Bachelor of Engineering, Hong Kong University Of Science & Technology (2010)
Doctor of Philosophy, Columbia University (2014)
Steven Chu, Postdoctoral Faculty Sponsor
Difference-Frequency Ultrasound Imaging With Non-Linear Contrast
IEEE TRANSACTIONS ON MEDICAL IMAGING
2020; 39 (5): 1759–66
Conventional ultrasound imaging is based on the scattering of sound from inhomogeneities in the density and the speed of sound and is often used in medicine to resolve pathologic compared to normal tissue. Here we demonstrate a difference-frequency ultrasound (dfUS) imaging method that is based on the interaction of two sound pulses that propagate non-collinearly and intersect in space and time. The dfUS signal arises primarily from the second-order non-linear coefficient, a contrast mechanism that differs from linear and harmonic US imaging. The distinct contrast mechanism allows dfUS to image anatomic features that are not identifiable in conventional US images of salmon and pig kidney tissue. Further, dfUS produces enhanced contrast of glioblastoma tumor implanted in the mouse brain, revealing its potential for improving medical diagnosis. Progress towards a real-time system is discussed.
View details for DOI 10.1109/TMI.2019.2957280
View details for Web of Science ID 000532214700043
View details for PubMedID 31804930
Optimization of the Trade-Off Between Speckle Reduction and Axial Resolution in Frequency Compounding
IEEE TRANSACTIONS ON MEDICAL IMAGING
2019; 38 (1): 107–12
We measured the reduction of speckle by frequency compounding using Gaussian pulses, which have the least time-bandwidth product. The experimental results obtained from a tissue mimicking phantom agree quantitatively with numerical simulations of randomly distributed point scatterers. For a fixed axial resolution, the amount of speckle reduction is found to approach a maximum as the number of bands increases while the total spectral range that they cover is kept constant. An analytical solution of the maximal speckle reduction is derived and shows that the maximum improves approximately as the inverse square root of the Gaussian pulse bandwidth. Since the axial resolution is proportional to the inverse of the pulse bandwidth, an optimized trade-off between speckle reduction and axial resolution is obtained. Considerations for the applications of the optimized trade-off are discussed.
View details for DOI 10.1109/TMI.2018.2856857
View details for Web of Science ID 000455110500011
View details for PubMedID 30028694
Optical Imaging and Spectroscopic Characterization of Self-Assembled Environmental Adsorbates on Graphene
2018; 18 (4): 2603–8
Topographic studies using scanning probes have found that graphene surfaces are often covered by micron-scale domains of periodic stripes with a 4 nm pitch. These stripes have been variously interpreted as structural ripples or as self-assembled adsorbates. We show that the stripe domains are optically anisotropic by imaging them using a polarization-contrast technique. Optical spectra between 1.1 and 2.8 eV reveal that the anisotropy in the in-plane dielectric function is predominantly real, reaching 0.6 for an assumed layer thickness of 0.3 nm. The spectra are incompatible with a rippled graphene sheet but would be quantitatively explained by the self-assembly of chainlike organic molecules into nanoscale stripes.
View details for DOI 10.1021/acs.nanolett.8b00348
View details for Web of Science ID 000430155900057
View details for PubMedID 29589951
- Two-dimensional models for the optical response of thin films 2D MATERIALS 2018; 5 (2)
- High-harmonic generation from an atomically thin semiconductor NATURE PHYSICS 2017; 13 (3): 262-?
High-order harmonics from bulk and 2D crystals
View details for Web of Science ID 000432564600541
- Ultrasensitive Plasmonic Detection of Molecules with Graphene ACS PHOTONICS 2016; 3 (4): 553-557
Measurement of Lateral and Interfacial Thermal Conductivity of Single- and Bilayer MoS2 and MoSe2 Using Refined Optothermal Raman Technique.
ACS applied materials & interfaces
2015; 7 (46): 25923-25929
Atomically thin materials such as graphene and semiconducting transition metal dichalcogenides (TMDCs) have attracted extensive interest in recent years, motivating investigation into multiple properties. In this work, we demonstrate a refined version of the optothermal Raman technique to measure the thermal transport properties of two TMDC materials, MoS2 and MoSe2, in single-layer (1L) and bilayer (2L) forms. This new version incorporates two crucial improvements over previous implementations. First, we utilize more direct measurements of the optical absorption of the suspended samples under study and find values ∼40% lower than previously assumed. Second, by comparing the response of fully supported and suspended samples using different laser spot sizes, we are able to independently measure the interfacial thermal conductance to the substrate and the lateral thermal conductivity of the supported and suspended materials. The approach is validated by examining the response of a suspended film illuminated in different radial positions. For 1L MoS2 and MoSe2, the room-temperature thermal conductivities are 84 ± 17 and 59 ± 18 W/(m·K), respectively. For 2L MoS2 and MoSe2, we obtain values of 77 ± 25 W and 42 ± 13 W/(m·K). Crucially, the interfacial thermal conductance is found to be of order 0.1-1 MW/m(2) K, substantially smaller than previously assumed, a finding that has important implications for design and modeling of electronic devices.
View details for DOI 10.1021/acsami.5b08580
View details for PubMedID 26517143
- Photonic and Plasmonic Guided Modes in Graphene-Silicon Photonic Crystals ACS PHOTONICS 2015; 2 (11): 1552-1558
Bright visible light emission from graphene
2015; 10 (8): 676-681
Graphene and related two-dimensional materials are promising candidates for atomically thin, flexible and transparent optoelectronics. In particular, the strong light-matter interaction in graphene has allowed for the development of state-of-the-art photodetectors, optical modulators and plasmonic devices. In addition, electrically biased graphene on SiO2 substrates can be used as a low-efficiency emitter in the mid-infrared range. However, emission in the visible range has remained elusive. Here, we report the observation of bright visible light emission from electrically biased suspended graphene devices. In these devices, heat transport is greatly reduced. Hot electrons (∼2,800 K) therefore become spatially localized at the centre of the graphene layer, resulting in a 1,000-fold enhancement in thermal radiation efficiency. Moreover, strong optical interference between the suspended graphene and substrate can be used to tune the emission spectrum. We also demonstrate the scalability of this technique by realizing arrays of chemical-vapour-deposited graphene light emitters. These results pave the way towards the realization of commercially viable large-scale, atomically thin, flexible and transparent light emitters and displays with low operation voltage and graphene-based on-chip ultrafast optical communications.
View details for DOI 10.1038/NNANO.2015.118
View details for PubMedID 26076467