Thermo-optic Effect Research Articles

Study of a dark core beam generated by nonlinear thermo-optical effect

Thermal lensing with a CW pump in materials with large extinction coefficient is used to convert a probe beam with Gaussian transverse intensity profile into a ring-shaped beam with dark core. The radius of the dark core can be controlled by varying the pump power. A Mach-Zehnder interferometer was used to study the beam’s phase profile. A theoretical analysis of the phase profile yields estimates of the threshold pump power needed to generate a dark core and the maximum change of the refractive index achieved in the experiment. As an application of the dark core beam, a numerical simulation of the effect has been combined with the experimental data to get a quick in situ estimate of thermal conductivity by a non-contact method.

Compact Modeling and Behavioral Simulation of an Optomechanical Sensor in Verilog A

Previous work has shown that optomechanical resonators are particularly well suited to the design of ultrasensitive mass sensors. They present an extremely low noise level, very high optical quality factor ( ${Q}>{10}^{5}$ ), excellent integration density and can resonate both in a gaseous and liquid environment. In order to reduce the long measurement time due to their small particle-capture area, several such resonators must be integrated onto the same chip. However, bulky laboratory equipment currently used to read a single optomechanical resonator cannot be practically scaled up to a large array of transducers. It is then required to design and eventually integrate a read-out interface that can process tens to thousands of resonators. To ease the design of such a circuit, this article presents a compact analytical model of an electrostatically actuated optomechanical resonator implemented in Verilog A. The proposed model includes both the optical and mechanical behaviors, as well as optomechanical coupling and thermo-optical effect. It was simulated in the Cadence Virtuoso environment and is consistent with the measured results.

Interference and electro-optical effects in cubic GaN/GaAs heterostructures prepared by molecular beam epitaxy

Cubic GaN (c-GaN) samples on GaAs (0 0 1) substrates were grown by RF-plasma-assisted molecular beam epitaxy, in which an As4 overpressure was employed for the nucleating layer. Photoreflectance spectra were obtained in the temperature range from 14 to 300 K. Two independent phenomena were noticed. The first one consisted in optical interference features below the c-GaN bandgap, whose origin is a thermo-optical effect: the ultraviolet perturbation beam changes the refractive index of the c-GaN. The second one represents electro-optical phenomena in which two classical band-to-band transitions occur: the first transition for c-GaN layer in which the temperature dependence reveals defects in the film attributed to a hexagonal fraction estimated previously between 3% and 10%, and a second transition for the GaAs substrate that shows Franz–Keldysh oscillations.

Carrier-induced optical bistability in the silicon micro-ring resonators under continuous wave pumping

The results of a study of the nonlinear frequency response of a silicon micro-ring resonator (MRR) manufactured using silicon-on-insulator technology are presented. Appearance of optical bistability caused by the frequency shift of the MRR resonant spectrum under continuous wave pumping is experimentally demonstrated. The experimental results are explained in the framework of an original theory, which accounts for the linear loss, Kerr nonlinearity, two-photon absorption, free-carrier-induced absorption and dispersion, and the thermo-optic effect. It is shown that the bistability of the frequency response originates from competition between frequency shifts of opposite signs that appear due to the presence of the thermo-optic and free-carrier effects, with the latter one being dominant.

Microsphere-Based Optical Frequency Comb Generator for 200 GHz Spaced WDM Data Transmission System

Optical frequency comb (OFC) generators based on whispering gallery mode (WGM) microresonators have a massive potential to ensure spectral and energy efficiency in wavelength-division multiplexing (WDM) telecommunication systems. The use of silica microspheres for telecommunication applications has hardly been studied but could be promising. We propose, investigate, and optimize numerically a simple design of a silica microsphere-based OFC generator in the C-band with a free spectral range of 200 GHz and simulate its implementation to provide 4-channel 200 GHz spaced WDM data transmission system. We calculate microsphere characteristics such as WGM eigenfrequencies, dispersion, nonlinear Kerr coefficient with allowance for thermo-optical effects, and simulate OFC generation in the regime of a stable dissipative Kerr soliton. We show that by employing generated OFC lines as optical carriers for WDM data transmission, it is possible to ensure error-free data transmission with a bit error rate (BER) of 4.5 × 10−30, providing a total of 40 Gbit/s of transmission speed on four channels.

High-speed thermally tuned electro-optical logic gates based on micro-ring resonators

This work proposes electro-optical design of logic gates using micro-ring resonators (MRR). We achieve this through various combinations of MRRs. In order to modulate the rings, thermo-optic effect is utilized. The reported devices are completely CMOS-compatible and can be fabricated using existing technologies. We also provide static responses to explain the device working principles. By plotting dynamic response spectra, the performance of each device is examined. The proposed logic elements are able to operate at 0.4 Mbps.

A Silicon Spatial Light Modulator Unit With High Extinction Ratios in C Band

A spatial light modulator (SLM) unit by using polycrystalline silicon (poly-Si) as resonant thin film is demonstrated here in order to improve the extinction ratios in C band (1530 to 1565 nm). The curves of the reflectivity of SLM unit versus wavelength with voltage OFF and ON are simulated and verified by experiments. The SLM unit presents high extinction ratios in C band with maximum of 42.2 dB at 1547.5 nm. The high extinction ratio is due not only to the excellent thermo-optic effect but also to the thermo-elastic deformation effect of the poly-Si thin film. The resonance wavelength has a red shift of 2 nm. The high-extinction-ratio C-band spatial light modulator unit has a giant application potential in the space optical communication field.

Broadband quasi-phase-matching in dispersion-engineered all-optically poled silicon nitride waveguides

Quasi-phase-matching (QPM) has become one of the most common approaches for increasing the efficiency of nonlinear three-wave mixing processes in integrated photonic circuits. Here, we provide a study of dispersion engineering of QPM second-harmonic (SH) generation in stoichiometric silicon nitride ( Si 3 N 4 ) waveguides. We apply waveguide design and lithographic control in combination with the all-optical poling technique to study the QPM properties and shape the waveguide dispersion for broadband spectral conversion efficiency inside Si 3 N 4 waveguides. By meeting the requirements for maximal bandwidth of the conversion efficiency spectrum, we demonstrate that group-velocity matching of the pump and SH is simultaneously satisfied, resulting in efficient SH generation from ultrashort optical pulses. The latter is employed for retrieving a carrier-envelope-offset frequency of a frequency comb by using an f − 2 f interferometric technique, where supercontinuum and SH of a femtosecond pulse are generated in Si 3 N 4 waveguides. Finally, we show that the waveguide dispersion determines the QPM wavelength variation magnitude and sign due to the thermo-optic effect.

Design of low loss 1 × 1 and 1 × 2 phase-change optical switches with different crystalline phases of Ge2Sb2Te5 films

On-chip photonics devices relying on the weak, volatile thermo-optic or electro-optic effects of silicon usually suffer from high insertion loss (IL) and a low refractive index coefficient. In this paper, we designed two novel 1 × 1 and 1 × 2 phase-change optical switches based on a signal-mode Si waveguide integrated with a Ge2Sb2Te5 (GST) top clad layer, respectively. The three-state switch including amorphous GST (a-GST), face centered cubic crystalline phase (FCC-GST) and hexagonal crystalline phase (HCP-GST) operated by utilizing the dramatic difference in the optical constants between the amorphous and two crystalline phases of GST. In the case of the 1 × 1 optical switch, an extinction ratio (ER) of 8.9 dB and an extremely low IL of 0.8 dB were achieved using an optimum GST length of only 2 μm. While for the 1 × 2 optical switch, low ILs in the range of 0.15 ∼ 0.35 dB for both ‘cross’ (a-GST) and ‘bar’ (FCC-GST and HCP-GST) states were also obtained. Additionally, we found that both ILs and mode losses of the switch with HCP-GST were about half lower than those with FCC-GST, which means FCC-GST could be instituted by HCP-GST in the traditional ovonic switch with the consideration of low loss. This research provides the fundamental understanding for the realization of low loss and non-volatile Si-GST hybrid optical switches, with potential for future communication networks.

Optoelectronic THz-Wave Beam Steering by Arrayed Photomixers With Integrated Antennas

We developed a thermo-optic effect based silica optical phased array (OPA) chip and evaluated its tunability of phases of lightwaves. The THz waves, whose phases are controlled by the OPA chip, are generated from four arrayed uni-travelling-carrier photodiodes (UTC-PDs) based on heterodyne photomixng. Meanwhile, the UTC-PDs, which integrated with 4×4 slot antennas, are fabricated on a monolithic InP substrate chip for 300-GHz wave beam combining and steering. Through the optoelectronic THz-wave phase control, the feasibility of 300-GHz beam continuous steering is successfully demonstrated in a range of 50°.

Gas identification in high-Q microbubble resonators.

A new, to the best of our knowledge, experimental mechanism is reported to realize the identification of gas by a microcavity sensor. Instead of measuring the change in the environment refractive index or absorption, the gas is detected indirectly and indentified by using the thermo-optics effect of a high-quality-factor microbubble resonator. When passing gas through the microbubble, the pressure induces a geometric deformation and thus an observable frequency shift, and the thermal bistability response varies due to the higher heat dissipation by gas molecules. With the two output parameters, we can unambiguously distinguish gas with different molecular weights, e.g., He, N2, and CO2. Our demonstration opens a new avenue of microcavity sensing by using indirect interaction between light and matter, realizing a multiple-parameter sensing approach for gas or solvent identification.

Silicon Optical Diode Based on Cascaded Opto-Mechanical Microring Resonators

We propose and experimentally demonstrate a silicon optical diode with high nonreciprocal transmission ratio (NTR) based on three cascaded opto-mechanical microring resonators (MRRs). As half waveguides of the opto-mechanical MRRs are free-hanging, the nonlinear effects (including the thermo-optic effect and opto-mechanical effect) are both enhanced. With low input powers, the transmission characteristics of the opto-mechanical MRRs could be flexibly manipulated by the strong nonlinear effects. More importantly, due to the optimized design of the cascaded MRRs, the NTRs are improved. In the experiment, a high NTR is successfully obtained with a low input power. The proposed optical diode provides a complementary metal-oxide semiconductor (CMOS) compatible solution for on-chip nonreciprocal transmission with dominant advantages of high NTRs, low-power consumption and compact size, which has many significant applications in on-chip optical computing and logic gates.

Laser-induced ultrasonics for detection of low-amplitude grating through metal layers with finite roughness.

We report on the use of laser-induced ultrasonics for the detection of gratings with amplitudes as small as 0.5 nm, buried underneath an optically opaque nickel layer. In our experiments, we use gratings fabricated on top of a nickel layer on glass, and we optically pump and probe the sample from the glass side. The diffraction of the probe pulse from the acoustic echo from the buried grating is measured as a function of time. We use a numerical model to show how the various physical phenomena such as interface displacement, strain-optic effects, thermo-optic effects, and surface roughness influence the shape and strength of the time-dependent diffraction signal. More importantly, we use a Rayleigh-Rice scattering theory to quantify the amount of light scattering, which is then used as in input parameter in our numerical model to predict the time-dependent diffracted signal.

Resonant opto-mechanical modulators and switches by femtosecond laser micromachining.

In this work we demonstrate novel integrated-optics modulators and switches, realized in a glass substrate by femtosecond laser pulses. These devices are based on oscillating microcantilevers, machined by water-assisted laser ablation. Single-mode optical waveguides are laser-inscribed inside the cantilever beam and continue in the substrate beyond the cantilever's tip. By exciting the resonant oscillation of the mechanical structure, coupling between the waveguide segments is varied in time. Operation frequencies are in the range of tens of kilohertz, thus they markedly overcome the response-time limitation of other glass-based modulators, which rely on the thermo-optic effect. These components may be integrated in more complex waveguide circuits or optofluidic lab-on-chips, to provide periodic and high-frequency modulation of the optical signals.

An invertible wavefront switching system with a high extinction ratio

We report the development of an invertible wavefront switching system, consisting of a spatial light modulator (SLM), a diffuser, a signal beam, and a control beam. The shaped optical signal beam can be switched between on and off states by disturbing the diffuser with a control beam via thermo-optic effect. During the wavefront shaping process, the extinction ratio of the signal beam is iteratively optimized. We demonstrate a high extinction ratio of nearly 20. In addition, when the control beam is on, a switch with either on or off state can be configured to via encoding different wavefront patterns to the signal beam. This work potentially advances many physical or bio-medical applications employing all-optical switches.

Simulation of the thermo-optic coupling effect in mid-infrared doubling frequency of AgGaSe2 crystal

The investigation of the thermo-optic coupling process of AgGaSe2 crystal was carried out via pulsed CO2 laser during doubling frequency at wavelength of 9.6 µm. To further detecting the thermal anisotropic property of AgGaSe2, a thermo-optic coupling model of the nonlinear crystal under IR frequency was constructed, following the coupling process was simulated according to this model. The simulation results present the intensities of the fundamental and harmonic beams, transformation efficiency and temperature distributions during the evolution process in detail. At the same time, temperature distribution has been identified that influence the conversion efficiency and light intensity significantly. Consequently, data provide fruitful evidences that compensation for the phase mismatch induced by the thermal effect is an outstanding approach in recovering the reduction of conversion efficiency compared with the temperature control.

All-Optical Switch by Light-to-Heat Conversion in Metal Deposited Si Ring Resonator

In this study, we propose a novel all-optical switch that is controlled by the thermo-optic effect in a silicon ring resonator with a metal cladding layer. A pump light is efficiently converted into heat through absorption by a metal layer, causing a resonant wavelength shift. The burst-switching measurement reveals a switching on/off ratio of 5.7 dB at the drop-port with a pump power of 20 mW.

Nonvolatile Electrically Reconfigurable Integrated Photonic Switch Enabled by a Silicon PIN Diode Heater.

Reconfigurability of photonic integrated circuits (PICs) has become increasingly important due to the growing demands for electronic-photonic systems on a chip driven by emerging applications, including neuromorphic computing, quantum information, and microwave photonics. Success in these fields usually requires highly scalable photonic switching units as essential building blocks. Current photonic switches, however, mainly rely on materials with weak, volatile thermo-optic or electro-optic modulation effects, resulting in large footprints and high energy consumption. As a promising alternative, chalcogenide phase-change materials (PCMs) exhibit strong optical modulation in a static, self-holding fashion, but the scalability of present PCM-integrated photonic applications is still limited by the poor optical or electrical actuation approaches. Here, with phase transitions actuated by in situ silicon PIN diode heaters, scalable nonvolatile electrically reconfigurable photonic switches using PCM-clad silicon waveguides and microring resonators are demonstrated. As a result, intrinsically compact and energy-efficient switching units operated with low driving voltages, near-zero additional loss, and reversible switching with high endurance are obtained in a complementary metal-oxide-semiconductor (CMOS)-compatible process. This work can potentially enable very large-scale CMOS-integrated programmable electronic-photonic systems such as optical neural networks and general-purpose integrated photonic processors.

Modeling spectral correlations of photon-pairs generated in liquid-filled photonic crystal fibers

The generation of photon-pairs with controllable spectral correlations is crucial in quantum photonics. Here we present the design of a photonic crystal fiber to generate widely-spaced four-wave mixing bands with spectral correlations that can be tuned through the thermo-optic effect after being infiltrated with heavy water. We present a theoretical study of the purity of the signal and idler photons generated as a function of temperature, pump spectral linewidth and the length of the fiber.

Thermally-Reconfigurable Silicon Photonic Devices and Circuits

Reconfigurable photonic integrated devices and circuits are very important for many applications. Among various mechanisms for realizing reconfigurability, thermo-optic effect is popular because of the availability for many materials, the design simplicity and the fabrication ease. In particular, silicon has a large thermo-optic coefficient as well as a high heat conductivity, and thus it is promising to realize efficient thermally-reconfigurable silicon photonic integrated devices and circuits. Recent progress is reviewed in this paper. First, thermally-tunable silicon photonic filters based on different structures are summarized, including microring resonators (MRRs) and MRRs-assisted Mach-Zehnder interferometers (MZIs). Second, silicon photonic switches based on MZIs and MRRs are reviewed. Finally, a review is given for thermally-reconfigurable silicon photonic integrated circuits developed for the applications in quantum photonics, microwave photonics, and optical interconnects.