Yu Takiguchi

High-quality generation of a multispot pattern using a spatial light modulator with adaptive feedback

We propose and demonstrate high-quality generation of a uniform multispot pattern (MSP) by using a spatial light modulator with adaptive feedback. The method iteratively updates a computer generated hologram (CGH) using correction coefficients to improve the intensity distribution of the generated MSP in the optical system. Thanks to a simple method of determining the correction coefficients, the computational cost for optimizing the CGH is low, while maintaining high uniformity of the generated MSP. We demonstrate the generation of a 28×28 square-aligned MSP with high uniformity. Additionally, the proposed method could generate an MSP with a gradually varying intensity profile, as well as a uniform MSP consisting of more than 1000 spots arranged in an arbitrary pattern.

Direct evidence for three-dimensional off-axis trapping with single Laguerre-Gaussian beam

Optical tweezers are often applied to control the dynamics of objects by scanning light. However, there is a limitation that objects fail to track the scan when the drag exceeds the trapping force. In contrast, Laguerre-Gaussian (LG) beams can directly control the torque on objects and provide a typical model for nonequilibrium systems such as Brownian motion under external fields. Although stable “mid-water” trapping is essential for removing extrinsic hydrodynamic effects in such studies, three-dimensional trapping by LG beams has not yet been clearly established. Here we report the three-dimensional off-axis trapping of dielectric spheres using high-quality LG beams generated by a special holographic method. The trapping position was estimated as ~ half the wavelength behind the beam waist. These results establish the scientific groundwork of LG trapping and the technical basis of calibrating optical torque to provide powerful tools for studying energy-conversion mechanisms and the nonequilibrium nature of biological molecules under torque.

Self-distortion compensation of spatial light modulator under temperature-varying conditions

Conventional methods of compensating for self-distortion in liquid-crystal-on-silicon spatial light modulators (LCOS-SLM) are based on aberration correction, where the wavefront of the incident beam is modulated to compensate for aberrations caused by the imperfect optical flatness of the LCOS-SLM surface. However, the phase distribution of an LCOS-SLM varies with changes in ambient temperature and requires additional correction. We report a novel phase compensation method under temperature-varying conditions based on an orthonormal Legendre series expansion of the phase distribution. We investigated the temperature dependency by controlling the ambient temperature with an incubator and successfully corrected for self-distortion in a temperature range of 20 °C to 50 °C. Our approach has the potential to be adopted in tight-focusing applications which require wavefront modulation with very high accuracy.

Effects of dielectric planar interface on tight focusing coherent beam: direct comparison between observations and vectorial calculation of lateral focal patterns

We report direct observation of lateral focal patterns through an acrylic material to investigate the effects of aberrations caused by a planar dielectric interface. Numerical analyses based on vectorial Huygens-Fresnel diffraction theory were also performed to examine the behavior of three-dimensional point spread functions. Experimental and numerical results showed agreement of the behavior of the peak position in the focal patterns with changes in the interface position. Our approach has the potential to predict the effects of aberrations in confocal laser scanning microscopes and super-resolution applications.

Improvement of laser dicing system performance I: high-speed, high-quality processing of thick silicon wafers using spatial light modulator

In the laser wafer dicing technique of stealth dicing (SD), a laser beam that is tightly focused inside a silicon wafer is scanned multiple times at different depths. The focused beam creates multilayered cracks that allow dry, debris-free dicing. To reduce the dicing time, it is desirable to produce longer cracks with each scan. However, when the laser beam is focused in a deep region of the wafer, the beam is blurred, and its power density decreases owing to spherical aberration caused by a refractive index mismatch between air and the wafer. Consequently, the generated cracks become shorter. We present an approach to making longer cracks deep within the wafer by correcting the spherical aberration. This correction is made using an SD machine incorporating a phase-only spatial light modulator to apply aberration correction patterns, which are calculated by a method based on inverse ray tracing. Experimental results using 300-µm wafers show that, when the aberration was corrected, the cracks formed during multidepth scans became longer even deep within the wafer and that the dicing speed with correction is more than twice that without correction. This is because each scan produced longer cracks, so fewer scans were necessary. We also demonstrated that the quality of dicing was improved.

Phase-modulating lasers toward on-chip integration

Controlling laser-beam patterns is indispensable in modern technology, where lasers are typically combined with phase-modulating elements such as diffractive optical elements or spatial light modulators. However, the combination of separate elements is not only a challenge for on-chip miniaturisation but also hinders their integration permitting the switchable control of individual modules. Here, we demonstrate the operation of phase-modulating lasers that emit arbitrarily configurable beam patterns without requiring any optical elements or scanning devices. We introduce a phase-modulating resonator in a semiconductor laser, which allows the concurrent realisation of lasing and phase modulation. The fabricated devices are on-chip-sized, making them suitable for integration. We believe this work will provide a breakthrough in various laser applications such as switchable illumination patterns for bio-medical applications, structured illuminations, and even real three-dimensional or highly realistic displays, which cannot be realised with simple combinations of conventional devices or elements.

Improvement of laser dicing performance II: dicing rate enhancement by multi beams and simultaneous aberration correction with phase-only spatial light modulator

“Stealth Dicing” laser processing is a dry and debris-free semiconductor wafer dicing method achieved by generating thermal micro-cracks inside a wafer with a tightly focused laser beam. This method has two practical issues: (1) the dicing speed is limited by the repetition rate of the pulsed laser, and (2) integrated circuits on the opposite side of the wafer from the laser light are potentially damaged by excessive laser intensity required to compensate for insufficient beam convergence. The insufficient beam convergence is a result of spherical aberration due to a refractive index mismatch between air and the wafer. These problems can be resolved by incorporating a phase-only spatial light modulator (SLM) into the laser dicing system. The SLM produces two types of wavefront configurations simultaneously for two different functions. One is for multi-beam generation with a phase grating pattern. This improves the dicing speed by a factor equal to the number of diffracted beams. The other is for aberration correction of the multiple beams using a pre-distorted wavefront pattern. By correcting aberrations, the focused multiple beams inside the wafer will become sufficiently convergent to avoid undesirable laser damage. We demonstrated these improvements by dicing sapphire wafers with a pulsed laser and a high-numerical-aperture objective lens.

Evaluation of Laguerre-Gaussian beam generated with integrable phase-modulating surface-emitting lasers

We demonstrated direct surface-emitting of Laguerre–Gaussian beams with wavefront modulated lasers. This integrable phase-modulating surface-emitting lasers has potential to emit arbitrarily configured beam patterns without requiring any optical elements or scanning devices. The fabricated devices are on-chip-sized, making them suitable for integration. We introduce a phase-modulating resonator in a semiconductor laser, which analogically behaves as phaseonly holograms, kinoform, to allow the concurrent realization of lasing and phase modulation. Particularly, this is promising in the use for free-space optical communications due to the fact that coaxial propagation of orbital angular momentum (OAM) properties with different OAM mode states are mutually orthogonal.

Single-Shot Optical Anisotropy Imaging with Quantitative Polarization Interference Microscopy

Optical anisotropy measurement is essential for material characterization and biological imaging. In order to achieve single-shot mapping of the birefringence parameters of anisotropic samples, a novel polarized light imaging concept is proposed, namely quantitative polarization interference microscopy (QPIM). QPIM can be realized through designing a compact polarization-resolved interference microscopy system that captures interferograms bearing samples linear birefringence information. To extract the retardance and the orientation angle maps from a single-shot measurement, a mathematical model for QPIM is further developed. The QPIM system is validated by measuring a calibrated quarter-wave plate, whose fast-axis orientation angle and retardance are determined with great accuracies. The single-shot nature of QPIM further allows to measure the transient dynamics of birefringence changes in material containing anisotropic structures. This application is demonstrated by capturing transient retardance changes in a custom-designed parallel-aligned nematic liquid crystal-based device.

Holographic 3D multi-spot two-photon excitation for fast optical stimulation in brain

We report here a holographic high speed accessing microscope of sensory-driven synaptic activity across all inputs to single living neurons in the context of the intact cerebral cortex. This system is based on holographic multiple beam generation with spatial light modulator, we have demonstrated performance of the holographic excitation efficiency in several in vitro prototype system. 3D weighted iterative Fourier Transform method using the Ewald sphere in consideration of calculation speed has been adopted; multiple locations can be patterned in 3D with single hologram. Standard deviation of intensities of spots are still large due to the aberration of the system and/or hologram calculation, we successfully excited multiple locations of neurons in living mouse brain to monitor the calcium signals.