Yasuyuki Ozeki

The three-dimensional (3D) stress distribution in a bulk GaN substrate with laser-induced indentations for laser slicing was characterized using stimulated Raman scattering (SRS) microscopy. Local shifts in the Raman peak corresponding to the E2H mode were detected around the indentations, indicating the induction of local stress. The spatial distribution of internal stress was successfully visualized, revealing a clear in-plane anisotropy with compressive and tensile stresses along the a- and m-axes, respectively, as well as its extension in the depth direction. SRS microscopy enables the 3D visualization of residual stress in bulk GaN substrates, facilitating the optimization of fabrication processes.

9-Cyanopyronin derivatives have been used as resonance Raman probes for highly sensitive and multiplexed vibrational biological imaging. Here, we synthesized a series of fourteen 9-cyanopyronin derivatives in which the 10th core atom is substituted with various group 13, 14, 15 or 16 elements, and systematically evaluated their properties using electronic pre-resonance stimulated Raman scattering (EPR-SRS) microscopy. Substitution at the 10th position modulates not only the vibrational frequency of the nitrile (C ≡ N) group in the silent region, but also the conjugated double-bond stretching frequency (C = C) in the fingerprint region, and additionally alters the subcellular localization of the probes. By employing different combinations of probes, we were able to utilize the distinct patterns of C ≡ N or C = C bond vibrations together with the different subcellular localizations of the probes to achieve live-cell three-color imaging. These 9-cyanopyronin derivatives greatly expand the vibrational palette available for multiplexed SRS imaging. The alt text for this image may have been generated using AI.

We investigate the optimal pump intensity in quantum-enhanced stimulated Raman scattering (SRS) microscopy using an asymmetric beamsplitter. Specifically, we derive the intensity that maximizes the signal-to-noise ratio (SNR) under the condition where the total optical intensity of the pump and Stokes pulses is constrained to prevent photodamage, and under the condition without such constraints. In the absence of power constraints, the highest SNR is achieved by simply maximizing the pump intensity without using squeezed light. Under power constraints, the optimal pump intensity in classical SRS is known to be one-third of the photodamage threshold. We reveal that, in the quantum-enhanced regime, the optimal pump intensity is lower than the classical optimum, and as the squeezing level increases, this deviation from the classical optimum grows more pronounced. Our analysis provides valuable guidelines for achieving high-sensitivity SRS imaging with quantum enhancement.

In recent years, quantum technology has expanded its reach across a wide range of fields. Pulsed squeezing, in particular, holds significant promise for advancing quantum-enhanced nonlinear microscopy as well as high-speed quantum computing and communication. However, the performance of pulsed squeezing using the widely adopted Ti:sapphire laser has been rarely investigated, especially in the picosecond regime. Here, we report record-level picosecond pulsed squeezing of −4.41 dB generated with a Ti:sapphire laser, together with a method for time-domain phase characterization of picosecond pulsed squeezed light. We anticipate that these results will promote the broader adoption of quantum systems based on the versatile, broadband-tunable Ti:sapphire platform and foster a deeper understanding of pulsed squeezed light.

We demonstrate vibrational imaging with stimulated Raman scattering microscopy for atomically thin layers and a strained nanostructure in 2D hexagonal boron nitride.

Emerging gallium nitride (GaN) vertical power devices require high-quality GaN crystals with reduced crystal defects, especially threading dislocations (TDs), to harness the high critical electric field and electron mobility of the material. A strong demand for characterizing TDs has driven the development of many imaging techniques, such as multiphoton excited photoluminescence (MPPL) microscopy and spontaneous Raman microscopy. However, all these techniques lack the capability of visualizing densely existing TDs in a non-destructive and three-dimensional (3D) way or distinguishing the types of TDs. Here, we propose multimodal 3D stimulated Raman scattering (SRS) and MPPL microscopy to non-destructively characterize the TDs in free-standing GaN substrates. Leveraging strong SRS signals, we achieve an imaging speed 10^2-10^3 times faster than that of spontaneous Raman microscopy. This acceleration of the imaging speed enables 3D SRS imaging of the strain fields that are induced by TDs and correlated with the edge components of the Burgers vectors. Multimodal 3D SRS and MPPL imaging reveals rich information on TD features and behavior, including propagation directions, Burgers vectors, dislocation reactions, and their relationships. Furthermore, we present a deterministic identification method for screw-type TDs, which are regarded as killer defects causing leakage current in p-n junctions, by exploiting no SRS peak shifts at screw dislocations in combination with MPPL patterns. Our multimodal imaging technique will greatly advance the understanding of dislocations toward realizing high-quality GaN crystals and high-performance GaN devices.

Three-dimensional integration is vital for advancing high-density electronic and photonic devices, yet managing residual stress and thermal hotspots in silicon (Si) remains a key challenge that impacts performance and reliability. To address this challenge, high-resolution, non-destructive methods capable of probing strain and temperature distributions within bulk Si are critically needed. In this work, we present a long-wavelength stimulated Raman scattering (SRS) microscopy. By employing an Er-doped fiber laser system generating a 1.56 µm pump and a 1.7 µm Stokes beam, we significantly enhance light penetration into Si, enabling accurate localization of mechanical stress and thermal gradients within the bulk.