Research Projects

Porous Silicon Biosensors | Silicon Photonic Biosensors | Photonic Structures for Optical Communication | Nanoscale Patterning | Porous Silicon Nanoparticles for Drug Delivery | Radiation Effects | Optomechanics

Past Projects

Porous Silicon Biosensors

Porous Silicon Bloch Surface and Sub-surface Waves Biosensors

PSi refractometric sensing applications have generally been size limited to molecules that diffuse into the porous matrix to cause a measurable change in effective optical thickness. Molecules approaching the average pore diameter clog the pore and hinder molecular infiltration, which significantly deteriorates the transduced signal. Hence there is a significant challenge in detecting biological entities such as viruses, bacteria, and blood cells that typically have sizes much larger than those of the pores. The PSi BSW/BSSW biosensor offers the possibility to detect both small molecules that infiltrate the pores and large molecules attached to the sensor’s surface. The BSW mode is a surface state excited within the truncated defect layer at the surface of a multilayer Bragg mirror. The novel BSSW mode is confined by a step or gradient refractive index within the multilayer and can selectively detect small molecules attached within the pores with an enhanced sensitivity (>2000 nm/RIU). The BSW and BSSW modes are each manifested as a distinct resonance peak in the reflectance spectrum, and the angular shift of each peak resulting from the light-matter interaction can be used to quantify the number of molecules attached to the sensor.

a) BSW/BSSW graphic with superimposed electric field profiles or a BSW (red), BSSW (blue), and band edge (purple) mode. b) Dispersion relation of a step index BSW/BSSW structure. The BSW appears within the band gap due to the surface defect state. The BSSW is introduced by the creation of a step refractive index profile. c) Schematic illustrating the location of large surface bound molecules and small molecules that can infiltrate the pores.

Porous Silicon Ring Resonators

The simple integration of a ring resonator into PSi allows for high sensitivity, high quality factor sensing applications that can be further assimilated with multiplexed microfluidics to create lab-on-a-chip technology. However, the majority of the electric field confined within the ring reduces the maximum detection sensitivity due to the small overlap between the optical mode and surface-bound molecules. In order to improve the capabilities of optical microring resonator biosensors, we fabricate microring resonators on a porous silicon (PSi) platform using the same fabrication techniques as used with traditional silicon-on-insulator (SOI) ring resonators. The simple integration allows for direct light-matter interaction of the analyte and optical mode leading to an enhanced sensitivity. The refractive index change caused by the molecules inside the pore are manifested as a shift of the spectral resonances. This molecular detection sensitivity is one order of magnitude greater than that reported for nucleic acid detection using SOI rings.

a) Top view of a ring resonator etched into a porous silicon waveguide. b) Spectral red shifts of the PSi ring resonances caused by the specific binding of a probe DNA to a small linker molecule, SPDP, and the hybridization of the probe DNA to its target PNA sequence.

Flow-Through Porous Silicon Membranes

Microfluidic systems are often integrated with sensors to facilitate real-time molecular detection. In a typical microfluidics system, the sample solution flows over the active binding surface of a sensor in a micro-channel. For sensors based on nanoporous materials, the flux into an individual pore can be as slow as a few molecules per pore per second due to the high aspect ratio of the nanopores. As a result, most molecules are swept downstream in the channel without reacting with the porous sensing surface. In this work, we present for the first time a label-free, flow-through sensing platform based on open-ended PSi microcavity membranes that is compatible with integration in on-chip sensor arrays. This flow-through platform, allowing analyte solutions to pass through the pores, greatly improves analyte transport efficiency, especially for larger molecules, and reduces sensor response time.

Schematic representation of flow-over (left) and flow-through (right) porous silicon membrane structures.

Quantum Dots as Signal Amplifiers for Dual-Mode Optical Biosensing in Porous Silicon Thin-Films

This work utilizes the high QY of colloidal QDs along with their high refractive index to engineer a novel, dual-mode optical detection scheme for small molecules in a tunable nanostructured porous matrix that offers the advantages of inherent size selectivity and extremely high active surface areas. We show, for the first time, quantum dot (QD) labeled sensing in a porous silicon host material with a sensitivity that is among the best reported in the literature under standard laboratory conditions. The work meticulously addresses the major challenges that come with using a three-dimensional nanostructured porous matrix for the selective capture of small molecules such as total surface area characterization and influence of pore size on target molecule infiltration. Beyond the material characterization and functionalization, the specific, dual-mode detection of biotin-QD conjugates in the porous silicon host matrix by reflective interferometric spectroscopy and fluorescence measurements is demonstrated. This approach combines the benefits of fluorescence based detection schemes via QD bioconjugation, including signal stability and high signal to noise ratio, with optical reflectance signal amplification achieved via the QD-enhanced refractive index change in the high surface area-to-volume porous silicon host matrix. Over a three order of magnitude improvement in the detection limit of biotin molecules is reported (from 2 pg/mm2 to 0.5 fg/mm2) when the QD labels are employed. These results demonstrate the opportunities offered by the utilization of QD-labeled sensing in a nanostructured three-dimensional matrix to efficiently capture target molecules.

a) Schematic representation of surface functionalized PSi film before and after attachment of QDs. Light reflecting off the top and bottom interfaces of the fi lm interfere and produce characteristic Fabry-Perot fringes. b) Reflectance fringe shifts and c) increase in EOT for an oxidized ~ 5.5 um thick PSi layer with 20 nm average pore diameter after APTES surface functionalization and immobilizing 2.8 nm PbS QDs. The increase in spectral shift and FFT amplitude demonstrate that QDs are being covalently bound to the functionalized pore walls.

a) Fluorescence spectra of QD-biotin conjugates in solution and bound inside a streptavidin-functionalized PSi film. The fluorescence of the conjugates inside the PSi film is modulated due to thin film interference effects and demonstrates that the conjugates are inside as opposed to only on top of the PSi film. Reflectance spectra for the detection of biotin b) with and c) without the use of QD-conjugates from two different PSi samples functionalized with streptavidin probes. Use of the QDs conjugated to biotin molecules significantly amplifies the sensor response.

Silicon Photonic Biosensors

Photonic Crystal Based Sensors

Photonic crystals, periodically patterned dielectric materials, have found many uses in optical waveguiding, switching, and sensing applications, among others. Useful properties of photonic crystals include the ability to engineer photonic bandgaps at specified frequencies/wavelengths, as well as the high field confinement in defect structures. In this research project, these properties are combined with effective index materials created by sub-wavelength holes fabricated inside PhC.

In order to achieve rapid, ultralow detection limits with microliter sample volumes, it is imperative to achieve a high surface density coverage of probe molecules that maximizes target capture with minimum incubation times. Without a sufficiently high probe molecule density, signal amplification, for example, through the use of a sandwich assay, must be implemented. We report on in situ, base-by-base synthesis of single-stranded DNA (ssDNA) probe molecules directly onto silicon photonic sensor surfaces, resulting in over 5-fold increased ssDNA probe surface coverage and a more than 5-fold increased detection sensitivity compared to sensors functionalized with traditional ssDNA probe attachment methods.

(a) SEM image of MHD photonic crystal device with a lattice hole radius of 100 nm and a lattice constant a of 410 nm. (b) Zoom-in SEM image of MHD cavity showing the defect holes, ∼ 50 nm in diameter, and neighboring right and left lattice holes that are shifted 0.15a outward to achieve lower mode profile perturbation at the cavity edge. (c) Simulated electric field distribution (TE mode) for the MHD cavity showing strong field confinement in the defect hole region due to a slot waveguide-like effect. (d) Average resonance wavelength shifts for probe and target binding on three MHD photonic crystals functionalized by the in situ ssDNA probe synthesis method and three MHD photonic crystals functionalized by the traditional ssDNA probe conjugation technique. (e) Kinetic binding curves for ssPNA target sequences using microfluidic channels as an analyte delivery system to 5 µm radius silicon ring resonators functionalized with either in situ synthesized or directly conjugated ssDNA probe molecules. The solid lines connecting data points taken before the rinse step indicate exponential fits of the kinetic binding rates of ssPNA for the two different rings. The faster response time of the ring functionalized with in situ synthesized probes is proportional to the increased probe surface coverage on that ring.

Suspended Ring Bio-Sensors

In order to further improve the sensitivity of optical sensors, it is necessary to increase the field overlap with captured target analytes. In this work, we characterize the sensing performance of suspended TM-mode silicon micro-ring resonators, 5 µm in radius, and demonstrate an enhanced sensitivity to molecular binding on the ring after suspension. In the TM-mode, the overall field intensity exists primarily outside of the waveguide core, with high electric field intensities present near the top and bottom surfaces. In traditional micro-ring resonators, only the top surface of the ring is available for surface analyte attachment, while the electric field intensity near the bottom surface dissipates by leaking into the underlying silicon dioxide substrate. In our approach, we suspend the TM-micro ring resonators in order to increase the surface area for binding events and increase the light-matter interaction with analytes. The suspended rings demonstrate excellent mechanical stability to multiple rinsing, soaking and nitrogen drying steps during the sensing procedure. We show that the resonance shift achieved by the suspended micro-rings after attachment of small chemical molecules and DNA is at least twice that of micro-rings supported by the silicon dioxide substrate.

(a-e) Scanning electron microscope images of a fabricated device. (d) Top view of a supporting truss. The width of truss is approximately 100 nm. (e) Tilted image of the supporting truss with a height of approximately 260 nm. The aspect ratio of 2.6 allows good mechanical support for the suspended ring structure. (f) Optical spectrum measurement after suspension process. The Q factor of 5 µm radius TM ring resonator is 15,000. (g) Bulk refractive index sensitivity of suspended ring resonator compared with traditional ring resonator on SiO2. The colored lines are linear fits to the data.

Photonic Structures for Optical Communication

Silicon Modulators Using Metal-Insulator Switching of Vanadium Dioxide

We present silicon (Si) vanadium-dioxide (VO2) hybrid optical modulators motivated by the need for compact Si-compatible optical switches operating at THz speeds. VO2 is a functional oxide undergoing metal-insulator transition (MIT) near 67C, with huge changes in electrical resistivity and near-infrared transmission. The MIT can be induced thermally, optically (by ultra-fast laser excitation in less than 100 fs), and possibly with electric field. VO2 is easily deposited on Si and its ultrafast switching properties in the near-infrared can be used to tune the effective index of ring resonators in the telecommunication frequencies instead of depending on the weak electro-optic properties of silicon. In an all optical geometry, we have demonstrated photothermal modulation in a ring resonator geometry and modulation in both linear and ring resonator geometries in response to nanosecond optical pulses. Also in a linear geometry, we have demonstrated the potential for electrically triggering the MIT in an electro-optic absorption based modulator. Currently, we are pursuing methods to continue increasing device performance speed while simultaneously achieving high modulation depths and maintaining small device footprints.

(a) Optical transmission of the 1.5um radius hybrid Si-VO2 ring resonator as a function of wavelength, before and after photothermal triggering the MIT with a 532nm pump laser. The lines are Lorentzian fits. Inset: IR camera images revealing vertical radiation at a fixed probe wavelength. (b) Time dependent optical transmission (measured at resonant frequency when VO2 is in its insulating state) of ring resonator in response to varying pulse fluences.

(a) Schematic and (b) SEM of the linear absorption electro-optic modulator. (c) Measured optical transmission for varying electrical pulse durations.

Nanoscale Patterning

Direct Imprinting of Porous Substrates (DIPS)

Porous materials offer unique properties that have lead to great interest in utilizing them as materials for many applications ranging from drug delivery and imaging, chemical and biomolecule sensing, to catalysis, light emission, biomaterials, optoelectronics, photovoltaics and many others. However, patterning these materials, which is necessary to fabricate many device structures and particles, 3-D morphologies, arrays, gratings and a variety of important optical structures, requires the use of lithographic techniques that add both time and cost to device fabrication. Example methods used to pattern porous materials such as porous silicon include photolithographic patterning and reactive ion etching, ion implantation, UV laser ablation, localized photo-oxidation, and dry removal soft lithography. Our technique, coined "Direct imprinting of porous substrates," or DIPS, is a direct-to-device technique for mechanically deforming porous nanomaterials to produce well-defined micro- and nano structures. While repeatably producing nanoscale features (<100nm) the overall process is remarkably simple, rapid, and low-cost.

Top view SEM image of porous silicon patterned by DIPS.

Free standing porous silicon microparticles fabricated by performing DIPS with a silicon grating stamp, rotating 90 degrees and stamping again. In this image, the particles are sitting on the original stamp used to define them.

Porous Silicon Nanoparticles for Drug Delivery

Porous Silicon Nanoparticle Therapeutics (Collaboration with Duvall Group)

Porous silicon nanoparticles (PSNPs) are excellent vehicles to improve delivery of drugs owing to their high porosity, large internal surface area and biocompatibility. In particular, our group is interested in using PSNPs to enhance the delivery and therapeutic efficacy of novel nucleic-acid drugs known as peptide nucleic acids (PNA). PNA have demonstrated enormous potential in treating a wide array of diseases (including viral infections, central nervous system disorders, cardiovascular disease, and cancer), but PNA therapeutic activity is limited by poor bioavailability and low-levels of cellular uptake. Our hypothesis is that the targeted delivery and therapeutic activity of PNA can be increased by packaging PNA into PSNP delivery vehicles.

Our initial studies show that PNA therapeutics can be synthesized in situ within a PSi substrate to yield therapeutic-loaded PSNPs. These arbitrarily shaped PSNPs significantly enhanced PNA intracellular uptake, were shown to be non-toxic, and ultimately improve PNA therapeutic efficacy. In addition, we have shown that Direct Imprinting of Porous Substrates (DIPS) can be used to size and shape-engineer PSNPs. Current studies are focused on examining the effect of PSNP geometry on the enhance delivery and cellular uptake of therapeutic PNA.

(Left) Cartoon describing in situ synthesis of PNA in PSi, a process used to fabricate PNA-loaded PSNPs, or “PNA-PSNPs.” (Right) PSNPs enhance the intracellular delivery of PNA therapeutics, which otherwise do not enter the cell.

(Left) PSNPs are more biocompatible than the positive control(green), and (Right) significantly enhance PNA therapeutic activity, as measured by % increase in luciferase activity in Huh7 human hepatocarcinoma cells. (*Note “AMO” references the positive control therapeutic oligonucleotide that was delivered with Fugene 6 transfection reagent; luciferase activity measured 48hrs after treatment)

(above) Examples of possible PSNP geometries produced by DIPS.

Radiation Effects

Impact of High Energy Radiation on Silicon and Silicon Based Devices

Influence of high energy radiation on electronics has been studied extensively over the past few years, as electronics are being used in radiation environments, such as in medical equipment, nuclear reactors or aboard satellites in space. In this initiative, we examine the influence of acute high dose radiation, such as 10-keV x-rays and 662-keV gamma rays, on the silicon surface and, silicon-based optical and electro-optical devices. We found that high energy radiation exposure enhances the oxidation rate of unpassivated silicon surfaces saturating at the typical native oxide thickness of ~ 2 nm. We applied this phenomenon to a silicon based optical device: a ring resonator. Additionally, a novel amorphous silicon solar cell structure will be fabricated by the incorporation of porous amorphous silicon and it’s radiation response will be determined.

Enhanced oxidation rate observed on silicon surface due to 10-keV x-ray irradiation. The sample was exposed to dose rates ranging from 5.8 krad(SiO2)/min to 67 krad(SiO2)/min for 180 min.

High Energy Radiation Effects on Monolayer QDs Integrated with Nanostructured Silicon Thin-Films

The effects of x-ray and gamma irradiation on the optical properties of colloidal CdTe/CdS quantum dots (QDs) immobilized in nanostructured silicon (NSSi) frameworks are currently under investigation with the goal of developing QD-based radiation scintillators and analyzing QD exciton dynamics as influenced by proximity to complex, disordered states of the substrate. Preliminary investigations of QD exciton lifetimes and photoluminescence intensities show a decrease with increasing exposure dose of x-rays and gamma-rays.

(Left) Illustration of CdTe/CdS QD attachment to PSi pore walls and a camera image of the PSi-QD sample under UV lamp (365 nm) excitation. (Right) CWPL spectra for PDDA functionalized PSi before (solid line) and after (dotted line) immobilization of CdTe QDs.

CWPL spectra of PSi-QD samples. a) QD fluorescence of the irradiated sample before X-ray irradiation (pre), followed by total exposure doses of 500 krad(SiO2), 4 Mrad(SiO2), 8 Mrad(SiO2) and 16 Mrad(SiO2). The QD peak fluorescence intensity decreases with X-ray irradiation total dose. b) Control experiment carried out for same time durations as the irradiations in (a) but the PSi-QD control sample was maintained in ambient environment without X-ray irradiation.

TRPL measured at an emission energy of 1.9 eV, showing QD fluorescence lifetime decay. a) Measurements of a PSi-QD sample pre- and post-irradiation at 5.4 Mrad(SiO2) demonstrate a decrease in lifetime from 37 ns to 11 ns ± 4 ns. b) The control sample exhibits a slight decrease in lifetime from 32 ns to 28 ns ± 4 ns after 147 min, a time duration equivalent to the irradiation experiment, compared to an initial measurement.

The effect of X-rays on irradiated materials is primarily classified as a point interaction with the creation of several secondary electron-hole pairs. The number of such secondary particles generated can be estimated from the average energy necessary to create an electron-hole pair, which is approximately 4.4 eV for cubic CdTe QDs. The ejection of an electron from the QD core will result in ionization and non-radiative Auger processes that occur on much faster time scales than radiative processes, will initiate a dark state in the QD. As a result, even in the event of absorption of a photon, an excited electron will not decay radiatively to the ground state and instead will transfer the acquired energy to a coupled hole through Coulombic interactions which may then rapidly relax through valence band states.


The research of optomechanics in our lab is to advance the understanding of light-matter interaction and gradient optical forces through the design, fabrication, and characterization of engineered photonic structures. Discovering strategic approaches for enhancing light-matter interaction and creating and controlling gradient optical forces is likely to open new avenues for enhancing the capture and detection of biomolecules, amplifying optical forces and optical sensitivity to mechanical motion, and realizing ultra-high sensitivity optical cavities that could form the basis for compact and energy efficient modulators and lasers.

(a) Dielectric constant plot of MHD simulation space, where black indicates ε = 12, white indicates ε = 1. Detailed MHD regions with effect radius (b) 0.2a, (c) 0.3a, and (d) 0.4a are also shown. The defect hole radius in all cases is 0.04a, with defect hole spacing 0.12a.

Past Projects:

White Light LEDs

With the potential for much longer lifetimes and lower energy consumption as compared to current lighting technologies, the use of white light-emitting diodes (LEDs) for general lighting, an area of research known as solid state lighting, is likely to be a disruptive technology in the lighting industry in the coming years. They have the potential to be more than twice as efficient as fluorescent lighting and ten times as efficient as incandescent lighting. Also, environmental benefits such as the reduction of carbon dioxide emissions and a reduced dependence on fossil fuels for lighting can be achieved.

Thus, there has recently been a great interest in the development of white-light phosphors for solid state lighting devices, specifically for use in light emitting diodes (LEDs). We have recently demonstrated cadmium selenide (CdSe) nanocrystals with a broad, balanced emission spectrum over the visible region due to their “magic size” diameter of 1.5 nm. When encapsulated in a perfluorocyclobutyl (BP-PFCB) polymer and coated on commercial UV LEDs, these magic-sized CdSe nanocrystals act as frequency downconverters to produce light from 420-710 nm, making them an excellent material to use as a white-light phosphor for LEDs.

Thus far, we have found that the choice of encapsulant plays a significant role in the optical properties of the nanocrystal films. Some encapsulants caused a quenching of the nanocrystal emission, a modification of the white-light spectrum, and/or did not provide protection for the nanocrystals where the films became discolored and the emission intensity decreased greatly in just a few days at temperatures found in an LED junction. The BP-PFCB polymer we have focused on provides a robust and stable environment for the nanocrystals while maintaining the unique optical properties of these ultra-small CdSe nanocrystals. With the color quality of these devices in the range of those desired for general lighting, we are now focusing on increasing the efficiency of our devices by optimization of the nanocrystal concentration, UV LED wavelength, and film thickness, investigation of their scattering properties, as well as the use of dielectric thin film mirrors to increase the extraction efficiency.

Absorption and emission of white-light CdSe nanocrystals, where the dashed lines are the absorbance curves and the blue lines are the emission curves. The red spectra are the nanocrystals in a toluene solution and the blue spectra are the BP-PFCB polymer encapsulating the nanocrystals in a film. The inset shows a color photograph of a UV LED coated with encapsulated nanocrystals.

The emission from white-light CdSe nanocrystals in different encapsulants, with the highest intensity seen for the BP-PFCB polymer.

Related Publications: M. A. Schreuder, J. D. Gosnell, N. J. Smith, S. M. Weiss, and S. J. Rosenthal, “Encapsulated white-light CdSe nanocrystals as nanophosphors for solid-state lighting,” J. Mater. Chem. 18, 970-975 (2008) (link)