Session K07: Optical Systems with Advanced Materials and Techniques

Band Engineering of Rare Earth Nickelates for Applications in Optoelectronics and Non-volatile Memory

The rich phase diagrams and exotic physical properties of rare earth nickelates (RNiO₃) have recently attracted much attention. Their band structures are highly sensitive to carrier density and bandwidth due to Mott physics. Here, we report the band engineering of RNiO₃ for applications in optoelectronics and non-volatile memory. Band structures of RNiO₃ have been tuned by controlling oxygen content during film deposition. Epitaxial GdNiO₃ films have been deposited on Nb-doped SrTiO₃ substrates to form heterojunctions. Under optimized condition, such designed heterojunctions work as self-powered photodetectors with high sensitivity. Under 365 nm illumination, the photo-dark ratio reaches 10³ when the power light density is as weak as 0.6 mW/cm² and the responsivity can be as high as 0.23 A/W at 0 V bias even light density is only 50 µW/cm², which are more competitive than some ultraviolet photodetectors based on GaN or ZnO. Also, further investigation confirms that oxygen vacancy migration across the interface of the heterojunction causes giant resistive switching behavior. An ON/OFF ratio of 10⁵ has been observed and no significant decay appears after more than 10⁴ write/read cycles. Our work promotes the development of multifunctional electronic devices based on nickelates.
  

100 THz-bandwidth difference frequency generation at LaAlO3/SrTiO3 nanoscale junctions

We investigate the broadband nonlinear generation and detection capabilities of nanoscale junctions created at the LaAlO3/SrTiO3 interface using conductive AFM lithography. Using the large non-resonant third-order nonlinear susceptibility in SrTiO3, strong difference frequency mixing occurs when the junction is biased, leading to induced polarization that can also be detected at the junction (Ma et al, Nano Lett 13, 2884 (2013)). Here we characterize the bandwidth of the resulting THz emission, which appears to be bandwidth limited to about 100 THz for the 7 fs Ti:Sapphire laser pulse used. The ultrawide tunable bandwidth opens pathways for a new generation of THz spectroscopy on nanomaterials.   

Abstract Withdrawn

ZnO based nitrogen oxide (NOx) sensors have come to focus due to the potentiality of it to detect toxic NOx gases at high temperature. Light-driven gas sensing is a most promising approach for room temperature low power operated sensors to avoid explosive hazards at high temperature. Furthermore, the performance of light-driven gas sensors can significantly be enhanced using light trapping mechanism. We have fabricated ordered ZnO nanostructures using different polystyrene colloid templates of diameters 300, 600, and 800 nm respectively. 300 nm colloids templated nanostructures have shown improved gas sensing response in the presence of white light than other sensors. The sensitivity of ordered sensors using 300 nm colloids is found to be enhanced 3 times at 10 ppm than the unordered ZnO nanorod sensors due to efficient light trapping within the structures. This observation suggests that the light trapping mechanism can be effectively used to design the sensors operated at room temperature with high sensing response.
  

Non-Hermitian Amplification via Four-Wave Mixing in Silicon Photonics

In nonlinear integrated optics, phase matching is a key requirement for practical device efficiency. Recently one new approach has been advanced, which goes beyond phase matching for amplification in nonlinear optical devices by harnessing the non-Hermitian properties of optical devices for four-wave mixing (FWM). Previously, we used parity-time (PT) symmetry breaking to show that by careful engineering of the dissipative spectral features in a nonlinear optical system, it is possible to achieve gain for the signal component in the absence of Hermitian phase-matching condition using a second or third-order nonlinear material by introducing optical losses to the idler component. In this work, our goal is to show numerically that a realistic device based on PT principles can be realized in a silicon-based material system. In particular, we investigate PT symmetry in FWM that provides signal gain while imposing idler loss in a waveguide-based directional coupler. The interaction of nonlinearity and non-Hermiticity can thus be applied to develop amplifiers and oscillators that do not require the stringent phase-matching condition between the different frequency components.
  

Effects of Viscus Damping on MEMS Sound Sensors with Integrated Comb Finger Capacitors1

The use of MEMS based sound sensors as miniature microphones operating in the audible frequency range is growing. In this work, MEMS sensors were designed to operate at their resonant frequency for applications in finding the direction of sound. The MEMS sensor consists of a 3x5 mm² cone shaped cantilever attached to the substrate at the narrow end. The thickness of the cantilever is about 25 μm. For electronic readout of the sensor response, an interdigitated comb finger capacitor was integrated at the far end. Two sensors were fabricated with 5 μm and 10 μm gaps between moving and fixed combs to assess how it affects sensor response. Measurements carried out using a laser vibrometer and electronic readout using comb finger capacitors found that the resonant frequency is at around 850 Hz. However, the peak widths and amplitudes were affected by the gap of the comb fingers indicating viscus damping when they interact with air. Finite element simulations, performed using COMSOL Multiphysics, found that damping is generated by both drag at the cantilever surface as well as movement of air between the comb fingers. In this presentation, measured and simulated responses of the two sensors will be addressed.   

Integration of Nanoscale Light Emitters and Hyperbolic Metamaterials: An Efficient Platform for the Enhancement of Random Laser Action

Hyperbolic metamaterials (HMM) have emerged as novel materials with exciting functionalities, especially for optoelectronic devices. Here, we provide the first attempt to integrate hyperbolic metamaterials with light emitting nanostructures, which enables to strongly enhance random laser action with reduced lasing threshold. Interestingly, compared with the reference sample based on Si substrate, the HMM sample achieves 6.2 times lasing intensity, lowers 20.7% of lasing threshold, shortens 44% lifetime, and enhances the differential quantum efficiency by 4.5 times. The underlying mechanism can be interpreted well based on the fact that the high-k modes excited by hyperbolic metamaterials can greatly increase the possibility of forming close loops decreasing the energy consumption for the propagation of scattered photons in the matrix. Moreover, rough interface between ZnO nanoparticles and the HMM can assist the propagation of the out-coupling power of the high-k modes to the far-field easily rather than let the emitted light get trapped inside the multilayer structure.   

Room-temperature Telecommunication Wavelength Lasing from GaN Multiple-quantum Wells

Near-infrared (NIR) semiconductor nanolasers have been attracting much attention for large-scale optoelectronic integration. Notwithstanding significant progress in III-V nanolaser in the NIR region at room-temperature, there are still relentless efforts to overcome low quantum efficiency and Auger process. Er-doped GaN multiple-quantum wells enable emitting strongly at technologically important wavelength of 1.5 micron which falls in minimum loss window of optical fibers and in the eye-safe wavelength region. Here, we report the realization of room-temperature stimulated emission at 1.5 micron wavelength regime from Er-doped GaN multiple-quantum wells on silicon and sapphire. Employing the well-acknowledged variable stripe technique, we have demonstrated an optical gain up to 170 cm-1 in these nanostructures. The emission intensity from our Er-doped GaN multiple-quantum wells increases significantly in comparison to that of a single layer. Achieving hybrid GaN-Si lasers opens up a new pathway towards full on-chip integration.   

Observations of novel acoustic waves in quartz.

A new design for a surface acousti wave resonator on quartz has been fabricated and tested. In place of the interdigital transducer electrode that is normally used to generate surface waves at a frequency determined by the pitch of the electrode fingers, a metallized piezo surface is fabricated with an oval opening in the metallization. A drive and a receiving electrode are placed within this opening. Surface waves launched from the drive electrode propagate through the open region and are quenched at the boundary of the outer metallized field. This geometry establishes conditions that support drumhead resonant modes within the device which are observed in the transmission and reflection properties of the device. Due to the geometry, these acoustic waves are inherently radial in nature and to the best of our knowledge have not been formally analyzed in the past.   

The Electronic Interface State characterization of Hyperbolic Metamaterials

The metal/insulator junction is the place where the breakage of translational symmetry in the crystal structure occurs. It contains a large density of partially bonded atoms that can be broken easily by applying a strong electrical stress at the interface. The result is a high concentration of defects at the interfacial layer in the insulator, the so-called interface states which can trap electrons at the interface for a short time, locally lower down the tunneling barrier there, and participate in the conduction and capacitance process in metal-insulator-metal (MIM) structures. A trapped electron shows photoresponse to a light source with a photon energy in the photoresponsive energy band of the structure which commonly is used to characterize the density of interface states and their energy distribution. In this work, we employ interface states to characterize photonic properties of subwavelength structures such as hyperbolic metamaterials made based on MIM junctions. Optical power dependence of interface states' photoresponse is used to characterize the slow-light and field confinement effects of nanoscale metal-dielectric multilayers in their epsilon-near-zero (ENZ) spectral region.   

Importance of oxide layer in plasmonic structures for optical enhancement applications

This work studies the effect of varying the SiO₂ thickness on a Si substrate containing plasmonic nanostructures. The goal of this oxide layer optimization is to improve surface enhancement for optical plasmonic applications. Optimization of plasmonic structure geometry and materials for specific combinations of incident light wavelength and polarization is crucial to the fabrication of ideal devices for light enhancement applications. It is also critical to investigate the effects of the oxide thickness on these devices, as many plasmonic structures are fabricated on Si substrates containing an oxide layer. Often, the thickness of this layer is not optimized, but doing so has been found to provide a potential eight-fold increase in enhancement. Here, we apply previous computational electromagnetic design results to the fabrication of nanodevices for improved signal strength, utilizing surface-enhanced Raman spectroscopy (SERS) and cathodoluminescence. SERS results are obtained using a custom-built Raman system, and plasmon resonance excitability is studied via cathodoluminescence to validate computational models that optimize the substrate for plasmonic enhancement.   

Piezo-optic coefficient of gallium nitride

Gallium nitride is a material of interest for a range of optical and optoelectronic applications, including its use in blue light LEDs, efficient solar cells, and high power optoelectronic devices for communications, radar, and power amplifiers. During high power operation, these devices develop temperature gradients which cause changes in the local refractive index due in part to the piezo optic effect, resulting in distortion of the output beam. Therefore, measurement of the piezo-optic coefficients is required to predict the performance of high power optoelectronic devices.
Uniaxial mechanical stress was applied to a sample of gallium nitride, which induced a birefringence in the sample. The magnitude of the induced birefringence was determined by shining polarized light from a HeNe laser through the sample at pressures between .5 and 10 MPa, passing the output beam through a rotating polarizer and measuring the intensity as a function of relative polarizer position. From the induced birefringence, the piezo-optic coefficient of gallium nitride was determined.
  

Temperature dependent refractive index measurements for gallium nitride with implications for phase matched and quasi-phase matched optical frequency conversion devices

Gallium nitride is a valuable material for optical and electronic applications due to its wide band gap and high thermal conductivity. Gallium nitride has the potential for being a valuable material in frequency conversion devices such as frequency doublers and optical parametric oscillators. Optical and mechanical characterization of gallium nitride, including the dependence of the refractive index on wavelength and temperature, is important for predicting the performance of devices using this material. The method of minimum deviation was used to measure the refractive index for wavelengths ranging from 0.400 to 5.20 microns and in temperatures ranging from 20 to 225°C. Results of this characterization will be presented along with calculations relating to phase matching and quasi-phase matching in optical parametric oscillators.   

Characterization of Indium Tin Oxide (ITO) PET Films for the Development of an Image Quality Tool (IQT)

To ensure the safety of travelers worldwide, airports use active millimeter wave Advanced Imaging Technology (AIT) systems to detect potential threats such as explosives. There is a need for an Image Quality Standard (IQT) to quantitatively guarantee the functionality of the AIT systems. Through the establishment of an IQT standard, it can be assured that an AIT is functioning the same when manufactured, pre- and post-certification, and at any point throughout its lifetime.

Our research proposes Indium Tin Oxide (ITO) coated polyethylene terephthalate (PET) films as an IQT standard for the range of 60-90 GHz, which overlaps with the frequency range of AIT systems currently in use. Five ITO-PET films of various resistivities are evaluated using free space reflection measurements. These films show stability over the 60-90 GHz range and an inverse relationship between surface resistivity and reflectivity is established. This stability confirms the potential for ITO-PET films to act as a standard in evaluating the functionality of IQT systems.   

Epsilon-Near-Zero Modes Enabling Field-Effect-Tunable Perfect Absorption

Light absorption with efficiency above 99% is in demand for light harvesting, high-resolution, and optical coating technologies. Near-IR perfect absorption is achievable in deep subwavelength (λ/100) transparent conducting oxides (TCO) layers through the excitation of plasmon-polariton modes at epsilon-near-zero (ENZ) frequencies. We show the perfect absorption in ultra-thin (< 30 nm) TCO nanolayers due to the excitation of the bound and radiative ENZ modes. TCO nanolayers are deposited by atomic layer deposition and RF sputtering at elevated temperatures to control their ENZ frequency. We also show that single layer ultra-thin films could be layered to create a broadband perfect absorber. We discuss post-fabrication tuning of ENZ perfect absorption by biasing a TCO metal-oxide-semiconductor device. The absorption tuning is enabled by the ENZ modes in TCO nanolayers with the thickness comparable to the electron accumulation Debye length (~1nm). The post-fabrication tuning of about 20% of the full width at half maximum of the absorption peak is predicted for the devices under study.