Center for Smart Structures & Materials

Two-photon lithography is useful for creating sub-micron scale 3D polymer structures. At CSSM, we have a commercial system from Nanoscribe that is used to create complex 3D structures in a wide range of projects in collaboration with several groups at NU and beyond. Applications include micro-optofluidic devices, metamaterials, and polymer micro-optic sensors.

PI: Sridhar Krishnaswamy

Many 3D microstructures require fabrication of a combination of micron-scale structures along with higher resolution sub-micron scale structues. The support structures of a micro-optical sensor, for instance, are typically several microns in dimension and do not require the high resolution that the optical waveguide components of the device require. While two-photon lithography provides high spatial resolution, creating coarser structures using such a high-resolution system is extremely time-consuming. We are currently developing a combined multi-photon direct laser writing system which enables the simultaneous writing of large area structures via one-photon writing along with selective high-resolution two-photon writing. The system is integrated into a single custom-built microscope and is of use in fabricating 3D metamaterials, 3D micro-optical sensors, and sensing platforms for biomedicine.

PI: Sridhar Krishnaswamy

We have developed photothermally assisted self-organization approaches to creating surface nanopatterns. The approach involves using photothermal assistance to cooperatively engage with the inherent tendency of a programmed surface to self-organize into nano/micro ridges or islands. Applications range from superhydrophobic surfaces to efficient photo-trapping for solar cells.

PI: Sridhar Krishnaswamy

Pulsed lasers have been exploited in different contexts to generate controlled mechanical deformation in materials at extreme pressures or stresses in the tens to thousands of MPa range and strain rates of up to 108 1/s. We are interested in understanding the role of inertia, elastic wave propagation, and rate dependent effects on the spontaneous delamination and blister deformation of thin films. Through these studies, we hope to exploit dynamic blister deformation of thin films as actuators in applications ranging from ballistic testing of protective materials using accelerated micro-particle projectiles, needle-less drug delivery using coated nano-particles, and manufacturing of complex surface patterns in flexible electronic structures.

PI: Oluwaseyi Balogun

In this work, we are investigating several azobenzene-functionalized polymers to identify suitable resists for direct laser writing of 3D microstructures using two-photon polymerization. These resists must retain the azobenzene trans-cis transformation for subsequent single-photon optically triggered reconfigurability. Applications to reconfigurable 3D micro-optofluidic systems are being developed.

PI: Sridhar Krishnaswamy

In this work, we are investigating several optically-active nanoparticles for integration into photoresists that enable fabrication of active micro-ring resonators and polymer optical amplifiers. Applications to sensing systems are being developed.

PI: Sridhar Krishnaswamy

Metamaterials for photonic and phononic control are being investigated both theoretically and experimentally.

PI: Oluwaseyi Balogun

We have developed shape-memory polymers for use in the fabrication of complex 3D structures. We have also developed systems for assisted surface nano-patterning. Thermomechanical response of shape memory polymers has been modeled using a physics-based two-phase model that has been integrated into large-deformation finite-element codes.

PI: Sridhar Krishnaswamy

In this project, we are developing Mesoscale Integrated Photonic Systems (MIPS) aimed at making photonic sensor systems robust enough to be integrated into real structures. The sensors are fabricated using two-photon lithography. We are developing polymer waveguide-based rigid and flexible MIPS and demonstrating their utility as strain and chemical sensors.

PI: Sridhar Krishnaswamy

We are developing chemical sensors based on nanocoated photonic crystal fibers that exhibit enhanced sensitivity and selectivity.

PI: Sridhar Krishnaswamy

We are developing fiber-optic dynamic strain sensors that are adaptive to quasistatic drift. These systems integrate two-wave mixing spectral demodulators with adaptive-sources to provide multiplexed sensing for acoustic emission and impact detection.

PI: Sridhar Krishnaswamy

We have developed graphene-oxide thin film chemical sensors for humidity detection. We have also developed graphene-based strain sensors with gage factors that are 3 orders of magnitude higher than conventional foil strain gauges.

PI: Sridhar Krishnaswamy

Recent advances in plasmonics have enabled unprecedented control of light matter interaction, making it possible to concentrate optical energy at the nanoscale. In this project, we explore plasmonic nanofocusing approaches to confine light at the apex of a scanning probe microscope tip . We use the confined light to create a near-field ultrafast optical probe for time-resolved detection of elastic waves with picosecond temporal resolution and nanoscale local spatial resolution. We also explore the nanofocusing approach to create near-field optical probes arrays using two dimensional arrays of metallic dimmers to enable parallel multi-point detection. This project advances the state-of-the art in optical imaging and laser based ultrasonic instrumentation and provides a tool for fundamental studies of nanoscale phonon transport, phonon and photon interaction, elastic wave propagation in nanostructures, and the dynamics of ultrafast nanomechanical resonators. This project has applications in areas such as nanomechanical signal processing, nondestructive materials characterization, and biophotonic sensing.

PI: Oluwaseyi Balogun

Additive manufacturing (AM) technologies have advanced rapidly over the last three decades to the point where they have the potential to fundamentally change the way that complex parts will be designed and fabricated in the future. In-situ sensor technologies that can be integrated with AM techniques are needed for monitoring process conditions such as local temperature distribution, melt pool size, product structural integrity (porosity, microstructure, and defect state), part shape precision, surface conditions, and depth dependent material properties. The objective of this project is to explore laser generated elastic waves in the ultrasonic frequency range for in-situ process quality control of metal additive manufacturing process. The project will provide the measurement science needed to enable a fast, reliable, and cost-effective ultrasonic methodology for real-time assessment of additive manufacturing product quality, process condition monitoring, and control of defects during component fabrication.

PI: Oluwaseyi Balogun

The small footprint and non-contact nature of laser ultrasonic methods make them especially useful for characterizing thin films. Several optical techniques have been devised and implemented. In our labs, we use pump-probe techniques in which very high frequency (GHz) acoustic waves are generated that propagate perpendicular to the film and reflect off the film/substrate interface. This bulk wave technique requires an ultrafast laser source, and material attenuation of high frequency ultrasound limits the useful measurement range to ultra-thin films. For thicker films, guided-wave ultrasonic techniques are more practical. Broadband guided wave methods can be used where surface-acoustic waves are generated with a simple pulsed laser point or line source which are then detected with an interferometer after some propagation distance along the film. Applications include thermal-barrier coatings and wear protection coatings.

PI: Sridhar Krishnaswamy

Photoacoustic methods can be used for ultrasonic defect imaging in structures such as turbine blades, GLARE composites, etc. The traditional mode of application of laser ultrasonics for defect imaging parallels conventional pitch-catch ultrasonic inspection mode, except that lasers are used to generate and detect the ultrasound. For detecting very small defects, the pitch-catch technique requires that the scatterrer reflect a significant fraction of the incident wave, and furthermore that the generating and receiving locations be in line and normal to the scatterrer. In our labs, we have developed a novel Scanning Laser Source (SLS) technique for detecting very small near surface scatterrers that are arbitrarily oriented with respect to the generating and detecting directions. The SLS technique has no counterpart in conventional ultrasonic inspection methodologies as it relies on near-field scattering and variations in thermoelastic generation of ultrasound in the presence and absence of defects. In the SLS technique, the ultrasound generation source, which is a point or a line-focused high-power laser beam, is swept across the test specimen surface and passes over surface-breaking flaws (see figure). The generated ultrasonic wave packet is detected using an optical interferometer or a conventional contact piezoelectric transducer either at a fixed location on the specimen or at a fixed distance between the source and receiver. The ultrasonic signal that arrives at the Rayleigh wave speed is monitored as the SLS is scanned. It can be seen that the amplitude and frequency of the measured ultrasonic signal have specific variations when the laser source approaches, passes over, and moves behind the defect.

PI: Sridhar Krishnaswamy

Research efforts in this area include probabilistic degradation models (SK & JDA), analytical modeling of scattering of ultrasonic waves by defects (JDA), and physics-based modeling of environmental degradation (JDA).

PIs: Jan D. Achenbach & Sridhar Krishnaswamy