Ian D. Lindsay

Wavelength-swept Yb-fiber master-oscillator-power-amplifier with 70nm rapid tuning range

A continuous-wave all-polarization maintaining ytterbium-doped fiber master oscillator power amplifier, with a tuning range of 70 nm addressable at tuning rates of up to 20 nm/ms, is described. Up to 10 W of linearly polarized output was generated with an amplified spontaneous emission content of less than 0.2% throughout the tuning range.

Balanced detection for interferometry with a noisy source

Optical properties of nanostructures depend on size, shape, material, and local environment. These characteristics can be probed interferometrically, given a broadband source. However, broadband supercontinuum sources are intrinsically noisy, limiting the measurement sensitivity. In this article we describe the application of an auto-balancing technique to reduce the noise in a broadband supercontinuum source, thus increasing the signal to noise ratio. We show a noise reduction of 41 dB allowing optical powers as small as 0.01 pW to be interferometrically detected with a 5 ms integration time.

Towards supercontinuum-driven hyperspectral microscopy in the mid-infrared

The extension of supercontinuum (SC) sources into the mid-infrared, via the use of uoride and chalcogenide optical fibers, potentially offers the high radiance of a laser combined with spectral coverage far exceeding that of typical tunable lasers and comparable to traditional black-body emitters. Together with advances in mid-IR imaging detectors and novel tunable filter designs, such supercontinua hold considerable potential as sources of illumination for spectrally-resolved microscopy targeting applications such as rapid histological screening. The ability to rapidly and arbitrarily select particular wavelengths of interest from a broad emission spectrum, covering a wide range of biologically relevant targets, lends itself to image acquisition only at key relevant wavelengths leading to more manageable datasets. However, in addition to offering new imaging modalities, SC sources also present a range of challenges to successful integration with typical spectral microscopy instrumentation, including appropriate utilisation of their high spatial coherence. In this paper the application of SC sources to spectrally-resolved microscopy in the mid-IR is discussed and systems-integration considerations specific to these sources highlighted. Preliminary results in the 3-5μm region, obtained within the European FP7 project MINERVA, are also presented here.

Towards the mid-infrared optical biopsy

We are establishing a new paradigm in mid-infrared molecular sensing, mapping and imaging to open up the midinfrared spectral region for in vivo (i.e. in person) medical diagnostics and surgery. Thus, we are working towards the mid-infrared optical biopsy (‘opsy’ look at, bio the biology) in situ in the body for real-time diagnosis. This new paradigm will be enabled through focused development of devices and systems which are robust, functionally designed, safe, compact and cost effective and are based on active and passive mid-infrared optical fibers. In particular, this will enable early diagnosis of external cancers, mid-infrared detection of cancer-margins during external surgery for precise removal of diseased tissue, in one go during the surgery, and mid-infrared endoscopy for early diagnosis of internal cancers and their precision removal. The mid-infrared spectral region has previously lacked portable, bright sources. We set a record in demonstrating extreme broad-band supercontinuum generated light 1.4 to 13.3 microns in a specially engineered, high numerical aperture mid-infrared optical fiber. The active mid-infrared fiber broadband supercontinuum for the first time offers the possibility of a bright mid-infrared wideband source in a portable package as a first step for medical fiber-based systems operating in the mid-infrared. Moreover, mid-infrared molecular mapping and imaging is potentially a disruptive technology to give improved monitoring of the environment, energy efficiency, security, agriculture and in manufacturing and chemical processing. This work is in part supported by the European Commission: Framework Seven (FP7) Large-Scale Integrated Project MINERVA: MId-to-NEaR- infrared spectroscopy for improVed medical diAgnostics (317803; www.minerva-project.eu).

Optical imaging beyond the diffraction limit by SNEM: Effects of AFM tip modifications with thiol monolayers on imaging quality

Tip-enhanced nanoscale optical imaging techniques such as apertureless scanning near-field optical microscopy (a-SNOM) and scanning near-field ellipsometric microscopy (SNEM) applications can suffer from a steady degradation in performance due to adhesion of atmospheric contaminants to the metal coated tip. Here, we demonstrate that a self-assembled monolayer (SAM) of ethanethiol (EtSH) is an effective means of protecting gold-coated atomic force microscopy (AFM) probe tips from accumulation of surface contaminants during prolonged exposure to ambient air. The period over which they yield consistent and reproducible results for scanning near-field ellipsometric microscopy (SNEM) imaging is thus extended. SNEM optical images of a microphase separated polystyrene-block-poly (methylmethacrylate) (PS-b-PMMA) diblock copolymer film, which were captured with bare and SAM-protected gold-coated AFM probes, both immediately after coating and following five days of storage in ambient air, were compared. During this period the intensity of the optical signals from the untreated gold tip fell by 66%, while those from the SAM protected tip fell by 14%. Additionally, gold coated AFM probe tips were modified with various lengths of alkanethiols to measure the change in intensity variation in the optical images with SAM layer thickness. The experimental results were compared to point dipole model calculations. While a SAM of 1-dodecanethiol (DoSH) was found to strongly suppress field enhancement we find that it can be locally removed from the tip apex by deforming the molecules under load, restoring SNEM image contrast.

Fast hyper-spectral imaging of cytological samples in the mid-infrared wavelength region

A prototype mid-infrared spectral imaging system for rapid assessment of cells for cytological diagnosis is reported. Based on a fibre optic super-continuum source that has large spectral brightness and is coupled in to an acousto-optic tuneable filter that can rapidly scan over a set of wavelengths that are chosen to give a high level of selectivity for a specific skin disease. The system has the potential to collect an image cube of 100 wavelengths and 300k pixels in 2 seconds so that cells on living people could be analysed. The system has been evaluated with colon cells over 2700- 3100 cm-1.

Tunable flat lenses in the mid-infrared

We present a stand-alone beam focusing flat lens for use in the mid-infrared wavelength range. An aperture and second-order grating structure in a thin gold layer is used to excite and then scatter a propagating surface plasmon polariton (SPP) to constructively interfere in the far-field to produce a narrow beam. A rapidly tunable optical parametric oscillator source is used to demonstrate how changing the wavelength of the incident light from 3.8 - 4.2 μm produces two interfering beams in the far-field caused by the scattering of the propagating SPP interfering with the incident light diffracted by the narrow (sub λ/2) aperture. We identify measured farfield patterns for varying wavelengths using the razor blade edge beam profiling method. The agreement between Finite-Difference Time-Domain (FDTD) modelled and measured results will allow the aperture/grating structures to be integrated directly onto the facets of edge-emitting lasers to dramatically reduce their beam divergence. As edge-emitting lasers have a fixed wavelength, the addition of a thin layer of material such as Silicon or Barium Strontium Titanate to the facets allows tuning by altering the SPP wavelength.

A frequency-swept source for broadband mid-infrared spectroscopy

Techniques for spectroscopic surface analysis with high spatial resolution, such as confocal microscopy and scanning near-field optical microscopy, require high-brightness coherent sources capable of rapidly addressing wide spectral ranges in regions corresponding to molecular resonances characteristic of specific functional groups. Here we describe the performance characteristics of a frequency-swept optical parametric oscillator (OPO) rapidly tuneable in the 2700 to 3600cm−1 region where many compounds show characteristic resonances associated with C-H, N-H and O-H stretching modes. The OPO was capable of rapid tuning (200nm/ms) over ranges of up to 900cm−1 within an overall 1440cm−1 tuning range and with output powers of up to 2.37W.

OPO based Off-Axis Integrated Cavity Output Spectroscopy for rapid chemical sensing

We presented rapid and sensitive trace gas detector based on a high power and fast scanning cw OPO in combination with Off-Axis Integrated Cavity Output Spectroscopy. A detection limit of 2×10^-9 cm^-1 Hz^-1/2 was obtained.

Application of mid-infrared attenuated total reflectance spectroscopy using broadly tunable OPOs

A singly resonant optical parametric oscillator based on MgO:PPLN is used as a tunable light source for attenuated total reflectance spectroscopy. Because of high emitted power spectral densities, diluting the sample can be avoided.