Photonic Crystal Reflectors Research Articles

Laser diode self-injection locking to an integrated high-Q Fabry-Perot microresonator.

Self-injection locking (SIL) of laser diodes to microresonators is a powerful technique that enables compact narrow-linewidth lasers. Here, we extend this technique to chip-integrated Fabry-Perot (FP) microresonators, which offer high-quality factors and large mode volumes in a compact footprint, reducing fundamental thermorefractive noise (TRN). The resonators consist of a silicon nitride waveguide terminated by two photonic crystal reflectors fabricated via scalable ultraviolet lithography. Evanescent side-coupling allows precise tuning of the SIL feedback mechanism. We present a theoretical model and experimentally demonstrate SIL, resulting in a fundamental thermorefractive-noise-limited laser. The experiments and the theoretical model are in excellent agreement. These results complement current SIL techniques and are relevant to chip-scale low-noise laser systems.

Integrated chirped photonic-crystal cavities in gallium phosphide for broadband soliton generation

Chirped mirrors have underpinned advances in ultra-fast lasers based on bulk optics but have yet to be fully exploited in integrated photonics, where they could provide a means to engineer otherwise unattainable dispersion profiles for a range of nonlinear optical applications, including soliton frequency comb generation. The vast majority of integrated resonators for frequency combs make use of microring geometries, in which only waveguide width and height are varied to engineer dispersion. Here, we present an integrated photonic-crystal Fabry–Pérot resonator made of gallium phosphide (GaP), a material exhibiting a Kerr nonlinearity 200 times larger than that of silicon nitride and a high refractive index that permits the creation of strongly chirped photonic-crystal mirrors. Leveraging the additional degrees of freedom provided by integrated chirped mirrors, we disentangle optical losses from dispersion. We obtain an overall dispersion that is more anomalous than that achievable in both silicon nitride and gallium phosphide ring resonators with the same free-spectral range (FSR), while simultaneously obtaining higher quality factors than those of GaP ring resonators. With subharmonic pulsed pumping at an average power of 23.6 mW, we are able to access stable dissipative Kerr frequency combs in a device with a FSR of 55.9 GHz. We demonstrate soliton formation with a 3-dB bandwidth of 3.0 THz, corresponding to a pulse duration of 60 fs. This approach to cavity design based on photonic-crystal reflectors offers nearly arbitrary dispersion engineering over the optical transparency window of the nonlinear material.

Photonic Crystal Reflectors with Ultrahigh Sensitivity and Discriminability for Detecting Extremely Low-Concentration Surfactants.

Developing a facile, intuitive, ultrahigh-sensitive sensor to detect harmful substances in water is critical. Here, an ultrahigh-sensitive sensor is fabricated using a quaternized lamellae-structured polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) block copolymer (BCP), capable of detecting the heavily used surfactants including sodium dodecyl sulfate (SDS) and sodium methyl sulfate (SMS) through direct visualization of the structural color change. Two distinct detecting mechanisms, including unexpected blue-shifting and red-shifting reflectance wavelengths, are found for low and high concentrations of the SDS surfactant, respectively, due to concentration-dependent compatibility between the quaternized P2VP (QP2VP) block chains and SDS molecules. As the SDS concentration is low (0-1 mM), the QP2VP chains undergo the counter anionic exchange with the hydrophobic alkyl chains of the SDS, resulting in a blue shift toward colorlessness. In contrast, as the SDS concentration is high (>1 mM), the nanoaggregation of the SDS molecules in the layered QP2VP microdomain leads to enhanced hydration nature and increased lamellar periodicity with the red-shifting reflectance wavelength. In contrast, SMS with weaker hydrophobicity results in unchanged and red-shifting reflectance wavelengths at low and high concentrations. Inspired by this, detecting the extremely low-concentration SDS surfactant (0.01 mM) by direct visualization is achieved. The structural color change for surfactant detection also exhibits excellent reversibility and discriminability, providing a straightforward method of detecting anionic surfactants.

Transformation one-dimensional photonic crystal omnidirectional reflector

In this paper, a concept of transformation one-dimensional photonic crystal is proposed to design omnidirectional reflector, which has a reduced thickness compared with that of the omnidirectional reflector using conventional one-dimensional photonic crystal structure. At first, the omnidirectional reflection band of an original one-dimensional photonic crystal is simulated using the transfer matrix method for comparison. Next, the transformation one-dimensional photonic crystal is designed based on transformation optics. Simulation results indicate that the transformation one-dimensional photonic crystal structures with arbitrary shrunken thickness can omnidirectionally reflect the incident TM waves that operates in the same omnidirectional reflection band of the original one-dimensional photonic crystal structure. Such a concept of transformation one-dimensional photonic crystal also can be expanded to any omnidirectional reflector designed by using other multilayer dielectric systems.

Kerr solitons in high-Q integrated Fabry-Pérot microresonators

Disspative Kerr Solitons are generated in an integrated standing wave Fabry-Pérot microresonator. Enabled by synthetic anomalous dispersion provided by a pair of Photonic Crystal Reflectors (PCR) forming a high-Q cavity, the generated soliton pulses exhibit a unique spectral profile extending beyond the PCRs’ bandgap.

High Light‐Utilization‐Efficiency Organic Photovoltaic Windows with Over 40% Transmittance Enabled by a Printable Bionic Photonic Reflector

AbstractSemitransparent organic solar cells (ST‐OSCs) have promising applications in building materials, automobiles, and green windows. However, photovoltaic building materials have a requirement for transmittance, which only reaches over 40% and can satisfy the condition of energy saving. In this work, a printable bionic photonic reflector is introduced to overcome the insufficient light utilization efficiency (LUE) of ST‐OSCs. The inverse‐opal photonic crystal reflectors can enhance the secondary absorption of the active layer by adjusting the particle size to reflect the wavelength of light, which can match with the absorption of the donor. Meanwhile, it allows other wavelengths of sunlight to be passed through for transmittance. With this strategy, power conversion efficiency (PCE) and average visible transmittance (AVT) are simultaneously improved. As a result, the PCE is improved to 9.93% with an AVT of 40.36%, and the LUE is significantly improved from 3.63% to 4.01%, which is a great advance in the performance of ST‐OSCs reported.

Monolithic Integrated Micro-Opto-Electromechanical Accelerometer Based On Michelson Interferometer Structure

In this paper, we design a monolithic integrated micro-opto-electromechanical system(MOEMS) accelerometer based on the Michelson interferometer structure. The accelerometer realizes the on-chip integration of the mode spot converter, the sensing optical path, the phase modulator and the photonic crystal reflector, and it senses the external acceleration through a “three-beam” structure which is composed of a central support beam and two tiny beams. The push-pull structure eliminates the coupling crosstalk caused by paraxial acceleration. Accurate results are demonstrated as follows: an accelerometer mechanical sensitivity of 3.638 nm/g, with a resonance frequency of 1742.2 Hz and a linear measurement range of ± 500 g, corresponding to a foot print of the core part with 960 μm×600 μm. With its high precision, large measurement range, and small size, the proposed accelerometer may pave the way for the applications in civil, industrial and military fields.

Multiscale Photonic Emissivity Engineering for Relativistic Lightsail Thermal Regulation.

The Breakthrough Starshot Initiative aims to send a gram-scale probe to our nearest extrasolar neighbors using a laser-accelerated lightsail traveling at relativistic speeds. Thermal management is a key lightsail design objective because of the intense laser powers required but has generally been considered secondary to accelerative performance. Here, we demonstrate nanophotonic photonic crystal slab reflectors composed of 2H-phase molybdenum disulfide and crystalline silicon nitride, highlight the inverse relationship between the thermal band extinction coefficient and the lightsail's maximum temperature, and examine the trade-off between minimizing acceleration distance and setting realistic sail thermal limits, ultimately realizing a thermally endurable acceleration minimum distance of 23.3 Gm. We additionally demonstrate multiscale photonic structures featuring thermal-wavelength-scale Mie resonant geometries and characterize their broadband Mie resonance-driven emissivity enhancement and acceleration distance reduction. More broadly, our results highlight new possibilities for simultaneously controlling optical and thermal response over broad wavelength ranges in ultralight nanophotonic structures.

Enhanced absorption in thin and ultrathin silicon films by 3D photonic band gap back reflectors

Since thin and ultrathin silicon films have limited optical absorption, we explore the effect of a nanostructured back reflector to recycle the unabsorbed light. As a back reflector, we investigate a three-dimensional (3D) photonic band gap crystal made from silicon that is readily integrated with the thin silicon films. We numerically obtain the optical properties by solving the 3D time-harmonic Maxwell equations using the finite-element method, and model silicon with experimentally determined optical constants. The absorption enhancement spectra and the photonic band gap generated current density are obtained by weighting the absorption spectra with the AM 1.5 standard solar spectrum. We study thin films in two different regimes, much thicker (L Si = 2400 nm) or much thinner (L Si = 80 nm) than the wavelength of light. For L Si = 2400 nm thin film, the 3D photonic band gap crystal enhances the spectrally averaged (λ = 680 nm to 880 nm) silicon absorption by 2.22 × (s −pol.) to 2.45 × (p −pol.), which exceeds the enhancement of a perfect metal back reflector (1.47 to 1.56 ×). The absorption is considerably enhanced by the (i) broadband angle and polarization-independent reflectivity in the 3D photonic band gap, and (ii) the excitation of many guided modes in the film by the crystal’s surface diffraction leading to greatly enhanced path lengths. For L Si = 80 nm ultrathin film, the photonic crystal back reflector yields a striking average absorption enhancement of 9.15 ×, much more than 0.83 × for a perfect metal. This enhancement is due to a remarkable guided mode that is confined within the combined thickness of the ultrathin film and the photonic crystal’s Bragg attenuation length. An important feature of the 3D photonic band gap is to have a broad bandwidth, which leads to the back reflector’s Bragg attenuation length being much shorter than the silicon absorption length. Consequently, light is confined inside the thin film and the remarkable absorption enhancements are not due to the additional thickness of the photonic crystal back reflector. We briefly discuss a number of high-tech devices that could profit from our results, including ultrathin film solar cells.

Resonant interaction between a core mode and two complementary supermodes in a honeycomb PCF reflector-based SPR sensor

In this paper, we show a new type of the resonant interaction between a core mode and two complementary supermodes in a honeycomb photonic crystal fiber reflector - based surface plasmon resonance sensor. The losses of the supermodes are close to the maximum of the loss of the core mode at a quasi-resonant wavelength (quasi loss matching point). The local spectral sensitivity is extremely sensitive to the radius of the gold layer. Thus, when the radius of the gold layer is increased with only 38.75 nm, the local spectral sensitivity of the maximum loss in the core mode is increased from 1000 nm/RIU to 4500 nm/RIU. Also, the local spectral sensitivity of the maximum imaginary part of the effective index in the core mode is increased from 1300 nm/RIU to 4950 nm/RIU. For a wavelength detection limit of 0.1 nm, the corresponding sensor resolutions are as fine as 2.2 × 10−5 RIU and 2.0 × 10−5 RIU, respectively. These values are better in comparison with other similar configurations.

Characterization of Waveguide Photonic Crystal Reflectors on Indium Phosphide Membranes

We present waveguide photonic crystal reflectors on the InP-membrane-on-silicon (IMOS) platform, and a method to accurately measure the reflectivity of those reflectors. The photonic crystal holes are patterned on a waveguide using electron-beam lithography and etched through the waveguiding layer to create a broadband distributed Bragg reflector. We show simulations of these reflectors and experimental results of fabricated devices, both showing a high, free-to-choose reflectivity, and high quality factor Fabry-Perot cavities. We experimentally show reflectivities higher than 95% for the reflectors and a quality factor as high as 15,911±511 for a Fabry-Perot cavity, using reflectors with a length of only 4 microns. For the first time, to our knowledge, two methods for measuring the reflectivity are used for characterization of on-chip reflectors to accurately determine the reflection. The first method is based on analysis of the transmission through a Fabry-Perot cavity, the second is based on a direct four-port measurement of the reflector. A systematic error is made in both methods, resulting in an upper and lower boundary for the actual reflection coefficient.

Three-dimensional macroporous photonic crystal enhanced photon collection for quantum dot-based luminescent solar concentrator

A luminescent solar concentrator (LSC) is a photon managing device that can harvest, direct and concentrate solar light to small areas, enabling subsequent coupling to photovoltaic devices (PVs) for enhanced solar energy conversion. However, the intrinsic photon loss through the so-called escape cone of the LSCs significantly limits their light harvesting and concentrating performance. In this work, we introduce a facile and low-cost approach for the fabrication of a three-dimensional (3D) macroporous photonic crystal (PC) filter as an efficient photon reflector, which can be coated onto quantum dot (QD) based LSC devices. We demonstrate that by controlling the PC reflection band to match the emission profile of the QD emitters, the light trapping efficiency of the PC coated LSC (PC-LSC) can be significantly improved from 73.3% to 95.1% as compared to the conventional PC-free LSC due to the reduced escape cone photon loss. In addition, we have developed a simulation model that considers the PC reflector effect. Both experimental and simulation results show that the enhancement in LSC device performance induced by the PC reflector increases with increasing dimension. In fact, simulation data predicts a maximum of 13.3-fold enhancement in external quantum efficiency (EQE) and concentration factor (C factor) of the PC-LSC under more ideal conditions. Moreover, the simulation result offers insight into the relationship between photon output efficiencies and the geometric design of the PC-LSC. Our study sheds light on future design and fabrication of LSC devices with enhanced photon collection and concentrating efficiencies through novel and wavelength-selective photon reflectors.

Photonic-Crystal-Reflector Nanoresonators for Kerr-Frequency Combs

We demonstrate Kerr-frequency-comb generation with nanofabricated Fabry–Perot resonators, which are formed with photonic-crystal-reflector (PCR) mirrors. The PCR group-velocity dispersion (GVD) is engineered to counteract the strong normal GVD of a rectangular waveguide, fabricated on a thin, 450 nm silicon nitride device layer. The reflectors enable resonators with both high optical quality factor and anomalous GVD, which are required for Kerr-comb generation. We report design, fabrication, and characterization of devices in the 1550 nm wavelength bands. Kerr-comb generation is achieved by exciting the devices with a continuous-wave laser. The versatility of PCRs enables a general design principle and a material-independent device infrastructure for Kerr-nonlinear-resonator processes, opening new possibilities for manipulation of light. Visible and multispectral-band resonators appear to be natural extensions of the PCR approach.

Reflecting AWG by Using Photonic Crystal Reflector on Indium-Phosphide Membrane on Silicon Platform

In this letter, a reflection-type arrayed waveguide grating (AWG) (de)multiplexer using high-reflection photonic crystal reflector (PCR) in the Indium-phosphide Membrane on Silicon (IMOS) platform is proposed and experimentally demonstrated for the first time. This reflection-type AWG enables a 35% size-reduced footprint compared with the traditional transmission-type AWG having the same spectral parameters. Considering the realized performance of silicon-nanowire R-AWGs, an acceptable performance ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$680\,\times \,190\,\mu \text {m}^{2}$ </tex-math></inline-formula> size, 6.7-dB loss, and 10-dB crosstalk) is obtained. By using the ultra-small <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$5.4\,\times \,0.7\,\mu \text {m}^{2}$ </tex-math></inline-formula> PCR, merely 1.1-dB power loss higher than the corresponding transmission-type AWG is implemented. The PCR is a standard building block with a reflectivity of >90%. Besides, the length reduction of the arrayed waveguide will contribute to minimize the accumulated phase error in fabrication.

Tunable optical assembly of subwavelength particles by a microfiber cavity

Optical assembly as a multiple optical trapping technique enables patterned arrangements of matter ranging from atoms to microparticles for diverse applications in biophysics, quantum physics, surface chemistry, and cell biology. Optical potential energy landscapes based on evanescent fields are conventionally employed for optical assembly of subwavelength particles, but are typically limited to predefined patterns and lacking in tunability. Here we present a microfiber photonic crystal cavity applicable for tunable optical assembly of subwavelength particles along a flexible path. This is enabled by excellent mechanical flexibility of the microfiber cavity as well as its broadband photonic crystal reflectors. By virtue of the broadband reflectors, the lattice constant of the assembled particles is precisely tunable via altering the wavelength of input light. Three-dimensional optical assembly is also realized by making use of the high-order transverse mode of the microfiber cavity. Moreover, the optical assembly process is detectable by simply monitoring the reflection/transmission spectrum of the microfiber cavity. The design of the microfiber cavity heralds a new way for tunable optical assembly of subwavelength particles, potentially applicable for development of tunable photonic crystals, metamaterials, and sensors.

Colored solar cells with spectrally selective photonic crystal reflectors for application in building integrated photovoltaics

In this article, we have reported a nanophotonic coating called Selectively Modulated Aesthetic Reflector Technology (SMART), which can be integrated to any matured solar cell technology to produce colored solar cells or modules. This technology has a potential significance in building integrated photovoltaics to improve aesthetics along with being commercially feasible. As a proof of concept, we have integrated this technology into crystalline silicon solar cells. Here we have used one-dimensional dielectric photonic crystals to selectively reflect the light, to realize multi-color appearances for the solar cells. Silicon Nitride (SiN) and Silicon Oxynitride (SiON) dielectric layers with varying compositions and thickness were used to fabricate these SMART coatings. The optoelectronic effects have been elucidated by spectroscopic, quantum efficiency and I-V measurements. The color perception has been quantitatively investigated using CIE (International Commission on Illumination) 1931 chromaticity diagram. The CIE color co-ordinates for the SMART coatings were (0.39, 0.32), (0.25, 0.38) and (0.20, 0.17) respectively for Red, Green and Blue (RGB) colours and (0.27, 0.29) for white. The cells show an integrated transmittance between 60 and 80% in the absorption regime of the solar cell depending on its color. The efficiency and the performance of different colored solar cells namely RGB and white have been shown on 156 cm2 cell area. The RGB colors have a relative power conversion efficiency of ∼70–80% based on different colors whereas white has a relative efficiency of 59% as compared to the standard reference silicon solar cell. This process is industrially feasible and could utilize the vertical area of buildings, which could have a significant application in urban landscaping.

Paintable temperature-responsive cholesteric liquid crystal reflectors encapsulated on a single flexible polymer substrate

Temperature-responsive photonic cholesteric liquid crystal reflectors deposited on flexible single substrates, encapsulated by protective polymer layers generated by photo-enforced stratification.

Enhancement of light trapping for thin film solar cells

ABSTRACTLight trapping is one of the key challenges for next generation thin film solar cells. In this work, we have identified the distinct light trapping effects for short and long wavelength solar spectrum range, by investigating lighting trapping structures on both sides of Si thin film solar cells. The sub-wavelength photonic front surface by wet etching and multi-layer photonic crystal reflector on the bottom surface are studied in detail for its solar energy absorption characteristics. Our study reveals the drastic difference of the light trapping effects within the solar spectrum wavelength. This work may provide guidance for the efficiency enhancementfor next generation thin film photovoltaic cells.

Impact of one-dimensional photonic crystal back reflector in thin-film c-Si solar cells on efficiency

In this work, the effect of one-dimensional photonic crystal on optical absorption, which is implemented at the back side of thin-film crystalline silicon (c-Si) solar cells, is extensively discussed. The proposed structure acts as a Bragg reflector which reflects back light to the active layer as well as nanograting which couples the incident light to enhance optical absorption. To understand the optical mechanisms responsible for the enhancement of optical absorption, quantum efficiency and current density for all structures are calculated and the effect of influential parameters, such as grating period is investigated. The results confirm that our proposed structure have a great deal for substantial efficiency enhancement in a broad range from 400 to 1100 nm.

Monolithic Two-Dimensional Photonic Crystal Reflectors for the Fabrication of Highly Efficient and Highly Transparent Dye-Sensitized Solar Cells.

The transparent characteristic of dye-sensitized solar cells (DSCs) makes them suitable for building integrated photovoltaic (BIPV) devices. However, the diffusive scattering layer, which is usually used to increase the efficiency of these devices, greatly lowers the transparency of the DSC. This paper described a two-dimensional (2D) photonic crystal (PC) reflector with a sub-micrometer characteristic length that can improve the efficiency of these devices while maintaining transparency. This 2D PCs were fabricated directly onto TiO2 photoelectrodes using colloidal lithography and have the structure of a nanopillar array. A nanopillar with a height of 430 nm was observed to selectively reflect up to 40% of the light of 400-500 nm wavelength. The perceived transparency of the 2D PC electrode was 52%, which is much higher than 0.3% of the conventional scattering layer. The DSC fabricated using the 2D PC electrode demonstrated a maximum photon-to-electric conversion efficiency of 8.23%, which is 18% higher than the pristine electrode. The 2D PC is a highly efficient and wavelength-selective reflector that can be applied to various photoelectric conversion devices.