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We developed a thermo-optically controlled nonmechanical optical beam scanner using a Si photonic crystal slow-light waveguide with a diffraction grating to achieve on-chip light detection and ranging (LIDAR). This Letter applies pre-emphasis signals to the thermo-optic control, and the cutoff frequency increases to 500 kHz. Observing the beam scanning in the space-time domain showed that the turn-on and turn-off times of the scanner for a rectangular drive voltage were 10 µs and reduced to 2.7 µs when the pre-emphasis signals were optimized. This new, to the best of our knowledge, result enables a frame rate of 29 fps for 12,800 resolution points in LIDAR.

Optical beam scanning is a widely utilized function in optical systems and a compact nonmechanical solid-state device has long been anticipated. Here, we have studied such a device consisting of photonic crystal slow-light waveguides and switch trees, fabricated by a Si photonics process, employing a bespoke prism lens for beam collimation. Further in this study, we particularly demonstrated the operation of the device only by the thermo-optic (TO) tuning of its components, at a fixed wavelength of light. A spot beam of ∼0.1° divergence, was scanned in two dimensions, in the angular range of 40° × 8.8° and average power consumption of < 0.7 W. Neglecting some disordered beams caused by the non-uniformity of the fabricated device, the estimated number of resolution points was 400 × 32 = 12,800, which required a significant effort in the device fabrication and calibration, utilizing optical-phased arrays, if the same performance was targeted.

We have studied frequency-modulated continuous-wave (FMCW) light detection and ranging which uses externally modulated light instead of a wavelength-chirped laser diode and a complicated clock sampling circuit. In this method, the FMCW beat signal is accompanied by a 3 dB average loss in principle when pseudo frequency-modulated light simply generated by an intensity modulation with frequency-swept signals is used. In this article, we first fabricated a Si in-phase/quadrature-phase modulator equipped with compact photonic-crystal phase shifters and generated carrier-suppressed single sideband modulation signals. Furthermore, we obtained almost ideal frequency modulation, with a side-mode suppression ratio of 22.3 dB, by temporally controlling the frequency and phase of the modulation signals. When the FMCW beat signal was produced with this modulated signal in a fiber delay interferometer, its intensity was increased by 5.3 dB, as compared with that using intensity modulation. This improvement corresponds to an 80% increase in the limit distance of FMCW ranging. We also generated similar signals using a single modulator with a bypass waveguide, which simplified modulation. The side-mode suppression ratio for the frequency-swept modulation was limited to 9.8 dB, while comparably large increase in the beat intensity was confirmed.

In this paper, we propose and demonstrate a frequency-modulated continuous-wave light detection and ranging (LiDAR) with a Si photonic crystal beam scanner, simultaneously enabling scanning laser Doppler measurements. This nonmechanical solid-state device can reduce the size of conventional scanning laser Doppler vibrometers, making LiDAR a multimodal imaging sensor, which can measure the distributions of distance, velocity, and vibration frequency. We fabricated this device using Si photonics process and confirmed the expected operations. Distance and velocity resolutions were less than 15 mm and 19 mm/s, respectively. The detection limit of the vibration amplitude determined by the signal-to-noise ratio was 2.5 nm.

The doubly periodic Si photonic crystal waveguide operates as a nonmechanical beam-steering device that can be applied in light detection and ranging. In this Letter, we develop and describe a prism lens that collimates a fan-shaped beam, emitted from the waveguide independent of the steering angle. Its fundamental profile is investigated using a theoretical analysis for thick lenses, and then, its detailed aspherical design is obtained. In ray tracing, this prism lens suppresses the beam divergence to less than the diffraction limit in most of the targeted beam-steering range. The prism lens is fabricated by acrylic cutting, and its expected characteristics are observed.

Refractive index fronts propagating in waveguides are special spatiotemporal perturbations. The interaction of light with such fronts can be described in terms of an indirect transition where the frequency and wavenumber of a guided mode both are changed. In recent years, front-induced transitions have been used in dispersion-engineered waveguides for frequency conversion, optical delays, and bandwidth and pulse duration manipulation. These concepts have originated from different research areas of photonics, such as nonlinear fibre optics, slow-light waveguides, plasma physics, moving media and relativistic effects. Here, we discuss these concepts, providing a unifying theoretical description and highlight the potential of this exciting research field for light manipulation in guided optics., Gaafar, M.A., Baba, T., Eich, M. et al. Front-induced transitions. Nat. Photonics 13, 737–748 (2019) doi:10.1038/s41566-019-0511-6

This paper proposes and demonstrates an optical leaky wave antenna (OWLA) excited by a beam expander consisting of a slab waveguide and parabolic photonic bandgap (PBG) reflector. Conventional OLWAs are usually excited through an adiabatic taper waveguide, but its large size is a drawback. The PBG reflector is easy to fabricate with the OLWA using Si photonics technology and its size is ten times smaller than that of the adiabatic taper waveguide. We confirmed the excitation of a shallow grating waveguide, which is a kind of OLWAs, through the beam expander in simulations and measurements.

Light detection and ranging (LiDAR) is drawing attention along with the topic of autonomous vehicles, but conventional LiDAR is large and expensive due to its mechanical scanner. Therefore, compact nonmechanical solid-state LiDAR, exploiting flash illumination, optical phased array, and slow light grating have been actively developed. This presentation reviews the status of the mechanical and nonmechanical LiDAR development and introduces the challenge to realize a solid-state one using Si photonic crystal slow light waveguide.

This paper presents the comparison of optical leaky waveguide antenna, i.e., grating waveguide (GWG), waffle waveguide (WWG) and waffle-iron waveguide (WIWG) by the optical experiment.

Moving refractive index fronts in waveguides with dispersion is a special type of spatio-temporal modulation leading to the change of signal frequency and wavenumber. The interaction of light with such fronts allows frequency conversion, light stopping, optical delays as well as bandwidth and pulse duration manipulation. We will present theoretical and experimental examples of signal transmission, reflection and trapping by the front and highlight special situations such as light stopping, time reversal or optical push broom effect. We will geometrically consider indirect transitions in the dispersion relation using the phase continuity relation at the front and present numerical solutions of the linear Schrodinger equation which follows from the slowly varying envelope approximation of the wave equation. In particular, for highly dispersive waveguides a temporal evolution of the spatial wave envelopes are considered in contrast to conventional spatial evolution of temporal envelopes. Further, we will present an overview of experimental results and estimate the maximal achievable effects for each of the application in different waveguide systems.

Moving refractive index fronts in waveguides with dispersion is a special type of spatio-temporal modulation. The interaction of light with that front allows frequency conversion, light stopping, optical delays, and bandwidth and pulse duration manipulation. Here, we present examples of signal transmission, reflection, trapping and stopping. We will geometrically consider indirect transitions in the dispersion relation using the phase continuity relation at the front and present numerical solutions of the linear Schrodinger equation which follows from the slowly varying envelope approximation of the wave equation.

We modeled the photonic bands of S i O 2 -cladded Si lattice-shifted photonic crystal waveguides via machine learning and found a structure that generates low-dispersion slow light with a group index of approximately 20 in the full C-band at telecom wavelengths. The normalized delay-bandwidth product is as large as 0.45, which is close to the theoretical upper limit. The transition structure between this waveguide and a Si-channel waveguide was designed using an evolutional optimization, and a C-band average loss of 0.116 dB/transition was calculated. These results prove the possibility of further enhancing the versatility of slow light.

This paper presents an optical leaky waveguide antenna, i.e., waffled waveguide (WWG). From both simulations and experiments, we evaluate the effectiveness of the WWG in terms of the antenna characteristics. The WWG radiates a narrow beam tilted by sweeping the incident wavelength, and consists of an array of small rectangular holes fabricated by silicon photonics. Compared to the conventional grating waveguide designed only with the longitudinal period and depth as free parameters, the lateral period of the rectangular holes in the WWG adds the design flexibility; consequently, the WWG achieves a large aperture size and high antenna gain. We reconstruct radiation pattern from aperture distribution and confirm the high antenna gain experimentally.

We theoretically optimized and demonstrated the periodic p–n junction in silicon photonic crystal slow-light modulators to balance the efficiency and speed of phase shifters and reduce the power consumption compared with those of previous linear and interleaved p–n junctions. In particular, sawtooth and wavy junctions, whose profiles match with the distribution of the slow-light mode, theoretically prove effective in achieving these objectives. However, the sawtooth junction requires a high-resolution process. Therefore, we finally employed the wavy junction and obtained 25- and 32-Gb/s operations in a 200-μm device with extinction ratios of 4 and 3 dB, respectively, for an excess modulation loss of 1 dB.

Semiconductor ion sensors that respond to the surface electric charge in a solution are used for chemical and biological sensing. Photonic sensors exploiting such a response in the photoluminescence intensity enable a simple system consisting only of a photopump source and a photodiode; however, their sensitivity is usually lower than that of electric sensors, such as ion-sensitive field-effect transistors. This study employed a GaInAsP semiconductor honeycomb photonic crystal slab as a photonic sensor structure and obtained a high ion sensitivity. The surface recombination, which is the origin of the ion sensitivity, was enhanced by increasing the surface-to-volume ratio and moderately suppressing the photopump level. Nevertheless, a sufficient signal-to-noise ratio was maintained by improving the light extraction efficiency. Moreover, a high pH sensitivity of 0.27 dB/pH, which is six times that without photonic crystals, was obtained and resulted in a pH resolution of 0.011 at pH ∼7 comparable with that of electric sensors.

We demonstrate 64 Gbps operation in a compact Si photonic crystal optical modulator that employs meander line electrodes and compensate for the phase mismatch between slow light and RF signals. Although low dispersion slow light increases the modulation efficiency, maintaining a sufficiently wide working spectrum, the phase mismatch becomes a limiting factor on the operation speed even when the phase shifter length is as short as 200 μm. Meander line electrodes broke this limit and enhanced the cutoff frequency by up to 31 and 38 GHz using 50 Ω and 20 Ω termination resistors, respectively. This allowed to use a group index of slow light higher than 20, and greatly improved the quality of the modulation characteristics at 25 and 32 Gbps. Clear open eye was observed even at 40–64 Gbps.

We have studied a Si photonics non-mechanical beam steering device for LiDARs. We exploit a doubly periodic Si photonic crystal waveguide (PCW) with a collimator lens, which emits a single-peaked optical beam. Thanks to the slow light effect in the PCW, wide range beam steering can be obtained in the longitudinal direction with maintaining a small beam divergence by a small change of the wavelength and/or index of the PCW. However, due to the symmetric crosssection of the PCW, the emission occurs in both upward and downward directions, which causes a 3-dB loss in the transmission of the optical beam. The downward beam is partly reflected by the substrate, and the reflected beam interferes with the upward beam and modifies the far field pattern, which further increases the loss at particular beam angles. In LiDARs, this loss is repeated at the reception of returned light, resulting in a severe loss penalty. In this study, we investigated the unidirectional upward emission in some PCW structures with vertical asymmetries. We found theoretically that a shallow etched grating on top of the Si layer, which overlaps with the PCW holes significantly increases the upward emission. We fabricated such a device using Si photonics CMOS process and observed 2-8 times stronger upward emission as compared with that of the symmetric PCW. Furthermore, we integrated 32 PCWs in parallel configuration and selected one working PCW so that its relative position against a collimator lens is switched and the beam is steered in the lateral direction. We observed over 400×32 resolution points.

The optical beam steering device is essential for LiDARs and non-mechanical ones have been developed extensively. We have studied the one based on a Si photonic crystal waveguide (PCW) that guides slow light. In LiDARs, the beam hits a distant object. Then, reflected light is scattered hemispherically and a part of it is returned and received by the PCW. In this process, a long PCW aperture is expected to increase the reception intensity. However, since the PCW has a propagation loss of the order of 10 dB/cm, the reception intensity is not increased by simply lengthening the PCW. In this study, in order to suppress the total loss of the PCW, we proposed and fabricated a serial array of PCWs, in which light is received by multiple and short PCWs and then summed by using Si wire waveguide and coupler. We first estimate the transmission and reception characteristics of the PCW array. The effective aperture radiating light is lengthened by dividing the PCW, so the beam divergence becomes small and the reception intensity is improved. Also, we measured the transmission characteristics of the PCW array. We obtained a 0.046° beam divergence by controlling the phase between the PCWs. In the beam steering by the wavelength scanning or heating, we confirmed that the phase matching angular step appears stepwise. If we use the angular step as a resolution point, we can obtain the beam steering without the phase control.

Slow light generated through silicon (Si) photonic crystal waveguides (PCWs) is useful for improving the performance of Si photonic devices. However, the accumulation of coupling loss between a PCW and Si optical wiring waveguides is a problem when slow-light devices are connected in a series in a photonic integrated circuit. Previously, we reported a tapered transition structure between these waveguides and observed a coupling loss of 0.46 dB per transition. This Letter employed particle swarm optimization to engineer the arrangement of photonic crystal holes to reduce loss and succeeded in demonstrating theoretical loss value of 0.12 dB on average in the wavelength range of 1540–1560 nm and an experimental one of 0.21 dB. Crucially, this structure enhances the versatility of slow light.

We demonstrate quadrature phase-shift keying (QPSK) modulation and four-level pulse-amplitude modulation (4-PAM) in Si Mach-Zehnder modulators whose phase shifter length is reduced to 300 μm by the slow-light effect in photonic crystal lattice-shifted waveguides and increased modal overlap with interleaved p-n junctions. The QPSK constellation patterns were observed up to 15 Gbaud (30 Gb/s) with an error-vector magnitude of

This paper describes a comparison of optical leaky-waveguide antennas, i.e., grating waveguide (GWG), waffle waveguide (WWG), and waffle-iron waveguide (WIWG), using both simulations and measurements. We simulate the antenna characteristics and experimentally confirm the high antenna gain.

The light emission characteristics of GaInAsP semiconductors in aqueous solutions are modified by the surface charge, depending on the balance between radiative and nonradiative recombination. This Letter demonstrates the application of a GaInAsP photonic crystal band-edge laser in sensing a solution pH, which reflects the surface charge. The sensitivity is enhanced by a hybrid of fractal honeycomb and close-packed structures with a large surface-to-volume ratio and a high quality factor ( Q ), which allows a low threshold carrier density. We experimentally obtained a maximum pH sensitivity of 1.9 dB/pH near the threshold and a signal-to-noise ratio of 52/pH (pH resolution of 0.019) at a pump power 2.4 times the threshold where the intensity noise diminished.

This paper presents high directive waffle waveguide of optical leaky wave antenna (OLWA) excited by photonic bandgap (PBG) reflector. For optical wireless communication, high antenna gain is required due to its high propagation loss. The waffle waveguide structure can realize high antenna gain and the reflector using PBG can excite wide-width OLWA efficiently. Our proposed structure obtains over 45 dBi directional gain in simulation and measurement results, which show good agreement. To our best knowledge, this the first demonstration of the WWG excited by parabolic reflector using PBG.

We demonstrate a nonmechanical, on-chip optical beam-steering device using a photonic-crystal waveguide with a doubly periodic structure that repeats the increase and decrease of the hole diameter. We fabricated the device using a complementary metal–oxide–semiconductor process. We obtained a beam-deflection angle of 24° in the longitudinal direction, while maintaining a divergence angle of 0.3°. Four such waveguides were integrated, and one was selected by a Mach–Zehnder optical switch. We obtained lateral beam steering by placing a cylindrical lens above these waveguides. By combining the lateral and longitudinal beam steering, we were able to scan the collimated beam in two dimensions, with 80 × 4 resolution points.

We demonstrate a non-mechanical on-chip optical beam steering device using the photonic crystal waveguide with a double periodic structure that repeats the increase and decrease of hole diameter. Guided slow light in this waveguide is radiated to be a light beam. Slow light shows strong dispersion, which allows a deflection angle of approximately 10 times that of a normal diffraction grating. We fabricated the device using complementary metal oxide semiconductor process and observed a beam deflection angle of 24° in the longitudinal direction with maintaining the divergence angle of 0.3° when the wavelength was changed by 27 nm. Four such waveguides were integrated, and one of them was selected by a Mach-Zehnder optical switch. Then, the lateral beam steering was obtained when a cylindrical lens was placed above these waveguides. By combining the longitudinal and lateral beam steering, the collimated beam was scanned two-dimensionally with 80 × 4 resolution points.

The double-periodic Si photonic crystal waveguide radiates guided slow light into free space as an optical beam. It also functions as a beam steering device, in which the steering angle is changed widely by the slight wavelength variant thanks to the large dispersion of slow light. A similar function is obtainable when the wavelength is fixed and the refractive index of the waveguide is changed. In this study, we integrated two kinds of heater structures in the waveguide and demonstrated the beam steering by the thermo-optic effect. For a p-i-p doped heater structure, we implanted a p-type dopant except around the waveguide core, and observed a beam steering angle Δθ = 26°, which is close to a theoretical value, with a relatively low heating power P = 1.6 W and high-speed response of 100 kHz order. However, the beam divergence increased up to δθ = 5°, which seemed to reflect the temperature nonuniformity in the Si slab. On the other hand, for the TiN heaters placed away from the waveguide core, we obtained a comparable steering angle with a narrower beam divergence of δθ < 0.3°. However, the required heating power was as large as P = 4.8 W, and the response speed was slow, reflecting its low heating efficiency and large heat capacity. We expect these problems to be solved by homogenizing the current and temperature distributions for the former and by optimizing the positioning of the heaters for the latter.

Time of flight LiDARs are used for auto-driving of vehicles, while FMCW LiDARs potentially achieve a higher sensitivity. In this study, we fabricated and tested each component of a FMCW LiDAR based on Si photonics and experimentally simulated the ranging action. Here, we drove a Si photonic crystal slow light modulator with linearly frequency-chirped signal in the frequency band of 500–1000 MHz and a repetition frequency of 100 kHz, to generate FM-signal light from a narrow-linewidth laser source. Next, we branched the signal light into two paths. One was inserted into a fiber delay line of 20–320 m and its output was irradiated to a photonic crystal slow beam steering device acting as an optical antenna via the free-space transmission. When the irradiation angle was optimized so that the antenna gain took maximum for a set laser wavelength, light was efficiently coupled into the antenna. We mixed the light output from the antenna with reference light of the other path with no delay, and detected it by balanced photodiodes. We observed a beat signal whose frequency well agreed with the theoretical value predicted from the length of the delay line. Thus, we succeeded in the experimental simulation of the FMCW LiDAR. We also observed a spectral sequence around the beat spectrum, in which the inter-frequency spacing equals the repetition frequency and corresponds to a range resolution of 30 cm which will be improved by expanding the modulation bandwidth.

We propose and demonstrate triangular-shaped coupled Si microrings for robust wavelength multi-/demultiplexing. Thanks to the geometry, stable interring couplings are obtained at straight waveguides, and heaters for thermal tuning can be isolated maximally with each other so that the thermal crosstalk is suppressed between rings. To suppress the bend loss and enhance the free spectral range (FSR), 2-μm-radius low-loss sharp bends with an optimum waveguide offset are employed. A CMOS-compatible process is used for fabricating second- and fourth-order coupled-microrings designed for the Butterworth and Chebyshev spectral responses, respectively. In the second-order rings, flat-top Box-like drop spectra with fluctuations 30 dB, bandwidth of 2.2 nm, FSR of 19 nm, and thermal tuning up to 8 nm are obtained experimentally. The efficiency of the thermal tuning is better and the thermal crosstalk is lower than those in conventional circular and race-track rings for such wide FSR.

This paper presents plane wave excitation for the optical leaky waveguide (OLWA) antenna by the arrayed slow taper. The proposed arrayed slow taper is composed of divided slow taper which is usually used for incident to the OLWA. The grating waveguide (GWG) is used to test the proposed structure. Our proposed arrayed slow taper enables to excite the GWG with half size of incident circuit as compared to conventional slow taper.

Silica-clad silicon photonic crystal waveguides (PCWs) are promising components for various applications because of their simple fabrication and generation of slow light. However, an optical loss higher than 4 dB occurs when they are simply coupled to input/output silicon wire waveguides. To reduce the optical loss, we proposed a junction structure in which light in the waveguide is first coupled to a high-group-velocity radiation mode at an expanded core and subsequently converted to the slow-light mode in a tapered core of the PCW. The coupling loss at a junction is calculated to be 0.28 dB at its minimum and less than 0.5 dB for the wavelength range of 12 nm. We measured a coupling loss of 0.46 dB for the device fabricated by the silicon photonics process. This low-loss junction well supports the practical use of PCWs.

This paper presents the plane wave excitation of the optical leaky waveguide antenna (OLWA) by the taper array and offset parabolic reflector to obtain high antenna gain. The taper array is composed of 12 slow tapers, and obtain high aperture efficiency. The parabolic reflector enables small size as compared with the taper array. We compare the performance of these excitation methods for the wide grating waveguide (GWG) in the simulation.

We propose an ultra-compact solid-state auto-correlator fabricated by Si photonics technology. The correlation waveform is obtained by detecting the overlap of two slow-light pulses counter-propagating in a photonic crystal waveguide integrated with two-photon absorption photodiode array. Since the device does not need a mechanical delay scanner, the full integration on a silicon-on-insulator, easy operation and high reliability are available. Different from conventional single-shot auto-correlators, it shows a much higher sensitivity for low-power pulses owing to the strong confinement and nonlinear enhancement of slow light. We succeeded in acquiring the auto-correlation waveform of pico-second pulses. The detection limit evaluated from the product of peak and average powers was of the order of 10−7 W2, which is equal to or even better than that of the commercial auto-correlators and 107 times better than the single-shot one's.

This paper presents a kind of optical leaky waveguide antenna (OLWA), circular waffle waveguide (CWWG), and demonstrates the gain enhancement by reducing perturbations. This antenna is fabricated by silicon photonics which is low-cost and mass-production. The CWWG has circular hole array on the waveguide surface instead of conventional linear grating. These holes reduce perturbations to extend effective antenna aperture, resulting in high antenna gain. To reduce more perturbations, we fabricate shallow hole array on waveguide surface. We confirm the antenna gain enhancement of the CWWG in both the simulation and the experiment.

Slow light in photonic crystal waveguides allows the phase shifters in Si MZ modulators to enhance the modulator efficiency. In this study, the group index and bandwidth product is maximized, the mode penetration into photonic crystal claddings and electrical series resistance are balanced, and the p-n junction profile is optimized. The device is fabricated on 200 mm wafer using KrF exposure, and practical performance (bitrates up to 32 Gbps, 3 dB extinction ratio, 1.75 V PPG voltage

A lattice-shifted photonic crystal waveguide (LSPCW) maintains slow light as a guided mode and works as an optical antenna when a kind of double periodicity is introduced. Selecting one LSPCW from its array and converting the fan beam to a spot beam using a collimator lens allows non-mechanical, two-dimensional beam steering. We employed a shallow-etched grating into the LSPCW as the double periodicity to increase the upward emission efficiency and designed a bespoke prism lens to convert the steering angle in a desired direction while maintaining the collimation condition for the steered beam. As a result, a sharp spot beam with an average beam divergence of 0.15° was steered in the range of 40 ∘ × 4.4 ∘ without precise adjustment of the lens position. The number of resolution points obtained was 4256. This method did not require complicated and power-consuming optical phase control like that in optical phased arrays, so it is expected to be applied in complete solid-state light detection and ranging.

We demonstrate an ultra-compact solid-state auto-correlator without delay scanning. We observe counter-propagating slow light pulses in a Si photonic crystal waveguide with a two-photon-absorption photodiode array. Sensitive detection is available for picosecond pules.

This paper presents the antenna pattern reconstruction of optical leaky waveguides antenna by phase-less measurement. In recent years, there has been tremendous increased in data traffic due to the widespread of mobile terminals, and high data transmission system for next generation is expected. Optical communication is considered as one of those systems. We have studied grating (GWG) and waffle waveguides (WWG) which are a type of optical leaky waveguide antennas (OLWA). These antennas can tilt a beam by sweeping incident wavelength. Each waveguide measurement is evaluated at near field because the antenna structure is very large with respect to the wavelength. To evaluate the antenna pattern from near field of the beam scanning antenna, amplitude and phase are essential. In the experimental system of optical device, measuring intensity of the light is easy. The measurement of phase is difficult. In this study, directivity of each waveguide is estimated by the aperture distribution method using approximate aperture distribution. The proposed method shows that estimate gain and radiation angle are in good agreement with calculated result.

We demonstrate label-free imaging of living cells using a GaInAsP semiconductor imaging plate. The photoluminescence (PL) intensity is changed by immersing the semiconductor wafer in different pH solutions and by depositing charged polyelectrolytes on the wafer. Various observations indicate that this phenomenon arises from the radiative and surface recombination rates modified by the Schottky barrier at the charged semiconductor surface. HeLa cancer cells were cultured on the semiconductor, and PL was observed using a near-infrared camera. The semiconductor areas with the cells attached exhibited characteristic PL profiles, which might reflect the attachment and surface condition of the cells, cellular matrix, and other substances.

The detection of low-concentration biomarkers is expected to facilitate the early diagnosis of severe diseases, including malignant tumors. Using photonic crystal nanolaser sensors, we detected prostate-specific antigen (PSA) from a concentration of 1 fM, which is difficult to detect by conventional enzyme-linked immunosorbent assay. The signal intensity and stability were improved by using a surfactant (i.e., ethanolamine). Even when a contaminant such as bovine serum albumin was mixed into the PSA sample, thereby increasing the concentration of the contaminant ten billion times, it was still possible to maintain a high level of detection.

We report what we call co- and counter propagating slow-light systems based on the high nonlinearity in Si lattice-shifted photonic crystal waveguides (LSPCWs). The intense slow-light pulse, as a control pulse, efficiently generates two-photon absorption and carrier plasma effects, which tunes the dispersion characteristics of the LSPCWs and spectrum of copropagating or counter-propagating slow-light pulse, as a signal pulse. Using the control pulse, we succeeded in experimentally demonstrating adiabatic wavelength conversion and its enhancement, delay tuning of up to 10 ps with a response time

We demonstrate 10 Gb/s modulation in a 200 μm photonic crystal silicon optical modulator, in both carrier-injection and depletion modes. In particular, this is the first demonstration of 10 Gb/s modulation in depletion mode and without pre-emphasis, in a Mach-Zehnder type modulator of this length, although moderate pre-emphasis can improve the signal quality. This is made possible by utilizing the slow-light of the photonic crystal waveguide, where the group index ng is ~ 30 and gives ~ 7 times enhancement in the modulation efficiency compared to rib-waveguide devices. We observe 10 Gb/s modulation at drive voltages as low as 1.6 V and 3.6 V peak-to-peak, in injection- and depletion-modes, respectively.

GaInAsP photonic crystal (PC) point-shift nanolasers operate at room temperature and under pulsed and continuous-wave condition by photopumping, with an effective threshold of ～1 μW, a single mode peak of over 40 dB, and a small modal volume of -0.2 times the cubic wavelength. We report the details of its design, fabrication process, measurement method, and lasing characteristics. In particular, we reveal that wide spectral broadening often observed for nanolasers under pulsed condition is caused by large thermal chirping. Then, we focus on a nanolaser-based liquid index sensor, which is also applicable to detecting gases and bio-molecules. The dependence of the laser's sensitivity and resolution on the modal profile and spectral linewidth are investigated. We also configure the laser in a large-scale array, and demonstrate spectrometer-free sensor system. Finally, we present a nanoslot nanolaser, which is particularly suitable for further reducing the modal volume and enhancing light-matter interaction and sensor performance. Additionally, we demonstrate that high sensitivity and spectral narrowing by the unique optical confinement in the nanoslot gives rise to.

Optical nonlinearity can be enhanced by the combination of highly nonlinear chalcogenide glass and photonic crystal waveguides (PCWs) providing strong optical confinement and slow-light effects. In a Ag-As(2)Se(3) chalcogenide PCW, the effective nonlinear parameter γeff reaches 6.3 × 10(4) W(-1)m(-1), which is 200 times larger than that in Si photonic wire waveguides. In this paper, we report the detailed design, fabrication process, and the linear and nonlinear characteristics of this waveguide at silica fiber communication wavelengths. We show that the waveguide exhibits negligible two-photon absorption, and also high-efficiency self-phase modulation and four-wave mixing, which are assisted by low-dispersion slow light.

We proposed and demonstrated a wavelength-division multiplexing (WDM) optical beam-steering device consisting of a thermally controlled doubly periodic Si two-dimensional bulk photonic crystal waveguide and coupled microring multiplexers. Beam forming and steering while maintaining a sharp profile is much easier in this device than with optical phased arrays which need the fine phase control. By dividing the range of beam-steering angles into different wavelength channels, it is possible to cover a wide range of angles, even when each angle is small. In this study, we fabricated a device with four wavelength channels, each of which showed beam steering of 4°-5° as a result of heating, resulting in a total of 16°. Two-dimensional steering is also achieved by loading a collimator lens and selecting one waveguide from those arrayed. We evaluated 112 resolution points with four wavelengths and 448 points in total by switching four waveguides. If this WDM concept is introduced into light detection and ranging and the number of wavelengths is increased, it will be possible to increase the sensing throughput, which is usually constrained by the round-trip time of light, by simultaneous parallel operation.

We demonstrate a Si photonic crystal waveguide Mach-Zehnder modulator that incorporates meander-line electrodes to compensate for the phase mismatch between slow light and RF signals. We first employed commonized ground electrodes in the modulator to suppress undesired fluctuations in the electro-optic (EO) response due to coupled slot-line modes of RF signals. Then, we theoretically and experimentally investigated the effect of the phase mismatch on the EO response. We confirmed that meander-line electrodes improve the EO response, particularly in the absence of internal reflection of the RF signals. The cut-off frequency of this device can reach 27 GHz, which allows high-speed modulation up to 50 Gbps.

Photonic crystals (PCs) are multi-dimensional periodic gratings, in which the light propagation is dominated by Bragg diffraction that appears to be refraction at the flat surfaces of the PC. The refraction angle from positive to negative, perfectly or only partially obeying Snell's law, can be tailored using photonic band theory. The negative refraction enables novel prism, collimation, and lens effects. Because PCs usually consist of two transparent media, these effects occur at absorption-free frequencies, affording significant design flexibility for free-space optics. The PC slab, a high-index membrane with a two-dimensional airhole array, must be carefully designed to avoid reflection and diffraction losses. Light focusing based on negative refraction forms a parallel image of a light source, facilitating optical couplers and condenser lenses for wavelength demultiplexing. A compact wavelength demultiplexer can be designed by combining the prism and lens effects. The collimation effect is obtainable not only inside but also outside of the PC by optimizing negative refractive condition.

We report an on-chip autocorrelator comprising a Si rib waveguide and two-photon-absorption photodiodes in which two pulses simultaneously propagate in opposite directions, and their correlation is acquired. The rib waveguide's broadband guiding mode and moderately low group index allow the device to operate over a wide wavelength range with high resolution. We confirm that the waveguide has a transmission band of at least 1300-1630 nm and observed correlation waveforms for pulses of the order of several hundred femtoseconds to several picoseconds, with a range of at least the entire C band region.

In this paper, we demonstrate state-of-the-art slow light in silicon-on-insulator photonic crystal coupled waveguide, which allows slow light pulse transmission and its tunable delay by means of structural chirping. The key idea of this study is the application of a folded chirping profile to the structure, instead of the conventional monotonous chirping. It suppresses unwanted spectral oscillation caused by structural disordering and expands the tuning range. By postprocessing an airhole-diameter-chirped device, we show that 0.9-ps-wide slow light pulses are delayed for 72 ps, corresponding to a buffering capacity of 80 bits. In a separate, unchirped device, we demonstrate a tunable delay by applying thermally induced index chirping. Here, a maximum tuning range of 103 ps and a tunable capacity of 22 bits are obtained.

Proposed is a simple silicon wire intersection with low diffraction loss and crosstalk. Finite-difference time-domain analysis shows the lowest loss to be as small as 0.14 dB. A nearly optimum structure was fabricated on SOI substrate, the measured loss being 0.24 dB, which is only 0.01 dB larger than the theoretical loss.