The Linearly Variable Filters (LVF) are complex optical devices that, integrated in a CCD, can realize a "single chip spectrometer". In the framework of an ESA Study, a team of industries and institutes led by SELEX-Galileo explored the design principles and manufacturing techniques, realizing and characterizing LVF samples based both on All-Dielectric (AD) and Metal-Dielectric (MD) Coating Structures in the VNIR and SWIR spectral ranges. In particular the achieved performances on spectral gradient, transmission bandwidth and Spectral Attenuation (SA) are presented and critically discussed. Potential improvements will be highlighted. In addition the results of a feasibility study of a SWIR Linear Variable Filter are presented with the comparison of design prediction and measured performances. Finally criticalities related to the filter-CCD packaging are discussed.
The main achievements reached during these activities have been:
- to evaluate by design, manufacturing and test of LVF samples the achievable performances compared with target requirements;
- to evaluate the reliability of the projects by analyzing their repeatability;
- to define suitable measurement methodologies
Small-dimension, low-mass spectrometers are useful for both Earth observation and planetary missions. A very compact multi-spectral mini-spectrometer that contains no moving parts, can be constructed combining a graded-thickness filter, having a spatially variable narrow-band transmission, to a CCD array detector. The peak wavelength of the transmission filter is moving along one direction of the filter surface, such that each line of a two-dimensional array detector, equipped with this filter, will detect radiation in a different pass band. The spectrum of interest for image spectrometry of the Earth surface is very wide, 400-1000nm. This requirement along with the need of a very small dimension, makes this filter very difficult to manufacture. Preliminary results on metal-dielectric wedge filters, with a gradient of the transmission peak wavelength equal to 60nm/mm, are reported.
Compact spectrometers are of interest for space applications for both Earth observation and analysis of planet soil. The
spectrometer here described is dedicated to Land imaging and is based on the use of linear variable filters for wavelength
selection. This kind of filter is able to transmit the radiation in a narrow band (<20 nm) centered on a wavelength that
changes along its surface, and to operate in a wide spectrum (visible-infrared). Both the variable filter characteristics and
the results of the breadboard spectrometer operation will be reported.
Metal thin film functional properties depend strongly on its nanostructure, which can be manipulated by varying nucleation and growth conditions. Hence, in order to control the nanostructure of aluminum thin films fabricated by RF magnetron sputtering, we made use of in-situ monitoring of electrical and optical properties of the growing layer as well as plasma characterization by mass and optical emission spectroscopy. The electrical conductivity and I-V characteristics were measured. The optical constants were obtained from optical monitoring based on spectral ellipsometry. The relevant models (based on one or two Lorentz oscillators and B-spline functions) were suggested to evaluate the data obtained from the monitoring techniques. The results of the in-situ monitoring were correlated with scanning electron microscope analyses. We demonstrated the monitoring was able to distinguish the growth mode in real-time. We could estimate the percolation threshold of the growing layer and control layer nanostructure. The nanostructure was effectively manipulated by RF power variation. Optical functions exhibiting plasmonic behavior in the UV range and a strong nonlinear character of I-V curves were obtained for an ultrathin Al film deposited at a lower growth rate.
Silver is widely used for fabrication of plasmonic devices because of its unique optical constants. The nanostructure of the Ag layer is mainly influenced during the initial stage of the silver nucleation. Therefore, we focus our attention on studying this stage of silver growth. Nanostructured ultrathin silver layers are prepared by means of magnetron sputtering. The initial stage of the nucleation and the layer growth is studied by optical monitoring, which is based on spectrophotometric measurement of the sample reflectivity. The measured data are compared to a model of layered structure. The noncontinual (Volmer-Weber) mode of the layer nucleation is clearly distinguished in the monitored data. Thus, we are able to estimate the point of noncontinual layer coalescence. The optical data are correlated with in situ monitoring of the electrical resistance. We find that the nucleation mode and resulting nanostructure can be significantly influenced by an ultrathin silver oxide interlayer.
Silver is widely used for a fabrication of plasmonic devices due to its unique optical constants. Nanostructured Ag layer
can exhibit strong localized surface plasmon resonance, which mainly affects its optical behavior in visible and near
infrared spectra. The nanostructure of the Ag layer is mainly influenced during the initial stage of the silver nucleation.
Therefore we focused our attention on the study of this stage of the silver growth. The nanostructured ultra-thin silver
layers were prepared by means of the magnetron sputtering. The nucleation mode and the resulting nanostructure was
controlled by the deposition conditions. The initial stage of the nucleation and the layer growth was studied by means of
an optical monitoring, which is based on a principle of spectrophotometric measurement of sample reflectivity. The
measured data were fitted to a model of layered structure. The non-continual (Volmer-Weber) mode of the layer
nucleation was clearly distinguished in the monitored data. Thus we were able to estimate the point of the non-continual
layer coalescence as well as the subsequent evolution of the surface roughness. The prepared nanostructured Ag layers
were analyzed by Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). Optical properties were
studied by spectroscopic ellipsometry and spectrophotometry.
Ultrathin nanostructured metal films exhibit unusual properties and performances. Film functional properties depend
strongly on the nanostructure that can be manipulated by varying nucleation and growth conditions.
Hence, in order to control the nanostructure of aluminium thin film fabricated by RF magnetron sputtering, we focus on
in-situ monitoring of electrical and optical properties of the growing layer as well as plasma characterization by mass and
optical emission spectroscopy. The electrical conductivity and I-V characteristics were measured. The optical constants
were obtained from optical monitoring based on a spectral ellipsometry. The relevant models (based on one or two
Lorentz oscillators and B-spline function) are suggested to evaluate the data obtained from the monitoring techniques.
The results of the in-situ monitoring are correlated with SEM analyses.
We demonstrate the monitoring can distinguish the growth mode in the real-time. We can estimate the percolation
threshold of the growing layer and control layer nanostructure. We show that the nanostructure can be manipulated by
RF power variation. Optical functions exhibiting plasmonic behaviour in the UV range and a strong nonlinear course of
I-V curves were obtained for ultrathin Al film deposited at lower growth rate.
A lightweight and compact spectrometer with spectral resolution in the order of 0.1 nm, in a spectral range from UV to
NIR, can be implemented using a spectral pre-selection technique at the spectrometer entrance, through transmission
variable filters. Such filters, based on thin-film optical coatings, have either a transmission peak or a transmission edge
that moves along one direction of the filter surface. Depending on the spectrometer design, different configurations of
the pre-selection device are possible. The operating spectral range (240-800 nm) is divided in a number of sub-ranges
and in each of them the filter transmission peak, or edge, is displaced from the minimum to the maximum wavelength
over a distance of few mm. Two cases are considered: a configuration with both a narrow-band transmission filter and a
band-pass filter having a linear spatial variation and a configuration with an edge filter having a non-linear spatial
variation. To obtain the required spatial profile of filter performance, a graded coating is deposited on a fused silica
substrate, by r.f. sputtering with a moving mask. Details on filter requirements and fabrication technology are reported.
The development of light-weight instrumentation for space-based measurements, involves an improvement in the
production techniques of optical components. In this work the design and manufacturing of small-dimension variable
filter is described. These visible-infrared filters are designed to be coupled to an array detector, in a way that each CCD
line is able to detect the radiation in a different narrow radiation band. In this way a compact low-mass optical sensor is
obtained that will be combined to a telescope for image spectrometry from space. The optical filters were manufactured
by radiofrequency magnetron sputtering, with the aid of a masking system inside the vacuum chamber. Testing at
cryogenic temperature proved their stability in the space environment conditions.
The thin-film technology allows the fabrication of optical filters which behaviour is not uniform along the surface of the
device. Such intentional non uniformity permits the selection of different wavelengths of the spectrum with a single
small-dimension optical component. Each wavelength is transmitted at a given position over the component surface and
this effect is obtained by depositing a graded coating on a glass substrate. This narrow-band transmission filter is based
on the combination of metal and dielectric thin films, in fact this choice allows a wide operating wavelength range with a
low number of layers. The optical filter coupled to an array detector gives a spectral sensor that is useful for the
fabrication of small-dimension and low-weight spectrometers.
The quality improvement of optical components for UV application demands increasing sensitivity of the instruments for optical losses measurement. In the optical region between 150 and 350 nm dedicated set-ups are normally needed to measure low-level scattering. Such instruments typically perform single-wavelength measurements, corresponding to that of the laser used. Another draw-back of such sophisticated tools is their elevated costs and the necessity of the staff specially prepared to handle with. Here we propose a novel instrument (patent pending) for measurement of spectral scattering from the high quality optical components in both UV and visible range. It permits spectral data acquisition being positioned as an attachment to a commercial spectrophotometer, and the wavelength range is limited mostly by the spectrophotometer characteristics. The advantage of low costs of a set-up constructed in the base of widely diffused commercial spectrophotometers is combined with the simplicity of its implementation. Moreover, the proposed instrument can be a base for the measurement of scattered light in a dedicated experimental set-up having the light source and the detectors different from those of a commercial spectrophotometer. Some examples of such ad-hoc set-ups are discussed here as well.
The improvement in production process of high quality optics needs routinary accurate characterization of the optical properties of the sample. Whereas some commercial instruments allow accurate measurements of the transmitted and reflected light, the measurement of the scattered light very often falls below the detection level when standard attachments, like the Ulbricht integrating sphere, are used. In order to characterize scattering levels below 10-3 dedicated set ups are usually used which are quite complicate to run and often do not fit the need of a production department. In this work we present the design and test of scatterometers based on the Coblentz sphere that fit inside a Perkin-Elmer λ-900 spectrophotometer. They are aimed to the measurement of light scattering in both the transmittion and the reflection hemisphere with a detection limit in the range of 50 ppm. The Coblentz sphere is a mirror like collector of the scattered light and does not suffer of the typical limitations of the Ulbricht sphere in terms of low collecting efficiency and spectral limitation in the UV edge. As an example, LaF3 films on silica samples were first characterized with both conventional photometric techniques (specular reflectance and direct transmittance) and ellipsometry and the film thicknesses and refractive indices, as well as inhomogeneity and interface layers, were determined. The scattering data were also independently analysed by considering the vector theory of scattering and the effect on the simulated spectral TS of several power spectrum densities obtained with different hypothesis on the correlation between interfaces.
This investigation deals with determination of optical parameters of thin PLZT films prepared by pulsed laser deposition on fused silica substrates at different oxygen pressure. Film composition and structure are investigated by WDX and XRD. Defects concentration in the films is studied using triboluminescence. Changes of film refractive index n((lambda) ), extinction k((lambda) ) with wavelength in the spectral region 0.3 - 1.1 micrometers and film thickness d are determined as a result of transmittance spectra processing. Waveguiding properties of the films are investigated.
Nitrogen-rich carbon nitride films were prepared by pulse laser deposition (PLD) combined with additional r.f. and hollow cathode discharges on fused silica, stainless steel and silicon substrates. The properties of combination of laser plasma with concentrated r.f. or HC discharges were studied using an optical emission spectroscopy (OES). The composition of the films was measured by WDX method. The transmission spectra and plastic microhardnes of the films were also measured.
The KGW thin films doped with Nd were prepared by pulsed laser deposition (PLD) on MgO, YAG and YAP substrates at substrate temperatures (Ts) varied from 400 degree(s)C to 800 degree(s)C in oxygen ambient atmosphere. The influence of the Ts and type of substrate on the film properties including the structure, luminescence and refractive index was studied. The best crystalline structure and strongest fluorescence signal was achieved at higher Ts on YAG substrate.
Thin CNx films were deposited by pulsed laser deposition (KrF excimer laser) with additional radio-frequency discharge of the nitrogen gas. Nitrogen pressure was in the range from 1 to 40 Pa and r.f. power was adjusted to 100 W. The substrate temperature was changed in the range from room temperature to 800 degree(s)C. An influence of the substrate temperature on the film composition and structure was studied. The composition of the films was measured by wavelength dispersive x-ray spectroscopy. The N/C ratio of films deposited at room temperature almost reach 1; however, it decreased with increasing substrate temperature. Structure was studied using Fourier transformed infrared spectroscopy analysis. Presence of CequalsC, CequalsN, CequalsVN, N-H and O-H groups was confirmed. Optical properties were analyzed using UV-VIS reflection spectroscopy.
Carbon nitride thin films are candidates for various technological applications. Special kind of a-Cnx film, as (beta) -C3N4, is supposed to be material exhibiting extreme hardness--higher than diamond. Many groups are trying to synthesize a-CNx films (with goal of creation (beta) -C3N4 phase), but concentration of nitrogen in films is still low. We have created series of nitrogenated amorphous carbon films in nitrogen atmosphere from graphite target by pulsed laser deposition. Additional DC and RF discharges were ignited in order to increase the reactivity of the nitrogen. Film properties were analyzed by X-ray diffraction and spectroscopic ellipsometry. The effects of the discharges on the C-N stoichiometry and on chemical bonding were studied. The N/C ratio increased with higher RF and DC plasma densities up to value of 0.25. Maximum values were reached at the nitrogen pressure twice lower for rf discharge than dc discharge assisted deposition. C-N stoichiometry and chemical bonding were investigated by Fourier transform infrared spectroscopy, X- ray Photoelectron Spectroscopy and Raman spectroscopy. Only films deposited at higher rf plasma power density showed the presence of triple bonded C equalsV N stretching mode.
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