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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.

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.

We have developed photonic crystal slow-light devices and photonic integration. Inverse design and compatibility to Si CMOS foundry process enable practically low loss components and demonstrations of LiDAR and high-speed modulator.

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.

In this chapter, the authors discuss some spontaneous emission characteristics in microcavity surface emitting lasers (SELs). Modified distribution of modes in the SEL is calculated and displayed over the optical wave vector space (k-space). The spontaneous emission factor is calculated to evaluate the amount of spontaneous emission control in the SELs and the possibility of the thresholdless operation. The spontaneous emission factor for the Bragg resonant mode in microcavity SELs is as large as 103 to 105 times that in conventional horizontal cavities. It is not so dependent on the vertical cavity quality but is almost inversely proportional to the cavity lateral size. To realize such an SEL experimentally, the reduction of the nonradiative recombination at the microcavity surface will be the important issue. In addition, investigation of the absolutely closed cavity structure will provide more remarkable effects of the microcavities.

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.

Wide-range nonmechanical beam steering is available by an array of Si photonic crystal slow-light waveguides and their switching without complicated control. FMCW LiDAR action is obtained with this beam steering on a Si photonics chip.

Photonic crystal has a history of 40 years, and some devices were commercialized recently. But they do not use a photonic band gap, which has been thought as the most important property of photonic crystals and actually produced various unique functions such as slow light. This presentation focuses on modulator and LiDAR, utilizing slow light in photonic crystals fabricated by silicon photonics technology.

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.

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.

We demonstrate that the lasing wavelength and emission intensity of GaInAsP photonic crystal nanolaser can be controlled by immersing it to an electrolyte and applying a bias voltage. This method gives an option to the control of semiconductor lasers by the iontronic effect.

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.

This paper presents a excitation method of optical leaky waveguide antenna (OLWA) by parabolic reflector using photonic bandgap. We evaluate the performance of OLWA by proposed and conventional excitation method in the simulation. Our proposed method enables to excite OLWA with small feeding circuit.

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.

We have developed GaInAsP semiconductor photonic crystal nanolaser biosensor, which detects the adsorption of proteins and DNAs from ultralow concentrations via the wavelength shift. In general, bio-chemical sensors based on photonic cavities are considered to detect the environmental refractive index change. In this study, however, we observed various evidence that the wavelength shift of the nanolaser sensor reflects not only the environmental index but also the surface charge of biomolecules and/or environmental ions. Different from the pH-sensitivity in the laser emission intensity we reported previously, the wavelength shift is extremely sensitive to the surface charge and ions so that we can detect the biomolecules from fM to aM. To investigate the mechanism, we exposed the device to plasma or DNA solution, and measured the zeta potential and flatband potential as well as the wavelength shift. From the consistent behaviors between them, we constructed a physical model that the surface charge modifies the Schottky barrier near the semiconductor surface, trap the excited carriers in the barrier, and produce the carrier plasma shift in the wavelength. Since the change of the flatband potential is already observed for fM DNA, we suppose a phenomenon such that a small number of charged molecules act as a trigger for changing the charged functional group on the device surface. These results also suggest the possibility that some other photonic sensors also involves similar electrochemical effects.

We demonstrate an adiabatic wavelength redshift using dynamic carrier depletion. Free carriers are first induced through two-photon absorption of a control pulse and then extracted by a reverse-biased p-i-n diode formed on a Si photonic crystal waveguide, resulting in rapid carrier depletion. A copropagating signal pulse is redshifted by the consequent increase in refractive index. We experimentally evaluated the dynamics of the carrier depletion by the pump-probe method and explored suitable conditions for adiabatic redshift. The signal's redshift was observed, and was confirmed to originate in the dynamic carrier depletion. The redshift was experimentally determined as 0.21 nm.

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.

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.

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.

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.

In this study, we fabricated a photoelectrochemical circuit using GaInAsP photonic crystal nanolasers as a working electrode. Then, we controlled the emission intensity and lasing wavelength of these nanolasers by applying a bias voltage in an ionic solution. The electrochemical working points for the emission intensity and wavelength were observed for the backward and forward biases, respectively. We confirmed that the emission intensity is primarily changed by the surface recombination, which is enhanced by the Schottky barrier near the solid–liquid interface. The wavelength shift is also assumed to be caused by the Pockels effect in the electric double layer of the solution. This control method can maximize and stabilize the performance of photonic biochemical sensors and also become an option in controlling the laser diode characteristics., This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in (Yoshito Saijo et al 2019 App. Phy. Let. 114 221105) and may be found at http://doi.org/10.1063/1.5098119.

We present the results of a theoretic study of the electromagnetic field imbalance in surface plasmon polaritons (SPPs), which reveal that the magnetic field components induced by the electric fields normal and parallel to a metal surface cancel each other in SPPs, resulting in an imbalance. A group velocity analysis shows that this imbalance contributes significantly to the slow propagation of SPPs. We also analyze the enhanced spontaneous emission and nonlinearity in plasmonic cavities, and the results indicate that the electromagnetic field imbalance must be considered to correctly estimate the interaction strength.

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 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

High-performance and low-cost sensors are critical devices for high-throughput analyses of bio-samples in medical diagnoses and life sciences. In this paper, we demonstrate photonic crystal nanolaser sensor, which detects the adsorption of biomolecules from the lasing wavelength shift. It is a promising device, which balances a high sensitivity, high resolution, small size, easy integration, simple setup and low cost. In particular with a nanoslot structure, it achieves a super-sensitivity in protein sensing whose detection limit is three orders of magnitude lower than that of standard surface-plasmon-resonance sensors. Our investigations indicate that the nanoslot acts as a protein condenser powered by the optical gradient force, which arises from the strong localization of laser mode in the nanoslot.

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.

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.

Light focusing characteristics of a negative refractive lens fabricated out of a silicon-on-insulator photonic crystal (PC) slab are investigated theoretically and experimentally. It focuses in the near infrared, but the focal spot is degraded by a lens aberration. To reduce the aberration, we designed a composite PC that gives rise to a narrower focal spot. In addition, two unique functions of this lens are demonstrated: refocusing outside of the PC and parallel focusing, enabling image transfer and real image formation, respectively. These results prove the feasibility of an in-plane free space optical network based on negative refraction.

A GaInAsP photonic crystal slab microlaser with an integrated passive waveguide was fabricated by a MOCVD butt-joint regrowth process. The boundary between the active and passive regions was optimized for high light transmission. In addition, the edge of the waveguide was designed to output a narrow beam, and the light detection was improved. Consequently, a maximum output power of 0.17 mW was obtained with a differential quantum efficiency of 20%, which would increase to more than 40% under ideal detection conditions.

Slow light with a remarkably low group velocity is a promising solution for buffering and time-domain processing of optical signals. It also offers the possibility for spatial compression of optical energy and the enhancement of linear and nonlinear optical effects. Photonic-crystal devices are especially attractive for generating slow light, as they are compatible with on-chip integration and room-temperature operation, and can offer wide-bandwidth and dispersion-free propagation. Here the background theory, recent experimental demonstrations and progress towards tunable slow-light structures based on photonic-band engineering are reviewed. Practical issues related to real devices and their applications are also discussed. The unique properties of wide-bandwidth and dispersion-free propagation in photonic-crystal devices have made them a good candidate for slow-light generation. This article gives the background theory of slow light, as well as an overview of recent experimental demonstrations based on photonic-band engineering.

Photonic crystals are multidimensional periodic gratings, in which the light propagation is dominated by Bragg diffraction that appears to be refraction at the flat surfaces of the crystals. The refraction angle from positive to negative, perfectly or only partially obeying Snell's law, can be tailored based on photonic band theory. Negative refraction enables novel prism, collimation, and lens effects. Because photonic crystals usually consist of two transparent media, these effects occur at absorption-free frequencies, affording significant design flexibility for free-space optics. The photonic-crystal slab, a high-index membrane with a two-dimensional airhole array, must be carefully designed to avoid unwanted reflection and diffraction. 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.

This paper presents experimental performance of Optical Leaky Wave Antenna (OLWA) using waffle waveguide (WWG). The antenna is fabricated by silicon photonics technology to achieve high directivity gain leaky waveguide antenna with small perturbation parameter of WWG. Narrow beam and one-dimensional beam scanning is demonstrated by sweeping input wavelength of OLWA. The measured WWG performance is also compared with a grating waveguide antenna.

We propose and demonstrate an on-chip optical correlator, in which two types of photonic crystal slow-light waveguides are integrated and operated as an optical delay scanner and a two-photon-absorption photodetector. The footprint of the device, which was fabricated using a CMOS-compatible process, was 1.0 × 0.3 mm(2), which is substantially smaller than that of conventional optical correlators with free-space optics. We observed optical pulses using this device and confirmed the correspondence of pulse waveforms with those observed using a commercial correlator when the pulse width was 5-7 ps. This device will achieve one-chipping of an optical correlator and related measurement instruments.

Doppler shift of signal light is obtainable by moving photonic bandgap mirror, which is dynamically formed by slow light pulse. Large wavelength shift from several 10 nm to a few 100 nm are numerically demonstrated.

We comprehensively investigated practical silica-clad photonic crystal waveguides, and found two lattice shifts optimum for low-dispersion slow light. Fabricated device shows a high group index of >50, which ensures twice higher performance than conventional ones.

We fabricated a compact on-chip optical correlator using two photonic crystal slow light waveguides, and succeeded in complete on-chip operation. Observed pulse waveform agreed well with the one observed by commercial optical correlator.

We observed the avalanche photodiode operation through defect levels at telecom band in Si photonic crystal slow light modulator. Maximum responsivity was 0.71 A/W with 350 avalanche gain. The eye opened at 20 Gbps.

We demonstrate on-chip pulse compression based on the dynamic tuning in a copropagating slow-light system. Low-dispersion slow-light control pulse in lattice-shifted photonic-crystal waveguides enhances two-photon absorption. Generating free carriers dynamically through the two-photon absorption so as to sweep a signal slow-light pulse, the spectral broadening occurs with a monotonic chirp. The signal pulse is then compressed through the compensation of the chirp by means of integrated heaters. An input pulse of 13.9 ps length was compressed to 1.4 ps, corresponding to a compression factor of 9.9.