Non-invasive continuous blood pressure monitoring remains elusive. There has been extensive research using the photoplethysmographic (PPG) waveform for blood pressure estimation, but improvements in accuracy are still needed before clinical use. Here we explored the use of an emerging technique, speckle contrast optical spectroscopy (SCOS), for blood pressure estimation. SCOS provides measurements of both blood volume changes (PPG) and blood flow index (BFi) changes during the cardiac cycle, and thus provides a richer set of parameters compared to traditional PPG. We found that features from the BFi waveforms were more significantly correlated with blood pressure than PPG features. Importantly, we also found that features combining BFi and PPG data were highly correlated with changes in blood pressure.

Speckle contrast optical spectroscopy for cuffless blood pressure estimation based on microvascular blood flow and volume oscillations

Biomedical Optics Express 16, 3004 (2025)

• Featured in Medical Xpress, "Researchers take major step toward cuff-free blood pressure monitoring" (July 2025)

• Featured in Earth.com, "Lasers and light replace the cuff in blood pressure breakthrough" (July 2025)

• Featured as Editor's Pick.

Simultaneous photoplethysmography and blood flow measurements towards the estimation of blood pressure using speckle contrast optical spectroscopy

Biomedical Optics Express 14, 1594 (2023)

Multispeckle diffuse correlation spectroscopy (DCS) is an indispensable tool for quantifying cerebral blood flow noninvasively by measuring the time statistics of the diffused light. However, the extremely high data rate from large array SPAD cameras goes beyond the data transfer rate commonly available and requires specialized high-performance computation to calculate large number of autocorrelators for real-time measurements. Here we used data compression scheme on-chip  (ATLAS, 2024) and in the readout field-programmable gate array (FPGA, 2023) of a large-pixel-count SPAD camera. This capability should democratize SPAD cameras and streamline system integration for multispeckle DCS.

ATLAS: A large array, on-chip compute SPAD camera for multispeckle diffuse correlation spectroscopy. Biomedical Optics Express 15, 6499 (2024)

• Featured in Special Issue, "Diffuse Optical Spectroscopy: Technology and Applications: introduction to the feature issue" (Oct 2024)

Field programmable gate array compression for large array multispeckle diffuse correlation spectroscopy. Journal of Biomedical Optics 28, 057001 (2023)

• Featured in SPIE News, "Data compression scheme facilitates measurement of blood flow to the brain" (May 2023)

Massively parallel, real-time multispeckle diffuse correlation spectroscopy using a 500x500 SPAD camera. Biomedical Optics Express 14, 703 (2023)

• Featured as Editor's Pick.

Speckle contrast optical spectroscopy (SCOS) is a noninvasive diffuse optical method for measuring microvascular blood flow in tissue by quantifying the spatial contrast of the laser speckle patterns. The state-of-the-art SCOS model however has yet to specify the impact of experimentally controllable parameters on the signal to noise ratio (SNR) of the measurements. Here we present a comprehensive noise model for a fiber based SCOS measurements, namely dynamic speckle model (DSM), that simulates the temporal evolution of speckle patterns and predicted the SNR of the measurements, to which speckle to pixel size ratio, shot noise, camera dark and read noise were included, and experimentally validated.

Model of dynamic speckle evolution for evaluating laser speckle contrast measurements of tissue dynamics

Biomedical Optics Express 13, 6533 (2022)

Variations in blood flow can be used to detect neuronal activities but its peak has a latency of a few seconds which is slow for real-time monitoring. Neuronal cells also deform during activation, which in principle can be utilized to detect neuronal activity on fast time-scales using diffuse correlation spectroscopy (DCS). Here we modeled the variations in the DCS signal that are expected to arise from neuronal activation using Monte Carlo simulations, including the impacts of neuronal cell motion, vessel wall dilation, and blood flow changes.

Measuring neuronal activity with diffuse correlation spectroscopy: a theoretical investigation

Neurophotonics 8, 035004 (2021)

A phonoriton is an elementary excitation in solids that is predicted to emerge from hybridization between exciton, phonon, and photon. Finding ways to create phonoritons and control over their properties is of great interest because they could serve as functional nodes in devices that utilize electronic, phononic, and photonic elements for energy conversion and thermal transport applications. Here, we predicted the emergence of phonoritons in a monolayer hexagonal boron nitride optical cavity.

Phonoritons in a monolayer h-BN optical cavity

Physical Review Letters 126, 227401 (2021)

• Featured in MPSD News, "Team hunts down elusive phonoriton with light trapped in a cavity" (Jun 2021)

Cerebral blood flow is an important biomarker of brain functions and health as it regulates the delivery of oxygen and key substrates to tissue. Moreover, blood flow changes in specific areas of the brain are correlated with neuronal activity in those areas, a key signature for neuroscience applications. Diffuse correlation spectroscopy (DCS) is a promising noninvasive optical technique for monitoring cerebral blood flow and for measuring cortex functional activation. However, the current state-of-the-art DCS adoption is hindered by a trade-off between sensitivity to the cortex and signal-to-noise ratio (SNR). Here we developed a scalable method that significantly increases the sensitivity of DCS instruments through measuring a thousand of speckles in parallel using a single-photon avalanche diode (SPAD) camera, thus enabling both high sensitivity to the brain cortex and high SNR. Our approach is scalable to even higher SNR since large-pixel-count SPAD cameras are becoming available, owing to the investments in LiDAR technology for automotive and augmented reality applications.

High-sensitivity multispeckle diffuse correlation spectroscopy

Neurophotonics 7, 035010 (2020)

• Featured in the Most Viewed article in Neurophotonics

• Featured in Facebook Research page

The electronic band structure of materials can be mapped out using angle-resolved photoemission spectroscopy (ARPES) and, when carried out using a time-resolved laser capability, it provides invaluable insights into the electron dynamics on femtosecond timescales. However, it has been difficult to access high momenta electrons with narrow energy resolution via laser-based ARPES. Here, we develop a new tabletop ARPES technique by combining a gas phase extreme ultraviolet (XUV) femtosecond light source, an XUV monochromator, and a time-of-flight (TOF) electron analyzer to develop this technique. Our technique can produce tunable photon energy between 24-33 eV with an unprecedented energy resolution of 30 meV and time resolution of 200 femtoseconds. This enables full access to the electronic band structure of all materials with narrow energy resolution on the femtosecond timescales.

Time-resolved XUV ARPES with tunable 24-33 eV laser pulses at 30 meV resolution

Nature Communications 10, 3535 (2019)

• Featured in MIT News, "Watching electrons using extreme ultraviolet light" (Aug 2019)

The electronic band structure of materials can be mapped out using angle-resolved photoemission spectroscopy (ARPES) and, when carried out using a time-resolved laser capability, it provides invaluable insights into the electron dynamics on femtosecond timescales. However, it has been difficult to access high momenta electrons with narrow energy resolution via laser-based ARPES. Here, we develop a new tabletop ARPES technique by combining a gas phase extreme ultraviolet (XUV) femtosecond light source, an XUV monochromator, and a time-of-flight (TOF) electron analyzer to develop this technique. Our technique can produce tunable photon energy between 24-33 eV with an unprecedented energy resolution of 30 meV and time resolution of 200 femtoseconds. This enables full access to the electronic band structure of all materials with narrow energy resolution on the femtosecond timescales.

Time-resolved XUV ARPES with tunable 24-33 eV laser pulses at 30 meV resolution

Nature Communications 10, 3535 (2019)

• Featured in MIT News, "Watching electrons using extreme ultraviolet light" (Aug 2019)

The emergence of massless Weyl fermions in some quantum materials results from the breaking of inversion symmetry. Hence, it can be sensitive to atomic-scale lattice distortions. Here we used terahertz light pulses to induce transitions between a topological and a trivial phase in the Weyl semimetal WTe2 through an interlayer shear strain. The structural changes were crystallographically measured using relativistic ultrafast electron diffraction, with sensitivity down to subpicometer length scale and subpicosecond time scale. The seemingly subtle interlayer displacement can lead to a two-fold increase of Weyl points separation. This method provides an ultrafast, energy-efficient means of manipulating the topological properties of materials.

An ultrafast symmetry switch in a Weyl semimetal

Nature 565, 61 (2019)

• Featured in News & Views, "Topological properties controlled by light", by Young-Woo Son (Jan 2019)

• Featured in SLAC News, "An ultrafast light switch for exotic materials", by Manuel Gnida (Jan 2019)

• Featured in Physics World, "Topological quantum materials switch up a gear", by Anna Demming (Jan 2019)

• Featured in APS March Meeting 2020 Invited Talk, presented by Prof. Aaron Lindenberg

The thesis work during my PhD years at MIT was recognized with a APS Richard L. Greene Dissertation Award (2019) and a Springer Nature Thesis Award (2017). Topics include the valley properties of monolayer transition-metal dichalcogenides, transient absorption spectroscopy, valley-selective optical Stark effect, Bloch-Siegert shift, intervalley biexcitons, biexcitonic optical Stark effect, Lennard-Jones potential for 2D excitons, and development of XUV-based time-resolved ARPES to probe large-wavevector electrons in quantum materials.

Coherent light-matter interactions in monolayer transition-metal dichalcogenides

Springer (2017)

• APS Richard L. Greene Dissertation Award in Experimental Condensed Matter Physics (2019)

• Springer Nature Thesis Award (2017)

Excitons have been perceived as the solid-state counterpart of atoms, but the analogy is usually drawn only for their similar internal structure. Here we observe that excitons in monolayer WS2 also exhibit mutual interactions where they effectively attract each other at long distance and repel at short distance. This mimics the Lennard-Jones potential between atoms despite the inherently complex many-body effects in solids that has eluded direct observation so far. 

Observation of exciton redshift-blueshift crossover in monolayer WS2

Nano Letters 17, 4210 (2017)

We observe a very rare phenomenon of the so-called "Bloch-Siegert shift" in solids. Monolayers WS2 have two valleys in their electronic structure. Shining visible light on these materials can modify the electronic levels in one valley but not the other. We found that in the infrared, if the frequency of the light was far away from the resonance, energy levels in both valleys are affected. The Bloch-Siegert effect could explain the shift in the "forbidden" valley. -Editor's Summary

Large, valley-exclusive Bloch-Siegert shift in monolayer WS2

Science 355, 1066 (2017)

• Featured in MIT News, "Toward valleytronic devices for data storage ....", by Helen Knight (Mar 2017)

• Featured in APS March Meeting 2018 Invited Talk, presented by Prof. Nuh Gedik

We observe a new type of optical Stark effect in monolayer WS2, one that is mediated by intervalley biexcitons under the blue-detuned driving with circularly polarized light. We find that such helical optical driving not only induces an exciton energy downshift at the excitation valley, but also causes an anomalous energy upshift at the opposite valley, which is normally forbidden by the exciton selection rules but now made accessible through the intervalley biexcitons.

Observation of intervalley biexcitonic optical Stark effect in monolayer WS2

Nano Letters 16, 7421 (2016)

Two excitons can bind to form a biexciton. It has orbital and spin states resembling those in diatomic molecules. In monolayer MoS2, the particle excitations can acquire a new valley degree of freedom from which a novel intervalley biexciton can be created. These biexcitons comprise two excitons from different valleys, which are distinct from those in conventional semiconductors and have no direct analog in atomic and molecular systems. In this work, we report the observation of these intervalley biexcitons that are generated coherently using ultrafast optical excitation.

Intervalley biexcitons and many-body effects in monolayer MoS2

Physical Review B 92, 125417 (2015)

Monolayer semiconductors such as WS2 have two different valleys in their electronic structures with corresponding energy gaps that are normally locked in equal magnitude. Separating these valleys in energy is of great interest because it would allow for control in valleytronic applications to carry information. In this work, we demonstrated that circularly polarized light can be used to shift the energy of one valley with respect to the other by means of the optical Stark effect. In particular, the effect was used to raise the exciton level in monolayer WS2 in a controllable valley selective manner. This result may provide a means to realize a new Floquet topological state of matter.

Valley-selective optical Stark effect in monolayer WS2

Nature Materials 14, 290 (2015)

• Selected for Cover Story in Nature Materials

• Featured in MIT News, "New findings could point the way to valleytronics", by David L. Chandler (Dec 2014)

• Featured in MIT News and MPC News, "Exploring valleytronics", by Denis Paiste (Dec 2015)

• Featured in IEEE Spectrum

• Featured in APS March Meeting 2016 Invited Talk, presented by Prof. Nuh Gedik