Top Cladding Layer Research Articles

Detection of dimethyl methylphosphonate (DMMP) with an interband cascade laser sensor

We demonstrate the sensitive detection of dimethyl methylphosphonate (DMMP, a hydrogen-bond (HB) basic phosphonate ester) using additional optical loss induced in an interband cascade laser with top optical cladding layer replaced by an exposed sensing window coated by a HB acidic sorbent layer. Thin coatings of the sorbents HCSFA2 and oapBPAF were deposited on the sensing window to allow reversible capture and concentration of DMMP for optical interrogation. Analyte levels down to 0.1 mg/m3 (∼20 ppb) were tested and successfully detected by monitoring the laser’s threshold or its output power at a fixed bias as a function of DMMP delivery concentration.

Integrating a Nanowire Laser in an on-Chip Photonic Waveguide.

We report a simple and facile integration strategy of a laser source in passive photonic integrated circuits (PICs) by deterministically embedding semiconductor nanowires (NWs) in waveguides. InP NWs laid on a SiN slab are buried by a polymer layer which also acts as an electron-beam resist. With electron-beam lithography, hybrid polymer-SiN waveguides are formed with precisely embedded NWs. The lasing behavior of the waveguide-embedded NWs is confirmed, and more importantly, the NW lasing mode couples into the hybrid waveguide and forms an in-plane guiding mode. Multiple waveguide-embedded NW lasers are further integrated in complex photonic structures to illustrate that the waveguiding mode supplied by the NW lasers could be manipulated for on-chip signal processing, including power splitting and wavelength-division multiplexing. This integration strategy of an on-chip laser is applicable to other PIC platforms, such as silicon and lithium niobate, and the top cladding layer could be changed by depositing SiN or SiO2, promising its CMOS compatibility.

Highly linear lithium niobate Michelson interferometer modulators assisted by spiral Bragg grating reflectors.

Highly linear electro-optic modulators are key components in analog microwave photonic links, offering on-chip direct mixing of optical and RF fields. In this work, we demonstrate a monolithic integrated Michelson interferometer modulator on thin-film lithium niobate (LN), that achieves linearized performance by modulating Bragg grating reflectors placed at the end of Michelson arms. The modulator utilizes spiral-shaped waveguide Bragg gratings on Z-cut LN with top and bottom electrodes to realize extensive reflectors, essential for linearized performance, in a highly integrated form. Optical waveguides are realized using rib etching of LN with precisely engineered bottom and top cladding layers made of silicon dioxide and SU-8 polymer, respectively. The compact design fits a 3 mm long grating in an 80 µm × 80 µm area, achieving a broad operating bandwidth up to 18 GHz. A spurious free dynamic range (SFDR) of 101.2 dB·Hz2/3 is demonstrated at 1 GHz, compared to 91.5 dB·Hz2/3 for a reference Mach-Zehnder modulator fabricated on the same chip. Further enhancement in SFDR could be achieved by reducing fiber-to-chip coupling loss. The proposed demonstration could significantly improve the linearity of analog modulator-based integrated optical links.

Efficient and compact sol-gel TiO2 thermo-optic microring resonator modulator

Thermo-optic (TO) modulators play an increasingly important role in wavelength routers, lidar, optical computing, and other reconfigurable photonic systems. Highly efficient TO tunable microring resonators (MRRs) were first demonstrated based on a sol-gel TiO2 platform in the 1310-nm waveband owing to the synergistic effect between the TiO2 core and SU-8 cladding with both the negative thermo-optical coefficients. The MRR modulator with SU-8 polymer as the top cladding layer exhibits a thermal tuning efficiency of 33.0 pm/mW, which is more than 14 times higher than that with silica top cladding. Its rise/fall times of 9.4 us/24 us and a Pπ power of 7.22 mW were achieved, indicating a relatively high TO modulator figure of merit among noncrystalline material platforms allowing monolithic integration on different substrates. These results yield a strong promise for applying the sol-gel TiO2 platform in photonic integrated circuits and suggest a new angle of view to design compact and efficient TO modulators in wearable devices, visible/infrared communication, and biophotonic applications.

Group-index-matched frequency conversion in lithium niobate on insulator waveguides

Sources of spectrally engineered photonic states are a key resource in several quantum technologies. Of particular importance are the so-called factorizable biphoton states, which possess no spectral entanglement and hence, are ideal for heralded generation of high-purity single photons. An essential prerequisite for generating these states through nonlinear frequency conversion is the control over the group indices of the photonic modes of the source. Here, we show that thin-film lithium niobate on insulator (LNOI) is an excellent platform for this purpose. We design and fabricate periodically poled ridge waveguides in LNOI to demonstrate group index engineering of its guided photonic modes and harness this control to experimentally realize on-chip group index matched type-II sum-frequency generation (SFG). Also, we numerically study the role of the top cladding layer in tuning the dispersion properties of the ridge waveguide structures and reveal a distinctive difference between the air and silica-clad designs which are currently among the two most common device cladding configurations in LNOI. We expect that these results will be relevant for various classical and quantum applications where dispersion control is crucial in tailoring the nonlinear response of the LNOI-based devices.

Исследование пространственных характеристик излучения поверхностно-излучающих квантово-каскадных лазеров с кольцевым резонатором

The results of studies of ring quantum-cascade lasers with surface emission due to a second-order grating formed in the top cladding layers are presented. Surface emission near 7.85 μm with a low threshold current density (3.8 kA/cm2), in comparison with ridge quantum-cascade lasers of the same cavity length is demonstrated. The results of measurements of the intensity distribution of the near and far fields at different pumping levels are presented. The estimated value of the angle of beam extraction relative to the surface normal is in the range (5.7–6.7)°.

Study of the spatial characteristics of emission of surface-emitting ring quantum-cascade lasers

The results of studies of ring quantum-cascade lasers with surface emission due to a second-order grating formed in the top cladding layers are presented. Surface emission near 7.85 μm with a low threshold current density (3.8 kA/cm2), in comparison with ridge quantum-cascade lasers of the same cavity length is demonstrated. The results of measurements of the intensity distribution of the near and far fields at different pumping levels are presented. The estimated value of the angle of beam extraction relative to the surface normal is in the range (5.7-6.7)o. Keywords: ring cavity, grating, focused ion beam (FIB) etching, superlattices, quantum-cascade laser, epitaxy, indium phosphide.

Material contact sensor with 3D coupled waveguides.

An evanescent field sensor to identify materials by contact has been demonstrated using a 3D coupled waveguide array. The array is formed by imbedding layered silicon nitride stripes as waveguide cores in polymer cladding and the top cladding layer is etched open for material sensing. When objects with different refractive indexes are placed on the surface of the sensor, the evanescent field is disturbed and both the local modal distribution and the coupling condition with the connecting segments are altered, leading to different interference patterns when light from the output facet is captured and focused onto a camera. We have chosen four conventional materials for test: polymer, silicon, aluminum and silver. The sensor is able to tell them apart with distinctive patterns. In addition, the sensor can identify the location of the contact, once the material is recognized. This simple and low-cost device, supported by the recent development of image recognition technology, may open up new possibilities in chip-based sensing applications.

Surface and interface characterization of Ru/C/Ru trilayer structure using grazing incidence X‐ray reflectivity and X‐ray fluorescence

In 8‐ to 20‐keV photon energy region, ruthenium and carbon thin films are used in multilayer monochromators. In the present study, this material combination is explored for X‐ray waveguide applications in hard X‐ray region. The structural parameters (thickness of each layer) of Ru/C/Ru waveguide structure are optimized to get maximum intensity enhancement of fundamental mode inside carbon guiding layer. A sample with optimized structural parameters is deposited using ion beam sputtering (IBS) technique and characterized using X‐ray reflectivity (XRR) and grazing incidence X‐ray fluorescence (GIXRF) techniques. The analysis suggests that the density of bottom Ru layer and carbon guiding layer is close to bulk density (~97% for Ru and ~95% for carbon), whereas density of top Ru layer is slightly lower (~93% of bulk density). A ~10% of thickness variation in top cladding layer along with marginal change in layer density deteriorate field enhancement in TE0 mode by more than three times. Effect of thickness and density variation on waveguide (Ru [7 nm]/C [18 nm]/Ru [20 nm]) performance is discussed.

Supercontinuum generation in tantalum pentoxide waveguides for pump wavelengths in the 900 nm to 1500 nm spectral region.

We characterize the spectral broadening performance in silica clad and unclad Tantalum pentoxide (Ta2O5) waveguides as a function of the input pulse central wavelength and polarization, sweeping over a wavelength range from 900 nm to 1500 nm, with an average incident power of 110 mW. The waveguides are 0.7 µm high and between 2.2 and 3.2 µm wide, and the SiO2 top cladding layer is 2 µm thick. We model the dispersion of the higher order spatial modes, and use numerical simulations based on the generalized nonlinear Schrödinger equation to analyze the nonlinear behaviour of the spatial modes within the waveguides as well as the dispersive effects observed in the experiments. We achieve octave spanning supercontinuum with an average power of 175 mW incident on the waveguide at 1000 nm pump wavelength.

Thermal dynamic imaging of mid-infrared quantum cascade lasers with high temporal–spatial resolution

The time-resolved (TR) temperature profile of actively biased mid-infrared quantum cascade lasers (MIR QCLs) was measured by using charge-coupled-device (CCD)-based thermoreflectance microscopy (TRM) with an ultrafast temporal resolution of 50 ns and a high spatial resolution of 390 nm. Based on the measured TR two-dimensional (2D) temperature profiles, the heat generation and dissipation dynamics within the lasers have been investigated. It is found that the active-region temperature increases quickly to a peak value (up to ∼100 °C above ambient room temperature) within 500 ns upon pulsed current injection of 6 A. The heat dissipation to the top and bottom cladding layers of the device is initially comparable, yet it evolves differently with time. Within 1–2 μs, the heat dissipation to the top cladding is substantially reduced and most of the heat is drained to the substrate through the bottom cladding layer. This constrained heat dissipation results in the elevated temperature in the active region, leading to thermal quenching of lasing operation, which is confirmed by experimental light–current–voltage measurement and theoretical thermal modeling. The TRM is an enabling tool for probing internal thermal dynamics of many active electronic and photonic devices, particularly for those needing special heat and thermal arrangement.

Ultra-sensitive refractive index gas sensor with functionalized silicon nitride photonic circuits

Portable and cost-effective gas sensors are gaining demand for a number of environmental, biomedical, and industrial applications, yet current devices are confined into specialized labs and cannot be extended to general use. Here, we demonstrate a part-per-billion-sensitive refractive index gas sensor on a photonic chip based on silicon nitride waveguides functionalized with a mesoporous silica top-cladding layer. Low-concentration chemical vapors are detected by monitoring the output spectral pattern of an integrated unbalanced Mach-Zehnder interferometer having one coated arm exposed to the gas vapors. We retrieved a limit of detection of 65 ppb, 247 ppb, and 1.6 ppb for acetone, isopropyl alcohol, and ethanol, respectively. Our on-chip refractive index sensor provides, to the best of our knowledge, an unprecedented limit of detection for low gas concentrations based on photonic integrated circuits. As such, our results herald the implementation of compact, portable, and inexpensive devices for on-site and real-time environmental monitoring and medical diagnostics.

Broadband Electro-Optical Crossbar Switches Using Low-Loss Ge2Sb2Se4Te1 Phase Change Material

This theoretical modeling and simulation paper presents designs and projected performance of non-volatile broadband on-chip 1 × 2 and 2 × 2 electro-optical switches operating in the telecommunication C -band and based on the silicon-on-insulator technological platform. These optical switches consist of an asymmetric two-waveguide directional coupler and a symmetric three-waveguide directional coupler, in which the optical phase change material Ge2Sb2Se4Te1 (GSST) is the top cladding layer for one of the silicon strip waveguides. Reversible crossbar switching is attained by the amorphous (Am) to crystalline (Cr) and Cr-to-Am phase transitions in the GSST induced by heating the GSST in contact with an indium tin oxide (ITO) microstrip through Joule heating. We examined device performance in terms of mid-band insertion loss (IL), crosstalk (CT), and 0.3-dB IL bandwidth (BW). The 2 × 2 results were IL = –0.018 dB, CT L c of 15.4 μm, and IL = 0.046 dB, CT L c of 17.4 μm. Simulations of the 1 × 2 devices at 16.7-μm L c revealed that IL = 0.083 dB and CT < 12.8 dB along with an expanded BW of 95 nm. Thermal simulations showed that a 5-V pulse train applied to 1019-cm−3 doped ITO would produce crystallization; however, the process of amorphization required a 24-V pulse of 2.9-μs duration to raise the GSST temperature above the melting temperature of 900 K.

Генерация квантово-каскадного лазера с тонкой верхней обкладкой

Experimental results on the manufacturing and study of the properties of 7-8 μm wavelength range injection quantum-cascade lasers (QCLs) with a waveguide with a thin top cladding layer based on indium phosphide are presented. The heterostructure is formed by molecular beam epitaxy on an InP substrate, with a In0.53Ga0.47As/Al0.48In0.52As active region. Lasing at a wavelength of 7.8 μm at a temperature of 300 K with a threshold current density of ~ 6 kA / cm2 is obtained. Characteristic temperature T0 and T1 values for the QCLs are about 150 K and 450 K, respectively. Obtained results confirm the promising design of the QCLs with a waveguide with a thin top cladding layer for detecting liquids and microfluidic devices development.

Zinc oxide clad limited area epitaxy semipolar III-nitride laser diodes.

We report continuous-wave (CW) blue semipolar (202¯1) III-nitride laser diodes (LDs) that incorporate limited area epitaxy (LAE) n-AlGaN bottom cladding with thin p-GaN and ZnO top cladding layers. LAE mitigates LD design limitations that arise from stress relaxation, while ZnO layers reduce epitaxial growth time and temperature. Numerical modeling indicates that ZnO reduces the internal loss and increases the differential efficiency of TCO clad LDs. Room temperature CW lasing was achieved at 445 nm for a ridge waveguide LD with a threshold current density of 10.4 kA/cm2, a threshold voltage of 5.8 V, and a differential resistance of 1.1 Ω.

Design of photonic crystal surface emitting lasers with indium-tin-oxide top claddings

Electrically pumped GaAs-based photonic crystal surface emitting lasers were fabricated using a simple fabrication process by directly capping the indium-tin-oxide transparent conducting thin film as the top cladding layer upon a photonic crystal layer. Optimization of the separate-confinement heterostructures of a laser structure is crucial to improving characteristics by providing advantageous optical confinements. The turn-on voltage, series resistance, threshold current, and slope efficiency of the laser with a 100 × 100 μm2 photonic crystal area operated at room temperature were 1.3 V, 1.5 Ω, 121 mA, and 0.2 W/A, respectively. Furthermore, we demonstrated a single-lobed lasing wavelength of 928.6 nm at 200 mA and a wavelength redshift rate of 0.05 nm/K in temperature-dependent measurements. The device exhibited the maximum output power of approximately 400 mW at an injection current of 2 A; moreover, divergence angles of less than 1° for the unpolarized circular-shaped laser beam were measured at various injection currents. Overall, the low threshold current, excellent beam quality, small divergence, high output power, and high-operating-temperature (up to 343 K) of our devices indicate that they can potentially fill the requirements for next-generation light sources and optoelectronic devices.

Observation of spin dependent electrochemical potentials at room temperature in a quantum well structure

Spin transport with a strong Rashba spin splitting is observed in a semiconductor channel. Using the current injection into the side of the quantum well, the potentiometric signal and the spin-torque-induced magnetization change are simultaneously detected. In this geometry, the spin current is directly injected from the ferromagnetic electrode into the quantum well channel and the electrochemical potential is monitored by the detection ferromagnet. For the side injection instead of vertical injection, the milling of the top cladding layer is not necessary, so the efficient spin transport and the undamaged Rashba spin splitting can be realized. The ferromagnetic detector senses the Rashba-effect-induced electrochemical potential shift brought by the charge current and this potentiometric signal is also modified by the magnetization rotation induced by the spin current. The large potential changes of 1.03 Ω and 0.96 Ω are observed at 1.8 K and 300 K, respectively.

Semipolar III-nitride laser diodes with zinc oxide cladding.

Incorporating transparent conducting oxide (TCO) top cladding layers into III-nitride laser diodes (LDs) improves device design by reducing the growth time and temperature of the p-type layers. We investigate using ZnO instead of ITO as the top cladding TCO of a semipolar (202¯1) III-nitride LD. Numerical modeling indicates that replacing ITO with ZnO reduces the internal loss in a TCO clad LD due to the lower optical absorption in ZnO. Lasing was achieved at 453 nm with a threshold current density of 8.6 kA/cm2 and a threshold voltage of 10.3 V in a semipolar (202¯1) III-nitride LD with ZnO top cladding.

Numerical simulation of low loss silicon photonic wire waveguide with multiple cladding layers

A different silicon photonic wire waveguide is proposed, which uses multiple thin cladding layers in order to reduce the index contrast between core and cladding interface. The reduced index contrast in the proposed waveguide has led to reduction in the scattering losses by 37% as compared to silicon wire waveguide for 400 nm × 220 nm waveguide dimension. The proposed waveguide has shown significant reduction in bending losses. It offers the bending loss of 0.0118 dB at the radius of 1 μm and 0.0063 dB for a radius of 2 μm at 1.55 μm wavelength as compared to 0.086 and 0.013 dB at the radius of 1 and 2 μm, respectively, offered by silicon photonic wire waveguide at 1.5 μm wavelength. The use of polymer material as top cladding layer resulted in decreasing the sensitivity of effective index against temperature for the designed waveguide by a factor of 2 as compared to silicon wire waveguide.

Plasmon-enhanced waveguide for dispersion compensation in mid-infrared quantum cascade laser frequency combs.

We demonstrate dispersion compensation in mid-infrared quantum cascade laser frequency combs (FCs) emitting at 7.8μm using the coupling of a dielectric waveguide to a plasmonic resonance in the top cladding layer of the latter. Devices with group velocity dispersion lower than 110 fs2/mm were fabricated, and narrow beatnotes with FWHM linewidths below 1kHz were measured on the entire operation range. At -20°C, the optical output power reaches 275mW, and the optical spectrum spans 60 cm-1. The multi-heterodyne beating spectrum of two devices was measured and spans 46 cm-1, demonstrating the potential of dispersion-engineered waveguides for the fabrication of highly stable and reliable quantum cascade laser FCs with high output power across the mid-infrared.