TeraX Labs

Under the wide scope of superconductor photonics, we exploit the potential of high-Tc superconductors in realizing low-loss, fully switchable metamaterials for a variety of exciting and useful phenomenon including sharp resonances, accessing low asymmetry regime of the Fano resonators and ultrathin metamaterials. We also harness the potential of the metamaterials by incorporating high-Tc superconductors into its design for ultrafast switching, and efficient modulation of THz waves. Upon photoexcitation with an optical pulse, the Cooper pairs dissociate and relax in an extremely short time scale, thereby resulting in an ultrafast switching from an excellent electrical conductor to a poor metal. We discovered that the superconductors show the existence of dual dissociation-relaxation channels within a single superconductivity restoration cycle. This peculiar observation leads to the discovery of superconducting dual-channel, ultrafast switchable devices which could be highly useful for several applications such as terahertz high-speed wireless communication, superconducting radiation sensors, and superconducting photodetectors.

References:
i) Y. K. Srivastava, M. Manjappa, L. Cong, H. N. S. Krishnamoorthy, V. Savinov, P. Pitchappa, R. Singh, Adv. Mater. 2018, 30, 1801257. [DOI]
ii) Y. K. Srivastava, M. Manjappa, H. N. Krishnamoorthy, and R. Singh, Adv. Opt. Mater., 2016, 4, 1875-1881. [DOI]
iii) R. Singh and N. Zheludev, Nature Photonics, 2014, 8, 679. [DOI]

MEMS is a specific class of structurally reconfigurable metasurfaces which provides a unique advantage for active manipulation of the near-fields in all the three spatial directions by exploiting sensitive changes to their micro scale movements. In this branch of research, we showed for the first time an experimental realization of multiple-input-output states in a CMOS compatible MEMS metamaterial performing composite logic gate operations using two electrical inputs and a THz readout beam.

References:
i) Manjappa, Manukumara; Pitchappa, Prakash; Singh, Navab; Wang, Nan; Zheludev, Nikolay; Lee, Chengkuo; Singh, Ranjan, Nat. Comm. 2018,9, 4056. [DOI]
ii) P. Pitchappa, M. Manjappa, H. N. S. Krishnamoorthy, Y. Chang, C. Lee, and R. Singh, Applied Physics Letters, 2017, 111, 261101. [DOI]
iii) L. Cong, P. Pitchappa, N. Wang, and R. Singh, [URL]

Dielectric resonators and metasurfaces offer a low-loss platform for efficient manipulation of electromagnetic waves ranging from microwaves to visible. Here, we design active supercavities using silicon to realize high Q resonances that could be actively controlled at an ultrafast time scale. Such supercavities can enable all-optical switching and modulation of extremely sharp resonances and thus could have numerous applications in lasing, mode multiplexing and biosensing.

References:
i)S. Han, L. Cong, Y. K. Srivastava, B. Qiang, M. V. Rybin, W. X. Lim, Q. Wang, Y. S. Kivshar, and R. Singh, [URL]

Metamaterial cavities provide an easy way to confine the THz waves at subwavelength length scales. Integrating semiconducting materials with the plasmonic metamaterials provide a unique platform to study the strong light-matter interactions by coupling the vibrational or lattice resonances with the metamaterial cavity resonances with highly confined fields. In our group, we investigate the strong polaritonic interactions and the quasiparticle formations in the metamaterial-semiconductor hybrid system possessing high confinement of the field within a small mode volume. These investigations are performed in the THz and the mid-infrared regime of electromagnetic spectrum, where we strong phonon-plasmon and lattice-plasmon polaritonic splitting are observed. Such studies would pave promising pathways for the development of a new class of quasiparticle formation and exploring the new physics beyond the strong and ultrastrong light-matter interactions.

References:
i) M. Manjappa, Y. K. Srivastava, A. Solanki, A. Kumar, T. C. Sum, and R. Singh, Advanced Materials, 2017, 29, 1605881. [DOI]
ii) T. C. Tan, Y. K. Srivastava, M. Manjappa, E. Plum, and R. Singh, Applied Physics Letters, 2018, 112, 201111. [DOI]
iii) G. Dayal, X. Y. Chin, C. Soci, and R. Singh, Advanced Optical Materials, 2017, 5, 1600559. [DOI]

Chalcogenide phase-change materials (PCMs) are identified as a promising material for tunable and reconfigurable nanophotonic applications which will enable next generation optical devices with multipurpose functionalities. This non-volatile platform has unique and far-reaching advantages such as, drastic optical contrast, fast switching speed, long term stability and substantial reconfigurability when compared to conventional static nano photonic platforms. Towards this end we are developing novel concepts in tunable micro and nano photonic devices in PCMs with applications such as, wide angle tunable perfect absorber using low‐loss phase change material Sb2S3 in the visible spectrum, dynamic color generation and electrical tuning of micro pixels using Sb2S3 and Ge2Sb2Te5(GST) thin films and tunable UV to visible plasmonics using micro heater integrated Sb2Te3/Si metasurfaces.

References:
i) Sreekanth, K.V., Prabhathan, P., Chaturvedi, A., Lekina, Y., Han, S., Zexiang, S., Tong Teo, E.H., Teng, J. & Singh, R. Wide-Angle Tunable Critical Coupling in Nanophotonic Optical Coatings with Low-Loss Phase Change Material. Small 18, 2202005 (2022).
ii) Sreekanth, K.V., Medwal, R., Srivastava, Y.K., Manjappa, M., Rawat, R.S. & Singh, R. Dynamic Color Generation with Electrically Tunable Thin Film Optical Coatings. Nano Letters 21, 10070-10075 (2021).
iii) Sreekanth, K.V., Medwal, R., Das, C.M., Gupta, M., Mishra, M., Yong, K.-T., Rawat, R.S. & Singh, R. Electrically Tunable All-PCM Visible Plasmonics. Nano Letters 21, 4044-4050 (2021).
iv) Sreekanth, K.V., Das, C.M., Medwal, R., Mishra, M., Ouyang, Q., Rawat, R.S., Yong, K.-T. & Singh, R. Electrically Tunable Singular Phase and Goos–Hänchen Shifts in Phase-Change-Material-Based Thin-Film Coatings as Optical Absorbers. Advanced Materials 33, 2006926 (2021).

Under this domain, we study light-matter interaction by integrating the metamaterials with different active and 2D materials to realize the active THz devices which are facile, cost-effective, and ultrafast in nature. In this regard, our initial finding with perovskites and Ge based hybrid metadevice showed low fluence and ultrafast switching of THz waves at flexible as well as rigid substrate platform.

References:
i) L. Cong, Y. K. Srivastava, A. Solanki, T. C. Sum, and R. Singh, ACS Photonics,2017, 4, 1595. [DOI]
ii) Y. K. Srivastava, A. Chaturvedi, M. Manjappa, A. Kumar, G. Dayal, C. Kloc, and R. Singh, Advanced Optical Materials,2017, 5, 1700762. [DOI]
iii) W. X. Lim, M. Manjappa, Y. K. Srivastava, L. Cong, A. Kumar, K. F. MacDonald, and R. Singh, Advanced Materials,2018, 30, 1705331. [DOI]

Recent development of ultrafast spintronics and ferromagnetism have paved the way towards a promising gapless THz emitter. The underlying mechanism is based on femtosecond laser pulse excitation of the ferromagnet and heavy metal heterostructure. It leads to an ultrafast spin scattering event in the Ferromagnet layer. Spins superdiffuse to adjacent heavy metal layer and undergo inverse spin hall effect to convert to transverse charge current and hence generate THz radiation. The processes have thus unveiled a novel research avenue in ultrafast science and introduce several exploratory grounds. In our lab, we aim to identify and answer a few of them through efficient spin manipulation methods.

We have a state-of-the-art spintronics THz emission set up. It consists of an ultrathin heterostructure of ferromagnet with heavy metal overlayers under the application of a magnetic field. A femtosecond laser pulse is used to illuminate the samples to produce an ultrafast spin current upon photoexcitation which undergoes superdiffusion from ferromagnet layer to the adjacent heavymetal layer. There it converts to a transverse charge current due to inverse spin hall effect to generate THz radiation. The emitted THz pulse is collected using the parabolic mirrors to focus onto the detector crystal (ZnTe) and thereby changing the polarization of low power optical pulse passing through it.

References:
i) Agarwal, Piyush, et al. "Terahertz spintronic magnetometer (TSM)." Applied Physics Letters 120.16 (2022): 161104. [DOI]
ii) Agarwal, Piyush, et al. "Ultrafast Photo‐Thermal Switching of Terahertz Spin Currents." Advanced Functional Materials 31.17 (2021): 2010453. [DOI]
iii) Agarwal, Piyush, et al. "Electric-field control of nonlinear THz spintronic emitters." Nature Communications 13.1 (2022): 1-8. [DOI]