Ultrafast laser pulses can be used to drive materials into nonequilibrium states that have unusual properties and are promising for technological applications. Different classes of phenomena are observed during and after the optical illumination. This Colloquium discusses the recent developments in this field, and the prospects for using these techniques to create materials with novel functionalities in a controlled way.

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Colloquium: Nonthermal pathways to ultrafast control in quantum materials

Exotic phenomena can be achieved in quantum materials by confining electronic states into two dimensions. For example, relativistic fermions are realized in a single layer of carbon atoms1, the quantized Hall effect can result from two-dimensional (2D) systems2,3, and the superconducting transition temperature can be considerably increased in a one-atomic-layer material4,5. Ordinarily, a 2D electronic system can be obtained by exfoliating the layered materials, growing monolayer materials on substrates, or establishing interfaces between different materials. Here we use femtosecond infrared laser pulses to invert the periodic lattice distortion sectionally in a three-dimensional (3D) charge density wave material (1T-TiSe2), creating macroscopic domain walls of transient 2D ordered electronic states with unusual properties. The corresponding ultrafast electronic and lattice dynamics are captured by time-resolved and angle-resolved photoemission spectroscopy6 and ultrafast electron diffraction at energies of the order of megaelectronvolts7. Moreover, in the photoinduced 2D domain wall near the surface we identify a phase with enhanced density of states and signatures of potential opening of an energy gap near the Fermi energy. Such optical modulation of atomic motion is an alternative path towards realizing 2D electronic states and will be a useful platform upon which novel phases in quantum materials may be discovered. Two-dimensional electronic states are observed at the induced domain walls of a three-dimensional charge density wave material by manipulating the periodic lattice distortion via femtosecond infrared pulses.

Optical manipulation of electronic dimensionality in a quantum material.

Multifunctional nanotheranostics for near infrared optical imaging-guided treatment of brain tumors.

It has been nearly a century since the original mechanism for charge density wave (CDW) formation was suggested by Peierls. Since then, the term has come to describe several related concepts in condensed matter physics, having their origin in either the electron–phonon or electron–electron interaction. The vast majority of CDW literature deals with systems that are metallic, where discussions of mechanisms related to the Fermi surface are valid. Recently, it has been suggested that semiconducting systems such as TiS3 and TiSe2 exhibit behavior related to CDWs. In such cases, the origin of the behavior is more subtle and intimately tied to electron–electron interactions. We introduce the different classifications of CDW systems that have been proposed and discuss work on the group IV transition metal trichalcogenides (TMTs) (ZrTe3, HfTe3, TiTe3, and TiS3), which are an exciting and emergent material system whose members exhibit quasi-one-dimensional properties. TMTs are van der Waals materials and can be readily studied in the few-layer limit, opening new avenues to manipulating collective states. We emphasize the semiconducting compound TiS3 and suggest how it can be classified based on available data. Although we can conjecture on the origin of the CDW in TiS3, further measurements are required to properly characterize it.

Collective states and charge density waves in the group IV transition metal trichalcogenides

We use broadband ultrafast spectroscopy to investigate the out-of-equilibrium dynamics of Ta$_{2}$NiSe$_{5}$. This layered chalcogenide material has recently been shown to host an excitonic insulating phase up to a critical temperature T$_{c}$ = 328 K, however the nature of the spontaneous symmetry breaking leading to the excitonic condensation, whether electronic or structural, is unknown. Here, we first demonstrate that the transient reflectivity (${\Delta}$R/R) in the near infrared range is connected to the low-energy dynamics of the excitonic order. In particular, ${\Delta}$R/R mirrors the behaviour of the order parameter of a symmetry broken phase, and changes sign as the temperature is raised across T$_{c}$. This enables us to track the status of the excitonic condensate and to show that the transient response above T$_{c}$ is that of a gapped state, supporting the recently proposed existence of a novel insulating phase of preformed excitons at high temperatures. We further demonstrate that the condensate is also suppressed at room temperature in the presence of high photoexcitation fluences. From the sub-50 fs, fluence dependent timescale of the condensate quenching and from the behaviour of the photoinduced coherent phonon modes, we conclude that electronic interactions prominently underlie the formation of the excitonic order. The observation of a sub-2 ps recovery of the symmetry broken phase confirms this conclusion and is of particular interest as this process is usually hindered by photoinduced heating in other strongly correlated materials. Our results therefore pave the way towards the ultrafast, complete control and manipulation of an exciton condensate at room temperature.

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Ultrafast melting and recovery of collective order in the excitonic insulator Ta$_{2}$NiSe$_{5}$

The clinical success of cisplatin ushered in a new era of the application of metallodrugs. When it comes to practice, however, drug resistance, tumor recurrence, and drug systemic toxicity make it implausible to completely heal the patients. Herein, we successfully transform an electron acceptor [1, 2, 5]thiadiazolo[3,4-g]quinoxaline into a novel second near-infrared (NIR-II) fluorophore H7. After PEGylation and chelation, HL-PEG2k exhibits a wavelength bathochromic shift, enhanced photothermal conversion efficiency (41.77%), and an antineoplastic effect against glioma. Its potential for in vivo tumor tracking and image-guided chemo-photothermal therapy is explored. High levels of uptake and high-resolution NIR-II imaging results are thereafter obtained. The hyperthermia effect could disrupt the lysosomal membranes, which in turn aggravate the mitochondria dysfunction, arrest the cell cycle in the G2 phase, and finally lead to cancer cell apoptosis. HL-PEG2k displays a superior biocompatibility and thus can be a potential theranostic platform to combat the growth and recurrence of tumors.

A Second Near-Infrared Ru(II) Polypyridyl Complex for Synergistic Chemo-Photothermal Therapy.

The origin of charge density waves (CDW) in TiSe$_2$ has long been debated, mainly due to the difficulties in identifying the timescales of how and when the excitonic pairing and electron-phonon coupling (EPC) come into play. Without a proper time resolution and microscopic mechanism, one has to assume simultaneous appearance of CDW and periodic lattice distortions (PLD). Here, we accomplish a complete separation of exciton and PLD dynamics and unravel their interplay in the ultrafast time domain in our real-time time-dependent density functional theory simulations. We find that laser pulses knock off the exciton order and induce a homogeneous bonding-antibonding transition in the initial 20 fs, then the weakened electronic order triggers ionic movements antiparallel to the original PLD. The EPC comes into play after the initial 20~fs, and the two processes mutually amplify each other leading to a complete inversion of CDW ordering. The self-amplified dynamics reproduces the evolution of band structures in excellent agreement with ultrafast photoemission experiment. Hence we resolve the key processes in the initial dynamics of CDW that help elucidate the mechanism underlying the long debated problem.

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Ultrafast charge ordering by self-amplified exciton-phonon dynamics in TiSe$_2$

Novel Diketopyrrolopyrrole NIR-II Fluorophores and DDR Inhibitors for in vivo Chemo-photodynamic Therapy of Osteosarcoma

Exosomes, a subgroup of extracellular vesicles secreted by multiple cells, have great potential as cancer biomarkers in clinical applications. However, enrichment and detection of exosomes from complex media remain a huge challenge due to their small size. Herein, we used iodixanol density gradient centrifugation for the isolation and purification of exosomes and label-free detection of exosomal PD-L1 using a biochip based on surface plasmon resonance (SPR-ExoPD-L1). The obtained exosomes are lipid-bilayer vesicles and the classical exosome markers CD9, CD63 and CD81 are highly enriched. Besides, PD-L1 is specifically expressed on exosomes instead of non-vesicular components or large extracellular vesicles. Compared with enzyme-linked immunosorbent assays, the SPR-ExoPD-L1 assay could better distinguish exosomes derived from melanoma cells with different levels of PD-L1. Accurate measurement of exosomal PD-L1 could provide critical clinical information for cancer diagnosis and personalized immunotherapy of cancer.

Isolation of circulating exosomes and identification of exosomal PD-L1 for predicting immunotherapy response.

Exosomal miRNAs play a critical role in cancer biology and could be potential biomarkers for cancer diagnosis. However, due to the low abundance of miRNAs in the exosomes, recognizing and detecting disease-associated exosomal miRNAs in an easy-to-operate way remain a challenge. Herein, we used a liposome-mediated membrane fusion strategy (MFS) to transfect CRISPR/Cas13a into exosomes, termed MFS-CRISPR, directly measuring exosomal miRNAs in plasma. Using the MFS-CRISPR platform for detection of the exosomal miR-21, we achieve a linear range spanning four orders of magnitude (104-108 particles/mL) and the method is able to detect the exosomal miR-21 in as low as 1.2 × 103 particles/mL. The liposome-mediated MFS could confine fluorescent signals in fused vesicles, which can be used for exosome heterogeneity analysis. Moreover, MFS-CRISPR assay was evaluated by measuring clinical samples, and the difference of miR-21 expression of breast cancer patients and healthy donors was significant. Because of high sensitivity and simplicity, the proposed method could have promising clinical potential for cancer diagnosis and treatment monitoring.

https://pubs.acs.org/doi/pdf/10.1021/acssensors.2c01683

Meng-Xue Guan

Papers

A Second Near-Infrared Ru(II) Polypyridyl Complex for Synergistic Chemo-Photothermal Therapy.

Ultrafast charge ordering by self-amplified exciton-phonon dynamics in TiSe$_2$

Novel Diketopyrrolopyrrole NIR-II Fluorophores and DDR Inhibitors for in vivo Chemo-photodynamic Therapy of Osteosarcoma

Isolation of circulating exosomes and identification of exosomal PD-L1 for predicting immunotherapy response.

Highly Effective Detection of Exosomal miRNAs in Plasma Using Liposome-Mediated Transfection CRISPR/Cas13a.