Thermo-optic Effect Research Articles

Towards low loss non-volatile phase change materials in mid index waveguides

Photonic integrated circuits currently use platform intrinsic thermo-optic and electro-optic effects to implement dynamic functions such as switching, modulation and other processing. Currently, there is a drive to implement field programmable photonic circuits, a need which is only magnified by new neuromorphic and quantum computing applications. The most promising non-volatile photonic components employ phase change materials such as GST and GSST, which had their origin in electronic memory. However, in the optical domain, these compounds introduce significant losses potentially preventing a large number of applications. Here, we evaluate the use of two newly introduced low loss phase change materials, Sb2S3 and Sb2Se3, on a silicon nitride photonic platform for future implementation in neuromorphic computing. We focus the study on Mach–Zehnder interferometers that operate at the O and C bands to demonstrate the performance of the system. Our measurements show an insertion loss below 0.04 dB μm−1 for Sb2S3 and lower than 0.09 dB μm−1 for Sb2Se3 cladded devices for both amorphous and crystalline phases. The effective refractive index contrast for Sb2S3 on SiNx was measured to be 0.05 at 1310 nm and 0.02 at 1550 nm, whereas for Sb2Se3, it was 0.03 at 1310 nm and 0.05 at 1550 nm highlighting the performance of the integrated device.

Ultrafast Silicon MZI Optical Switch With Periodic Electrodes and Integrated Heat Sink

The compact, high-speed, low-power consumption Mach-Zehnder interferometer (MZI)-type optical switch using thermo-optical effect is a key device in future optical integrated circuits. In this study, we achieved ultra-high-speed switching operation of 0.4 μs and low electrical power operation of 23 mW with a small device footprint by loading an asymmetric doped MZI structure and an integrated heat sink. This optical switch can be fabricated in the standard manufacturing process of silicon photonic devices, has high practicality and will be a useful tool in various optical integrated circuits.

Gaussian-Shaped Gain-Dopant Distributed Fiber for High Output Power Fiber Amplifier

The remarkable evolution of ytterbium-doped fiber (YDF) lasers and amplifiers is interrupted by a limiting thermo-optical effect called transverse mode instability (TMI). Hereon, we propose a Gaussian-shaped gain-dopant distributed (GSGDD) YDF, which is fabricated by a modified chemical vapor deposition (MCVD) process combined with solution doping technique (SDT). By regulating the solution concentrations of soot layers, the content of Yb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> ions presents Gaussian-shaped distribution in the transverse direction while the refractive index profile (RIP) exhibits a stepped profile. The laser performance of this fiber is verified by a bidirectional pumped master oscillator power amplifier (MOPA). Over 3 kW near-single-mode laser output is obtained with the slope efficiency of 84.9%. At the highest power output, there are no Stokes light components in the spectrum and the beam quality M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> factor is ~1.45 These results suggest that the GSGDD fiber owns great potential to achieve high power output with excellent beam quality.

Design of GaN-Based PCSEL With Temperature-Insensitive Lasing Wavelength

Considering the compensation of thermo-optic coefficients (TOC) between GaN-based cavity and TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> photonic crystal (PhC) layer, we have theoretically designed a photonic crystal surface emitting laser (PCSEL) with temperature-insensitive wavelength. The structure includes a freestanding GaN membrane with embedded multiple InGaN/GaN quantum well (QW) layer and the top TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> PhC layer. The high refractive index (RI) of PhC and the freestanding membrane cavity structure cooperate to enable a stronger field-coupling in PhC, thus fulfill the athermal coupling condition. Interestingly, the wavelength temperature dependence is only 0.0015 nm/°C for such PCSEL with a 82% PhC confinement factor, and its athermal property is proportional to the field confinement factor in TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> PhC layer. To illustrate, a general formula has been developed after the investigations of thermo-optic effect and thermal expansion effect.

Liquid Crystal-Embedded Hollow Core Fiber Temperature Sensor in Fiber Ring Laser

An optical fiber temperature sensor based on Mach–Zehnder interferometer and thermo-optic effect of the liquid crystal (LC) in fiber ring laser (FRL) system is proposed and experimentally demonstrated. The LC is infiltrated into the core of hollow core fiber, and the resonant wavelength is more sensitive to temperature variation due to the interaction between the incident light and the cavity infiltrating liquid crystal with high thermal light coefficient. Meanwhile, the FRL system was further used to make the sensor have good performance in the case of high signal-to-noise ratio (∼35 dB), narrow half-height width (FWHM = 0.15 nm), and high sensitivity in the temperature range from 20 °C to 50 °C, with the maximum sensitivity of 1.318 nm/°C. As far as we know, in the FRL system, the liquid crystal material has a better temperature sensing performance than the previous fiber. Nevertheless, the system has the advantages of good repeatability, low cost, simple production, small volume, high sensitivity. In marine microbial culture and detection, it is necessary to carry out high sensitivity measurement within a small temperature variation range. This reliable and excellent temperature performance has a potential application prospect.

Design of an electric-driven nonvolatile low-energy-consumption phase change optical switch

Traditional optical switches relying on the weak, volatile thermo-optic or electro-optic effects of Si or SiN waveguides show a high consumption and large footprint. In this paper, we reported an electric-driven phase change optical switch consisting of a Si waveguide, Ge2Sb2Te5 (GST) thin film and graphene heater suitable for large-scale integration and high-speed switching. The reversible transition between the amorphous and crystalline states was achieved by applying two different voltage pulses of 1.4 V (SET) and 4 V (RESET). The optical performance of the proposed switch showed a high extinction ration of 44–46 dB in a wide spectral range (1525–1575 nm), an effective index variation of Δn eff = 0.49 and a mode loss variation of Δα = 15 dB μm−1 at the wavelength of 1550 nm. In thermal simulations, thanks to the ultra-high thermal conductivity of graphene, the proposed switch showed that the consumption for the SET process was only 3.528 pJ with a 1.4 V pulse of 5 ns, while a 4 V pulse of 1.5 ns was needed for RESET process with a consumption of 1.05 nJ. Our work is helpful to analyze the thermal-conduction phase transition process of on-chip phase change optical switches, and the design of the low-energy-consumption switch is conducive to the integrated application of photonic chips.

1.5 and 2.0 µm all-optical modulators based on niobium-carbide (Nb2C)-PVA film

In this work, a niobium carbide (Nb2C) was proposed and demonstrated as an optical modulator for operation in the 1.5 and the 2.0 µm wavelength region. The Nb2C solution was first prepared using the liquid phase exfoliation technique, and then embedded into a polyvinyl alcohol (PVA) polymer to form a thin film. Based on the thermo-optic effect exhibited by the Nb2C-PVA film, successful modulations of the 1.5 and the 2.0 µm signals were observed, where the signals could be modulated up to a frequency of 5 kHz. The modulators could also be employed in an all-optical signal processing system, in which information codes from the control light could be conveyed through both signals. The integration of the Nb2C-PVA film into the all-optical modulation system shows the material efficiency to modulate the transmission light with high stability, showing significant potential of the device in an all-optical signal processing system.

Securing Practical Quantum Communication Systems with Optical Power Limiters

Controlling the energy of unauthorized light signals in a quantum cryptosystem is an essential criterion for implementation security. Here, we propose a passive optical power limiter device based on thermo-optical defocusing effects providing a reliable power limiting threshold which can be readily adjusted to suit various quantum applications. In addition, the device is robust against a wide variety of signal variations (e.g. wavelength, pulse width), which is important for implementation security. Moreover, we experimentally show that the proposed device does not compromise quantum communication signals, in that it has only a very minimal impact (if not, negligible impact) on the intensity, phase, or polarization degrees of freedom of the photon, thus making it suitable for general communication purposes. To show its practical utility for quantum cryptography, we demonstrate and discuss three potential applications: (1) measurement-device-independent quantum key distribution with enhanced security against a general class of Trojan-horse attacks, (2) using the power limiter as a countermeasure against bright illumination attacks, and (3) the application of power limiters to potentially enhance the implementation security of plug-and-play quantum key distribution.

Overcoming thermo-optical dynamics in broadband nanophotonic sensing

Advances in integrated photonics open up exciting opportunities for batch-fabricated optical sensors using high-quality-factor nanophotonic cavities to achieve ultrahigh sensitivities and bandwidths. The sensitivity improves with increasing optical power; however, localized absorption and heating within a micrometer-scale mode volume prominently distorts the cavity resonances and strongly couples the sensor response to thermal dynamics, limiting the sensitivity and hindering the measurement of broadband time-dependent signals. Here, we derive a frequency-dependent photonic sensor transfer function that accounts for thermo-optical dynamics and quantitatively describes the measured broadband optomechanical signal from an integrated photonic atomic force microscopy nanomechanical probe. Using this transfer function, the probe can be operated in the high optical power, strongly thermo-optically nonlinear regime, accurately measuring low- and intermediate-frequency components of a dynamic signal while reaching a sensitivity of 0.7 fm/Hz1/2 at high frequencies, an improvement of ≈10× relative to the best performance in the linear regime. Counterintuitively, we discover that a higher transduction gain and sensitivity are achieved with lower quality-factor optical modes for low signal frequencies. Not limited to optomechanical transducers, the derived transfer function is generally valid for describing the small-signal dynamic responses of a broad range of technologically important photonic sensors subject to the thermo-optical effect.

Integrated Thermally Tuned Mach-Zehnder Interferometer in Z-Cut Lithium Niobate Thin Film

We propose and demonstrate a type of highly efficient tunable Mach-Zehnder interferometer (MZI) in Z-cut lithium niobate thin film (TFLN) based on the thermo-optical effect. By placing the electrode directly above the TFLN waveguide, a more uniform and higher heat distribution was obtained, compared with the conventional horizontal structure. Both the principle and design strategies of the proposed scheme are described. The process flow of the device fabrication is demonstrated in Z-cut TFLN. The measured static results accord well with the simulations. About 0.121 nm/V tuning efficiency is observed, which is nearly double the tuning efficiency based on the electro-optic effect, and may prove useful when high switching speed is not the primary consideration. The presented thermo-optic tuning scheme can be extended to other scenarios easily and may satisfy many different application requirements in integrated lithium niobate photonics (LNPs).

Attack Mitigation of Hardware Trojans for Thermal Sensing via Micro-ring Resonator in Optical NoCs

As an emerging role in new-generation on-chip communication, optical networks-on-chip (ONoCs) provide ultra-high bandwidth, low latency, and low power dissipation for data transfers. However, the thermo-optic effects of the photonic devices have a great impact on the operating performance and reliability of ONoCs, where the thermal-aware control with accurate measurements, e.g., thermal sensing, is typically applied to alleviate it. Besides, the temperature-sensitive ONoCs are prone to be attacked by the hardware Trojans (HTs) covertly embedded in the counterfeit integrated circuits (ICs) from the malicious third-party vendors, leading to performance degradation, denial-of-service (DoS), or even permanent damages. In this article, we focus on the tampering and snooping attacks during the thermal sensing via micro-ring resonator (MR) in ONoCs. Based on the provided workflow and attack model, a new structure of the anti-HT module is proposed to verify and protect the obtained data from the thermal sensor for attacks in its optical sampling and electronic transmission processes. In addition, we present the detection scheme based on the spiking neural networks (SNNs) to implement an accurate classification of the network security statuses for further high-level control. Evaluation results indicate that, with less than 1% extra area of a tile, our approach can significantly enhance the hardware security of thermal sensing for ONoC with trivial costs of up to 8.73%, 5.32%, and 6.14% in average latency, execution time, and energy consumption, respectively.

Ultrafast Third-Order Nonlinear Optical Response Excited by fs Laser Pulses at 1550 nm in GaN Crystals.

The ultrafast third-order optical nonlinearity of c-plane GaN crystal, excited by ultrashort (fs) high-repetition-rate laser pulses at 1550 nm, wavelength important for optical communications, is investigated for the first time by optical third-harmonic generation in non-phase-matching conditions. As the thermo-optic effect that can arise in the sample by cumulative thermal effects induced by high-repetition-rate laser pulses cannot be responsible for the third-harmonic generation, the ultrafast nonlinear optical effect of solely electronic origin is the only one involved in this process. The third-order nonlinear optical susceptibility of GaN crystal responsible for the third-harmonic generation process, an important indicative parameter for the potential use of this material in ultrafast photonic functionalities, is determined.

Dual slot-mode NOEM phase shifter.

Photonic system component counts are increasing rapidly, particularly in CMOS-compatible silicon photonics processes. Large numbers of cascaded active photonic devices are difficult to implement when accounting for constraints on area, power dissipation, and response time. Plasma dispersion and the thermo-optic effect, both available in CMOS-compatible silicon processes, address a subset of these criteria. With the addition of a few back-end-of-line etch processing steps, silicon photonics platforms can support nano-opto-electro-mechanical (NOEM) phase shifters. Realizing NOEM phase shifters that operate at CMOS-compatible voltages (≤ 1.2 V) and with low insertion loss remains a challenge. Here, we introduce a novel NOEM phase shifter fabricated alongside 90 nanometer transistors that imparts 5.63 radians phase shift at 1.08 volts bias over an actuation length of 25μm with an insertion loss of less than 0.04 dB and 3 dB bandwidth of 0.26 MHz.

Ultrafast all-optical diffraction switching using semiconductor metasurfaces

Ultrafast all-optical switching using Mie resonant metasurfaces requires both on-demand tunability of the wavefront of the light and ultrafast time response. However, devising a switching mechanism that has a high contrast between its “on” and “off” states without compromising speed is challenging. Here, we report the design of a tunable Mie resonant metasurface that achieves this behavior. Our approach utilizes a diffractive array of semiconductor resonators that support both dipolar and quadrupolar Mie resonances. By balancing the strengths of the dipole and quadrupole resonances, we can suppress radiation into the first diffraction order, thus creating a clearly delineated “off”-state at the operating wavelength. Then, we use optical injection of free- carriers to spectrally shift the multipoles and rebalance the multipole strengths, thereby enabling radiation into the diffraction order—all on an ultrafast timescale. We demonstrate ultrafast off-to-on switching with Ion/Ioff ≈ 5 modulation of the diffracted intensity and ultrafast on-to-off switching with Ion/Ioff ≈ 9 modulation. Both switches exhibit a fast τtr ≈ 2.7 ps relaxation time at 215 μJ cm−2 pump fluence. Further, we show that for higher fluences, the temporal response of the metasurface is governed by thermo-optic effects. This combination of multipole engineering with lattice diffraction opens design pathways for tunable metasurface-based integrated devices.

Plasmonic sensor for magnetic field detection with chalcogenide glass and ferrofluid materials under thermal variation in near infrared

In this paper, surface plasmon resonance based magnetic field sensor is simulated and analyzed in near infrared wavelength region (1557 nm) . The magnetic fluid is a colloidal material that can transform into different states depending on the strength of the external magnetic field (H). The structure of the sensor consists of chalcogenide (Ge 28 Sb 12 Se 60 ) prism, silver layer, graphene monolayer, and a magnetic fluid (ferrofluid) film. Analysis is carried out using temperature-dependent complex refractive index of graphene (as per Kubo formulation) and silver layer (as per phonon-electron and electron-electron scatterings) along with the thermo-optic effect in prism and ferrofluid media. Angular interrogation method has been used. An extremely fine detection limit of 0.0037 mT at room temperature can be achieved. As large magnitude of figure-of-merit (FOM) as 1.7263 (mT) −1 can be achieved with the proposed magnetic field sensor. High refractive index of Ge 28 Sb 12 Se 60 glass is helpful in ascertaining high spatial resolution while the non-linearity in Ge 28 Sb 12 Se 60 may not be an issue as a low power laser source is required for the practical implementation of the proposed magnetic field sensor scheme. • Chalcogenide glass and ferrofluid materials have been used in plasmonic sensor for magnetic field measurement. • Angular interrogation method is applied while utilizing graphene layer that acts as a protective layer atop Ag layer. • Effect of temperature in near infrared (1557 nm) is studied for sensing performance enhancement. • Proposed probe provides significantly large sensitivity and fine detection limit of magnetic field detection.

All‐fibre phase shifter based on tapered fibre coated with MoWS 2 ‐rGO

In this work, an all-fibre phase shifter using tapered fibre coated with molybdenum tungsten disulfide - reduced graphene oxide (MoWS2-rGO) is demonstrated. The large evanescent field of tapered fibre allows MoWS2-rGO to absorb injected light from a 1400-nm pump. It subsequently modifies the refractive index of both an MoWS2-rGO solution and tapered fibre because of the thermo-optic effect. A maximum wavelength shift of 2.489 nm is observed and the equivalent phase shift is 0.91π. The efficiency of the phase shifter is calculated at 0.0175 nm/mW. The proposed phase shifter is beneficial for a variety of applications, specifically for future optical beam-forming networks and all optical signal processing devices.

All-Optical Control of a Single Resonance in a Graphene-On-Silicon Nanobeam Cavity Using Thermo-Optic Effect

We propose and demonstrate on-chip all-optical control of a single resonance in a graphene-on-silicon nanobeam cavity using thermo-optic (TO) effect. In the design, the graphene sheet covered on the nanobeam cavity is patterned in a “U” shape, which enables the device to maintain a constant quality factor and a high extinction ratio (ER) after graphene transfer. Benefiting from the ultra-small optical mode volume of the nanobeam cavity and the high thermal conductivity of graphene, efficient all-optical control of a single resonance is implemented with a tuning efficiency of 0.0346 nm/mW. All-optical switching is also demonstrated with a switching power of 47 mW at a high ER of ~16 dB. Moreover, the 10%-90% switching times are 3.24 μs and 5.52 μs for the rising edge and the falling edge, respectively. Such graphene-assisted device potentially could be a good candidate to realize efficient on-chip all-optical control of an individual resonance with a high ER.

Narrow linewidth VCSEL based on resonant optical feedback from an on-chip microring add-drop filter

We demonstrate a narrow linewidth vertical-cavity surface-emitting laser (VCSEL) by injecting resonant optical feedback into the lasing cavity. A single longitudinal mode VCSEL with a Lorentzian linewidth of 32.6 kHz and a purified optical spectrum is experimentally achieved by an on-chip microring add-drop filter with a quality factor of 1.36 million, where the feedback level is ${-}{47.77}\;{\rm{dB}}$. The frequency noise spectrum of the VCSEL demonstrates that the thermo-optic effect in the microring resonator can also stabilize the lasing frequency. A VCSEL with narrow linewidth and stable frequency provides a high-performance light source for a single VCSEL or VCSEL array-based application.

Miniaturized integrated spectrometer using a silicon ring-grating design.

We introduce and experimentally demonstrate a miniaturized integrated spectrometer operating over a broad bandwidth in the short-wavelength infrared (SWIR) spectrum that combines an add-drop ring resonator narrow band filter with a distributed Bragg reflector (DBR) based broadband filter realized in a silicon photonic platform. The contra-directional coupling DBR filter in this design consists of a pair of waveguide sidewall gratings that act as a broadband filter (i.e., 3.9 nm). The re-directed beam is then fed into the ring resonator which functions as a narrowband filter (i.e., 0.121 nm). In this scheme the free spectral range (FSR) limitation of the ring resonator is overcome by using the DBR as a filter to isolate a single ring resonance line. The overall design of the spectrometer is further simplified by simultaneously tuning both components through the thermo-optic effect. Moreover, several ring-grating spectrometer cells with different central wavelengths can be stacked in cascade in order to cover a broader spectrum bandwidth. This can be done by centering each unit cell on a different center wavelength such that the maximum range of one-unit cell corresponds to the minimum range of the next unit cell. This configuration enables high spectral resolution over a large spectral bandwidth and high extinction ratio (ER), making it suitable for a wide variety of applications.

Laser ray bundle: a thermometry method for HIFU heating

High-intensity focused ultrasound (HIFU) is a therapeutic modality that heats tissue by focusing a sound beam into a narrow, localized region. This paper is concerned with the thermo-optic lensing effect observed by Al-Qraini et al (Ultrasound Med. Biol. 39 647–59) when visible light was shone at a transparent tissue-mimicking phantom heated by HIFU. Thermal lensing was caused by light refraction as it passed through the thermal gradient region. HIFU thermal lensing was reproduced experimentally and studied analytically using the geometric (eikonal) ray equation to devise a method for temperature measurement. Under the Gaussian thermal field assumption, explicit closed form solutions were found for the forward thermo-optic ray deflection problem, and for the inverse opto-thermal conversion problem that yielded the underlying temperature field from measured ray deflections. Neither temperature calibration nor iterative computations were needed. In the experimental setup, a bundle of thin laser rays illuminated the HIFU-heated spot in a clear tissue-mimicking phantom. The ray deflections were recorded and directly converted into temperature with very good temporal and spatial resolution. These compared reasonably well against the output of an embedded thermocouple (TC)—considering the absorptive and viscous heating artifacts which were also investigated. The TC metallic tip junction was carefully positioned to coincide with the axis of peak heating. It was found that the TC stem (wire) transferred heat by conduction from the tip into the tissue located off the HIFU heating axis. The proposed thermometry method can be useful in elucidating heating from focused ultrasound sources, or from other sources of localized heating such as lasers.