TECHNICAL FIELD

The invention relates to the field of transmission optical filters, optical detectors and spectrometers comprising such filters and optical detectors.

More precisely the invention relates to highly transmitting optical filters and such filter arrays having a band-pass with low bandwidth, and comprising a nanostructured substrate and waveguides arranged on such structured substrates. The invention relates also to a miniature spectrometer configured for the market of consumer electronics. The invention encompasses also a method of fabrication of an optical transmission filter having a low bandwidth.

The invention also relates to a method of fabrication of the transmission optical filter and optical detector.

BACKGROUND OF THE ART

The use of optical filters, in particular transmission filters, is important in optical components and optical instruments and systems. For example optical filters combined are often combined with optical detectors, in vision systems such as for example telescopes or in spectroscopes, medical instruments or industrial control and inspection systems. It is indeed useful to have a material or a device that selectively transmits light only in a narrow range of frequencies within a broad spectral range.

Optical transmission filters may be realized in different ways. For example in the far IR (infrared) spectrum filters may be made from wire-mesh or metallic grids. IN the ultraviolet, visible and near-infrared conventional filters may be absorption filters which have in general a very broad transmission spectrum. In order to make transmission filters having a small spectral bandwidth filters may be made by realizing stacks of dielectric thin-films configured as interference filters. Such filters are made up of a great number of layer, typically more than 30, deposited one on the other by vapor deposition techniques for example. The problem with interference based filters is that the wavelength can only be varied by tilting the filter and/or varying the layer thicknesses, which is not only a complex and expensive process, but also makes it difficult to realize an array of different filters side by side within a small dimension.

Optical filters may also be made by thin nanostructured metal layers, in particular metal layers that have arrays of sub-wavelength apertures. Such filters are described in for example the publication by T. W. Ebessen et al. [Ref.1].

In the documents US2012129269 and US2010046077, a sensor based on nanostructured optic filter arrays is disclosed. It consists in a subwavelength array of metallic nanostructures (mainly Al) which transmits a portion of the light spectrum. This approach is interesting because only a variation of the pitch or the fill factor of the nanostructures in the array induces a change of the transmitted color. It enables a fast fabrication with e.g. embossing where the filters are generated all at once. However, the bandwidth is very large due to losses in the metallic structures.

In the document WO2012007147, a bandpass filter based on complementary metallic nanostructures is disclosed. Although this approach has a potential to reduce the bandwidth of plasmonic filters, it still remains large to be competitive of spectrometric or spectral imaging applications.

Mazulquim et al. in Ref. [2] report on a binary Al grating on an Al2O3waveguide which filters light in transmission. A variation of the grating pitch leads to a variation of the filter spectral position, which is sought. However, the grating is in direct contact with the waveguide, which leads to a broadening of the transmission band and a relatively high background transmission (20-30%). A related document U.S. Pat. No. 9,081,150 discloses a grating layer, a sublayer of thickness less than 200 nm. The structure disclosed in U.S. Pat. No. 9,081,150 is complex to fabricate as it requires a development and etching step into an Al film to generate the grating.

Wang et al. in Ref. [3] propose to add a low index MgF2cladding between the Al grating and the waveguide made of Si3N4. However, the structure is a membrane and is not supported by a substrate, which complicates even more the fabrication. In addition, the fact that the structure is suspended in air, or vacuum, largely increases the bandwidth. A related document CN106896436 presents similar limitations than the filter described in Wang et al.

SUMMARY OF THE INVENTION

It is an objective of the invention to provide an optical transmission filter solving the limitations of prior art optical transmission filters.

To the contrary of purely plasmonic filters, wherein light propagates at the surface of metallic structures, the optical filter of the invention comprises a dielectric waveguide that supports the resonance provided by a nanostructured metallic layer. This reduces considerably the loss of the optical filter and allows also providing an optical filter having a narrow bandwidth, such as for example lower than 20 nm.

Also, to the contrary of other devices of prior art, the optical filter comprises a low index cladding on the waveguide which enable to reduce considerably losses during the propagation of light in the waveguide of the optical filter. Without using a low index cladding it is not possible to reduce the bandwidth of an optical filter that is based on a nanostructured metallic layer.

The filter of the invention is also easy and cheap to fabricate to the contrary of optical transmission filters of prior art that use binary metallic structures which are complex to fabricate, as they need a complex etching step.

More precisely, the invention is achieved by an optical transmission filter, configured to transmit a portion of the spectrum of a light beam incident on said filter, comprising a substrate having a structured first substrate surface and a second substrate surface opposite to said first substrate surface, and comprising also a nanostructured metallic layer and a waveguide arranged between said substrate and said nanostructured metallic layer.

The first substrate surface is a patterned surface comprising a one- or two-dimensional array of nanostructures consisting of adjacent ridges and grooves. A high refractive index dielectric waveguide is arranged on said first substrate surface and has a patterned shape defined by said first substrate surface, said dielectric waveguide having a refractive index n1between 1.45 and 3.3.

A low index dielectric layer is arranged on top of said dielectric waveguide and has a patterned shape defined by said dielectric waveguide, said patterned shape defining adjacent low index dielectric ridges and grooves said dielectric layer having a refractive index n2between 1.15 and 1.7, n2being lower than n1.

An array of metallic nanostructures is arranged on at least a portion of said low index dielectric layer and having, at least partially, a patterned shape defined by said dielectric waveguide The advantage of using a structured metallic payer on top of a waveguide is that light incoupled by diffraction from the structured metallic layer is outcoupled after propagation over a certain length, typically 5 μm, out of the waveguide and interferes with the incident light so that a window of transmission is obtained at resonance.

In an embodiment of the invention said array of metallic nanostructures comprises an array of apertures each of said apertures facing one of the grooves of said low index dielectric layer. The advantage of using apertures in the structured metallic layer is to reduce the full width at half maximum (FWHM) of the transmission band(s) of the filter while keeping a high peak transmission level.

In an embodiment of the invention said array of metallic nanostructures has a non-uniform thickness and has a thickness of the metallic nanostructures on the ridges of said low index dielectric layer that is greater than its thickness on the grooves of said low index dielectric layer. Using a structured metallic layer having portions that have a partial light transmission allows to provide an optical filter that is cheaper to make and allows providing additional design flexibility of the filter.

In an embodiment of the invention said array comprises a plurality of N subarrays having different periodicities P1-PNin at least one dimension, N being preferably greater than 10. Using a plurality of subarrays allows providing a filter having a plurality of predetermined transmission spectral bands.

In an embodiment of the invention said metallic nanostructures only covers at least a portion of the ridges of said low index dielectric layer.

In an embodiment of the invention said metallic nanostructures only cover at least a portion said grooves. Covering only a portion of the ridges and/or grooves of the low index dielectric layers allows providing a wide range of design alternatives having different transmission properties. The transmission characteristics are also less sensitive to fabrication defects in the uncovered areas of the low index dielectric layer.

In an embodiment of the invention the thickness of said dielectric waveguide is between 20 nm and 150 nm, preferably between 30 nm and 100 nm.

In an embodiment of the invention the thickness of said low index dielectric layer is between 80 nm and 200 nm, preferably between 30 nm and 200 nm.

In an embodiment of the invention the period of said array of metallic nanostructures is between 20 nm and 500 nm.

In an embodiment of the invention said array of metallic nanostructures is a linear array of nanometer sized lamellas. The advantage of using lamellas for the metallic nanostructures is to provide a simple linear optical filter that is easy to manufacture. The transmission spectrum of the linear optical filter depends on the polarization of incident light. Measuring the polarization state of light may be useful for some applications.

In an embodiment of the invention said first substrate surface is a binary patterned surface. The binary structure enables a higher control of the area of the apertures during fabrication.

In an embodiment of the invention said first substrate surface is a sinusoidal patterned surface. The sinusoidal patterned surface has the advantage that it can originated with standard grating fabrication techniques such as laser interference lithography.

The invention concerns also an optical filter system comprising the optical transmission filter as described and comprises a detector array is fixed to said substrate Adapting a detector array such as a CMOS detector array allows proving a simple and compact optical filter systems that may be applied into a cheap spectrometer and/or provide a multispectral imager.

The invention relates also to a spectrometer comprising at least one optical transmission filter and/or at least one said optical transmission filter system said spectrometer having a spectral resolution lower than 30 nm, preferably lower than 20 nm, for incident light having a wavelength between 300 nm and 790 nm. A spectrometer based on the optical filter and filter system of the invention allows providing a compact, reliable and performant spectrometer at very low cost. The optical filter can be directly fabricated on a detector array without damaging its functionality.

The invention is also achieved by a method of fabrication of an optical transmission filter as described and comprises the following steps a-g:

a) providing a substrate having a contact surface;

b) providing a mold master comprising a nanostructured surface;

c) applying on said contact surface a layer of sol-gel;

d) realizing a nanoimprinted layer of said sol-gel layer by using light illumination of said layer of sol-gel;

e) depositing a high refractive index thin film coating on said nanoimprinted layer;

f) depositing a low refractive index thin film coating on said high refractive index thin film coating;

g) depositing, by oblique coating, on said low refractive index thin film coating a metallic coating.

The method of the invention allows providing a simple and cheap process flow that does not require expensive equipments. The method provides also an easy and precise reproducible process over large areas, such as wafer scale or on a roll-to-roll production line.

