David N. Sharp

Fabrication of photonic crystals for the visible spectrum by holographic lithography

The term ‘photonics’ describes a technology whereby data transmission and processing occurs largely or entirely by means of photons. Photonic crystals are microstructured materials in which the dielectric constant is periodically modulated on a length scale comparable to the desired wavelength of operation. Multiple interference between waves scattered from each unit cell of the structure may open a ‘photonic bandgap’—a range of frequencies, analogous to the electronic bandgap of a semiconductor, within which no propagating electromagnetic modes exist. Numerous device principles that exploit this property have been identified. Considerable progress has now been made in constructing two-dimensional structures using conventional lithography, but the fabrication of three-dimensional photonic crystal structures for the visible spectrum remains a considerable challenge. Here we describe a technique—three-dimensional holographic lithography—that is well suited to the production of three-dimensional structures with sub-micrometre periodicity. With this technique we have made microperiodic polymeric structures, and we have used these as templates to create complementary structures with higher refractive-index contrast.

AN INFRARED SPECTRUM OF THE MOLECULAR DICATION (DOUBLY POSITIVELY CHARGED MOLECULE), D35CL2+

We have observed an infrared spectrum within the X 3Σ− state of DCl2+ using a fast-ion-beam/laser-beam spectrometer. A preliminary analysis shows good agreement with the rotational constants and tunneling lifetimes calculated by Bennett and McNab for the ν=1–2 vibrational band, although the calculated band origin appear to be in error by 21.1 cm−1.

Registration of single quantum dots using cryogenic laser photolithography

We have registered the position of single InGaAs quantum dots using a cryogenic laser photolithography technique. This is an important advance towards the reproducible fabrication of solid-state cavity quantum electrodynamic devices, a key requirement for commercial exploitation of quantum information processing. The quantum dot positions were registered with an estimated accuracy of 50 nm by fabricating metal alignment markers around them. Photoluminescence spectra from quantum dots before and after marker fabrication were identical except for a small redshift (~1 nm), probably introduced during the reactive ion etching.

Photonic crystals for the visible spectrum by holographic lithography

We describe a general and flexible technique for the fabrication of three-dimensional photonic crystals that is particularly well adapted to the production of structures with the sub-micron periodicity required for applications in the visible optical spectrum. Three-dimensional microstructure is generated by using a four-beam laser interference pattern to expose a thick layer of photoresist. Exposed areas are rendered insoluble; unexposed areas are dissolved away leaving a three-dimensional photonic crystal formed of cross-linked polymer with air-filled voids. The polymeric structure may be used as a template for the production of photonic crystals with higher refractive index contrast. Photonic crystals made of polymer and of TiO2 have been characterized by scanning electron microscopy and by optical diffraction measurements.

Electrically conducting, ultra-sharp, high aspect-ratio probes for AFM fabricated by electron-beam-induced deposition of platinum.

We report on the fabrication of electrically conducting, ultra-sharp, high-aspect ratio probes for atomic force microscopy by electron-beam-induced deposition of platinum. Probes of 4.0 ±1.0 nm radius-of-curvature are routinely produced with high repeatability and near-100% yield. Contact-mode topographical imaging of the granular nature of a sputtered gold surface is used to assess the imaging performance of the probes, and the derived power spectral density plots are used to quantify the enhanced sensitivity as a function of spatial frequency. The ability of the probes to reproduce high aspect-ratio features is illustrated by imaging a close-packed array of nanospheres. The electrical resistance of the probes is measured to be of order 100 kΩ.

Replicated Photonic Crystals by Atomic Layer Deposition within Holographically Defined Polymer Templates

We report the replication of holographically defined photonic crystals using multistage atomic layer deposition. Low- and high-temperature atomic layer depositions were combined with selective etching to deposit and remove multiple conformal thin films within three-dimensional polymer templates. Using intermediate Al2O3 inverse replicas, temperature-sensitive SU-8 photonic crystal templates were faithfully replicated with TiO2 and GaP, greatly increasing the dielectric contrasts of the photonic crystals. Optical measurements are in good agreement with the calculated band structures.

Cryogenic two-photon laser photolithography with SU-8

We have shown that photolithography can be used to create alignment markers on a semiconductor substrate at cryogenic temperatures. The epoxy resist SU-8 can be exposed effectively by two-photon absorption at a temperature of 4K. By this means a spectroscopy apparatus can be used to find the positions of randomly distributed structures at low temperatures, such as InGaAs∕GaAs quantum dots, and mark their positions. We present a systematic study of the optical exposure parameters for cryogenic two-photon laser photolithography with SU-8.

Study of two-photon laser photolithography with SU-8 at cryogenic temperatures

We show that the SU-8 epoxy resist can be exposed by two-photon absorption at 4 K, allowing spectroscopy and lithography to be performed by the same apparatus. We present a systematic study of the exposure parameters.

Accuracy of single quantum dot registration using cryogenic laser photolithography

We have registered the position of single InGaAs quantum dots using a novel cryogenic laser photolithography technique. This would be useful in realizing solid state cavity quantum electrodynamics. By fabricating metal alignment markers around the quantum dot, it was registered with an accuracy of 50 nm. Following the marker fabrication process we demonstrated that the same quantum dot was reacquired, with an accuracy of 150 nm. The photoluminescence spectra from the quantum dots before and after processing were identical except for a small red shift (~1 nm), probably introduced during the reactive ion etching.

Device Fabrication in High-Index 3D Photonic Crystals

Holographic lithography (HL) is a flexible technique for the fabrication of three-dimensional (3D) photonic crystals with the submicron periodicity required for optical and near-IR applications. We demonstrate two key steps towards the creation of integrated optical devices based on waveguides and microcavities operating within a complete photonic band gap: 1) infiltration of a holographically-defined polymeric 3D photonic crystal template with high-index dielectric by Atomic Layer Deposition (ALD) [1]; and 2) creation of localised structural defects embedded in, and in registration with, a 3D photonic crystal by direct two-photon laser writing [2]. Structural and optical characterisation of TiO2 photonic crystals produced by infiltration and removal of the polymer template demonstrates the high quality of the negative replica. Structural characterisation of photonic crystals with embedded defects shows a faithful rendering of the designed structure in the developed polymer photonic crystal. The combination of these three techniques (HL, two-photon writing and ALD) maps out a clear route to device fabrication in high-index 3D photonic crystals.