arXiv subjects

Wen-Hao Chang

Publications and source records attributed to Wen-Hao Chang.

At least 19 recordsLinked to original sources

arXiv2025

Cryogenic scanning photocurrent spectroscopy for materials responses to structured optical fields

Circular dichroism spectroscopy is known to provide important insights into the interplay of different degrees of freedom in quantum materials, and yet spectroscopic study of the optoelectronic responses of quantum materials to structured optical fields, such as light with finite spin and orbital angular momentum, has not yet been widely explored, particularly at cryogenic temperature. Here we demonstrate the design and application of a novel instrument that integrates scanning spectroscopic photocurrent measurements with structured light of controlled spin and orbital angular momentum. For structured photons with wavelengths between 500 nm to 700 nm, this instrument can perform spatially resolved photocurrent measurements of two-dimensional materials or thin crystals under magnetic fields up to $\pm$ 14 Tesla, at temperatures from 300 K down to 3 K, with either spin angular momentum $\pm \hbar$ ororbital angular momentum $\pm \ell \hbar$ (where $\ell$=1,2,3... is the topological charge), and over a (35 $\times$ 25) $μm^2$ area with ~ 1 $μm$ spatial resolution. These capabilities of the instrument are exemplified by magneto-photocurrent spectroscopic measurements of monolayer 2H-$MoS_2$ field-effect transistors, which not only reveal the excitonic spectra but also demonstrate monotonically increasing photocurrents with increasing |$\ell $| as well as excitonic Zeeman splitting and an enhanced Landé g-factor due to the enhanced formation of intervalley dark excitons under magnetic field. These studies thus demonstrate the versatility of the scanning photocurrent spectrometry for investigating excitonic physics, optical selection rules, and optoelectronic responses of novel quantum materials and engineered quantum devices to structured light.

cond-mat.other↗

arXiv2025

Indistinguishable Single Photons from Nanowire Quantum Dots in the Telecom O-Band

On-demand single-photon sources operating at telecom wavelengths are crucial for quantum communication and photonic quantum technologies. In this work, we demonstrate high-purity, indistinguishable single-photon generation in the telecom O-band from an InAsP/InP nanowire quantum dot. We measured a single-photon purity of $g^2(0)=0.006(3)$ under aboveband excitation. Furthermore, we characterize two-photon interference via Hong-Ou-Mandel measurements and achieve a photon indistinguishability of $94.6\,\%$ with a temporal postselection of 100 ps time window and $5.58\,\%$ without temporal postselection. We measure a first-lens source efficiency of $\sim28\,\%$. These results highlight the potential of nanowire quantum dots as a promising source of telecom single photons for photonic quantum applications, offering deterministic positioning, efficient photon extraction, and scalable production.

quant-ph↗

arXiv2025

On-demand generation of entangled photons pairs in the telecom O-band from nanowire quantum dots

On-demand entangled photon pairs at telecom wavelengths are crucial for quantum communication, distributed quantum computing, and quantum-enhanced sensing and metrology. The O-band is particularly advantageous because of its minimal chromatic dispersion and transmission loss in optical fibers, making it well-suited for long-distance quantum networks. Site-controlled nanowire quantum dots have emerged as a promising platform for the on-demand generation of single and entangled photons, offering high extraction efficiency and the potential for scalable fabrication in large uniform arrays. However, their operation has been largely restricted to the visible and first near-infrared (NIR-I) windows. Here, we demonstrate an on-demand bright source of entangled photon pairs with high fidelity in the telecom O-band based on site-controlled nanowire quantum dots. We measure a fine-structure splitting of 4.6 $μ$eV, verifying the suitability of the quantum dot for generating high-fidelity polarization-entangled photon pairs. Full quantum state tomography of the two-photon state generated by the biexciton\hyph exciton cascade reveals a maximum fidelity of $85.8\% \pm 1.1\%$ to the $Φ^+$ Bell state, and a maximum concurrence of $75.1\% \pm 2.1\%$. We estimate the source efficiency at the first lens to be 12.5$\%$. This bright, scalable, and deterministic source of entangled photons in the telecom range represents a valuable step forward in advancing practical quantum applications at telecom wavelengths.

quant-ph↗

arXiv2024

Magnetic Field-Induced Polar Order in Monolayer Molybdenum Disulfide Transistors

In semiconducting monolayer transition metal dichalcogenides (ML-TMDs), broken inversion symmetry and strong spin-orbit coupling result in spin-valley lock-in effects so that the valley degeneracy may be lifted by external magnetic fields, potentially leading to real-space structural transformation. Here, we report magnetic field (B)-induced giant electric hysteretic responses to back-gate voltages in ML-MoS2 field-effect transistors (FETs) on SiO2/Si at temperatures < 20 K. The observed hysteresis increases with |B| up to 12 T and is tunable by varying the temperature. Raman spectroscopic and scanning tunneling microscopic studies reveal significant lattice expansion with increasing |B| at 4.2 K, and this lattice expansion becomes asymmetric in ML-MoS2 FETs on rigid SiO2/Si substrates, leading to out-of-plane mirror symmetry breaking and the emergence of a tunable out-of-plane ferroelectric-like polar order. This broken symmetry-induced polarization in ML-MoS2 shows typical ferroelectric butterfly hysteresis in piezo-response force microscopy, adding ML-MoS2 to the single-layer material family that exhibit out-of-plane polar order-induced ferroelectricity, which is promising for such technological applications as cryo-temperature ultracompact non-volatile memories, memtransistors, and ultrasensitive magnetic field sensors. Moreover, the polar effect induced by asymmetric lattice expansion may be further generalized to other ML-TMDs and achieved by nanoscale strain engineering of the substrate without magnetic fields.

cond-mat.str-el↗

arXiv2023

Dielectric breakdown and sub-wavelength patterning of monolayer hexagonal boron nitride using femtosecond pulses

Hexagonal boron nitride (hBN) has emerged as a promising two-dimensional (2D) material for many applications in photonics. Although its linear and nonlinear optical properties have been extensively studied, its interaction with high-intensity laser pulses, which is important for high-harmonic generation, fabricating quantum emitters, and maskless patterning of hBN, has not been investigated. Here we report the first study of dielectric breakdown in hBN monolayers induced by single femtosecond laser pulses. We show that hBN has the highest breakdown threshold among all existing 2D materials. This enables us to observe clearly for the first time a linear dependence of breakdown threshold on the bandgap energy for 2D materials, demonstrating such a linear dependency is a universal scaling law independent of the dimensionality. We also observe counter-intuitively that hBN, which has a larger bandgap and mechanical strength than quartz, has a lower breakdown threshold. This implies carrier generation in hBN is much more efficient. Furthermore, we demonstrate the clean removal of hBN without damage to the surrounding hBN film or the substrate, indicating that hBN is optically very robust. The ablated features are shown to possess very small edge roughness, which is attributed to its ultrahigh fracture toughness. Finally, we demonstrate femtosecond laser patterning of hBN with sub-wavelength resolution, including an isolated stripe width of 200 nm. Our work advances the knowledge of light-hBN interaction in the strong field regime and firmly establishes femtosecond lasers as novel and promising tools for one-step deterministic patterning of hBN monolayers.

physics.optics↗

arXiv2023

Tomography Scan of Charge Density Wave in NbSe2

Charge density wave (CDW) resulted from a small distortion in the lattice is able to create new orders beyond the original lattice. In 2H-NbSe2, one of the layered transition metal dichalcogenides (TMD), the 3x3 charge order appears in two-dimensional (2D) layers. Although CDW is usually described by a sine wave, the spatial distribution within a 2D layer has never been systematically visualized. Here by using scanning tunneling microscopy (STM) and density functional theory (DFT), we have monitored the evolution of 3x3 CDW along c-axis and realized a nearly tomography scan of CDW of the topmost layer. The results show that the strength of 3x3 charge order varies while increasing the tunneling current. The 3x3 charge order is relatively strong at the outermost Se level and decreases while probing in between Se and Nb levels. Interestingly, the 3x3 charge order gets strong again as reaching Nb level but along with a phase shift. We further calculated the orbital charge distributions and found that both CDW intensity modulation and phase shift are strongly correlated with the distribution of Se p orbitals and Nb d orbitals.

physics.app-ph↗

arXiv2022

Efficient Multiple Exciton Generation in Monolayer MoS2

Utilizing the excess energy of photoexcitation that is otherwise lost as thermal effects can improve the efficiency of next-generation light-harvesting devices. Multiple exciton generation (MEG) in semiconducting materials yields two or more excitons by absorbing a single high-energy photon, which can break the Shockley-Queisser limit for the conversion efficiency of photovoltaic devices. Recently, monolayer transition metal dichalcogenides (TMDs) have emerged as promising light-harvesting materials because of their high absorption coefficient. Here, we report efficient MEG with low threshold energy and high (86%) efficiency in a van der Waals (vdW) layered material, MoS2. Through different experimental approaches, we demonstrate the signature of exciton multiplication and discuss the possible origin of decisive MEG in monolayer MoS2. Our results reveal that vdW-layered materials could be a potential candidate for developing mechanically flexible and highly efficient next generation solar cells and photodetectors.

cond-mat.mtrl-sci↗

arXiv2022

Homoepitaxy of rhombohedral-stacked MoS2 with room temperature switchable ferroelectricity

The discovery of interfacial ferroelectricity in two-dimensional rhombohedral (3R)-stacked semiconductors opens up a new pathway for achieving ultrathin computing-in-memory devices. However, exploring ferroelectricity switching in natural 3R crystals is difficult due to lack of co-existing 3R stacking domains. Here, we present that MoS2 homoepitaxial patterns with 3R polytypic domains can manifest switchable ferroelectricity at room-temperature. Based on the diffusion limited aggregation theory, such MoS2 patterns are formed under the low Mo chemical potential and low temperature with respect to common chemical vapor deposition synthesis. The alternation of 3R polytypes in the MoS2 homoepitaxial patterns, observed by scanning transmission electron microscopy, accounts for ferroelectricity switching. The MoS2 field-effect transistors with 3R polytypic domains exhibit a repeatable counterclockwise hysteresis with gate voltage sweeping, an indication of ferroelectricity switching, and the memory window exceeds those measured for compact-shaped 3R bilayer devices. This work provides a direct growth concept for layered 3R-based ferroelectric memory.

cond-mat.mtrl-sci↗

arXiv2022

Exciton-Driven Renormalization of Quasiparticle Band Structure in Monolayer MoS2

Optical excitation serves as a powerful approach to control the electronic structure of layered Van der Waals materials via many-body screening effects, induced by photoexcited free carriers, or via light-driven coherence, such as optical Stark and Bloch-Siegert effects. Although theoretical work has also pointed to an exotic mechanism of renormalizing band structure via excitonic correlations in bound electron-hole pairs (excitons), experimental observation of such exciton-driven band renormalization and the full extent of their implications is still lacking, largely due to the limitations of optical probes and the impact of screening effects. Here, by using extreme-ultraviolet time-resolved angle-resolved photoemission spectroscopy together with excitonic many-body theoretical calculations, we directly unmask the band renormalization effects driven by excitonic correlations in a monolayer semiconductor. We revealed a surprising bandgap opening, increased by 40 meV, and a simultaneous enhancement of band effective mass. Our findings unmask the novel exciton-driven mechanism towards the band engineering in photoexcited semiconducting materials, opening a new playground to manipulate the transient energy states in layered quantum materials via optical controls of excitonic many-body correlations.

cond-mat.mtrl-sci↗

arXiv2022

Quantitative determination of interlayer electronic coupling at various critical points in bilayer MoS2

Tailoring interlayer coupling has emerged as a powerful tool to tune the electronic structure of van der Waals (vdW) bilayers. One example is the usage of the moire pattern to create controllable two-dimensional electronic superlattices through the configurational dependence of interlayer electronic couplings. This approach has led to some remarkable discoveries in twisted graphene bilayers, and transition metal dichalcogenide (TMD) homo- and hetero-bilayers. However, a largely unexplored factor is the interlayer distance, d, which can impact the interlayer coupling strength exponentially. In this letter, we quantitatively determine the coupling strengths as a function of interlayer spacing at various critical points of the Brillouin zone in bilayer MoS2. The exponential dependence of the coupling parameter on the gap distance is demonstrated. Most significantly, we achieved a 280% enhancement of K-valley coupling strength with an 8% reduction of the vdW gap, pointing to a new strategy in designing a novel electronic system in vdW bilayers.

cond-mat.mtrl-sci↗

arXiv2021

Ultrafast Multi-Shot Ablation and Defect Generation in Monolayer Transition Metal Dichalcogenides

Transition metal dichalcogenides are known to possess large optical nonlinearities and driving these materials at high intensities is desirable for many applications. Understanding their optical responses under repetitive intense excitation is essential to improve the performance limit of these strong-field devices and to achieve efficient laser patterning. Here, we report the incubation study of monolayer MoS${}_{2}$ and WS${}_{2}$ induced by 160 fs, 800 nm pulses in air to examine how their ablation threshold scales with the number of admitted laser pulses. Both materials were shown to outperform graphene and most bulk materials; specifically, MoS${}_{2}$ is as resistant to radiation degradation as the best of the bulk thin films with a record fast saturation. Our modeling provides convincing evidence that the small reduction in threshold and fast saturation of MoS${}_{2}$ originates in its excellent bonding integrity against radiation-induced softening. Sub-ablation damages, in the forms of vacancies, lattice disorder, and nano-voids, were revealed by transmission electron microscopy, photoluminescence, Raman, and second harmonic generation studies, which were attributed to the observed incubation. For the first time, a sub-ablation damage threshold is identified for monolayer MoS${}_{2}$ to be 78% of single-shot ablation threshold, below which MoS${}_{2}$ remains intact for many laser pulses. Our results firmly establish MoS${}_{2}$ as a robust material for strong-field devices and for high-throughput laser patterning.

physics.optics↗

arXiv2021

Ultrafast Laser Ablation, Intrinsic Threshold, and Nanopatterning of Monolayer Molybdenum Disulfide

Laser direct writing is an attractive method for patterning 2D materials without contamination. Literature shows that the femtosecond ablation threshold of graphene across substrates varies by an order of magnitude. Some attribute it to the thermal coupling to the substrates, but it remains by and large an open question. For the first time the effect of substrates on femtosecond ablation of 2D materials is studied using MoS$_{2}$ as an example. We show unambiguously that femtosecond ablation of MoS$_{2}$ is an adiabatic process with negligible heat transfer to the substrates. The observed threshold variation is due to the etalon effect which was not identified before for the laser ablation of 2D materials. Subsequently, an intrinsic ablation threshold is proposed as a true threshold parameter for 2D materials. Additionally, we demonstrate for the first time femtosecond laser patterning of monolayer MoS$_{2}$ with sub-micron resolution and mm/s speed. Moreover, engineered substrates are shown to enhance the ablation efficiency, enabling patterning with low-power femtosecond oscillators. Finally, a zero-thickness approximation is introduced to predict the field enhancement with simple analytical expressions. Our work clarifies the role of substrates on ablation and firmly establishes femtosecond laser ablation as a viable route to pattern 2D materials.

physics.optics↗

arXiv2021

Single-crystal hexagonal boron nitride monolayer epitaxially grown on Cu (111) thin film across a wafer

We demonstrate single crystal growth of wafer-scale hexagonal boron nitride (hBN), an insulating atomic thin monolayer, on high-symmetry index surface plane Cu(111). The unidirectional epitaxial growth is guaranteed by large binding energy difference, ~0.23 eV, between A- and B-steps edges on Cu(111) docking with B6N7 clusters, confirmed by density functional theory calculations.

cond-mat.mtrl-sci↗

arXiv2020

Momentum resolved ground/excited states and the ultra-fast excited state dynamics of monolayer MoS2

The emergence of transition metal dichalcogenides (TMD) as crystalline atomically thin semiconductors has created a tremendous amount of scientific and technological interest. Many novel device concepts have been proposed and realized (1-3). Nonetheless, progress in k-space investigations of ground/excited state electronic structures has been slow due to the challenge to create large scale, laterally homogeneous samples. Taking advantage of recent advancements in chemical vapor deposition, here we create a wafer-size MoS2 monolayer with well-aligned lateral orientation for advanced electron spectroscopy studies (4-6). Low energy electron diffraction and scanning tunneling microscopy (STM) demonstrate atomically clean surfaces with in-plane crystalline orientation. The ground state and excited state electronic structures are probed using scanning tunneling spectroscopy (STS), angle-resolved photoemission (ARPES) and time-resolved (tr-)ARPES. In addition to mapping out the momentum-space quasiparticle band structure in the valence and conduction bands, we unveil ultrafast excited state dynamics, including inter- and intra-valley carrier scattering and a rapid downward energy shift by ~ 0.2eV lower than the initial free carrier state at Sigma point.

cond-mat.mes-hall↗

arXiv2020

Ultralow Schottky Barriers in hBN-Encapsulated Monolayer WSe$_2$ Tunnel Field-Effect Transistors

To explore the potential of field-effect transistors (FETs) based on monolayers of the two-dimensional semiconducting channel(SC) for spintronics, the two most important issues are to ensure the formation of variable low resistive tunnel ferromagnetic contacts(FC), and to preserve intrinsic properties of the SC during fabrication. Large Schottky barriers lead to the formation of high resistive contacts and methods adopted to control the barriers often alter the intrinsic properties of the SC. This work aims at addressing both issues in fully encapsulated monolayer WSe$_2$ FETs by using bi-layer h-BN as a tunnel barrier at the FC/SC interface. We investigate the electrical transport in monolayer WSe$_2$ FETs with current-in-plane geometry that yields hole mobilities $\sim$ 38.3 $cm^{2}V^{-1}s^{-1}$ at 240 K and On/Off ratios of the order of 10$^7$, limited by the contact regions. We have achieved ultralow effective Schottky barrier ($\sim$ 5.34 meV) with encapsulated tunneling device as opposed to a non-encapsulated device in which the barrier heights are considerably higher. These observations provide an insight into the electrical behavior of the FC/h-BN/SC/h-BN heterostructures and such control over the barrier heights opens up the possibilities for WSe$_2$-based spintronic devices.

physics.app-ph↗

arXiv2019

Moiré Potential Impedes Interlayer Exciton Diffusion in van der Waals Heterostructures

The properties of van der Waals (vdW) heterostructures are drastically altered by a tunable moiré superlattice arising from periodic variations of atomic alignment between the layers. Exciton diffusion represents an important channel of energy transport in semiconducting transition metal dichalcogenides (TMDs). While early studies performed on TMD heterobilayers have suggested that carriers and excitons exhibit long diffusion lengths, a rich variety of scenarios can exist. In a moiré crystal with a large supercell size and deep potential, interlayer excitons may be completely localized. As the moiré period reduces at a larger twist angle, excitons can tunnel between supercells and diffuse over a longer lifetime. The diffusion length should be the longest in commensurate heterostructures where the moiré superlattice is completely absent. In this study, we experimentally demonstrate that the moiré potential impedes interlayer exciton diffusion by comparing a number of WSe2/MoSe2 heterostructures prepared with chemical vapor deposition and mechanical stacking with accurately controlled twist angles. Our results provide critical guidance to developing 'twistronic' devices that explore the moiré superlattice to engineer material properties.

cond-mat.mes-hall↗

arXiv2019

Tailoring excitonic states of van der Waals bilayers through stacking configuration, band alignment and valley-spin

Excitons in monolayer semiconductors have large optical transition dipole for strong coupling with light field. Interlayer excitons in heterobilayers, with layer separation of electron and hole components, feature large electric dipole that enables strong coupling with electric field and exciton-exciton interaction, at the cost that the optical dipole is substantially quenched (by several orders of magnitude). In this letter, we demonstrate the ability to create a new class of excitons in transition metal dichalcogenide (TMD) hetero- and homo-bilayers that combines the advantages of monolayer- and interlayer-excitons, i.e. featuring both large optical dipole and large electric dipole. These excitons consist of an electron that is well confined in an individual layer, and a hole that is well extended in both layers, realized here through the carrier-species specific layer-hybridization controlled through the interplay of rotational, translational, band offset, and valley-spin degrees of freedom. We observe different species of such layer-hybridized valley excitons in different heterobilayer and homobilayer systems, which can be utilized for realizing strongly interacting excitonic/polaritonic gases, as well as optical quantum coherent controls of bidirectional interlayer carrier transfer either with upper conversion or down conversion in energy.

cond-mat.mes-hall↗

arXiv2018

Single Crystalline Silver Films for Plasmonics: From Monolayer to Optically Thick Film

Epitaxial growth of single crystalline noble metals on dielectric substrates has received tremendous attention recently due to their technological potentials as low loss plasmonic materials. Currently there are two different growth approaches, each with its strengths and weaknesses. One adopts a sophisticated molecular beam epitaxial procedure to grow atomically smooth epitaxial Ag films. However, the procedure is rather slow and becomes impractical to grow films with thickness > 50 nm. Another approach adopts a growth process using rapid e-beam deposition which is capable of growing single crystalline Ag films in the thick regime (> 300 nm). However, the rapid growth procedure makes it difficult to control film thickness precisely, i.e., the method is not applicable to growing thin epitaxial films. Here we report a universal approach to grow atomically smooth epitaxial Ag films with precise thickness control from a few monolayers to the optically thick regime, overcoming the limitations of the two aforementioned methods. In addition, we develop an in-situ growth of aluminum oxide as the capping layer which exhibits excellent properties protecting the epitaxial Ag films. The performance of the epitaxial Ag films as a function of the film thickness is investigated by directly measuring the propagation length of the surface plasmon polaritons (SPPs) as well as their device performance to support a waveguide plasmonic nanolaser in infrared incorporating an InGaAsP quantum well as the gain media.

cond-mat.mtrl-sci↗