Plasmonic sensors show great potential in healthcare with nanohole arrays being most suited to highly sensitive, multiplexed label-free biosensing in point-of-care diagnostics. Extraordinary optical transmission (EOT) occurs when incident light is coupled to plasmonic modes in a nanohole array. When biomolecules bind to the surface of the nanohole array the refractive index surrounding it changes. We use optical microspectrometry and imaging to measure the EOT resonance wavelength shift due to the presence of these biomolecules. We demonstrate the biosensing capabilities of our plasmonic sensor with the detection of molecular and protein binding events, and cancer biomarkers.
KEYWORDS: Near field, Imaging spectroscopy, Plasmonics, Spectroscopy, Surface plasmons, Electron microscopy, Near field optics, Optical parametric oscillators, Metals, Dielectrics
Photoemission electron microscopy (PEEM) is an attractive and advantageous technique in the field of plasmonics. Whilst surface plasmons are excited at the metal dielectric interface by light, it is the near-field photoelectron distribution that is imaged, with <40 nm resolution, and thereby the optical diffraction limit is overcome. Additionally parallel acquisitioning makes time-resolved (TR) PEEM1 possible. PEEM therefore allows us to investigate light-matter interactions in localized, propagating and hybridized surface plasmons leading to advances in fundamental research and technological applications.
In addition to near-field imaging it is also possible to perform near-field spectroscopy. A tunable short pulse optical parametric oscillator (OPO) light source can be combined with PEEM. We demonstrate this technique with arrays of whispering gallery mode (WGM) cavities2 fabricated with focused ion beam milling (FIB) on gold surfaces. Characteristic spectral peaks and near-field mode distributions result from the coherent excitation of different plasmon resonances. This near-field interference of modes allows us to control the emission from these WGM cavities3. Additionally recent advances in ultrafast near-field microscopy and spectroscopy will be discussed.
[1] M. Bauer, C. Wiemann, J. Lange, D. Bayer, M. Rohmer and M. Aeschlimann, Appl. Phys. A 88 473 (2007)
[2] E. J. Vesseur, F. J. García de Abajo and A. Polman Nano Letters 9 3147 (2009)
[3] P. Melchior, D. Kilbane, E. J. Vesseur, A. Polman and M. Aeschlimann Optics Express 23, 31619 (2015)
We demonstrate a table-top strong band emission water window source based on laser-produced high-Z plasmas.
Resonance emission from multiply charged ions merges to produce intense unresolved transition arrays in the 2 to 4
nm region, extending below the carbon K edge (4.37 nm). Arrays resulting from n = 4-n = 4 transitions are overlaid
with n = 4-n = 5 emission and shift to shorter wavelength with increasing atomic number. Under spectral analysis a
guideline for microscope construction design for single-shot live cell imaging is proposed based on the use of a
bismuth plasma source, coupled with multilayer mirror optics.
We demonstrate an efficient extreme ultraviolet (EUV) source for operation at λ = 6.7 nm by optimizing the optical
thickness of gadolinium (Gd) plasmas. Using low initial density Gd targets and dual laser pulse irradiation, we
observed a maximum EUV conversion efficiency (CE) of 0.54% for 0.6% bandwidth (BW) (1.8% for 2%BW),
which is 1.6 times larger than the 0.33% (0.6%BW) CE produced from a solid density target. Enhancement of the
EUV CE by use of a low-density plasma is attributed to the reduction of self-absorption effects.
We have demonstrated a laser-produced plasma extreme ultraviolet source operating in the 6.5-6.7 nm region based
on rare-earth targets of Gd and Tb coupled with a Mo/B4C multilayer mirror. Multiply charged ions produce strong
resonance emission lines, which combine to yield an intense unresolved transition array. The spectra of these
resonant lines around 6.7 nm suggest that the in-band emission increases with increased plasma volume by
suppressing the plasma hydrodynamic expansion loss at an electron temperature of about 50 eV, resulting in
maximized emission. We also have investigated the dependence of the spectral behavior and conversion efficiencies
of rare-earth plasma extreme ultraviolet sources with peak emission at 6.7 nm on laser wavelength and the initial
target density. The maximum conversion efficiency was 1.3% at a laser intensity of 1.6 × 1012 W/cm2 at an operating
wavelength of 1064 nm, when self-absorption was reduced by use of a low initial density target.
An engineering prototype high average power 13.5-nm source has been shipped to semiconductor facilities to permit
the commencement of high volume production at a 100 W power level in 2011. In this source, UTA (unresolved
transition array) emission of highly ionized Sn is optimized for high conversion efficiency and full recovery of the
injected fuel is realized through ion deflection in a magnetic field. By use of a low-density target, satellite emission
is suppressed and full ionization attained with short pulse CO2 laser irradiation. The UTA is scalable to shorter
wavelengths, and Gd is shown to have similar conversion efficiency to Sn (13.5 nm) at a higher plasma temperature,
with a narrow spectrum centered at 6.7 nm, where a 70% reflectivity mirror is anticipated. Optimization of short
pulse CO2 laser irradiation is studied, and further extension of the same method is discussed, to realize 100 W
average power down to a wavelength of 3 nm.
The search for a source of EUV radiation for photolithography in the 13.5 nm region has been narrowed down to laser produced or pulsed discharge plasmas containing either xenon or tin. Higher conversion efficiencies can however be obtained with tin based plasmas within this wavelength regime. It is known that EUV photoabsorption by the lower ion stages of xenon reduces the photon flux from a xenon source. This is due to shape resonances from 4d-epsilonf transitions within Xe I-IV. The corresponding resonances for Sn I-IV have been obtained by means of the dual laser plasma (DLP) technique. It is also found that above the 4d ionisation threshold the spectra of Sn I-IV are dominated by a 4d-epsilonf shape resonance which peaks at close to 65 eV in each case. A transfer of oscillator strength from the shape resonance to pseudo-discrete 4d→nf transitions with increasing ionisation is clearly evident. Hartree-Fock with configuration interaction and relativistic time dependent local density approximation calculations successfully account for this behaviour and also permit identification of the discrete features.
The photoabsorption spectra of Te I-Te IV have been recorded and analysed in the XUV spectral region using the dual laser produced plasma technique. Photoexcitation from the 4d subshell is the dominant process in the 35-150 eV energy region. For photon energies between 35-45 eV discrete structure corresponding to 4d-np (n>4) transitions were obtained. Above the 4d ionisation threshold the spectra of Te I-Te III were found to be dominated by a 4d-ef shape resonance, which peaks at ~88 eV in each case. A transfer of oscillator strength from the resonance to discrete 4d-nf (n>3) transitions with increasing ionisation is clearly evident, and the 4d-4f transitions are the strongest features in the Te IV spectrum. Hartee-Fock with configuration interaction and time dependent local density approximation calculations successfully account for this behaviour and permit identification of the discrete features. The use of a prepulse to maximise the brightness of a tungsten continuum emitting plasma was also investigated.
We report here some observations and preliminary findings from a study focussed on the vacuum-UV (λ, 40-60 nm) radiation emitted during the interaction of 150 ps laser pulses (100-400 mJ) with copper pre-plasmas formed by an electro-optically synchronised (0.1 - 0.8 J, 8 ns) long pulse laser. We have observed significant gains in VUV flux that scale with inter-laser delay. We also report preliminary observations on total X-ray emission from the interaction of a superintense 80 fs, 200 mJ laser pulse at the UK ASTRA laser facility with a similar pre-plasma at irradiances approaching 1019 W/cm2
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