Arbitrary spectro-temporal pulse-shaping algorithm.

Measurement applications in optical fields require arbitrary spectro-temporal pulse shaping. However, conventional pulse-shaping algorithms are limited to controlling only the shape of the temporal intensity waveform. To overcome this limitation, we introduce the concept of short-time Fourier transformation into the conventional iterative Fourier transform algorithm, enabling it to introduce spectro-temporal constraints using a spectrogram image as a target. We numerically demonstrate that the proposed algorithm can find an appropriate spectral phase modulation pattern to realize arbitrarily controlled spectro-temporal pulse waveforms by testing the algorithm with different spectro-temporal multi-pulse waveforms. The algorithm benefits from reducing computational costs for generating spectro-temporal waveforms.

Multi-focusing surface-emitting lasers.

Complete control of a beam pattern requires not only projecting a two-dimensional (2D) pattern but also focusing on a three-dimensional (3D) point cloud, which is typically achieved utilizing holography under the framework of diffraction. We previously reported direct focusing from on-chip size surface-emitting lasers that utilize a holographically modulated photonic crystal cavity based on 3D holography. However, this demonstration was of the simplest 3D hologram with a single point and single focal length, and the more typical 3D hologram with multiple points and multiple focal lengths has not yet been examined. Toward direct generation of a 3D hologram from the on-chip size surface-emitting laser, we here examined a simple 3D hologram featuring two different focal lengths with a single off-axis point in each to reveal the fundamental physics. Two types of holography, one based on superimposition and the other on random tiling, successfully demonstrated the desired focusing profiles. However, both types caused a spot noise beam in the far field plane due to interference between focusing beams with different focal lengths, especially in the case of the superimposing method. We also found that the 3D hologram based on the superimposing method consisted of higher order beams including the original hologram due to the manner of the holography. Secondly, we demonstrated a typical 3D hologram with multiple points and focal lengths and successfully showed the desired focusing profiles by both methods. We believe our findings will bring innovation to mobile optical systems and pave the way to developing compact optical systems in areas such as material processing, micro fluidics, optical tweezers, and endoscopy.

On-chip size, low-noise fringe pattern projector offering highly accurate 3D measurement.

Fringe pattern projectors are quite useful for highly accurate three-dimensional (3D) measurement when a projector or LED array is used for illumination. We have fabricated a 0.2 mm × 0.2 mm structured light source, which was an on-chip size surface-emitting laser that utilized a holographically modulated two-dimensional (2D) photonic crystal (PC). This will make possible an extremely compact 3D measurement system that will positively impact mobile systems. However, the fringe pattern tends to cause speckle-like noise that leads to severe positional error in 3D measurement. Here we present a simple approach to projecting a low-noise fringe pattern from our surface-emitting lasers by using a one-dimensional (1D) focusing hologram. This method improves the flatness of the fringe pattern by around four times.

Complex-Amplitude-Modulation Vectorial Excitation Beam for High-Resolution Observation of Deep Regions in Two-Photon Microscopy.

In two-photon microscopy, aberration correction is an essential technique for realizing high resolution in deep regions. A spatial light modulator (SLM) incorporated into an optical system for two-photon microscopy performs pre-compensation on the wavefront of the excitation beam, restoring the resolution close to the diffraction limit even in the deep region of a biological sample. If a spatial resolution smaller than the diffraction limit can be achieved along with aberration correction, the importance of two-photon microscopy for deep region observation will increase further. In this study, we realize higher resolution observations in the deep region by combining two resolution-enhancement methods and an aberration correction method. Therefore, a z-polarizer is added to the aberration-correction optical system, and the SLM modulates the amplitude and phase of the excitation beam; in other words, complex-amplitude modulation is performed. The lateral resolution is found to be approximately 20% higher than the diffraction limit obtained using a circularly polarized beam. Verification was conducted by simulation and experimentation using model samples and ex vivo biological samples. The proposed method has the potential to be effective for live imaging and photostimulation of the deep region of the sample, although it requires only minor changes to the conventional optical system that performs aberration correction.

Measurement of group refractive indices of glass using a color-selective multi-pulse generated with a spatial light modulator.

We demonstrate a method of measuring group refractive indices (ng) by using only a temporal waveform of a color-selective multi-pulse (CSMP) generated with a spatial-light-modulator-based optical pulse shaper. Each pulse of the CSMP is characterized by different spectral components. Thus, ng of a sample can be estimated by calculating the group delay of the spectrally different pulses in the CSMP after passing through the sample. As a proof-of-concept, we measured ng of a glass plate by using a CSMP. We confirmed that the measured ng values were in agreement with those derived from Sellmeier's formula with an accuracy on the order of 10-4.

Energy adjustment pulse shaping algorithm part I: accuracy improvement of phase retrieval IFTA.

Techniques to generate a targeted temporal waveform with high accuracy are desirable to extend the application range for pulse shapers. In this study, a target energy adjustment mechanism is applied to the input-output iterative Fourier transform algorithm (IFTA). It is numerically demonstrated that, considering multi-pulse temporal waveforms, the developed algorithm provides a suitable spectral phase modulation pattern and improves the shape of the temporal waveform compared to that of the input-output IFTA.

Energy adjustment pulse shaping algorithm part II: realization of a spectral intensity design.

An accurately controlled arbitrary temporal waveform is required for many applications. To realize accurate pulse shaping, many optimization algorithms have been proposed to design spectral phase modulation patterns. However, as far as the authors know, no intensity optimization algorithm has been proposed. Therefore, in this paper, an algorithm is proposed to design an optimal spectral intensity modulation pattern for shaping short laser pulses. Consequently, it is numerically demonstrated that the proposed algorithm provides suitable spectral intensity modulation patterns, which create more accurate shapes of temporal waveform than those of spectral phase-only modulation.

Generation of arbitrarily chirped and CEP-controlled terahertz pulses for dispersion compensation using an optical pulse shaping technique and a fan-out periodically poled crystal.

We constructed a system that can generate phase-controlled terahertz (THz) pulses using a fan-out periodically poled lithium tantalate crystal and an optical pulse shaper containing a spatial light modulator. The phase of each THz frequency components could be controlled by manipulating the delay time of the corresponding optical pulses. Using the system, we generated arbitrarily group-velocity-dispersion-controlled THz pulses, where the chirp parameter was 2.53 ps2/rad between 0.6 and 1.5 THz. In addition, we generated arbitrarily carrier-envelope-phase-controlled THz pulses in the same system. Phase-controlled THz pulses may be useful for applications such as dispersion compensation.

High-resolution simultaneous microscopy of refractive index and fluorescent intensity distributions by using localized surface plasmons.

We propose a localized surface plasmon microscope that provides simultaneous imaging of refractive index and fluorescent intensity distributions. We show experimental images of fluorescent and transparent particles under circular pupil illumination to confirm simultaneous high-resolution imaging. Furthermore, we investigate applicability of annular pupil illumination employing two axicons to improve energy efficiency in the fluorescent imaging and find that a brighter image is obtainable by maintaining high spatial resolution for both imaging modes.

Localized surface plasmon microscopy of submicron domain structures of mixed lipid bilayers.

We propose scanning localized surface plasmon microscopy of mixed lipid bilayers with submicron domain structures. Our observation technique, which employs localized surface plasmons excited on a flat metal surface as a sensing probe, provides non-label and non-contact imaging with the spatial resolution of ∼ 170 nm. We experimentally show that submicron domain structures of mixed lipid bilayers can be observed. A detailed analysis finds that the domains are classified into two groups.

Scanning and non-scanning surface plasmon microscopy to observe cell adhesion sites.

We observe adhesion sites of a cell on a substrate with high resolution. Since this observation requires interfacial measurements between the cell and the substrate, we employ scanning localized surface plasmon microscopy. We experimentally show that focal adhesion sites of a mouse muscle cell can be observed without fluorescent labeling. We also show that a non-scanning surface plasmon microscope combined with the scanning localized surface plasmon microscope contributes to observing an entire cell adhesion site and identify regions of interest.

Model for measurement of water layer thickness under lipid bilayers by surface plasmon resonance.

A multilayered-substrate model is proposed for surface plasmon resonance (SPR)-based measurement of the thickness of a water layer sandwiched between a lipid bilayer and an underlying support. To calculate sensitivity, a 473 nm-wavelength excitation source and a silver layer are used for the SPR sensor. It is theoretically shown that the multilayered substrate design achieves sufficient sensitivity for such measurements and that sensitivity is enhanced with a SiO(2) layer of appropriate thickness and a buffer solution of high refractive index.

High resolution imaging of patterned model biological membranes by localized surface plasmon microscopy.

We report on microscopic imaging of phospholipid membranes. To achieve nonlabel, noncontact, and high spatial resolution imaging of the membranes, we use optically excited localized surface plasmons as a virtual measurement probe to obtain the local refractive index. This enables significantly higher lateral resolution of approximately 170 nm. We reveal that the developed microscope has the capability of observing lipid bilayers with thickness of 3.0 nm deposited into the gaps in a patterned lipid bilayer with thickness of 4.6 nm. We find that the thickness resolution against the deposited lipid bilayer is approximately 0.33 nm.