Thermo-optic Modulation Research Articles

Frequency modulated hybrid photonic crystal laser by thermal tuning.

We demonstrate frequency modulation (FM) in an external cavity (EC) III-V/silicon laser, comprising a reflective semiconductor optical amplifier (RSOA) and a silicon nitride (SiN) waveguide vertically coupled to a 2D silicon photonic crystal (PhC) cavity. The PhC cavity acts as a tunable narrowband reflector giving wavelength selectivity. The FM was achieved by thermo-optical modulation of the reflector via a p-n junction. Single-mode operation was ensured by the short cavity length, overlapping only one longitudinal laser mode with the reflector. We investigate the effect of reflector modulation theoretically and experimentally and predict a substantial tracking of the resonator by the laser frequency with very small intensity modulation (IM).

Mid-Infrared Slow Light Engineering and Tuning in 1-D Grating Waveguide

The design, fabrication, and characterization of 1-D grating waveguide as slow light structure working in mid-infrared (MIR) region are demonstrated for the first time. The effects of various structural parameters on slow light properties are investigated through theoretical analysis, simulation and experimental verification, providing guidance on slow light engineering in 1-D grating waveguide. By adjusting structural parameters, average group indices of 9.4–15.5 with bandwidths of 23–66 nm and normalized delay-bandwidth products of 0.093–0.164 are obtained. Thermo-optic tuning is also demonstrated with π phase shift and group index tuning from 23 to 33 at the wavelength of 3.9024 μm by applying 1.4 mA current. The proposed tunable 1-D grating slow light waveguide provides a promising platform for various MIR applications such as on-chip absorption-based biochemical sensors, tunable optical buffers, and thermo-optic modulators.

Integrated Silicon-on-Insulator Spectrometer With Single Pixel Readout for Mid-Infrared Spectroscopy

Mid-infrared spectroscopy in the 2–4 $\mu$ m wavelength range is of cardinal value for many sensing applications. Current solutions involve bulky and expensive systems to operate. Silicon-on-Insulator (SOI) waveguide technology offers means to miniaturize the different parts of the spectrometer. However, the development of on-chip detectors for the mid-infrared wavelength range is in its infancy and the characteristics are not on par with their discrete (cooled) counterparts. In this work, a compact and cheap mid-infrared spectrometer is realized by integrating a SOI arrayed waveguide grating (AWG) spectrometer operating in the 2.3 $\mu$ m wavelength range with a high performance mid-infrared photodiode. The AWG has 12 output channels with a spacing of 225 GHz (4 nm) and a free spectral range of 3150 GHz (56 nm), which are simultaneously collected by a single, transistor outline (TO)-packaged extended InGaAs PIN photodiode. The response of each AWG channel is discerned by time-sequentially modulating the optical power in each output channel using integrated Mach-Zehnder based thermo-optic modulators with a $\pi$ -phase shift power consumption of $\approx$ 50 mW. As an example, the absorption spectrum of a 0.5 mm thick polydimethylsiloxane sheet is sampled and compared to a benchtop spectrometer to good agreement.

Tunable multiband plasmonic response of indium antimonide touching microrings in the terahertz range.

In this paper, we propose a terahertz (THz) plasmonic structure that supports three resonance modes, including the charge transfer plasmon (CTP), the bonding dipole-dipole plasmon, and the antibonding dipole-dipole plasmon, which can be strongly tuned by geometrical parameters, passively, and the temperature, actively. The structure exhibits a considerable thermal sensitivity of more than 0.01THz/K. The introduced multiband and tunable THz plasmonic structures offer important applications in thermal switches, thermo-optical modulators, broadband filters, design of multifunctional molecules originating from the multiband specification of the proposed structure, and improvement in plasmonic sensor applications stemming from a detailed study of the CTP mode.

Demonstration of an optical directed half-subtracter using integrated silicon photonic circuits.

An integrated silicon photonic circuit consisting of two silicon microring resonators (MRRs) is proposed and experimentally demonstrated for the purpose of half-subtraction operation. The thermo-optic modulation scheme is employed to modulate the MRRs due to its relatively simple fabrication process. The high and low levels of the electrical pulse signal are utilized to define logic 1 and 0 in the electrical domain, respectively, and the high and low levels of the optical power represent logic 1 and 0 in the optical domain, respectively. Two electrical pulse sequences regarded as the operands are applied to the corresponding micro-heaters fabricated on the top of the MRRs to achieve their dynamic modulations. The final operation results of bit-wise borrow and difference are obtained at their corresponding output ports in the form of light. At last, the subtraction operation of two bits with the operation speed of 10 kbps is demonstrated successfully.

Experimental realization of a CMOS-compatible optical directed priority encoder using cascaded micro-ring resonators

AbstractIn this paper, we propose and experimentally demonstrate an integrated optical device that can implement the logical function of priority encoding from a 4-bit electrical signal to a 2-bit optical signal. For the proof of concept, the thermo-optic modulation scheme is adopted to tune each micro-ring resonator (MRR). A monochromatic light with the working wavelength is coupled into the input port of the device through a lensed fiber, and the four input electrical logic signals regarded as pending encode signals are applied to the micro-heaters above four MRRs to control the working states of the optical switches. The encoding results are directed to the output ports in the form of light. At last, the logical function of priority encoding with an operation speed of 10 Kbps is demonstrated successfully.

Vanadium oxide thin films with high midwave & longwave infrared thermo-optic coefficients and high temperature coefficients of resistance

The development of thermo-optic modulator elements for midwave infrared (MWIR) and longwave infrared (LWIR) electro-optic systems, and the development of resistive thermometer elements for infrared imaging microbolometers require materials with high thermo-optic coefficients (TOC) and high temperature coefficients of resistance (TCR), respectively. In this paper, we synthesize novel vanadium oxide (VxOy) thin film structures and measure their MWIR/LWIR thermo-optic coefficients (TOCs) and their temperature coefficients of resistance (TCRs). The VxOy thin film are synthesized by sputter depositing interchanging, 5 nm-thick, layers of vanadium sesquioxide (V2O3) and vanadium (V) reaching a final thin film thickness of 95 nm. The sputter deposited multilayer structures are then ex-situ annealed in N2 and O2 atmospheres at 300 °C for 30 min. Infrared spectroscopic ellipsometry was used to measure the optical constants of the thin films as a function of temperature across the MWIR and LWIR bands (3000–14000 nm). The synthesized VxOy thin films exhibited high TOCs and high TCRs when operated in their semiconducting phase. A high TOC was measured reaching a maximum of 0.0278 °C−1 and 0.119 °C−1 at λ = 4000 nm for N2 and O2 annealed VxOy thin films respectively; and reaching a maximum of 0.0634 °C−1 and 0.139 °C−1 at λ = 10,000 nm for N2 and O2 annealed VxOy thin films respectively. Moreover, sheet resistance versus temperature measurements were conducted revealing room temperature sheet resistances of 1.495 k Ω/sq. and 1.516 k Ω/sq. and TCRs of −3.54%/°C and −3.46%/°C for N2 and O2 annealed VxOy thin films respectively.

High-Speed, Phase-Dominant Spatial Light Modulation with Silicon-Based Active Resonant Antennas

Spatiotemporal control of optical wavefronts is of great importance in numerous free-space optical applications including imaging in 3D and through scattering media, remote sensing, and generation of various beam profiles for microscopy. Progress in these applications is currently limited due to lack of compact and high-speed spatial light modulators. Here we report an active antenna comprising a free-space coupled asymmetric Fabry–Perot resonator that produces a phase-dominant thermo-optic modulation of reflected light at frequencies approaching tens of kilohertz. As a proof of concept for spatial light modulation, we demonstrate a 6 × 6 array of such active antennas with beam deflection capability. The robust design of our silicon-based active antenna will enable large-scale integration of high-speed, phase-dominant spatial light modulators.

Experimental demonstration of an optical Feynman gate for reversible logic operation using silicon micro-ring resonators

Abstract Currently, the reversible logic circuit is a popular research topic in the field of information processing as it is a most effective approach to minimize power consumption, which can achieve the one-to-one mapping function to identify the input signals from its corresponding output signals. In this letter, we propose and experimentally demonstrate an optical Feynman gate for reversible logic operation using silicon micro-ring resonators (MRRs). Two electrical input signals (logic operands) are applied across the micro-heaters above MRRs to determine the switching states of MRRs, and the reversible logic operation results are directed to the output ports in the form of light, respectively. For proof of concept, the thermo-optic modulation scheme is used to achieve MRR’s optical switching function. At last, a Feynman gate for reversible logic operation with the speed of 10 kbps is demonstrated successfully.

Demonstration of a Silicon Photonic Circuit for Half-Add Operations Using Cascaded Microring Resonators

We report a silicon photonic circuit that can perform half-add operations using cascaded microring resonators (MRRs). Electrical pulse sequences regarded as the operands are applied to the corresponding MRRs to achieve their dynamic modulations. The final operation results are directed to the corresponding outputs in the form of light. For proof of principle, the thermo-optic modulation scheme that needs simpler fabrication processes is employed to modulate MRRs. Finally, the circuit is fabricated on an 8-in silicon-on-insulator (SOI) substrate using complementary metal-oxide-semiconductor (CMOS) technology, and the half-add operations with the speed of 10 Kb/s are demonstrated successfully.

Benchmarking System-Level Performance of Passive and Active Plasmonic Components: Integrated Circuit Approach

Using criteria of bandwidth and energy consumption for signal guiding and processing, system-level figures of merit (FOMs) for both passive and active plasmonic circuit components are introduced, benchmarking their performance for the realization of high-bandwidth optical data communication on a chip. The FOM for passive plasmonic interconnects has been derived in terms of the system-level performance of the plasmonic circuitry, emphasising the bandwidth and power dissipation densities. These parameters are linked to the local waveguide characteristics, such as the mode propagation length, bend radius, and mode size. The FOM enables a comparison of the main types of plasmonic waveguides and can serve as a benchmark for future designs of photonic integrated circuits. A FOM for active photonic or plasmonic electro-optical, thermo-optical, and all-optical modulators is also derived to reflect the same benchmarking principles. A particular emphasis is made on establishing a practically oriented benchmark where the integral performance of the circuit, not the size or energy consumption of individual components, plays the defining role.

Influence of the preparation conditions of erbium-doped bismuth germanate glasses on its optical response

The influence of the glass preparation conditions (melting temperature and crucible material) and of post-preparation heat treatments on the refractive index, optical absorption and near infrared luminescence of an Erbium-doped Bi2O3-GeO2 glass has been studied. The optical absorption and emission are strongly dependent on these conditions and Bi to Er energy transfer is observed to occur. In some cases, a high absorption due to the formation of Bi nanoparticles is obtained. Consequently transmission-temperature hysteresis loops during heating-cooling cycles are observed due to the reversible melting and solidification of nanoparticles. Our results indicate that, with adequate composition and preparation conditions, bismuth germanate glasses can be very useful for a large variety of optical devices, such as amplifiers and thermo-optical modulators and filters.

Optical Voltage Sensors Based on Integrated Optical Polarization-Rotated Reflection Interferometry

Optical voltage sensors based on reflection interferometry of two orthogonal polarizations have been demonstrated by using a polymeric integrated optic device, in which a thermooptic phase modulator, a polarization converter, and a polarizer are integrated on a single chip. The sensing probe for electric field measurement was prepared by assembling an LiTaO3 electrooptic crystal along with a polarization maintaining collimator, a Faraday rotator, and a dielectric mirror. Spectral bandwidth of the light source was optimized to be 16.5 nm in order to provide low noise and good extinction ratio in the reflection interferometry. For an applied voltage of 60 Hz, 4 ${\rm kV}_{\rm pp} $ , the sensor exhibited linear response with a phase retardation sensitivity of 0.5°/ ${\rm kV}$ .

Novel Mach–Zehnder Interferometer Waveguide Design as a Light Delivery System for Heat-Assisted Magnetic Recording

We propose a light delivery system to the near-field transducer (NFT) of a heat-assisted magnetic recording (HAMR) head using a Mach–Zehnder interferometer (MZI) waveguide arrangement with a transverse electric (TE) mode coupled to the planar waveguide from a single mode fiber. The proposed design offers great flexibility in the coupling of light to the NFT in order to match the desired radiation pattern of the antenna and thus to optimize the energy transfer. Furthermore, it allows excitation of particular surface plasmon (SP) resonances of the transducer (quadrupole or higher) by controlling the coupling angle and the phase of the two beams of light. The optimum phase shift between the TE waveguide modes incident on the NFT can be achieved either statically by making one of the MZI arms longer/shorter compared with the other one, or dynamically by changing the mode effective index of the MZI waveguide arm through electro-optic or thermo-optic modulation. In addition, the proposed MZI waveguide arrangement enables easier coupling from a laser with coupling losses from laser to MZI waveguide being below 3 dB and the design enables an arrangement of the NFT placed either on the top of the waveguide at the termination side of the MZI or between the rib and the ridge at the maximum of the electric field of the waveguide, which maximizes the SP resonance enhancement of the NFT. Another advantage is that the magnetic-write pole, in our design, can be easily integrated on the same chip, preferably above the center of the MZI, between the two arms of the interferometer and away from the propagating mode, thus avoiding blocking of the light path.

Recursive multiport schemes for implementing quantum algorithms with photonic integrated circuits

We present recursive multiport schemes for implementing quantum Fourier transforms and the inversion step in Grover's algorithm on an integrated linear optics device. In particular, each scheme shows how to execute a quantum operation on $2d$ modes using a pair of circuits for the same operation on $d$ modes. The circuits operate on path-encoded qudits and realize $d$-dimensional unitary transformations on these states using linear optical networks with $O\left(d^2\right)$ optical elements. To evaluate the schemes against realistic errors, we ran simulations of proof-of-principle experiments using a simple fabrication model of silicon-based photonic integrated devices that employ directional couplers and thermo-optic modulators for beam splitters and phase shifters, respectively. We find that high-fidelity performance is achievable with our multiport circuits for $2$-qubit and $3$-qubit quantum Fourier transforms, and for quantum search on four-item and eight-item databases.

Fast Adaptive Thermal Camouflage Based on Flexible VO₂/Graphene/CNT Thin Films.

Adaptive camouflage in thermal imaging, a form of cloaking technology capable of blending naturally into the surrounding environment, has been a great challenge in the past decades. Emissivity engineering for thermal camouflage is regarded as a more promising way compared to merely temperature controlling that has to dissipate a large amount of excessive heat. However, practical devices with an active modulation of emissivity have yet to be well explored. In this letter we demonstrate an active cloaking device capable of efficient thermal radiance control, which consists of a vanadium dioxide (VO2) layer, with a negative differential thermal emissivity, coated on a graphene/carbon nanotube (CNT) thin film. A slight joule heating drastically changes the emissivity of the device, achieving rapid switchable thermal camouflage with a low power consumption and excellent reliability. It is believed that this device will find wide applications not only in artificial systems for infrared camouflage or cloaking but also in energy-saving smart windows and thermo-optical modulators.

Analysis of thermo-optic effect-based refractive index dynamic modulation in microspherical resonator.

The thermo-optic effect has been utilized to modulate the refractive index dynamically within a whispering gallery mode resonator. Modulation with a large tuning range is mostly performed for mode locking and dynamic control of the optical path at a modulation frequency as low as several hertz, while high-frequency modulation up to megahertz is mainly exploited in optical switching devices with small tuning range. Here, we introduce the response functions theoretically to describe the dynamic response of temperature changes in the mode volume and the resonator body, respectively. This result is verified experimentally in silica microspherical resonators. The dependence of the tuning range on the modulation frequency is achieved. This knowledge could pave the way toward more practical control of refractive index in microresonators.

Stress-optic modulator in TriPleX platform using a ezoelectric lead zirconate titanate (PZT) thin film.

We will demonstrate a stress-optic phase modulator in the passive SiN-based TriPleX platform using a layer of piezoelectric material. Regarding the stress-optic effect, the piezoelectric layer deposited on top of an optical waveguide is employed to control the phase of propagating light in the structure by applying an electrical field across the layer. In this work, it is demonstrated that the stress-optic effect lowers the power consumption by a factor of one million for quasi-DC operation and increases the modulation speed by three orders of magnitude, compared to currently used thermo-optic modulation in the TriPleX platform.

Electronic modulation of infrared radiation in graphene plasmonic resonators.

All matter at finite temperatures emits electromagnetic radiation due to the thermally induced motion of particles and quasiparticles. Dynamic control of this radiation could enable the design of novel infrared sources; however, the spectral characteristics of the radiated power are dictated by the electromagnetic energy density and emissivity, which are ordinarily fixed properties of the material and temperature. Here we experimentally demonstrate tunable electronic control of blackbody emission from graphene plasmonic resonators on a silicon nitride substrate. It is shown that the graphene resonators produce antenna-coupled blackbody radiation, which manifests as narrow spectral emission peaks in the mid-infrared. By continuously varying the nanoresonator carrier density, the frequency and intensity of these spectral features can be modulated via an electrostatic gate. This work opens the door for future devices that may control blackbody radiation at timescales beyond the limits of conventional thermo-optic modulation.

A highly efficient thermo-optic microring modulator assisted by graphene.

Graphene's remarkable electrical and optical properties afford great potential for constructing various optoelectronic devices, including modulators, photodetectors and pulse lasers. In particular, graphene-based optical modulators were demonstrated to be featured with a broadband response, small footprint, ultrafast speed and CMOS-compatibility, which may provide an alternative architecture for light-modulation in integrated photonic circuits. While on-chip graphene modulators have been studied in various structures, most of them are based on a capacitance-like configuration subjected to complicated fabrication processes and providing a low yield of working devices. Here, we experimentally demonstrate a new type of graphene modulator by employing graphene's electrical and thermal properties, which can be achieved with a simple fabrication flow. On a graphene-coated microring resonator with a small active area of 10 μm(2), we have obtained an effective optical modulation via thermal energy electrically generated in a graphene layer. The resonant wavelength of the ring resonator shifts by 2.9 nm under an electrical power of 28 mW, which enables a large modulation depth of 7 dB and a broad operating wavelength range of 6.2 nm with 3 dB modulation. Due to the extremely high electrical and thermal conductivity in graphene, the graphene thermo-optical modulator operates at a very fast switching rate compared with the conventional silicon thermo-optic modulator, i.e. 10%-90% rise (90%-10% fall) time of 750 ns (800 ns). The results promise a novel architecture for massive on-chip modulation of optical interconnects compatible with CMOS technology.