Ganapathi S. Subramania

Polarization-Independent Silicon Metadevices for Efficient Optical Wavefront Control

We experimentally demonstrate a functional silicon metadevice at telecom wavelengths that can efficiently control the wavefront of optical beams by imprinting a spatially varying transmittance phase independent of the polarization of the incident beam. Near-unity transmittance efficiency and close to 0-2π phase coverage are enabled by utilizing the localized electric and magnetic Mie-type resonances of low-loss silicon nanoparticles tailored to behave as electromagnetically dual-symmetric scatterers. We apply this concept to realize a metadevice that converts a Gaussian beam into a vortex beam. The required spatial distribution of transmittance phases is achieved by a variation of the lattice spacing as a single geometric control parameter.

Fabrication of three-dimensional photonic crystal with alignment based on electron beam lithography

We demonstrate the fabrication of a three-dimensional woodpile photonic crystal in the near-infrared using a layer-by-layer approach involving electron beam lithography and spin on glass planarization. The alignment accuracy between the first and the fifth layer is within 10% of the lattice spacing as measured from cross section scanning-electron-microscopy images. Optical reflectivity measurements reveal peaks consistent with the photonic gap frequency. The method offers a way of rapid prototyping full three-dimensional photonic band gap devices with considerable flexibility of materials choice. Moreover, lattice structure that can operate at wavelengths into the visible can be fabricated using this approach.

Log-Pile TiO2 Photonic Crystal for Light Control at Near-UV and Visible Wavelengths

Three-dimensional photonic crystals (3DPCs) with an omnidirectional photonic bandgap (OPBG) are an important class of materials for the manipulation of photons. By creating a complete electromagnetic vacuum over a desired wavelength range, a 3DPC with an OPBG opens up new regimes of light–matter interaction, such as photon–atom bound states, spontaneous emission control, ultralow-loss waveguiding, and negative refraction, with significant impact in areas such as all-optical computing and circuitry, energy conversion, and subdiffraction optics. In particular, 3DPCs with OPBGs in the visible frequency regime can potentially enhance performance and efficiency of solid-state lighting, solar-energy conversion, and other applications by controlling energy transfer via modification of the photonic density of states (PDOS), as well as enable real-time subdiffraction imaging via control of photonic dispersion. Here, we demonstrate a 9-layer TiO2 log-pile 3DPC with a lattice constant of 250 nm and a rod size of 70 nm, fabricated using an optimized layer-by-layer processing approach involving TiO2 deposition, electron-beam patterning, and a reactive ion etch sequence. The 3DPC exhibits a stackingdirection bandgap characterized by a strong reflectance between 380and 500-nm wavelength. The realization of a 3DPC with uniform sub-100-nm features and a bandgap spanning the near-UV and blue–green regime is an important milestone because the potential complete control of the electromagnetic environment in the visible region offered by OPBG-3DPC structures is extremely attractive for a variety of high-impact applications. Fabrication of visible-frequency PCs has always posed an immense challenge because of the need to simultaneously satisfy the requirement of nanoscale dimensions and high refractive index (n) contrast in the PC structure. Since n of most non-absorbing materials in the visible region is less than 2.8, only diamondlike crystal structures such as the log pile enable 3DPCs with an OPBG. In fact, because a log pile can exhibit an OPBG with n as low at 2.0, the use of a log-pile 3DPC maximizes the bandwidth of the OPBG in the visible frequency region. TiO2 is often the material of choice for visible 3DPCs because it is transparent over the entire frequency range (extinction coefficient k 0) and exhibits a very high n 2.8 for a single-crystalline rutile phase. Many submicrometer lithographic techniques have been utilized for single-step, multiple-layer fabrication of 3DPCs, including holographic/ interference lithography, multiphoton polymerization, as well as direct laser writing. Researchers have successfully demonstrated templates with diamondlike crystal structures, some of which were subsequently infiltrated with high-n materials to create a negative replica. However, in all cases the fabricated device was not optimized for a visible OPBG due to either a lattice constant>500 nm or n<2.0. In contrast, a layer-by-layer lithographic technique based on a multilevel electron-beam direct-write method offers the capability to obtain small feature sizes and periodicity utilizing a high-nmaterial, which is required to fabricate PCs with OPBG in the visible regime. Two layer-by-layer processing methods (methods 1 and 2) were employed to fabricate the TiO2 log-pile 3DPCs. In method 1, we begin by depositing a planar TiO2 film by reactive sputtering. The TiO2 film exhibits n and k ranging from 2.25 and 0.002 at 800 nm to 2.55 and 0.01 at 400 nm (Fig. 1a), respectively, as expected. TiO2 is reactive ion etched through an electron-beam-patterned poly(methyl methacrylate) (PMMA) mask, backfilled and planarized with evaporated and spin-coated SiO2, and the process is repeated for additional layers (see Experimental section for more details). Using this process flow, shown schematically in Figure 1b, we fabricated a 4-layer log-pile 3DPC. Figure 2 shows a top-view scanning electronmicroscopy (SEM) image of a 4-layer structure with a 350-nm lattice constant. The image shows an average rod width of 140 nm and also reveals the underlying layers (out of focus), which are shifted by half of a lattice constant. Cross-section images taken along two perpendicular cut directions (1 and 2) reveal the four layers with accurate alignment between successive layers. We also note a distinct waviness in the nanorod appearance. This is due to our decision to over-etch the backfilled SiO2 film by 10% during the planarization step to ensure connectivity between different layers of TiO2 rods, which causes overlapping between layers and the wavy appearance. A relatively thick layer of PMMA ( 350 nm) was used to ensure sufficient thickness (>100 nm) of resist was left behind after the reactive ion etch step in order to have a clean lift-off after backfilling with SiO2. This is due to the fact that the etch selectivity of TiO2, as defined by the etch rate of TiO2 versus the etch rate of the PMMA mask, is only 0.5. However, the use of thicker PMMA results in poorer electron-beam dose control, which, in turn, leads to variation in rod width between layers (Fig. 2). We addressed this problem using a complementary pattern and evaporation of TiO2, as described in the next section

Nano-lithographically fabricated titanium dioxide based visible frequency three dimensional gap photonic crystal

Photonic crystals (PC) have emerged as important types of structures for light manipulation. Ultimate control of light is possible by creating PCs with a complete three dimensional (3D) gap [1, 2]. This has proven to be a considerable challenge in the visible and ultraviolet frequencies mainly due to complications in integrating transparent, high refractive index (n) materials with fabrication techniques to create ~ 100nm features with long range translational order. In this letter, we demonstrate a nano-lithography approach based on a multilevel electron beam direct write and physical vapor deposition, to fabricate four-layer titania woodpile PCs that potentially exhibit complete 3D gap at visible wavelengths. We achieved a short wavelength bandedge of 525nm with a 300nm lattice constant PC. Due to the nanoscale precision and capability for defect control, the nanolithography approach represents an important step toward novel visible photonic devices for lighting, lasers, sensing and biophotonics.

Quantum-Size-Controlled Photoelectrochemical Fabrication of Epitaxial InGaN Quantum Dots

We demonstrate a new route to the precision fabrication of epitaxial semiconductor nanostructures in the sub-10 nm size regime: quantum-size-controlled photoelectrochemical (QSC-PEC) etching. We show that quantum dots (QDs) can be QSC-PEC-etched from epitaxial InGaN thin films using narrowband laser photoexcitation, and that the QD sizes (and hence bandgaps and photoluminescence wavelengths) are determined by the photoexcitation wavelength. Low-temperature photoluminescence from ensembles of such QDs have peak wavelengths that can be tunably blue shifted by 35 nm (from 440 to 405 nm) and have line widths that narrow by 3 times (from 19 to 6 nm).

Near-infrared surface plasmon polariton dispersion control with hyperbolic metamaterials

We demonstrate experimentally signatures and dispersion control of surface plasmon polaritons from 1 to 1.8 µm using periodic multilayer metallo-dielectric hyperbolic metamaterials. The fabricated structures are comprised of smooth films with very low metal filling factor. The measured dispersion properties of these hyperbolic metamaterials agree well with calculations using transfer matrix, finite-difference time-domain, and effective medium approximation methods despite using only 2.5 periods. The enhancement factor in the local photonic density of states from the studied samples in the near-infrared wavelength region is determined to be 2.5-3.5. Development of this type of metamaterial is relevant to sub-wavelength imaging, spontaneous emission and thermophotovoltaic applications.

Emission modification of CdSe quantum dots by titanium dioxide visible logpile photonic crystal

Air band modes of three-dimensional photonic crystals (3DPCs) have a higher photonic density of states, potentially enabling greater emission enhancement. However, it is challenging to introduce emitters into the “air” region without significantly disturbing the photonic band structure of the PC. Here, we overcome this difficulty by introducing a low refractive index aerogel matrix containing CdSe quantum dots (625 nm peak emission) into a titanium dioxide logpile PC. We observe that the aerogel infiltration indeed preserves the bandstructure. We measure an emission suppression of ∼0.25 times inside and an enhancement of approximately three times outside the bandgap with only one vertical unit cell.

Chemoselective gas sensors based on plasmonic nanohole arrays

We have demonstrated a binary chemoselective gas sensor using a combination of plasmonic nanohole arrays and a voltage-directed assembly of diazonium chemistry. The employment of a voltage-directed functionalization allows for the realization of a multiplexed sensor. The device was read optically and was fabricated using a combination of electron-beam and conventional lithography; it contains several regions each electrically isolated from each other. We used calibrated gas dosage delivery to confirm the selectivity of the sensor and observed reversible spectral shifts of several nm upon gas exposure. The resulting spectral shift indicates the potential for use in chemical arrayed detection for low concentration gas sensing

Tuning the microcavity resonant wavelength in a two-dimensional photonic crystal by modifying the cavity geometry

High-quality-factor microcavities in two-dimensional photonic crystals at optical frequencies have a number of technological applications, such as cavity quantum electrodynamics, optical switching, filtering, and wavelength multiplexing. For such applications, it is useful to have a simple approach to tune the microcavity resonant wavelength. In this letter, we propose a microcavity design by which we can tune the resonant wavelength by changing the cavity geometry while still obtaining a high quality factor.

Silicon-based near-visible logpile photonic crystal

A nanocavity structure is embedded inside a silicon logpile photonic crystal that demonstrates tunable absorption behavior at near visible wavelengths well beyond the absorption edge of silicon. This is due to silicon’s indirect bandgap resulting in a relatively slow increase in the absorption of silicon with decreasing wavelength. Our results open up the possibility of utilizing the wide, complete three dimensional photonic gap enabled by the large refractive index of silicon to create three dimensional photonic crystal based devices well into the visible regime.