Compact thin film lithium niobate folded intensity modulator using a waveguide crossing

A small footprint, low voltage and wide bandwidth electro-optic modulator is critical for applications ranging from optical communications to analog photonic links, and the integration of thin-film lithium niobate with photonic integrated circuit (PIC) compatible materials remains paramount. Here, a hybrid silicon nitride and lithium niobate folded electro-optic Mach Zehnder modulator (MZM) which incorporates a waveguide crossing and 3 dB multimode interference (MMI) couplers for splitting and combining light is reported. This modulator has an effective interaction region length of 10 mm and shows a DC half wave voltage of roughly 4.0 V, or a modulation efficiency (Vπ ·L) of roughly 4 V·cm. Furthermore, the device demonstrates a power extinction ratio of roughly 23 dB and shows .08 dB/GHz optical sideband power roll-off with index matching fluid up to 110 GHz, with a 3-dB bandwidth of 37.5 GHz.

Instantaneous microwave-photonic spatial-spectral channelization via k-space imaging

The ability to both spatially and spectrally demultiplex wireless transmitters enables communication networks with higher spectral and energy efficiency. In practice, demultiplexing requires sub-millisecond latency to map the dynamics of the user space in real-time. Here, we present a system architecture, referred to as k-space imaging, which channelizes the radio frequency signals both spatially and spectrally through optical beamforming, where the latency is limited only by the speed of light traversing the optical components of the receiver. In this architecture, a phased antenna array samples radio signals, which are then coupled into electro-optic modulators (EOM) that coherently up-convert these signals to the optical domain, preserving their relative phases. The received signals, now optical sidebands, are transmitted in optical fibers of varying path lengths, which act as true time delays that yield frequency-dependent optical phases. The output facets of the optical fibers form a two-dimensional optical phased array in an arrangement preserving the phases generated by the angle of arrival (AoA) and the time-delay phases. Directing the beams emanating from the fibers through an optical lens produces a two-dimensional Fourier transform of the optical field at the fiber array. Accordingly, the optical beam formed at the back focal plane of the lens is steered based upon the phases, providing the angle of arrival and instantaneous frequency measurement (IFM), with latency determined by the speed of light over the optical path length. We present a numerical evaluation and experimental demonstration of this passive AoA- and frequency-detection capability.

Log-periodic temporal apertures for grating lobe suppression in k-space tomography

Millimeter-wave (mmW) imaging receivers have demonstrated the ability to sense radio-frequency (RF) waves using traditional phased antenna array techniques, and, through a coherent photonic up-conversion process, image these waves using free-space optical systems. Building upon the idea of coherent up-conversion, k-space tomography extends the functionality of the millimeter-wave imaging receiver as a two-dimensional spatial processing unit to three-dimensional sensing with the addition of frequency detection. In this configuration, an arrayed waveguide grating, or temporal aperture, is implemented following the photonic up-conversion of RF signals received by the phased array. These waveguides of varying length add a spectral beam-forming network to the existing spatial beam-forming of the mmW-imaging receiver. The introduction of three-dimensional phase information to the imaging system disrupts the ability to directly image the RF signal distribution on a photo-detector array, requiring the application of tomographic algorithms to reconstruct the power distribution of the received signals. In order to receive and properly recover the spatial-spectral distribution of RF sources, the antenna array and temporal array must be sampled adequately to avoid introduction of grating artifacts into the system response. Grating lobes, an artifact of regular spacing of elements within a grating, restrict the alias-free field of regard for antenna arrays, or the free spectral range for time-delay based arrays, thus limiting the spatial-spectral monitoring of RF sources via the k-space imaging modality. To alleviate this constraint, we present a non-uniform log-periodic array sampling for the k-space tomographic time-delay based aperture, greatly increasing the free spectral range of the system while maintaining the number of existing channels.

Subvolt electro-optical modulator on thin-film lithium niobate and silicon nitride hybrid platform

A low voltage operation electro-optic modulator is critical for applications ranging from optical communications to an analog photonic link. This paper reports a hybrid silicon nitride and lithium niobate electro-optic Mach-Zehnder modulator that employs 3 dB multimode interference couplers for splitting and combining light. The presented amplitude modulator with an interaction region length of 2.4 cm demonstrates a DC half-wave voltage of only 0.875 V, which corresponds to a modulation efficiency per unit length of 2.11 V cm. The power extinction ratio of the fabricated device is approximately 30 dB, and the on-chip optical loss is about 5.4 dB.

High-performance racetrack resonator in silicon nitride - thin film lithium niobate hybrid platform

In this paper, we propose an electro-optic modulator design in a hybrid Si₃N₄-X-cut LiNbO₃. The modulator is based on a modified racetrack resonator and performs at both DC and heightened frequencies. Here the driving electrodes are defined along the straight section of the racetrack. This is done to maximize modulation and minimize modulation-cancelation that occurs in a conventional X-cut LiNbO₃-based resonator due to the directional change of the electric field in the micro-ring. The single bus racetrack resonator is formed in a hybrid Si₃N₄-LiNbO₃ platform, to guide the optical mode. The fabricated device is characterized and has a measured tunability and intrinsic quality factor (Q) of 2.9 pm/V and 1.3 × 10⁵, respectively. In addition, the proposed racetrack device exhibits enhanced electro-optic conversion efficiency at modulation frequencies that match with the racetrack's optical free spectral range (FSR). For example, at the modulation frequency of 25 GHz, which corresponds to the fabricated device's optical FSR frequency, a ∼10 dB increase in electro-optic conversion efficiency is demonstrated. With the enhancement, our measured device demonstrates a conversion efficiency comparable to non-resonant thin-film LiNbO₃ devices that possess RF electrodes that are 10 times longer in length.

Tunable hybrid silicon nitride and thin-film lithium niobate electro-optic microresonator

This Letter presents, to the best of our knowledge, the first hybrid Si₃N₄-LiNbO₃-based tunable microring resonator where the waveguide is formed by loading a Si₃N₄ strip on an electro-optic (EO) material of X-cut thin-film LiNbO₃. The developed hybrid Si₃N₄-LiNbO₃ microring exhibits a high intrinsic quality factor of 1.85×10⁵, with a ring propagation loss of 0.32 dB/cm, resulting in a spectral linewidth of 13 pm, and a resonance extinction ratio of ∼27  dB within the optical C-band for the transverse electric mode. Using the EO effect of LiNbO₃, a 1.78 pm/V resonance tunability near 1550 nm wavelength is demonstrated.

Vertical mode transition in hybrid lithium niobate and silicon nitride-based photonic integrated circuit structures

This Letter presents an optical mode transition structure for use in Si₃N₄/LiNbO₃-based hybrid photonics. A gradual modal transition from a Si₃N₄ waveguide to a hybrid Si₃N₄/LiNbO₃ waveguide is achieved by etching a terrace structure into the sub-micrometer thick LiNbO₃ film. The etched film is then bonded to predefined low pressure chemical vapor deposition Si₃N₄ waveguides. Herein we analyze hybrid optical devices both with and without the aforementioned mode transition terrace structure. Experimental and simulated results indicate that inclusion of the terrace significantly improves mode transition compared to an abrupt transition, i.e., a 1.78 dB lower mode transition loss compared to the abrupt transition. The proposed transition structure is also applied to the design of hybrid Si₃N₄-LiNbO₃ micro-ring resonators. A high-quality factor (Q) resonator is demonstrated with the terrace transition which mitigates undesired resonances.

Thin film lithium niobate electro-optic modulator with terahertz operating bandwidth

We present a thin film crystal ion sliced (CIS) LiNbO₃ phase modulator that demonstrates an unprecedented measured electro-optic (EO) response up to 500 GHz. Shallow rib waveguides are utilized for guiding a single transverse electric (TE) optical mode, and Au coplanar waveguides (CPWs) support the modulating radio frequency (RF) mode. Precise index matching between the co-propagating RF and optical modes is responsible for the device's broadband response, which is estimated to extend even beyond 500 GHz. Matching the velocities of these co-propagating RF and optical modes is realized by cladding the modulator's interaction region in a thin UV15 polymer layer, which increases the RF modal index. The fabricated modulator possesses a tightly confined optical mode, which lends itself to a strong interaction between the modulating RF field and the guided optical carrier; resulting in a measured DC half-wave voltage of 3.8 V·cm^-1. The design, fabrication, and characterization of our broadband modulator is presented in this work.

Photonic probing of radio waves for k-space tomography

We harness coherent optical processing to simultaneously sense the angle of arrival and the frequency of radio waves. Signals captured by a distributed antenna array are up-converted to optical domain using electro-optic modulators coupled to individual antennas. Employing a common laser source to feed all the modulators ensures spatially coherent up-conversion of radio-frequency (RF) waves to optical beams carried by optical fibers. Fiber-length dispersion extends the spatial aperture of the distributed antenna array into the temporal dimension. The interference of beams emanating from the fibers is captured by a CCD and used to computationally reconstruct RF waves in k-space.

110 GHz CMOS compatible thin film LiNbO3 modulator on silicon

In this paper we address a significant limitation of silicon as an optical material, namely, the upper bound of its potential modulation frequency. This arises due to finite carrier mobility, which fundamentally limits the frequency response of all-silicon modulators to about 60 GHz. To overcome this limitation, another material must be integrated with silicon to provide increased operational bandwidths. Accordingly, this paper proposes and demonstrates the integration of a thin LiNbO₃ device layer with silicon and a novel tuning process that matches the propagation velocities between the propagating radio-frequency (RF) and optical waves. The resulting lithium niobate on silicon (LiNOS) modulator is demonstrated to operate from DC to 110 GHz.

Thin LiNbO3 on insulator electro-optic modulator

This Letter presents a method for the fabrication and integration of a thin LiNbO3 substrate with a Si handle wafer. An inverted ridge structure guides a single optical mode in an electro-optic modulator fabricated on a mechanically thinned substrate. To define an optical waveguide, a ridge structure is first patterned on a 500 μm thick X-cut LiNbO3 wafer; then a low dielectric constant adhesive layer is used to bond the etched LiNbO3 to Si. The LiNbO3 is mechanically thinned to 4 μm, and planar electrodes are patterned. Experimental results demonstrating modulation with a V(π)L of 7.1 V-cm were shown, optical loss was low enough, and film quality high enough, to enable an interaction length of 0.8 cm.

Packaging and design of a heterogeneous dual laser chip for a widely tunable spectrally pure optical RF source

In this paper, we report the results of the efforts to extend our previous work through the packaging and redesign of a heterogeneously integrated silicon-photonic circuit for use in a modulation side-band injection-locked optical RF generation system. Towards that effort, we attempted to improve the RF spectrum coverage of our design by decreasing the laser cavity length. Despite the unintended formation of an additional parasitic cavity in that device, we demonstrated increased spectrum coverage between 5 and 50 GHz in a packaged module with an ∼ 1-Hz linewidth.

Improved configuration and reduction of phase noise in a narrow linewidth ultrawideband optical RF source

In this Letter, we report on the improved configuration of a widely tunable optical RF generation system, particularly for the generation of low-frequency RF, as well as the reduction of phase noise in that same system. Using an amplitude modulator, a simplified system design was demonstrated with fewer components and improved phase noise performance, especially at RF frequencies below ∼36 GHz. Excess phase noise due to acoustic vibrations of the optical fibers was also successfully eliminated by mechanical isolation. A minimum phase noise of -124 dBc/Hz at 10 kHz offset was demonstrated at 4 GHz.

Measured comparison of contrast and crossover periods for passive millimeter-wave polarimetric imagery

Several targets are set-up outside and imaged by a passive millimeter-wave sensor over a 24 hour period. The sensor is capable of measuring two linear polarization states simultaneously and the contrasts of the targets are compared for the different polarizations. The choice of polarization is shown to have an impact on the contrast of different targets throughout the day. In an extreme case the contrast of a target experiences a crossover event and disappears for one polarization while it presents a strong contrast (9 K) with the other polarization. Experimental results are shown along with a simulation of the scene using a ray tracing program.

Higher-order computational model for coded aperture spectral imaging

Coded aperture snapshot spectral imaging systems (CASSI) sense the three-dimensional spatio-spectral information of a scene using a single two-dimensional focal plane array snapshot. The compressive CASSI measurements are often modeled as the summation of coded and shifted versions of the spectral voxels of the underlying scene. This coarse approximation of the analog CASSI sensing phenomena is then compensated by calibration preprocessing prior to signal reconstruction. This paper develops a higher-order precision model for the optical sensing in CASSI that includes a more accurate discretization of the underlying signals, leading to image reconstructions less dependent on calibration. Further, the higher-order model results in improved image quality reconstruction of the underlying scene than that achieved by the traditional model. The proposed higher precision computational model is also more suitable for reconfigurable multiframe CASSI systems where multiple coded apertures are used sequentially to capture the hyperspectral scene. Several simulations and experimental measurements demonstrate the benefits of the discretization model.

Full spectrum millimeter-wave modulation

In recent years, the development of new lithium niobate electro-optic modulator designs and material processing techniques have contributed to support the increasing need for faster optical networks by considerably extending the operational bandwidth of modulators. In an effort to provide higher bandwidths for future generations of networks, we have developed a lithium niobate electro-optic phase modulator based on a coplanar waveguide ridged structure that operates up to 300 GHz. By thinning the lithium niobate substrate down to less than 39 µm, we are able to eliminate substrate modes and observe optical sidebands over the full millimeter-wave spectrum.

Passive 77 GHz millimeter-wave sensor based on optical upconversion

A passive millimeter-wave (mmW) sensor operating at a frequency of 77 GHz is built and characterized. The sensor is a single pixel sensor that raster scans to create an image. Optical upconversion is used to convert the incident mmW signal into an optical signal for detection. Components were picked to be representative of a single element in a distributed aperture system. The performance of the system is analyzed, and the noise equivalent temperature difference is found to be 0.5 K (for a 1 s integration time) with a diffraction limited resolution of ~8 mrad. Representative images are shown that demonstrate the phenomenology associated with this spectrum.

Dynamically induced nonlinearity in a resonant-cavity interferometric intensity modulator

The frequency dependence of the spur-free dynamic range (SFDR) in a modulator based on an injection-locked laser is analyzed. It is shown that as the modulation frequency approaches half of the locking range, the SFDR of the modulator approaches that of a standard Mach-Zehnder configuration. At low frequencies, the SFDR degrades by 2 dB for every octave of frequency increase.

Development of a digital-micromirror-device-based multishot snapshot spectral imaging system

We report on the development of a digital-micromirror-device (DMD)-based multishot snapshot spectral imaging (DMD-SSI) system as an alternative to current piezostage-based multishot coded aperture snapshot spectral imager (CASSI) systems. In this system, a DMD is used to implement compressive sensing (CS) measurement patterns for reconstructing the spatial/spectral information of an imaging scene. Based on the CS measurement results, we demonstrated the concurrent reconstruction of 24 spectral images. The DMD-SSI system is versatile in nature as it can be used to implement independent CS measurement patterns in addition to spatially shifted patterns that piezostage-based systems can offer.

Nanomembrane transfer process for intricate photonic device applications

We demonstrate a process for the fabrication and transfer of silicon nanomembranes (Si-NMs) that have been released from their host substrates and redeposited on foreign flexible or flat substrates. The transfer process developed allows intricate photonic devices to be transferred via NMs to a variety of new substrate materials. This allows the transferred devices to benefit from the material properties of both substrate and NM. Our process is designed to transfer and stack large-area photonic devices without compromising their optical performance. The process has been used to transfer large-area unpatterned silicon NMs, in excess of 2.5 cm(2), and photonic devices with intricate device designs containing various fill factors. We have also demonstrated transferred photonic crystal devices that have maintained structural integrity and functionality.

Fabrication of Large Area Fishnet Optical Metamaterial Structures Operational at Near-IR Wavelengths

In this paper, we demonstrate a fabrication process for large area (2 mm × 2 mm) fishnet metamaterial structures for near IR wavelengths. This process involves: (a) defining a sacrificial Si template structure onto a quartz wafer using deep-UV lithography and a dry etching process (b) deposition of a stack of Au-SiO₂-Au layers and (c) a 'lift-off' process which removes the sacrificial template structure to yield the fishnet structure. The fabrication steps in this process are compatible with today's CMOS technology making it eminently well suited for batch fabrication. Also, depending on area of the exposure mask available for patterning the template structure, this fabrication process can potentially lead to optical metamaterials spanning across wafer-size areas.

Experimental demonstration of an optical-sectioning compressive sensing microscope (CSM)

In this paper we present the design and implementation of a Compressive Sensing Microscopy (CSM) imaging system, which uses the Compressive Sensing (CS) method to realize optical-sectioning imaging. The theoretical aspect of the proposed system is investigated using the mathematical model of the CS method and an experimental prototype is constructed to verify the CSM design. Compared to conventional optical-sectioning microscopes (such as Laser Scanning Confocal Microscopes (LSCMs) or Programmable Array Microscopes (PAMs)), the CSM system realizes optical-sectioning imaging using a single-pixel photo detector and without any mechanical scanning process. The complete information of the imaging scene is reconstructed from the CS measurements numerically.

Free-carrier absorption modulation in silicon nanocrystal slot waveguides

Free-carrier absorption (FCA) has proven to be an important obstacle in the development of a silicon-based laser; however, FCA may serve as a potential advantage in active silicon-based switches or modulators. In this work, we present FCA modulation in slot waveguides with silicon nanocrystals (Si-ncs) embedded in SiO(2) as the low-index slot material. Slot waveguides were fabricated with and without Si-ncs, and the presence of Si-ncs was shown to increase the pump-induced FCA loss in the waveguides by a factor of 4.5. We modeled the Si-nc material using a four-level rate equation analysis to estimate the excited population of Si-ncs, allowing us to extract a value of 2.6 × 10(-17) cm(2) for the FCA cross section of the Si-nc material.

Mitigation of Si nanocrystal free carrier absorption loss at 1.5 microm in a concentric microdisk structure

We study the mitigation of Si nanocrystal (Si-nc) free carrier absorption (FCA) loss at telecom wavelengths in a concentric microdisk design. The concentric microdisk design relies on using the Si-nc emission as an optical pump for the surrounding Er-based lasing media without subjecting the lasing mode to the FCA loss present in the Si-ncs. We analyze the FCA loss as a function of overhang width in this design and show that for large enough overhang width the FCA loss is negligible. We also compute the FCA cross section from the FCA loss and number of excited Si-ncs, modeled by a four-level system, and show sigma(FCA)=1.08 +/- 2.3 x 10(-17) cm(2), which is in good agreement with reported cross sections for similar films.

Comparison of diurnal contrast changes for millimeter-wave and infrared imagery

Far-infrared outdoor imagery has a lower contrast in the morning/afternoon relative to the highest contrast, which is observed at 14:00. Millimeter-wave (mmW) imagery can also follow this pattern. However, in this paper, we show that the opposite can occur for mmW imagery, wherein a higher contrast can occur in the morning/afternoon and lower contrast at 14:00. To this end, we show that a wood and rubber sample are observed to have a difference in mmW radiometric temperature of 17 degrees C at 9:00 and a difference of only 7 degrees C at 14:00. Details of our observations are presented.

High yield fabrication of low threshold single-mode GaAs/AlGaAs semiconductor ring lasers using metallic etch masks

We demonstrate a novel high yield fabrication process for single-mode ridge-waveguide GaAs/AlGaAs ring lasers with significantly lower threshold currents than previously reported for similar devices. In this fabrication process, the ridge waveguide structure is patterned using a metallic etch mask, which survives ensuing fabrication steps to form a continuous metallic cover over the entire resonator structure. This metallic cover improves the uniformity of electrical contact between the resonator structure and the metallic biasing layer deposited at the conclusion of the fabrication process. This leads to optimum electrical pumping of the fabricated devices. This fabrication process also allows for the passivation of the ridge-waveguide device sidewalls and separation of the metallic biasing layer from the optical mode.

Pulsed pumping of silicon nanocrystal light emitting devices

Typical silicon nanocrystal light emitting devices (LEDs) operate under direct current (DC) biasing conditions that require high electric fields or high current densities. The electroluminescence (EL) under these conditions relies on impact excitation that can be damaging to the material. In this work, we present bipolar injection into silicon nanocrystal LEDs using a pulsed pumping scheme. We measured the frequency dependence of the integrated and time-resolved EL of the LEDs. The frequency dependent behavior of the time-resolved characteristics is used to explain the integrated EL measurements. In addition, the light output of the device was measured under pulsed excitation and was found to increase by a factor of 18 as compared to the case of DC excitation.

Comparison of raised-microdisk whispering-gallery-mode characterization techniques

We compare the two prevailing raised-microdisk whispering-gallery-mode (WGM) characterization techniques, one based on coupling emission to a tapered fiber and the other based on collecting emission in the far field. We applied both techniques to study WGMs in Si nanocrystal raised microdisks and observed dramatically different behavior. We explain this difference in terms of the radiative bending loss on which the far-field collection technique relies and discuss the regimes of operation in which each technique is appropriate.

Integration of silicon nanocrystals and erbium ring cavities for a silicon pumped Er:SiO2 laser

In this work we present a novel two-stage approach to achieve electrically pumped lasing on a CMOS compatible material platform in the telecom region. The proposed design consists of an electrically pumped silicon nanocrystal (Si-nc) light source acting as an optical pump for an Erbium doped silicate (Er:SiO2) lasing cavity. The integrated design, based on concentric disks of Si-nc and Er:SiO2, provides a means of coupling the Si-nc pump signal to the Er ions without requiring overlap of the Er based lasing mode with the Si-nc material. We present an electromagnetic analysis of the pump and lasing modes in the proposed configuration. We also present fabrication and characterization of Si-nc and Er:SiO2 microdisks as components of the integrated design.

Coupling Si nanocrystal microdisk emission to whispering-gallery modes in a concentric SiO2 ring

We present a concentric microdisk design in which luminescence from an inner disk of Si nanocrystals (Si-ncs) contributes to resonant modes in an outer ring of SiO2. Photoluminescence from fabricated structures reveals the excitation of whispering-gallery modes (WGMs) with quality factors as high as 2850, limited by the spectral resolution of our spectrometer. Two-dimensional finite-difference time-domain simulations provide insight into the WGM properties and the role of disk and ring geometry. The presented concentric disk structure provides a means to use the efficient visible luminescence of Si-ncs as an optical pump for an extrinsic lasing material such as Er:SiO2.

Phase modulation using dual split ring resonators

In this paper, we studied phase modulation numerically using metamaterials such as stacked structures of dual split ring resonators (DSRRs). To demonstrate the modulation, a vertical and a planar design were considered, where the wave vectors were parallel and perpendicular to the proposed structures creating 70 degrees and 80 degrees of phase change, respectively. In both of the designs modulation was brought about by changing the effective index of the structure through switching between the open and short states of the DSRRs while maintaining high transmission. One of the attractive features of our design was the thin layers of DSRRs, where for the vertical and planar models the DSRRs layers were 5 mm and 2.28 mm respectively. The numerical results obtained by simulation matched well with the theoretical prediction.

Thin film solar cell design based on photonic crystal and diffractive grating structures

In this paper we present novel light trapping designs applied to multiple junction thin film solar cells. The new designs incorporate one dimensional photonic crystals as band pass filters that reflect short light wavelengths (400 - 867 nm) and transmit longer wavelengths(867 -1800 nm) at the interface between two adjacent cells. In addition, nano structured diffractive gratings that cut into the photonic crystal layers are incorporated to redirect incoming waves and hence increase the optical path length of light within the solar cells. Two designs based on the nano structured gratings that have been realized using the scattering matrix and particle swarm optimization methods are presented. We also show preliminary fabrication results of the proposed devices.

Electromagnetic modeling of active silicon nanocrystal waveguides

In this paper we propose an electromagnetic analysis of active silicon nano-crystal (Si-nc) waveguide devices. To account for the nonlinearity in the active medium we introduce a four level rate equation model whose parameters are based on experimentally reported material properties. The electromagnetic polarization serves to couple the quantum mechanical and electromagnetic behavior within the ADE-FDTD scheme. The developed modeling tool is used to simulate waveguide amplifiers, enhanced spontaneous emission microcavities, and the temporal lasing dynamics of active Si-nc based devices.

Lasing dynamics of a silicon photonic crystal microcavity

In this paper we propose a novel silicon microcavity design based on the dispersion engineered photonic crystals (PhCs). With the unique self-collimation property of PhCs, we optimize the passive cavity by tuning the design parameters, such as coupling gap size and array size, to achieve higher Q factor and drop efficiency. Highest cavity mode below the band edge is of particular interest. The strong mode confinement in the low index active material offers an opportunity to realize a lasing mechanism. To investigate the lasing dynamics we introduce the rate equations of atomic system into the electromagnetic polarization to fully describe the nonlinearity of active medium. With these auxiliary differential equations we solve the time evolutions of the electromagnetic waves and atomic populations by using the FDTD method.

Calculation of effective permittivity, permeability, and surface impedance of negative-refraction photonic crystals

We consider the eigen-fields of a two-dimensional negative-refraction photonic crystal and obtain negative effective permittivity and negative effective permeability. Effective permittivity, permeability, and surface impedance are calculated by averaging the eigen-fields. The value of the surface impedance is shown to be location-dependent and is validated by finite-difference time-domain simulations. The unique power propagation mechanism in the photonic crystal is demonstrated through time-evolution of eigen-fields.

Negative refraction imaging in a hybrid photonic crystal device at near-infrared frequencies

We present the experimental demonstration of imaging of a point source by negative refraction at near-infrared frequencies using a hybrid photonic crystal device. The photonic crystal device, fabricated by patterning holes in 260nm silicon-on-insulator, integrates a triangular-lattice photonic crystal with a large photonic bandgap and square-lattice photonic crystal with negative refraction. Experimental results show that the output of a line-defect photonic bandgap waveguide provides a nearly ideal point source and then is imaged through the photonic crystal by negative refraction.

Multiscale free-space optical interconnects for intrachip global communication: motivation, analysis, and experimental validation

The use of optical interconnects for communication between points on a microchip is motivated by system-level interconnect modeling showing the saturation of metal wire capacity at the global layer. Free-space optical solutions are analyzed for intrachip communication at the global layer. A multiscale solution comprising microlenses, etched compound slope microprisms, and a curved mirror is shown to outperform a single-scale alternative. Microprisms are designed and fabricated and inserted into an optical setup apparatus to experimentally validate the concept. The multiscale free-space system is shown to have the potential to provide the bandwidth density and configuration flexibility required for global communication in future generations of microchips.

Two bit optical analog-to-digital converter based on photonic crystals

In this paper, we demonstrate a 2-bit optical analog-to-digital (A/D) converter. This converter consists of three cascaded splitters constructed in a self-guiding photonic crystal through the perturbation of the uniform lattice. The A/D conversion is achieved by adjusting splitting ratios of the splitters through changing the degree of perturbation. In this way, output ports reach a state of '1' at different input power levels to generate unique states desired for an A/D converter. To validate this design concept, we first experimentally characterize the relation between the splitting ratio and the degree of lattice perturbation. Based on this understanding, we then fabricate the 2-bit A/D converter and successfully observe four unique states corresponding to different power levels of input analog signal.

Optically tunable silicon photonic crystal microcavities

We demonstrate the use of silicon photonic crystal based microcavity structures to perform light modulation at potentially giga-Hertz speeds through the use of optically induced plasma dispersion. The cavity configurations considered have the potential to operate at low pump power when the Q of the cavity involved is maximized.

Experimental demonstration of self-collimation inside a three-dimensional photonic crystal

We present our experimental demonstration of self-collimation inside a three-dimensional (3D) simple cubic photonic crystal at microwave frequencies. The photonic crystal was designed with unique dispersion property and fabricated by a high precision computer-controlled machine. The self-collimation modes were excited by a grounded waveguide feeding and detected by a scanning monopole. Self-collimation of electromagnetic waves in the 3D photonic crystal was demonstrated by measuring the 3D field distribution, which was shown as a narrow collimated beam inside the 3D photonic crystal but a diverged beam in the absence of the photonic crystal.

Electromagnetic simulation of quantum well structures

We present an auxiliary differential equation Finite-difference Time-domain (ADE-FDTD) approach to numerically model the wave propagation within a gain or absorbing medium such as quantum well structures. Start from traditional quantum electronics theory, the macroscopic susceptibility of the semiconductor is derived and expressed by a multiple-Lorentz-like model based on Prony's method. With the auxiliary differential equation method each Lorentz-like model can be simulated in the time domain and the induced polarization is then determined by summing all the models. By incorporating the induced polarization into the time-domain Maxwell's equations, electromagnetic wave propagation in the quantum well medium can be accurately modeled using the FDTD method.

Perfect lens makes a perfect trap

In this work, we present for the first time a new and realistic application of the "perfect lens", namely, electromagnetic traps (or tweezers). We combined two recently developed techniques, 3D negative refraction flat lenses (3DNRFLs) and optical tweezers, and experimentally demonstrated the very unique advantages of using 3DNRFLs for electromagnetic traps. Super-resolution and short focal distance of the flat lens result in a highly focused and strongly convergent beam, which is a key requirement for a stable and accurate electromagnetic trap. The translation symmetry of 3DNRFL provides translation-invariance for imaging, which allows an electromagnetic trap to be translated without moving the lens, and permits a trap array by using multiple sources with a single lens. Electromagnetic trapping was demonstrated using polystyrene particles in suspension, and subsequent to being trapped to a single point, they were then accurately manipulated over a large distance by simple movement of a 3DNRFL-imaged microwave monopole source.

Three-dimensional subwavelength imaging by a photonic-crystal flat lens using negative refraction at microwave frequencies

We experimentally demonstrate subwavelength resolution imaging at microwave frequencies by a three-dimensional (3D) photonic-crystal flat lens using full 3D negative refraction. The photonic crystal was fabricated in a layer-by-layer process. A subwavelength pinhole source and a dipole detector were employed for the measurement. By point-by-point scanning, we obtained the image of the pinhole source shown in both amplitude and phase, which demonstrated the imaging mechanism and subwavelength feature size in all three dimensions. An image of two pinhole sources with subwavelength spacing showed two resolved spots, which further verified subwavelength resolution.

All-optical switching in silicon photonic crystal waveguides by use of the plasma dispersion effect

Silicon photonic crystals offer new ways of controlling the propagation of light as well as new tools for the realization of high-density optical integration on monolithic substrates. However, silicon does not possess the strong nonlinearities that are commonly used in the dynamic control of optical devices. Such dynamic control is nevertheless essential if silicon is to provide the higher levels of functionality that are required for optical integration. We demonstrate that the combination of the refractive index change caused by the presence of photoexcited carriers, or so-called plasma dispersion, and photonic crystal properties such as photonic bandgaps, constitutes a powerful tool for active control of light in silicon integrated devices. We show close to 100% modulation depth near the photonic crystal band edge.

Efficient terahertz coupling lens based on planar photonic crystals on silicon on insulator

We present a promising coupling device, namely, a terahertz (THz) planar photonic crystal (PhC) lens based on the effective refractive-index contrast between the PhC and the surrounding unpatterned area. Three-dimensional finite-difference time-domain calculations show a 90% power transfer from a 100-microm silicon waveguide to a 10-microm waveguide, and 45% coupling efficiency is confirmed experimentally. These results demonstrate the utility of the PhC lens as an effective approach to coupling into PhC THz circuits.

Design of two-dimensional polarization-selective diffractive optical elements with form-birefringent microstructures

We describe a design methodology for synthesizing polarization-sensitive diffractive optical elements based on two-dimensional form-birefringent microstructures. Our technique yields a single binary element capable of producing independent phase transformations for horizontally and vertically polarized illumination. We designed two elements for operation at 10.6 microm and fabricated them in silicon. Qualitative experimental results agree with design predictions.

Plane-wave expansion method for calculating band structure of photonic crystal slabs with perfectly matched layers

We present a new algorithm for calculation of the band structure of photonic crystal slabs. This algorithm combines the plane-wave expansion method with perfectly matched layers for the termination of the computational region in the direction out of the plane. In addition, the effective-medium tensor is applied to improve convergence. A general complex eigenvalue problem is then obtained. Two criteria are presented to distinguish the guided modes from the PML modes. As such, this scheme can accurately determine the band structure both above and below the light cone. The convergence of the algorithm presented has been studied. The results obtained by using this algorithm have been compared with those obtained by the finite-difference time-domain method and found to agree very well.

Total internal reflection-evanescent coupler for fiber-to-waveguide integration of planar optoelectric devices

We present a method for parallel coupling from a single-mode fiber, or fiber ribbon, into a silicon-on-insulator waveguide for integration with silicon optoelectronic circuits. The coupler incorporates the advantages of the vertically tapered waveguides and prism couplers, yet offers the flexibility of planar integration. The coupler can be fabricated by use of either wafer polishing technology or gray-scale photolithography. When optimal coupling is achieved in our experimental setup, the coupler can be packaged by epoxy bonding to form a fiber-waveguide parallel coupler or connector. Two-dimensional electromagnetic calculation predicts a coupling efficiency of 77% (- 1.14-dB insertion loss) for a silicon-to-silicon coupler with a uniform tunnel layer. The coupling efficiency is experimentally achieved to be 46% (-3.4-dB insertion loss), excluding the loss in silicon and the reflections from the input surface and the output facet.