Research Projects

Current Projects: Waveguide Biosensors | Nanocrystal Light Emitters | Grating-coupled Sensors
Membrane Waveguides | Size-Dependent Infiltration | Direct Imprinting (DIPS)

Past Projects: White Light LEDs | Photonic Crystal Sensors | VO2 Modulators | Radiation Sensors

Porous Silicon Biosensor

Porous silicon (PSi) is an excellent material for biosensing due to its large surface area and its capability for molecular size selectivity. Our research is on a label-free nanoscale PSi resonant waveguide biosensor. The PSi waveguide consists of two porous silicon layers with pores having an average diameter of 20-30 nm. Molecular binding in the PSi is detected optically based on a shift of the waveguide resonance angle. The magnitude of the resonance shift is directly related to the quantity of biomolecules attached to the pore walls. We have demonstrated that the PSi waveguide sensor can selectively discriminate between complementary and non-complementary DNA. The advantages of the PSi waveguide biosensor include strong field confinement and a sharp resonance feature, which allow for high sensitivity measurements with a low detection limit. Simulations indicate that the sensor has a detection limit of 50nM DNA concentration or equivalently, 5 pg/mm2.

Fig. 1. (a) PSi waveguide consisting of a low porosity (high index) in the low porosity layer. A prism is used to couple light at a specific waveguide increase its effective refractive index and change the 310 nm low porosity layer and 1330 nm high porosity layer.

Fig. 2. (a) PSi waveguide resonance after each functionalization step: after oxidation, after silanization, and after glutaraldehyde + probe + ethanolamine(GA + probe + EA). (b) PSi waveguide resonance shift for complimentary DNA, demonstrating the recognition of DNA binding inside the PSi waveguide. (c) Negligible PSi waveguide resonance shift for non-complimentary DNA, demonstrating the PSi waveguide can distinguish complimentary and non-complimentary DNA. (d) No waveguide resonance shift upon exposure of PSi waveguide to buffer solution. The resolution of the prism coupler is 0.002◦, and random angular variations of the rotary table of the prism coupler (±0.004◦) are averaged out by taking multiple measurements.

Nanoporous materials hold great potential for improving the performance of biosensors due to their large available surface area and their capability for size-selective infiltration. One of the principal challenges associated with the use of nanopores is achieving efficient infiltration of molecules whose size is of the same order of magnitude as the pore radius. There is an intrinsic trade-off between ease of infiltration and level of sensor response for the detection of biomolecules in nanopores. DNA molecules of lengths between approximately 1.5 and 5.5 nm have been exposed to porous silicon waveguide sensors with nanopore diameters of approximately 30 nm in order to experimentally determine the size-dependent sensitivity. While theoretical calculations that do not take into account the infiltration challenges of biomolecules in nanopores suggest that longer DNA molecules yield a stronger sensor response, experimental results show that shorter molecules produce a larger response due to their ease of infiltration.

Fig. 3. Simulated resonance shift for different lengths of DNA at different pore wall coverages, where 8 x 1013 probes/cm2 corresponds to 100% coverage. The slope of the linear curve is the sensitivity of detection. The 24-base DNA molecules can be detected most sensitively because their infiltration into the porous silicon waveguide causes the largest refractive index change. The simulation ignores practical infiltration challenges and assumes that the DNA molecules can infiltrate into all pores with diameters larger than the DNA length.

Fig. 4. Experimental results showing the resonance shift of porous silicon waveguides at different concentrations (25-100 μM) and different lengths (8-24 bases) of DNA. The slopes of the linearly fitted curves are the sensitivities of detection. Each data point includes an error bar, and the detection sensitivity with associated error range for each DNA length is also shown. In contrast to Figure 3, we have higher sensitivity for shorter DNA molecules.

In addition, experiments involving other types of molecules (such as spherical nanocrystals) will provide insights about shape-dependent infiltration properties, and experiments in which molecules are infiltrated into larger pores (such as n-type porous silicon with average pore diameter of 60-100 nm) will suggest whether a general biomolecule-pore size ratio can be established for efficient infiltration into nanoscale voids. The research work has significant impact on design of biosensors utilizing various porous materials, such as porous silicon, porous SiO2, porous TiO2, porous resin, sol-gel, porous ZnO, etc.

Porous Silicon Nanocrystal Light Emitters

This initiative investigates how the advantages of solid state lighting can be incorporated into a silicon-compatible platform. Silicon-based devices have the advantage of easy integration with standard microelectronic devices, which extend the utility of the light sources to applications such as optical interconnects. Traditionally, silicon is not an efficient light-emitter due to its indirect band gap. Incorporating efficient, light emitting nanocrystals into a porous silicon matrix allows efficient light emission from a silicon-based device. This project investigates the ability of a porous silicon resonant cavity to modify and enhance the emission spectrum of lead sulfide and cadmium selenide nanocrystals. Additionally, this project studies methods for the passivation of the porous structure and characteristics of quantum dot infiltration.

Grating-coupled Porous Silicon

Porous silicon (PSi) waveguides, since first appearing in the early 1990\u2019s, have special structural and optoelectronic properties. These properties enable it to be used in many ways, such as optical interconnection, photonics and biosensing. There are several ways to couple light into a PSi waveguide, like end-fire coupling and prism coupling. Now we use the grating coupling method.

The PSi waveguide is fabricated by electrochemical etching of silicon wafers in an HF electrolyte. Then we use E-beam lithography to fabricate gratings on top the waveguides. Thanks to the presence of the grating, which provides the necessary momentum, the incident light can be coupled into the PSi waveguide\u2019s guided mode. At a certain incidence angle, the guided mode can also be coupled out of the waveguide and form a resonance peak in the reflection spectrum. The resonance peak can be shifted if we add some linkers to the surface of the PSi. Therefore, it can be used in the biosensing area.

Grating coupling PSi waveguide.

The Metricon Model 2010/M Prism Coupler (Metricon Corp., USA) is used in the non-contact, VAMFO mode to monitor the reflectance of the PSi waveguide with the grating coupler. A 1550 nm diode laser was used as the light source because silicon has low absorption losses in the near-infrared wavelength. A schematic of the measurement configuration for the grating coupled porous silicon waveguides is shown. Light from the laser is incident on the waveguide at variable angle by rotating the stage, and the reflected light intensity is measured by a photodetector.

Schematic of the measurement configuration for the grating coupled PSi waveguides

In order to attach probe DNA oligos to oxidized porous silicon waveguides, an organofunctional silane, 3-APTES, and a cross-linker, Sulfo-SMCC, were used to modify the silica surface. Then, thiol modified probe DNA oligos were attached. Experimental results show that it can distinguish specific DNA sequences with a selectivity of approximately 6:1. Reflectance measurements were taken after each functionalization step in order to confirm the attachment of the silane, cross-linker, probe DNA and antisense molecules.

Grating coupled PSi waveguide reflectance spectra after oxidation and attachment of 3-APTES, Sulfo-SMCC, probe DNA and antisense.

Porous Silicon Membrane Waveguides (Kretschmann configuration)

We fabricated a polymer-cladded porous silicon (PSi) waveguide sensor, which consists of an approximately 700 nm thick polymer as cladding layer and a 1.55 \u03bcm thick PSi membrane as the waveguide layer. Based on this single layer PSi waveguide structure, we have investigated the relationship between design parameters and small molecule detection sensitivity of PSi waveguides. Perturbation theory calculations suggest that the sensitivity improves as the porosity of the PSi waveguide approaches to a critical porosity or, equivalently, the resonance angle approaches 90°. Experimental verification of the trend was performed and a 120°/RIU detection sensitivity, corresponding to nanomolar detection limits, was demonstrated at a coupling angle of ~68°. Much higher detection sensitivity is predicted for higher angle resonances.

(a) Experimentally measured sensitivities of polymer-cladded PSi waveguides (solid symbols) as a function of the porosity of a 1.55 \u03bcm thick PSi waveguide layer at wavelength of 1550 nm. Curve fitting of the data points clearly demonstrates the trend of increasing sensitivity with decreasing porosity. The resonances corresponding to the circled data points are shown in (b), where the higher angle resonance gives the highest sensitivity of detecting small 3-APTES molecules. The inset shows a schematic of the polymer-cladded PSi waveguide sensor.

Size-dependent Infiltration of Nanoscale Molecules into Nanoscale Voids

The relationship between porous silicon pore size and biomolecule size must be quantitatively determined to enable reliable infiltration and molecular binding, and to ensure the highest sensitivity detection of target biomolecules. The pore openings must be sufficiently large to allow efficient infiltration of biomolecules. When the infiltration efficiency is compromised, fewer receptor molecules are attached inside the pores and therefore fewer target species can be captured and detected. However, if the pore size is too large, then the refractive index change resulting from the incorporation of small biomolecules in the porous host material will be too small to detect and scattering losses may dominate. Moreover, choosing an oversized pore compromises the size-exclusion and filtering benefits afforded by the porous matrix. We studied the efficiency of biomolecule attachment in nanoscale porous silicon by exposing PSi membrane waveguides with three distinct average pore diameters (15 nm, 30 nm, and 60 nm) to DNA molecules with different lengths (8-bases [~1.8 nm] to 24 bases [~5.3 nm]). The figure below shows our experimental measurements of the waveguide resonance shifts after DNA attachment. The largest pores accommodate all DNA sequences investigated but the shortest sequences are more sensitively detected in the smaller pores.

Resonance angle shifts of single-layer porous silicon membrane waveguide with different pore sizes for detection of 100-\u03bcM DNA molecules of different lengths (~2.2 Å /base). For optimal performance as a sensor, the pore size must be appropriately tuned based on the size of the molecule of interest.

Direct Imprinting of Porous Substrates

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.

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)

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 material created by sub-wavelength holes fabricated inside PhC defects to create biosensors with increased surface area, and therefore, sensitivity.

Simulations were carried out using L1 and L3 photonic crystal defect designs with various multi-hole defect (MHD) structures, using FDTD software. It was found that the high quality factor (Q) associated with PhC defects was not degraded significantly by the presence of MHD. The high surface area inside the MHD increases the resonance shift compared to single surface-based sensors. Future work will involve the fabrication of these devices using standard microfabrication techniques.

(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.

(a) Photonic bands for the photonic crystal with hole radii of 0.4a. A photonic bandgap exists for only for the TE polarization between 0.2462 and 0.4052. (b) Resonance frequency for varied defect hole dielectric constant and MHD effective radius.

Related Publications: C. Kang and S. M. Weiss, “Photonic crystal with multiple-hole defect for sensor applications,” Opt. Express 16, 18188 (2008). [Also appears in Virtual Journal for Biomedical Optics vol. 3, issue 12 (2008)] (link)

Silicon Modulators Using Metal-Insulator Switching of Vanadium Dioxide

We present an optical modulator based on a silicon ring resonator coated with vanadium-dioxide (VO2) motivated by the need for compact silicon-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 silicon 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. The VO2-silicon hybrid ring resonator is expected to operate at speeds up to 10 THz at low Q-factor and with shorter cavity lifetimes, thus enabling compact, faster, more robust devices. We have made ring resonator structures on SOI substrates with rings varying in diameter from 3-10 ?m coupled to 5 mm-long waveguides at separations of 200 nm. Rings were coated with 60 nm of VO2 by pulsed laser deposition. As proof-of-concept, by switching the VO2 top layer thermally, we were able to modulate the resonance frequency of the ring to match with the predictions from our FDTD simulations.

Our recent effort has been concentrated on inducing the MIT in VO2 electrically in both linear absorption and ring resonator electro-optic modulator geometries. Gold contacts were deposited on top of the VO2 structure and a voltage applied across them. VO2 undergoes MIT at the threshold voltage and sudden jump in electrical current is observed, as well a decrease in optical transmission through the waveguide. Increasing the current though the structure after MIT occurs, increases the modulation depth making implementing analog electro-optic modulators possible.

Optical transmission of the 1.5um radius hybrid Si-VO2 ring resonator as a function of wavelength, before and after 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.

Schematic and SEM of the linear absorption electro-optic modulator

Example of electro-optic modulation induced by voltage pulses

Impact of High Energy Radiation on Silicon and Silicon based materials

In this initiative, we examine the influence of acute high dose radiation on silicon-based materials like, crystalline and amorphous silicon, porous silicon and silicon nanofibers. The impact on their surface states, surface area, doping, crystallinity, oxide formation, etc. will be monitored. The results of this investigation could be potentially used to make an optical lightweight dosimeter.

Our initial study of x-ray irradiation on bare silicon showed almost three times higher oxidation rate, as opposed to the control sample placed under room light. Porous silicon has also shown an 80% higher oxidation rate under high energy radiation. The large internal surface area of porous silicon makes it an attractive material for highly responsive dosimetry applications.

Study of native oxide growth on an irradiated bare silicon sample and a control sample. 10keV X-rays were used at a dose rate of 31.5krad/min to radiate the sample, while the control sample was placed under room light. Radiation was done for 355 minutes.