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There is an ever-growing need for compact sources which can be used for the cooling process in high accuracy atomic clocks. Current systems make use of large, expensive lasers which are power-hungry and often require frequency doubling in order to hit the required wavelengths. Distributed feedback (DFB) lasers have been fabricated at a number of key wavelengths which would allow chip scale atomic devices with very high accuracy to become a reality. Two key atomic transitions analysed here are 88 Sr + and 87 Rb which require cooling at 422 nm and 780.24 nm, respectively. The vital parameter of the DFB lasers for this application is the linewidth, as very narrow linewidths are required in order for the atomic cooling process to occur. The lasers realised here produce the required power levels, with high side-mode suppression ratios and show good single mode tuning which is important for hitting precise wavelengths. This work will present the latest techniques and results using the DFB lasers at both wavelengths.

Owing to increased performance capabilities over their LED counterparts, Gallium Nitride (GaN) based Laser Diodes (LDs) are becoming more prevalent in several applications, including lighting, visible light communications, medical spectroscopy, and atom cooling. For many of these applications, a tunable, precise wavelength is necessary such that chromatic aberrations are minimized and that specific wavelengths with narrow linewidth can be utilized for atomic transitions or filtered communications systems. This paper discusses one method of achieving single-mode lasing operation in GaN LDs, namely the deeply-etched sidewall grating Distributed Feedback (DFB) LD. Optical characteristics of such devices will be discussed, as well as their feasibility for optical communications.

Metasurfaces, which are the 2D version of metamaterials, have revolutionised compact optics. Using subwavelength periodic nanostructured dielectrics, the refractive index and absorption properties of metasurfaces can manipulate light to a degree surpassing conventional bulk materials. Using metasurfaces, the phase, polarization, spin (for circularly polarised light), amplitude and wavelength of light can all be arbitrarily tailored to imitate a lens, which we refer to as a metalens (ML). MLs allow a larger choice of materials for optical components and have five major advantages over traditional refractive lenses – superior resolution, miniaturisation, lighter weight, multifunctionality and cost. In recent years, numerous metasurfaces with useful functionalities have been proposed, and although novel in their approach they still have very few real-world applications. One such application is within infrared laser systems, which have real-world use such as laser designators. In this work, we demonstrate polarisation-insensitive metalenses working at λ = 1064 nm, with a d = 1 mm aperture size and four different F-numbers (f# = 0.5 - 5). The lenses are made using amorphous silicon (a-Si) pillars on top of a fused silica substrate, in order to function with a high efficiency (>60%) and little loss – where previous metal-based (plasmonic) metalens devices suffered from low efficiency (

Over the past 20 years, research into Gallium Nitride (GaN) has evolved from LED lighting to Laser Diodes (LDs), with applications ranging from quantum to medical and into communications. Previously, off-the-shelf GaN LDs have been reported with a view on free space and underwater communications. However, there are applications where the ability to select a single emitted wavelength is highly desirable, namely in atomic clocks or in filtered free-space communications systems. To accomplish this, Distributed Feedback (DFB) geometries are utilised. Due to the complexity of overgrowth steps for buried gratings in III-Nitride material systems, GaN DFBs have a grating etched into the sidewall to ensure single mode operation, with wavelengths ranging from 405nm to 435nm achieved. The main motivation in developing these devices is for the cooling of strontium ions (Sr+ ) in atomic clock applications, but their feasibility for optical communications have also been investigated. Data transmission rates exceeding 1 Gbit/s have been observed in unfiltered systems, and work is currently ongoing to examine their viability for filtered communications. Ultimately, transmission through Wavelength Division Multiplexing (WDM) or Orthogonal Frequency Division Multiplexing (OFDM) is desired, to ensure that data is communicated more coherently and efficiently. We present results on the characterisation of GaN DFBs, and demonstrate their capability for use in filtered optical communications systems.

Gallium nitride (GaN) sources are becoming a regular part of today's world and are now key devices for lighting infrastructures, communications systems and quantum applications, amongst others. In particular, many applications have seen the shift from LEDs to laser diodes to make use of higher powers, higher bandwidths and increased transmission distances. Laser communication systems are well established, however there are applications where the ability to select a single emitted wavelength is highly desirable, such as quantum atomic clocks or in filtered communication systems. Distributed feedback (DFB) lasers have been realised emitting at a single wavelength where the grating structure is etched into the sidewall of the ridge. The main motivation in developing these lasers is for the cooling of ions in atomic clocks; however their feasibility for optical communications is also explored. Narrow linewidth lasers are desirable and this paper will explore how this is achieved. Data rates in excess of 1 Gbit/s have also been achieved in a directly modulated, unfiltered system. These devices lend themselves towards wavelength division multiplexing and filtered optical communications systems and this will be analysed further in the work presented here.

Metasurfaces have revolutionized the definition of compact optics. Using subwavelength periodic structures of nanostructured dielectrics, the refractive index and absorption properties of metasurfaces – which are 2D metamaterials – can manipulate light to a degree not possible with conventional bulk glasses and crystals. The phase, polarization, spin (for circularly polarized light), amplitude and wavelength of light can all be manipulated and crafted to user-specified values to mimic the action of a lens, which we refer to as a metalens (ML). MLs have four major advantages over traditional refractive lenses – superior resolution, lighter weight, miniaturization and cost. Many metasurfaces with useful functionalities have been proposed in recent years, yet although novel in their approach have few real-world applications. One such market is the use within infrared laser systems, such as laser designators. In this work, we demonstrate metasurface lenses working at a wavelength of λ = 1064 nm, with aperture d = 1 mm and four different Fnumbers (focal length f = 0.5, 1, 2 and 5 mm). The lenses are composed of 700nm high a-Si pillars – ranging from 70- 360 nm diameter – which are fabricated using electron beam lithography (EBL) and reactive ion etching processes, on top of a fused silica substrate. Such lenses are shown to have diffraction-limited performance, with focal spot-size agreeing with theoretical values of λ‧f/d. Furthermore, we have designed large area lenses with aperture d = 10 mm, where the number of pillars per lens exceeds 550 million. By using an efficient Python script, we are able to produce these 100 mm2 samples with just 14 hours of EBL writing time.

We demonstrate a single wavelength operation from an InGaN/GaN distributed feedback (DFB) blue laser at 42X nm. The 39th order grating is etched in the sidewall to achieve single wavelength. The laser has a FWHM of ~ 25 pm at 500mA pulsed current with 15 mW output power.

We have realised InGaN/GaN distributed feedback laser diodes emitting at a single wavelength in the 42X nm wavelength range. Laser diodes based on Gallium Nitride (GaN) are useful devices in a wide range of applications including atomic spectroscopy, data storage and optical communications. To fully exploit some of these application areas there is a need for a GaN laser diode with high spectral purity, e.g. in atomic clocks, where a narrow line width blue laser source can be used to target the atomic cooling transition. Previously, GaN DFB lasers have been realised using buried or surface gratings. Buried gratings require complex overgrowth steps which can introduce epi-defects. Surface gratings designs, can compromise the quality of the p-type contact due to dry etch damage and are prone to increased optical losses in the grating regions. In our approach the grating is etched into the sidewall of the ridge. Advantages include a simpler fabrication route and design freedom over the grating coupling strength.Our intended application for these devices is cooling of the Sr+ ion and for this objective the laser characteristics of SMSR, linewidth, and power are critical. We investigate how these characteristics are affected by adjusting laser design parameters such as grating coupling coefficient and cavity length.

Gallium nitride (GaN) laser diodes are becoming popular sources not only for lighting but for applications ranging from communications to quantum. This paper presents the use of a commercial, off-the-shelf laser diode, with an emission wavelength of 450 nm, for visible light communication, both in free space and for underwater scenarios. Data rates up to 15 Gbit/s have been achieved by making use of orthogonal frequency division multiplexing (OFDM). In addition, distributed feedback (DFB) lasers have been realised emitting at a single wavelength which lend themselves towards applications where high spectral purity is crucial such as atomic clocks or filtered free space transmission systems. These devices have the grating structure etched into the sidewall of the ridge and work is ongoing to measure the linewidth of these lasers with the intended application of cooling Sr+ ions.

We report on the design, fabrication, and characterization of InGaN/GaN distributed feedback laser diodes emitting at a single wavelength around 430 nm. Third-order sidewall-etched gratings were used which have the advantage of a simple fabrication route with no need for overgrowth. We carried out waveguide modeling to find the effective modal indices of the grating and calculate the coupling coefficient. The laser ridge and the grating were formed by electron beam lithography followed by inductively coupled plasma etching. The as-cleaved lasers emitted in the pulsed regime with an SMSR of 22 dB and a peak single-mode output power of 40 mW. Slope efficiency was similar for both Fabry–Perot and DFB chips demonstrating that performance is not compromised by the addition of the grating.

We report on InGaN/GaN distributed feedback laser diodes with high order gratings emitting at a single wavelength around 428 nm. The 39th order notched gratings have the advantage of a simplified fabrication route with no need for overgrowth. The laser ridge and grating were formed by electron beam lithography followed by ICP etching. The as-cleaved lasers emitted in the pulsed regime with a peak single-mode output power of 15 mW. Optimization of the grating design should lead to higher power single wavelength operation.

We report on deeply etched sidewall grating DFB lasers in the InGaN/GaN material system emitting at a single wavelength around 434 nm. GaN lasers have a wide range of applications in communications, displays and storage. The availability of a single wavelength device with a good side mode suppression ratio (SMSR) would allow further applications to be addressed such as sources for laser cooling and Fraunhofer line operation for solar background free communications. Sidewall etched gratings have the advantage of fabrication with no need for overgrowth and have been demonstrated in a range of other material systems and wavelengths. Importantly for GaN based devices, this design has the potential to minimise fabrication induced damage to the epi structure. We investigated two laser designs, one with 80 % duty-cycle 3rd order gratings and another with 39th order partial gratings. Simulation of the 2D waveguide sections was carried out to find the optimal grating width. For fabrication, the laser ridge and gratings were patterned in a single step using electron beam lithography and ICP etched to a depth of 500 nm. Contact metal was deposited and the sample thinned and cleaved into 1 mm long cavities. The as-cleaved 3rd order lasers emit in the pulsed regime with a SMSR of 20 dB and a peak single-mode output power of 40 mW. The output power is similar to that of parallel processed FP lasers. The 39th order lasers also exhibit narrow spectral width at an output power of 10 mW.

We have developed a novel batch fabrication process for cantilever array aperture probes used in scanning near-field optical microscopy (SNOM). The array probes, consisting of 16 parallel cantilevers with each tip having an identical aperture, are proposed to dramatically reduce the scanning time for imaging or increase the throughput of near-field optical microscopy and lithography. In this study, apertures are defined by direct-write electron beam lithography and subsequent pattern transfer by reactive ion etch. Better quality and reproducibility of apertures are demonstrated through optical throughput measurement of the probes. The fabrication of apertures having complex shapes is also demonstrated. The process includes such novel elements as the use of local oxidation of silicon and the use of an aluminium - compatible release etch. The process has demonstrated very high yield with a measured reproducibility of aperture diameter of 5nm. Thermal oxide cantilevers allow the use of the probes at wavelengths as short as 250nm.

An additional marker can predict alignment errors in electron beam lithography.Correlation marker search using Penrose tiles can avoid damaging markers.Mark locate with defocus and undersampling can achieve 1nm repeatability. Alignment between lithography layers is essential for device fabrication. A minor defect in a single marker can lead to incorrect alignment and this can be the source of wafer reworks. In this paper we show that this can be prevented by using extra alignment markers to check the alignment during patterning, rather than inspecting vernier patterns after the exposure is completed. Accurate vernier patterns can often only be read after pattern transfer has been carried out. We also show that by using a Penrose tile as a marker it is possible to locate the marker to about 1nm without fully exposing the resist. This means that the marker can be reused with full accuracy, thus improving the layer to layer alignment accuracy. Lithography tool noise limits the process.

Correlation-based alignment is an alternative alignment method for electron beam lithography. Using complex marker patterns, such as Penrose patterns, which contain more positional information, greater alignment accuracy can be achieved. Correlation-based alignment with Penrose patterns is less susceptible to marker edge defects, such as rat bites, roughness and flagging, since many more edges contribute to determining the marker position. There are however other defects associated with fabricating markers and this paper investigates how defects that result in parts of the pattern being omitted or obscured affect the correlation process when using Penrose pattern markers. We show that in both cases severely damaged markers can be used successfully and demonstrate fabricated structures with sub-5nm alignment using markers with up to 80% of the marker pattern missing.

This paper examines the desirable properties of marker patterns for use in correlation-based alignment systems and demonstrates alignment accuracies of better than 1nm. A framework for evaluating different classes of marker patterns has been developed and a figure of merit for marker patterns used in correlation-based alignment has been defined. We show that Penrose tilings have many desirable properties for correlation-based alignment. An alignment system based on correlation and using marker patterns derived from Penrose tilings has been developed and implemented on a commercial Vistec VB6 UHR EWF electron beam lithography tool. A new method of measuring alignment at the sub-nm level using overlaid gratings and a Fourier Transform based analysis scheme is introduced.

The development of nanofabrication techniques for creating high aspect ratio (∼50:1) sub-10 nm silicon nanowires (SiNWs) with smooth, uniform, and straight vertical sidewalls using an inductively coupled plasma (ICP) etching process at 20 °C is reported. In particular, to improve the quality and flexibility of the pattern transfer process for high aspect ratio SiNWs, hydrogen silsesquioxane, a high-resolution, inorganic, negative-tone resist for electron-beam lithography has been used as both the resist for defining sub-10 nm patterns and the hard mask for etching the underneath silicon material. The effects of SF6/C4F8 gas flow rates, chamber pressure, platen power and ICP power on the etch rate, selectivity, and sidewall profile are investigated. To minimize plasma-induced sidewall damage, moderate plasma excitation power (ICP power of 600 W) and low ion energy (platen power of 6–12 W) were used. Using the optimized etch process at room temperature (20 °C), the authors have successfully fabricated sub-1...

This paper describes a robust process for the fabrication of highly doped Silicon-On-Insulator nanowires and devices. The process uses electron-beam lithography, lowdamage dry etch and controlled thermal oxidation to deliver consistent, reproducible and reliably nanowires of nominal widths from 100 nm down to sub-5 nm etched to a depth of 55 nm in silicon. Initial electrical measurements indicate metallic behavior for the widest wires and below a particular width, the wires become depleted showing electrical behaviour consistent with Coulomb blockade at room temperature.

We present a new class of interferometer system that is capable of simultaneous measurement of absolute position and rotation in all six degrees of freedom (DOF) with nanometer precision. This novel capability is due to the employment of a system of interference fringes that is not periodic. One of the key strengths offered by this new approach is that the absolute position of the system can be determined with a single measurement, rather than by counting fringes during displacement from a known location. The availability of a simultaneous measurement of all six DOF eliminates many problems associated with conventional interferometry.