X. Meng, Pandey, T., Fu, S., Yang, J., Jeong, J., Chen, K., Singh, A., He, F., Xu, X., Singh, A. K., Lin, J. - F., and Wang, Y., “Giant Thermal Conductivity Enhancement in Multilayer MoS2 under Highly Compressive Strain,” eprint arXiv:1708.03849, 2017. Publisher's VersionAbstract
Multilayer MoS2 possesses highly anisotropic thermal conductivities along in-plane and cross-plane directions that could hamper heat dissipation in electronics. With about 9% cross-plane compressive strain created by hydrostatic pressure in a diamond anvil cell, we observed about 12 times increase in the cross-plane thermal conductivity of multilayer MoS2. Our experimental and theoretical studies reveal that this drastic change arises from the greatly strengthened interlayer interaction and heavily modified phonon dispersions along cross-plane direction, with negligible contribution from electronic thermal conductivity, despite its enhancement of 4 orders of magnitude. The anisotropic thermal conductivity in the multilayer MoS2 at ambient environment becomes almost isotropic under highly compressive strain, effectively transitioning from 2D to 3D heat dissipation. This strain tuning approach also makes possible parallel tuning of structural, thermal and electrical properties, and can be extended to the whole family of 2D Van der Waals solids, down to two layer systems.
X. Xu, Zheng, X., He, F., Wang, Z., Subbaraman, H., Wang, Y., Jia, B., and Chen, R. T., “Observation of Third-order Nonlinearities in Graphene Oxide Film at Telecommunication Wavelength,” Scientific Reports, vol. 7, pp. 9646, 2017. Publisher's VersionAbstract
All-optical switches have been considered as a promising solution to overcome the fundamental speed limit of the current electronic switches. However, the lack of a suitable third-order nonlinear material greatly hinders the development of this technology. Here we report the observation of ultrahigh third-order nonlinearity about 0.45 cm2/GW in graphene oxide thin films at the telecommunication wavelength region, which is four orders of magnitude higher than that of single crystalline silicon. Besides, graphene oxide is water soluble and thus easy to process due to the existence of oxygen containing groups. These unique properties can potentially significantly advance the performance of all-optical switches.
K. Chen, Ghosh, R., Meng, X., Roy, A., Kim, J. - S., He, F., Mason, S. C., Xu, X., Lin, J. - F., Akinwande, D., Banerjee, S. K., and Wang, Y., “Experimental evidence of exciton capture by mid-gap defects in CVD grown monolayer MoSe2,” npj 2D Materials and Applications, vol. 1, pp. 1-15, 2017. Publisher's VersionAbstract
In two dimensional (2D) transition metal dichalcogenides, defect-related processes can significantly affect carrier dynamics and transport properties. Using femtosecond degenerate pump-probe spectroscopy, exciton capture, and release by mid-gap defects have been observed in chemical vapor deposition (CVD) grown monolayer MoSe2. The observed defect state filling shows a clear saturation at high exciton densities, from which the defect density is estimated to be around 0.5 × 1012/cm2. The exciton capture time extracted from experimental data is around ~ 1 ps, while the average fast and slow release times are 52 and 700 ps, respectively. The process of defect trapping excitons is found to exist uniquely in CVD grown samples, regardless of substrate and sample thickness. X-ray photoelectron spectroscopy measurements on CVD and exfoliated samples suggest that the oxygen-associated impurities could be responsible for the exciton trapping. Our results bring new insights to understand the role of defects in capturing and releasing excitons in 2D materials, and demonstrate an approach to estimate the defect density nondestructively, both of which will facilitate the design and application of optoelectronics devices based on CVD grown 2D transition metal dichalcogenides.
K. Chen, Sheehan, N., He, F., Meng, X., Mason, S. C., Bank, S. R., and Wang, Y., “Measurement of Ambipolar Diffusion Coefficient of Photoexcited Carriers with Ultrafast Reflective Grating-Imaging Technique,” ACS Photonics, 2017. Publisher's VersionAbstract
A novel ultrafast reflective grating-imaging technique has been developed to measure ambipolar carrier diffusion in GaAs/AlAs quantum wells and bulk GaAs. By integrating a transmission grating and an imaging system into the traditional pump–probe setup, this technique can acquire carrier diffusion properties conveniently and accurately. The fitted results of the diffusion coefficient and diffusion length in bulk GaAs agree well with the literature values obtained by other techniques. The diffusion coefficient and diffusion length of GaAs/AlAs quantum wells are found to increase with the well layer thickness, which suggests that interface roughness scattering dominates carrier diffusion in GaAs/AlAs quantum wells. With the advantages of simple operation, sensitive detection, rapid and nondestructive measurement, and extensive applicability, the ultrafast reflective grating-imaging technique has great potential in experimental study of carrier diffusion in various materials.
W. Wu, Chai, Z., Gao, Y., Kong, D., He, F., Meng, X., and Wang, Y., “Carrier dynamics and optical nonlinearity of alloyed CdSeTe quantum dots in glass matrix,” Optical Materials Express, vol. 7, no. 5, pp. 1547-1556, 2017. Publisher's VersionAbstract
Size and pump-fluence dependent ultrafast carrier dynamics of CdSeTe QDs are investigated using femtosecond pump-probe techniques operating at two different repetition rates: 1 kHz (low-repetition rate), and 76 MHz (high-repetition rate). With a low-repetition rate laser and 3.1 eV excitation photon energy, multiple exciton generation (MEG) is observed and the optical responses of alloyed QDs clearly show three components: a fast decay ascribed to carrier recombination, an intermediate component associated with MEG decay, and a slow decay associated with radiative Auger recombination. With a high-repetition rate laser and excitation photon energy resonant with band-edge energy, obvious coherent phonon oscillations are observed in 4 nm CdSeTe QDs due to impulsive stimulated Raman scattering. Open-aperture Z-scan measurement is used to clarify the size and pump-fluence dependence of optical nonlinearity under femtosecond laser excitation. With increasing laser power, an evolution from saturable absorption to reverse saturable absorption in CdSeTe QDs is observed. The transition process is analyzed using a phenomenological model based on nonlinear absorption coefficient and saturation intensity. These results indicate that CdSeTe QDs in a glass matrix are a class of materials for potential application in all-optical switching devices.
X. Xu, Pan, Z., Jia, B., Wang, Y., and Chen, R. T., “All-optical switch with 1 ps response time enabled by graphene oxide infiltrated subwavelength grating waveguide,” Silicon Photonics XII, vol. 10108. San Francisco, California, United States, pp. 1010805, 2017. Publisher's VersionAbstract
In this paper, we propose an all-optical switch using graphene oxide (GO) infiltrated subwavelength grating (SWG) waveguide. Benefiting from the extremely large Kerr coefficient of GO (four orders of magnitude larger than conventional materials) and large mode volume overlap factor of the SWG (4~10 times larger than conventional strip waveguides), the switch is capable of achieving THz speed with less than 1 fJ energy consumption per bit, which is more than three orders of magnitude smaller than THz switches reported so far.
K. Chen, Yogeesh, M. N., Huang, Y., Zhang, S., He, F., Meng, X., Fang, S., Sheehan, N., Tao, T. H., Bank, S. R., Lin, J. - F., Akinwande, D., Sutter, P., Lai, T., and Wang, Y., “Non-destructive measurement of photoexcited carrier transport in graphene with ultrafast grating imaging technique,” Carbon, vol. 107, pp. 233-239, 2016. Publisher's VersionAbstract
Graphene has great potential for fabrication of ultrafast opto-electronics, in which relaxation and transport of photoexcited carriers determine device performance. Even though ultrafast carrier relaxation in graphene has been studied vigorously, transport properties of photoexcited carriers in graphene are largely unknown. In this work, we utilize an ultrafast grating imaging technique to measure lifetime (τr), diffusion coefficient (D), diffusion length (L) and mobility (μ) of photoexcited carriers in mono- and multi-layer graphene non-invasively. In monolayer graphene, D∼10,000 cm2/s and μ∼120,000 cm2/V have been observed, both of which decrease drastically in multilayer graphene, indicating that the remarkable transport properties in monolayer graphene originate from its unique Dirac-Cone energy structure. Mobilities of photoexcited carriers measured here are several times larger than the Hall and Field-Effect mobilities reported in literature (<15,000 cm2/V), due to the high energy of photoexcited carriers. Our results indicate the importance of obtaining monolayer graphene to realize high-performance graphene devices, as well as the necessity to use transport properties of photoexcited carriers for predicting the performance of graphene-based opto-electronics.
W. Wu, He, F., and Wang, Y., “Reversible ultrafast melting in bulk CdSe,” Journal of Applied Physics, vol. 119, pp. 055701, 2016. Publisher's VersionAbstract
In this work, transient reflectivity changes in bulk CdSe have been measured with two-color femtosecond pump-probe spectroscopy under a wide range of pump fluences. Three regions of reflectivity change with pump fluences have been consistently revealed for excited carrier density, coherent phonon amplitude, and lattice temperature. For laser fluences from 13 to 19.3 mJ/cm2, ultrafast melting happens in first several picoseconds. This melting process is purely thermal and reversible. A complete phase transformation in bulk CdSe may be reached when the absorbed laser energy is localized long enough, as observed in nanocrystallineCdSe.
Z. Wang, Xu, X., Fan, D., Wang, Y., and Chen, R., “Low-loss Subwavelength Grating Waveguide Bends Based on Index Engineering,” in CLEO: Science and Innovations 2016, San Jose, California, United States, 2016. Publisher's VersionAbstract
We report the design, simulation and experimental demonstration of low loss subwavelength grating waveguide (SWG) bends. With trapezoidal shape silicon pillars, the average insertion loss of a 5μm SWG waveguide bend is reduced drastically from 5.43dB to 1.10dB per 90°bend for quasi-TE polarization.
F. He, Wu, W., and Wang, Y., “Direct measurement of coherent thermal phonons in Bi2Te3/Sb2Te3 superlattice,” Applied Physics A, vol. 122, pp. 777, 2016. Publisher's VersionAbstract
Coherent thermal phonons (CTPs) play an important role in thermal transport in superlattice (SL) structures. To have a profound understanding of CTP transport in SL, direct measurement of CTP properties is necessary. In this study, coherent phonon spectroscopy has been utilized to generate and detect CTP in Bi2Te3/Sb2Te3 SL. Phonon lifetimes have been extracted from experimental data, with which mode-wise thermal conductivities have been calculated. Comparing with bulk Bi2Te3, the estimated mode-wise thermal conductivity of longitudinal acoustic phonons shifts to higher frequencies, due to constructive coherent phonon interference. Our results suggest that it is possible to use SL structure to manipulate coherent phonon propagation and to tailor thermal conductivity.
Z. Wang, Xu, X., Fan, D., Wang, Y., Subbaraman, H., and Chen, R. T., “Geometrical tuning art for entirely subwavelength grating waveguide based integrated photonics circuits,” Scientific Reports, vol. 6, pp. 24106, 2016. Publisher's VersionAbstract
Subwavelength grating (SWG) waveguide is an intriguing alternative to conventional optical waveguides due to the extra degree of freedom it offers in tuning a few important waveguide properties, such as dispersion and refractive index. Devices based on SWG waveguides have demonstrated impressive performances compared to conventional waveguides. However, the high loss of SWG waveguide bends jeopardizes their applications in integrated photonic circuits. In this work, we propose a geometrical tuning art, which realizes a pre-distorted refractive index profile in SWG waveguide bends. The pre-distorted refractive index profile can effectively reduce the mode mismatch and radiation loss simultaneously, thus significantly reduce the bend loss. This geometry tuning art has been numerically optimized and experimentally demonstrated in present study. Through such tuning, the average insertion loss of a 5 μm SWG waveguide bend is reduced drastically from 5.43 dB to 1.10 dB per 90° bend for quasi-TE polarization. In the future, the proposed scheme will be utilized to enhance performance of a wide range of SWG waveguide based photonics devices.
Z. Wang, Xu, X., Fan, D., Wang, Y., and Chen, R. T., “High quality factor subwavelength grating waveguide micro-ring resonator based on trapezoidal silicon pillars,” Optics Letters, vol. 41, no. 14, pp. 3375-3378, 2016. Publisher's VersionAbstract
Subwavelength grating waveguide-based micro-ring resonators (SWGMRs) are a promising platform for research in light–matter interaction. However, it is extremely difficult to achieve small radius SWGMR devices (e.g., 5 μm) with satisfying quality factors (e.g., ∼10,000). One major issue is the large bend loss of small radius SWGMRs. In this work, we report the use of trapezoidal silicon pillars instead of conventional rectangular silicon pillars as building blocks of SWGMRs. We found that an asymmetric effective refractive index profile created by trapezoidal silicon pillars can significantly reduce the bend loss and therefore increase the quality factors of SWGMRs. For the first time to the best of our knowledge, we have experimentally demonstrated a 5 μm radius SWGMR made of trapezoidal silicon pillars (T-SWGMR) with an applicable quality factor as high as 11,500, 4.6 times of that (∼2800) offered by a conventional SWGMR made of rectangular silicon pillars, which indicates an 81.4% reduction of the propagation loss. This approach can also be readily employed to enhance SWGMRs with larger radii. We have also experimentally demonstrated a 10 μm radius T-SWGMR with a quality factor as high as 45,000, which indicates a propagation loss as low as 6.07 dB/cm.
W. Wu and Wang, Y., “

Ultrafast carrier dynamics and coherent acoustic phonons in bulk CdSe

,” Optics Letters, vol. 40, no. 1, pp. 64-67, 2015. Publisher's VersionAbstract
The femtosecond pump-probe technique is used to study the dynamics of photoexcited carriers and coherent acoustic phonons in bulk CdSe semiconductor. A turning point from fast to slow decay is observed, whose amplitude decreases with pump fluences and eventually flips the sign of differential reflectivity. The maximum change of differential reflectivity shows a saturation at high pump fluences, which is attributed to the optical energy gap dependent on carrier density. Long-lasting coherent oscillations of acoustic phonons have also been detected, and their amplitude and lifetime have a strong dependence on pump fluences. Our results can facilitate the understanding of ultrafast carrier and phonon dynamics in CdSe nanocrystals.
J. Covey, Finke, A. D., Xu, X., Wu, W., Wang, Y., Diederich, F., and Chen, R. T., “

All-optical switching with 1-ps response time in a DDMEBT enabled silicon grating coupler/resonator hybrid device

,” OPTICS EXPRESS, vol. 22, no. 20, pp. 24532, 2014. Publisher's Version
Y. Zhang and Wang, Y., “

The Effect of Coherent Optical Phonon on Thermal Transport

,” Appl. Phys. A, 2014.PDF icon PDF
Y. Wang, Guo, L., Xu, X., Pierce, J., and Venkatasubramanian, R. a, “The Origin of Coherent Phonons in Bi2Te3 Excited by Ultrafast Laser Pulses,” Phys. Rev. B, vol. 88, pp. 064307 , 2013.PDF icon PDF
Y. Wang, Qiu, B., McGaughey, A. J. H., Ruan, X., and Xu, X., “Mode-Wise Thermal Conductivity of Bismuth Telluride,” J. of Heat Transfer, vol. 135, pp. 091102 , 2013.PDF icon PDF
Y. Wang and Xu, X., “Molecular dynamics studies of ultrafast laser-induced nonthermal melting,” Appl. Phys. A, vol. 110, pp. 617, 2012.PDF icon PDF
C. Yang, Wang, Y., and Xu, X., “Molecular dynamics studies of ultrafast laser-induced phase and structural change,” International Journal of Heat and Mass Transfer, vol. 55, pp. 6060–6066, 2012.PDF icon PDF
Y. Wang, Yu, X., and Venkatasubramanian, R., “Acoustic Phonon Scattering in Bi2Te3 /Sb2Te3 Superlattices,” Appl. Phys. Lett., vol. 97, pp. 083103, 2010.PDF icon PDF