1] K. Han, S. Kim, J. Wirth, M. Teng, Y. Xuan, B. Niu and M. Qi, " Strip-Slot Direct Mode Converter," Optics Express, 24, 6532-6541 (2016).
[2] T. Min, S. Kim, K. Han, B. Niu, Y. Lee, and M. Qi, "Silicon Nitride Polarization Beam Splitter Based on MMI with Phase Delay Line," CLEO, San Jose, CA, May. 14-19, 2017.
[3] K. Han, M. Teng, B. Niu, Y. Lee, S. Kim, and M. Qi, "Double slot fiber-to-chip coupler using direct strip-slot mode coupling," OFC, Los Angeles, CA, USA, Mar. 19–23, 2017.
[4] S. Kim and M. Qi "Post-Fabrication Trimming on Silicon Nitride Photonic Bragg Grating Add-Drop Filter," FiO, Tucson, AZ, Oct. 19-23, 2014

Plasmonic Metal Nanostructures

Plasmonics has attracted substantial research interest with its ability to squeeze light down to the subwavelength scale and to enhance the field intensity for a strong light-matter interaction. Plasmonics also gives a non-trivial phase variation due to the complex dielectric constants (ε = ε' + iε”), and people have been utilizing such properties to design a metallic nanostructure for arbitrary phase control. However, a huge metallic loss and a lack in large-scale fabrication technique have prevented them from practical applications in optics. Nowadays, researchers have shifted their approaches to utilize optical losses, instead of opposing them, through other forms of energy (heat or frequency conversion).

Here, we are interested in exploring a non-trivial phase variation with different combinations of materials and their applications to the heat transfer and other thin film optical components.

[1] S. Kim, M. Man, M. Qi, and K. J. Webb, " Angle-Insensitive and Solar-Blind Ultraviolet Bandpass Filter," Optics Letters, 39, 5784-5787 (2014).
[2] S. Kim, Y. Xuan, V. P. Drachev, L. T. Varghese, L. Fan, M. Qi, and K. J. Webb, " Nanoimprinted Plasmonic Nanocavity Arrays," Optics Express, 21, 15081-15089 (2013).
[3] L. T. Varghese, L. Fan, Y. Xuan., C. Tansarawiput, S. Kim, and M. Qi, " Resistless Nanoimprinting in Metal for Plasmonic Nanostructures," Samll, 9, 3778-3783 (2013).