GB2558574A

GB2558574A - High efficiency grating

Info

Publication number: GB2558574A
Application number: GB1700160.3A
Authority: GB
Inventors: Riis Erling; S Arnold Aidan; F Griffin Paul
Original assignee: University of Strathclyde
Current assignee: University of Strathclyde
Priority date: 2017-01-05
Filing date: 2017-01-05
Publication date: 2018-07-18
Also published as: WO2018127684A1; GB201700160D0

Abstract

An optical element comprising a plurality of optical features 60a-e, wherein each optical feature has a profile in 1, 2 or 3 dimensions. The optical element may be a diffraction grating, hologram or comprise concentric rings. The profile of the optical features may be: rounded, chamfered, pyramidal, beveled, sinusoidal, triangular, curved, stepped, saw-tooth and manufactured onto an optical element that is planar, curved or sheet like. The optical element may have a second side with optical features. The optical element may emit no zeroth order beam and at least one first order beam (70, Fig. 1C). The optical element may have a hole, aperture or window to let light or particles in or out and may be part of a differentially pumped vacuum system, magneto-optical trap, atomic clock or cooling lattice.

Description

(71) Applicant(s):

University of Strathclyde (Incorporated in the United Kingdom)

McCance Building, 16 Richmond Street, GLASGOW, G1 1XQ, United Kingdom (72) Inventor(s):

Erling Riis Aidan S Arnold Paul F Griffin (74) Agent and/or Address for Service:

Marks & Clerk LLP

Aurora, 120 Bothwell Street, GLASGOW, G2 7JS, United Kingdom (51) INT CL:

G02B 5/18 (2006.01) G21K 1/06 (2006.01) (56) Documents Cited:

US 5473471 A US 4552435 A

Nature Nanotechnology, Vol. 8, 7th April 2013, NSHII et al, A surface patterned chip as a strong source of ultra-cold atoms for quantum technologies, pp 321-324.

Journal of the Optical Society of America, Vol. 30, Issue 11,1st November 2013, LEE et al., Sub-Doppler cooling of neutral atoms in a grating magneto-optic trap, pp 2869-2874.

Optics Express, Vol. 23, Issue 7, 6th April 2016, MCGILLIGAN et al., Phase-space properties of magneto-optical traps utilising mirco-fabricated gratings, pp. 8948-8959.

Journal of the Optical Society of America, Vol. 33, Issue 6,1st June 2016, MCGILLIGAN et al.,

Diffraction grating characterisation for cold-atom experiments, pp. 1271-1277.

(58) Field of Search:

INT CL G02B, G21K

Other: WPI, EPODOC, INSPEC (54) Title of the Invention: High efficiency grating

Abstract Title: Optical element suitable for trapping or cooling atoms (57) An optical element comprising a plurality of optical features 60a-e, wherein each optical feature has a profile in 1,2 or 3 dimensions. The optical element may be a diffraction grating, hologram or comprise concentric rings. The profile of the optical features may be: rounded, chamfered, pyramidal, beveled, sinusoidal, triangular, curved, stepped, saw-tooth and manufactured onto an optical element that is planar, curved or sheet like. The optical element may have a second side with optical features. The optical element may emit no zeroth order beam and at least one first order beam (70, Fig. 1C). The optical element may have a hole, aperture or window to let light or particles in or out and may be part of a differentially pumped vacuum system, magneto-optical trap, atomic clock or cooling lattice.

60a 60b 60c 60d 60e

Figure 1D

At least one drawing originally filed was informal and the print reproduced here is taken from a later filed formal copy.

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Figure 1A

Figure 1B

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06 18

Figure 1C

Figure 1D

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06 18

Figure 2

4/5

105 ^a)|····· ····· ····· >···· <····

110 115

120

Figure 3A

Figure 3B Figure 3C

Figure 3D

06 18

Figure 3E

130

Figure 3F

5/5

06 18

250a

247

High Efficiency Grating

Field

Described herein is an optical element, which, optionally but not essentially, may be used to create diffraction beams from an incident beam which can be used to trap and/or cool atoms. The present invention also relates to associated methods of manufacturing and using an optical element.

Background

A magneto-optical trap (MOT) can be created by shining laser beams, e.g. three pairs of opposing laser beams along three perpendicular axes on a single point which is within a magnetic field. Another example of a MOT can also be created with four laser beams directed towards a centre point in a tetrahedral arrangement. These setups require careful alignment of multiple laser beams.

Instead of using multiple laser beams directed towards a centre point, it is possible to use a single laser beam incident on an optical element. The other required beams can be generated by diffraction. The incident and diffracted beams can spatially overlap, and the region of spatial overlap may be used to trap and/or cool atoms. Energy loss during the diffraction process can limit the temperatures to which atoms can be cooled. It is also generally important to maintain the volume of the region of spatial overlap of the beams.

Binary 2D gratings for use in cold-atom experiments are described in “Diffraction grating characterisation for cold-atom experiments by. J. P. McGilligan et al; Journal of the Optical Society of America B, Vol. 33, No. 6, p. 1271-1277.

It is at least one objective of one embodiment of the present invention to provide an improved optical element, such as an improved optical element for trapping and/or cooling atoms.

Summary

According to a first aspect there is provided an optical element, the optical element comprising a plurality of optical features, wherein each optical feature has a profile, and each profile comprises one or more heights, thicknesses or depths between a maximum height, thickness or depth and a minimum height, thickness or depth; and/or each profile defines or results in a range of optical path lengths for radiation incident on and/or scattered by and/or diffracted by the optical element.

52700552-1 -IROBERTSON

The optical element may be configured for use in an optical trap such as a magneto-optical trap. The optical element may be or may be comprised in a diffraction grating or holographic element.

The optical element may be configured to receive and/or interact with radiation in order to produce an optical effect such as diffraction or a holographic effect. Radiation may be electromagnetic radiation of any suitable wavelength, e.g. from ultraviolet to infrared, such as wavelengths less than or equal to 0.1cm or less than or equal to 1000 nm or between 0.1 cm and 10 nm. The optical element and/or the optical features may be configured to interact with electromagnetic radiation of any wavelength, in use. Similarly, light, optical radiation or radiation may be used interchangeably herein and may refer to electromagnetic radiation of any wavelength, and it not limited to visible light or electromagnetic radiation of any particular wavelength or wavelength range.

The profile of an optical feature may describe the surface of the optical feature along, in the direction of and/or parallel to the incident radiation. The profile of an optical feature may describe the shape of the optical feature along, in the direction of and/or parallel to the incident radiation.

The range of optical path lengths of the incident and/or scattered radiation may result in the incident and/or scattered radiation diffracting.

One or more or each of the optical features may be or comprise at least one of: rounded, chamfered, bevelled, radiused, sinusoidal, triangular, pyramidal, smooth, nonsquare, curved, stepped, sawtooth, discontinuous, non-binary, analogue and/or multilevelled. All of the optical features may be the same, or at least some of the optical features may be different to other optical features.

The optical element may have a base surface, and the surface may be at least one of flat, planar, curved and/or smooth. The optical element may be planar, curved, or in the form of a plate or sheet. The optical element may be macroscopically flat, planar, curved and/or smooth. The plurality of optical features may be microscopic or nanoscale. The surface of the optical element may be optically active, e.g. the surface may interact with radiation. The surface may comprise the optical features, which may be configured to interact with the radiation to produce an optical effect, such as diffraction or a holographic effect.

The optical features of the optical element may be on and/or protrude from the surface of the optical element. The optical features of the optical element may be

52700552-1 -IROBERTSON integral with the optical element, for example the optical element and the optical features may be made from a single piece of material and/or unitary.

The optical element may have at least one operational direction, e.g. at least one direction in and/or along which the optical element interacts with radiation. The at least one operational direction may be a direction in and/or along which the optical element interacts with the radiation, for example it may be the direction in which the optical element scatters and/or diffracts radiation. The operational direction may be perpendicular to one or more longitudinally extending optical feature. For example, the optical element may be a 1D diffraction grating, which may comprise at least one, e.g. a plurality of, longitudinally extending optical features, and the operational direction may be the direction perpendicular to the longitudinal direction of the longitudinal optical features. Radiation travelling along the at least one operational direction may interact with the optical element. At least a component of radiation incident on the optical element along the at least one operational direction may interact with the optical element.

The at least one operational direction may be the direction in and/or along which radiation is scattered and/or diffracted by the optical element and/or the optical features.

The at least one operational direction may be defined by at least one of the size, shape, configuration, layout, pattern, arrangement, periodicity and/or duty cycle of the optical features.

The optical element may be configured to scatter radiation. The optical element may be configured to diffract radiation, for example the optical element may be or comprise a diffraction grating or element. The optical element may be a 1D grating, such as a 1D diffraction grating. The optical element may be a 2D grating, such as a 2D diffraction grating.

The optical element may comprise more than one grating, such as more than one diffraction grating, such as more than one 1D grating. The more than one grating may be arranged rotationally symmetrically. The more than one grating may meet at a central point. One or more or each of the gratings may be angled or oriented differently to one or more or each other grating. At least one or more or each of the gratings may scatter or diffract radiation in a different direction to one or more or each other grating. At least one or more or each grating may scatter or diffract radiation towards one or more or each other grating, for example some or each of the gratings may each scatter

52700552-1 -IROBERTSON or diffract radiation towards a central point, which may be a point where the gratings meet.

The plurality of optical features may be configured to scatter radiation. The plurality of optical features may be arranged such that the optical features and/or the optical element diffract radiation. The plurality of optical features may be arranged in a regular or irregular pattern. The optical features may be arranged such that the optical element is holographic.

The optical features may be arranged such that the optical element is or comprises a plurality of ring shaped or circular optical features, wherein the ring shaped or circular optical features may be concentric and may increase or sequentially increase in width over a radial dimension from the outside towards the centre of the optical element, similarly to a binary Fresnel lens, which may be used for guiding and/or trapping chemical entities, e.g. in a conservative potential.

The optical element may be a hologram or holographic surface. A holographic optical element may be used to create an arbitrary optical lattice or arbitrary optical potentials.

The optical element comprising the plurality of ring shaped or circular features may be an expansion of a binary Fresnel lens into its energy efficient and aberrationfree continuous counterparts e.g. the rings of the Fresnel lens may be rounded or smoothed or otherwise adapted as the optical features of the optical element. The optical element comprising the plurality of ring shaped or circular optical features may be used to trap and/or cool atoms in a vacuum. The optical element may be a large numerical aperture lens. The optical element may optically tweeze atoms and/or dielectric particles, for example the optical element comprising the plurality of ring shaped or circular optical features may optically tweeze atoms and/or dielectric particles in a vacuum, gas or liquid e.g. by designing the radii of the ring shaped or circular optical features using the wavelength of the incident radiation, e.g. in a liquid (λ/η, where n is the refractive index of the liquid). At least one or each optical element or at least one surface thereof, e.g. an outer surface, may be sloped, slanted or oblique.

The profile of an optical feature may comprise the perpendicular distance from the surface of the optical element to the surface of the optical feature. The profile of an optical feature may be the cross-sectional profile of the optical feature and/or the optical element. The profile of an optical feature may be along at least one operational direction of the optical element and or along an operational direction of the optical

52700552-1 -IROBERTSON feature. The profile of an optical feature may be a planar profile through the optical feature, for example through the centre of the optical feature. The profile of an optical feature may describe the height, thickness or depth of that optical feature For example the height profile may describe the height of an optical feature along a cross-section of the optical feature. The thickness profile may describe the thickness of an optical feature along a cross-section of the optical feature. The depth profile may describe the depth of an optical feature along a cross-section of the optical feature.

Each profile may be at least one of linear, rounded, radiused, chamfered, bevelled, sinusoidal, triangular, pyramidal, smooth, non-square, curved, stepped, sawtooth, discontinuous, non-binary, analogue and/or multi-levelled.

The minimum height of each optical feature may be the points of the optical feature which lie on the base surface of the optical element. The minimum height points may be the points of the optical feature which do not extend perpendicularly or do not protrude from the surface of the optical element.

The minimum depth of each optical feature may be the points of the optical feature which lie closest to the base surface of the optical element. The minimum thickness of each optical feature may be the points of the optical feature which are thinnest, for example along the edges of the optical feature.

The maximum height of each optical feature may be the point or points of the optical feature furthest from the base surface of the optical element, such as an apex and/or plateau.

The maximum depth of each optical feature may be the point or points of the optical feature furthest from the base surface of the optical element.

The maximum thickness of each optical feature may be the thickest point or points of the optical feature.

The height, thickness or depth of at least one or more or each optical feature may be the distance between the minimum height, thickness or depth and the maximum height, thickness or depth of an optical feature along the direction perpendicular to the tangent of the optical element, e.g. perpendicular to the base surface of the optical element, which may be at the location of the optical feature. The height, thickness or depth of one or more or each optical feature, e.g. in a binary optical element, may be a fraction of the wavelength of the radiation, e.g. a quarter of the wavelength of radiation incident on the optical features and/or optical element. For example, in a rubidium-MOT operating at an incident radiation wavelength of 780 nm, the height of at least one of the optical features may be 195 nm. The height, thickness

52700552-1 -IROBERTSON or depth of one or more of each optical feature, e.g. in a non-binary optical element, may be a fraction of and/or less than the wavelength of the radiation incident on the optical features and/or optical element, e.g. a quarter of the wavelength of radiation incident on the optical features and/or optical element.

The optical element may be configured to be irradiated by one or more than one wavelength of light (e.g. simultaneously). Some of the optical features may be configured to be irradiated by one wavelength of radiation, while other optical features may be configured to be irradiated with at least one other wavelength of light. The optical element may be able to trap and/or cool one or more than one chemical entity, e.g. with the one or more than one wavelength of light.

The optical features and/or the profiles of the optical features may be such that incoming radiation that is incident on the optical element is not reflected and/or there is no zero order diffraction beam of the incident radiation. The optical features and/or the profiles of the optical features may be such that less than 20%, preferably less than 10%, further preferably less than 5% and most preferably less than 1% of the energy and/or light of the incident radiation is reflected and/or diffracted in the zero order diffraction beam. For example, it may be the size, shape, arrangement or other properties of the optical features, and/or the features of the profiles which prevent reflection and/or zero order diffraction of the incident radiation.

The optical features may be arranged in a regular pattern, such as a square, triangular or hexagonal pattern. The optical features may be arranged symmetrically, for example the optical features may be arranged such that the optical features have rotational and/or mirror symmetry. The optical features may be arranged periodically. The periodicity of the optical features may be 1 to \2 wavelengths of the incident radiation, which may ensure no diffraction orders beyond the first order are generated when the incident radiation is diffracted by the optical element. For periodic patterns, e.g. square patterns, of optical features, the orders of diffraction are solutions of:

dsin0 = Jn_x ²+n_y ² λ where d is the period of the optical features, λ is the wavelength of the radiation incident on the optical element, Θ is the angle between the incident radiation and the normal of the optical element, and n_x and n_y are the diffraction orders of the diffracted beams in perpendicular directions.

The optical element may comprise a hole. The hole may be less than 1 cm in diameter, for example of the order of 1 mm in diameter, preferably of the order of 0.1 mm. The hole may allow radiation, such as at least some of the incident radiation, to

52700552-1 -IROBERTSON pass through the optical element. The hole may allow radiation to pass but prevent matter from passing through the optical element, for example the hole may comprise a window of transparent material in the optical element.

The hole may allow matter, such as atomic clouds, to pass through the optical element. The optical element may comprise a component in a differentially-pumped vacuum system. The optical element may separate a high pressure side of a vacuum system from a low pressure side. Matter, such as atoms, may be loaded on the high pressure side of the optical element, pass through a hole in the optical element to a low pressure side of the optical element, where the matter may become cooled and/or trapped. The matter may be delivered or driven through the hole, e.g. by at least one of: gravity, magnetic, optical dipole and/or radiation pressure transfer. The hole may be a differential pumping aperture. One side, e.g. the low pressure side, of the optical element may be a more secure side than at least one other side of the optical element. Matter on the one side, e.g. the low pressure side, of the optical element may remain in the trap or in the region of spatial overlap of the beams longer than for the other side, for example it may exist in a single state for a longer time. Matter on the one side, e.g. the low pressure side, of the optical element may be trapped and/or cooled, such as trapped and/or cooled in a single point or location or within the trap or within the region of spatial overlap of the beams for a longer time than for the other side. The matter on the low pressure side of the optical element may be evaporatively cooled to temperatures above or below the quantum degeneracy regime.

The optical element may be opaque. The optical element may be made from, coated and/or covered by an opaque material. The optical element may be made from silicon, gallium arsenide, and/or any other material in which sub-micrometre/nanometre features can be made. The optical element may be metal coated, for example gold and/or aluminium coated.

The optical element may be at least partially transparent. The optical element may comprise transparent and/or non-transparent regions. The transparent regions may comprise a transparent material. The transparent regions may comprise or consist of holes or gaps in the optical element, which may be gas filled, such as air filled, or may be liquid filled, or may be a vacuum. The optical features may be separated, such as spatially separated, by transparent regions. The optical features may be nontransparent. The optical features and the regions between the optical features may both be transparent with different refractive indexes, and radiation incident on the optical element may propagate through the optical features and the regions between

52700552-1 -IROBERTSON the optical features with different path lengths, which may result in diffraction. The optical features may be within the optical element and/or below the surface of the optical element. The optical element may operate in transmission.

Radiation which is incident on the optical element may be incident on the optical element at an angle, such as an acute angle, or may be incident perpendicular to the optical element. Radiation which is incident on the optical element may be incident along an operational direction, such as at an acute angle in a plane which is perpendicular to the optical element.

Radiation which is incident on the optical element may be formed in a beam, which may be a beam of light, such as laser light. Incident radiation may be at least one of x-ray, ultra violet, visible, infrared, microwave and/or radio waves. Incident radiation may be of a narrow wavelength range, such as 100 nm, 50 nm, 20 m, 10 nm, 5 nm, 1 nm, such as a range in the order of pico, femto or atto metres, e.g. a fraction of a nanometre, for example a thousandth or a millionth of a nanometre, or of the order of a thousandth or a millionth of a nanometre.

Incident radiation may or may not be shaped or modified, for example by optics, such as lens, and/or opaque obstructions, such as beam blocks, before being incident on the optical element.

Incident radiation which is incident on the optical element may be scattered by the optical element and/or optical features. Radiation which is incident on the optical element may be diffracted by the optical element and/or optical features.

Incident radiation may be diffracted by the optical element into at least one diffracted beam, for example, 1, 2, 3, 4, or more diffracted beams, which may all be first order diffracted beams. All the diffracted beams may have the same energy and/or profile. Over 80% of the energy or light of the incident radiation, preferably over 90% of the energy or light of the incident radiation, further preferably over 95% of the energy or light of the incident radiation, and most preferably over 99% of the energy or light of the incident radiation may be diffracted, preferably in first order diffracted beams. The incident radiation may be diffracted into N beams. Each of the N beams may have 1/N the energy of the incident radiation. The diffracted energy and/or light of the incident radiation may be equally diffracted into each diffracted beam. Incident radiation which is not diffracted may be due to diffractive loss, which may result in energy lost as heat. The optical element and/or the optical features may be configured to minimise diffractive loss.

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The energy and/or light diffracted in each diffracted beam may be controlled by at least partially derating the optical element and/or at least some of the optical features. The optical element and/or the optical features may be at least partially derated by controlling the reflectivity of the optical element and/or at least some of the optical features, for example by at least partially coating the optical element and/or at least some of the optical features with a partially reflective material. The derating may comprise reducing the energy and/or brightness of the diffracted beams. For example, a gold-coated optical element will reflect more radiation, such as visible and infrared radiation, than an aluminium-coated optical element, and so the diffracted beams from a gold-coated optical element will be brighter and/or more powerful than diffracted beams from an aluminium-coated optical element.

The number of diffracted beams may be limited such that the spatial overlap of the incident radiation and the diffracted beams has a large volume, such that a large number of chemical entities may be trapped and/or cooled. Increasing the number of diffracted beams, such as first order diffracted beams, may reduce the volume of the spatial overlap of the incident radiation and the diffracted beams, which may be undesirable.

The optical element may be a 1D diffraction grating, and the incident radiation may be diffracted into positive and negative first order diffraction beams. The optical element may be a 2D diffraction grating, and the incident radiation may be diffracted into 4 first order diffraction beams, which may be positive and negative first order diffraction beams for each of the 2 dimensions of the 2D diffraction grating.

The incident radiation and/or the at least one diffracted beam may be polarised, e.g. circularly polarised. The polarisation of the incident radiation and/or the at least one diffracted beam may be arranged such that radiation travelling in opposing directions is oppositely polarised and/or opposed. The polarisation of the incident radiation and/or the at least one diffracted beam may be arranged such that chemical entities in the region of spatial overlap of the incident radiation and the at least one diffracted beam experience opposing forces due to the polarisation of the opposing radiation.

The spatial overlap of the incident radiation and the at least one diffracted beam may define an overlap region. The overlap region may comprise or at least partially define a light crystal or lattice, which may be created by the interference of the incident beam and the at least one diffracted beam.

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The properties of the optical crystal or lattice may be determined by at least one of the properties of the incident radiation, such as the wavelength or beam profile of the incident radiation, and/or the properties of the optical element, such as the configuration or arrangement of the optical features. The optical features may be configured or arranged such that the optical crystal or lattice is tailored or designed. For example, the optical crystal or lattice may be deep, versatile and/or aberration free. A non-periodic arrangement of optical features, such as optical features arranged such that the optical element is a hologram, may produce an optical crystal or lattice containing arbitrary optical dipole potentials.

The optical element may be optically active on one or more sides. The optically active sides of the optical element may each comprise optical features.

The optical element may be double or multiple sided. The optical element may comprise a first side and a second side, and both the first and second sides may be optically active. Both the first and second sides may comprise optical features. The first and second sides may be parallel. Matter and/or radiation may be able to pass from the first side to the second side and/or vice versa through a hole in the optical element.

According to a second aspect of the present invention is a method of trapping and/or cooling chemical entities using the optical element of the first aspect, comprising:

providing a beam of radiation incident on the optical element, the beam of incident radiation being diffracted into at least one diffracted beam, such that the incident beam and the at least one diffracted beam spatially overlap; wherein an optical lattice is created by the spatial overlap of the incident beam and the at least one diffracted beam; and the chemical entities are trapped and/or cooled by the optical lattice.

The method may comprise applying a magnetic field to the region of spatial overlap of the incident beam and at least one diffracted beam. The magnetic field, such as a magnetic quadrupole field, may be generated by a magnetic field generator, which may be or comprise anti-Helmholtz coils, for example. The external magnetic field may create a magneto-optical trap.

The incident beam of radiation may be laser light or narrow linewidth radiation, and may be perpendicularly incident on the optical element. The at least one diffracted beams may be all first order diffraction beams. The incident radiation may be diffracted into first order diffracted beams only.

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The chemical entities may become trapped and/or cooled in the optical potential of the optical lattice. The trapping and/or cooling of the chemical entities may be at least partially determined by the properties of the optical crystal or lattice, which may be determined by the arrangement and/or configuration of the optical features of the optical element. The trapping and/or cooling of the chemical entities may be at least partially determined by the properties of the magnetic field, which may be determined by the properties of the magnetic field generator. The chemical entities may be trapped in 2-dimensions and free to move in the third dimension, or the chemical entities may be trapped in 3-dimensions.

The optical features of the optical element may be configured such that the incident beam is not back-reflected and/or there is no zero-order diffraction beam.

Chemical entities which are trapped and/or cooled by the optical element may be at least one of molecules and/or atoms.

According to a third aspect of the present invention is a magneto-optical trap comprising the optical element of the first aspect, a light source and a magnetic field generator.

The magnetic field generator may be or comprise anti-Helmholtz coils.

The light source may be or comprise a laser or LED or a narrow linewidth radiation source.

According to a fourth aspect of the present invention is a measurement tool comprising the optical element of the first aspect or the magneto-optical trap of the third aspect.

The measurement tool may be a precision measurement tool, such as an atomic based precision measurement tool e.g. an atomic clock.

According to a fifth aspect of the present invention is a method of manufacturing the optical element of the first aspect.

The method of manufacture may comprise etching or abrading a material to form the optical features. The material may be selectively etched or abraded, for example a mask may be used to selectively protect portions of the material whilst unmasked portions are selectively etched or abraded. A selective pattern may be made in the mask by electron-beam lithography, photolithography and/or nanoimprint lithography.

The method of manufacture may comprise depositing the optical features on the surface of the optical element. The optical features may be selectively deposited on the surface of the optical element, for example a mask may be used to selectively

52700552-1 -IROBERTSON protect portions of the optical element whilst optical features are selectively deposited on unmasked portions of the optical element.

A patterned stamp may be used to selectively deposit, mould and/or shape mask material and/or optical feature material.

The method may comprise an etching step, such as an ion etch and/or wet chemical etch.

It should be understood that the individual features and/or combinations of features defined above in accordance with any aspect of the present invention or below in relation to any specific embodiment of the invention may be utilised, either separately and individually, alone or in combination with any other defined feature, in any other aspect or embodiment of the invention.

Furthermore, the present invention is intended to cover apparatus configured to perform any feature described herein in relation to a method and/or a method of using or producing, using or manufacturing any apparatus feature described herein.

Brief Description of the Drawings

At least one embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1A shows a prior art binary diffraction grating, and Figures 1B-D show non-binary diffraction gratings;

Figure 2 shows a schematic of the operating principle of an optical element;

Figures 3A-C show prior art binary diffraction gratings, and Figures 3D-F show non-binary diffraction gratings; and

Figures 4A-C show a schematic of the operating principle of optical elements .

Detailed Description of the Drawings

Throughout the following description, identical reference numerals will be used to identify identical parts.

Figure 1A shows a binary diffraction grating 5. The binary diffraction grating 5 is formed from silicon and comprises a substrate 6 that defines a base surface 7, wherein a plurality of optical features 10 protrude perpendicularly from the base surface 7. Although the exemplary grating is formed from silicon, it will be appreciated that other suitable etchable or otherwise processable substrate materials such as GaAs may be used. It will be appreciated that, in a side cross sectional profile, the binary diffraction grating 5 alternates between two heights, namely the height of the base surface 7 and

52700552-1 -IROBERTSON the height of the optical features 10. The plurality of optical features 10 have a generally square profile and are together configured to diffract 17 an incident beam 15 of radiation. The generally square profile may result in shadowing 20, which can reduce the efficiency of the diffraction process. Less than 100% of the energy of the incident light beam 15 is therefore diffracted. The energy of the trapped light 20 is lost due to the shape of the diffraction grating 5, for example as heat and/or diffuse scattered light. An incident beam diffracted by a 2D diffraction pattern may have up to 20% of the energy and/or radiation of the incident beam in each of the four first order diffraction beams, with the remaining energy lost as heat at the sharp edges of the troughs of the diffraction grating 5. The net imbalance in radiation pressure perpendicular to the grating (i.e. towards the grating) creates a limit to the minimum temperature which can be achieved by laser cooling using binary diffraction grating 5. This limit can be improved by modifying the beam intensity profile of the incident beam 15 of radiation.

Figure 1B shows a non-binary diffraction grating 55. The non-binary diffraction grating comprises a substrate 56 such as a silicon substrate, having a base surface 57 and a plurality of optical features 60 that are at least partially curved in a side cross sectional profile, e.g. being rounded or shaped similarly to the optical features 10 in the example of Figure 1A but with upper edges of the optical features 60 being radiused or curved. It will be appreciated that the optical features 60 have a non-binary profile. The optical features are configured to diffract the incident beam 65 of radiation. Less or no diffracted radiation 70 is trapped in the trough of the non-binary diffraction grating 55 compared to the binary grating 5 of Figure 1A and the radiation is instead diffracted from the diffraction grating 55. The proportion of energy of the incident beam 65 of radiation that is diffracted is much higher than for the arrangement of Figure 1A, with improved efficiencies compared to the arrangement of Figure 1A, which may approach 100% of the incident radiation potentially being diffracted. For a 1D diffraction grating, close to 50% of the incident energy and/or radiation may be diffracted in each of the two first order diffraction beams. For a 2D diffraction grating, close to 25% of the incident energy and/or light may be diffracted in each of the four first order diffraction beams. As there is little or no energy loss in the diffraction process, it is possible to achieve laser cooling with the non-binary diffraction grating 55 to a lower temperature than binary diffraction grating 5 without shaping the intensity of the incident beam 65 of radiation or compromising the volume of the spatial overlap region of the incident beam 65 of radiation and the diffracted radiation 70. The height profile 62 of one of the optical features 60 is highlighted. The height 63 of one of the optical features 60 is highlighted.

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Figure 1C shows a preferred non-binary diffraction grating 55. The non-binary diffraction grating comprises a substrate 56 such as a silicon substrate, having a base surface 57 and a plurality of sinusoidal optical features 60. It will be appreciated that the optical features 60 have a non-binary profile. The optical features are configured to diffract the incident beam 65 of radiation. The optical features 60 comprise no sharp corners at which diffracted radiation 70 can be trapped. The sinusoidal optical features 60 diffract the energy of the incident beam 65 more efficiently than the optical elements 5, 55 of Figures 1A and 1B. A similar level of efficiency of diffraction could be achieved with triangular optical features 60.

Figure 1D shows different examples of non-binary optical features 60 on a diffraction grating 55. The optical features 60 are chamfered 60a, partially rounded 60b, stepped 60c, fully rounded 60d and triangular 60e.

Figures 2 shows a beam 65 of radiation that is incident on a diffraction grating 55, wherein the incident beam 65 is diffracted into positive and negative first order diffraction beams 75a, 75b. The incident beam 65 and the outgoing diffraction beams 75a, 75b partially spatially overlap, creating an overlap region 80. The radiation of the incident beam 65 and outgoing diffracted beams 75a, 75b interfere to create a light lattice, which is an ordered spatial arrangement of maxima and minima of light intensity. Atoms may be trapped and/or cooled in the light lattice. The large volume of the overlap region 80 allows for a large number of atoms to be cooled, which can result in an increased sensitivity of any device comprising an optical element with diffraction grating 55. As only one incident beam 65 of radiation and only one diffraction grating 55 are needed to produce a large overlap region, diffraction grating 55 can be used to make optically compact and simple laser cooling systems.

Figures 3A-C show examples of binary diffraction gratings used to trap and/or cool atoms. The binary diffraction gratings consist of two different heights. The optical features have areas of maximum height, denoted black, and rest of the optical elements 105, 110, 115 are a planar level of minimum height denoted white. The difference in height of the minimum height and maximum height is a quarter wavelength of the wavelength of the incident radiation, which ensures there is no backreflected or zero order diffracted radiation. The diffraction patterns are based on separate circle optical feature pattern 105, separate square optical feature pattern 110 and adjacent square optical feature pattern 115. The areas of maximum height and minimum height both occupy 50% of the area of the optical elements 105, 110, 115.

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The unit cell of each pattern of the optical elements 105, 110, 115 are shown as squares in the lower left corner of the pattern.

Figure 3D shows a non-binary optical element 120 according to the present invention. The optical element 120 comprises optical features which have areas of maximum height, denoted black, and areas of minimum height, denoted white. The areas denoted grey comprise areas of different heights which lie between the maximum and minimum heights. The height H of the optical features of optical element 120 are of the form:

where h is approximately 0.77 χ λ, which is a fraction of the wavelength of radiation incident on the optical element, and h is the difference in height between the maximum height and the minimum height of the optical elements. The value of h is much less critical in the non-binary optical element 120 than in binary optical elements such as optical elements 105, 110, 125 shown in Figures 3A to 3C. The value of h is chosen to minimise the amount of back-reflected or zero order diffracted radiation.

Figure 3E shows a non-binary optical element 125 which comprises three different heights, denoted white, grey, and black. The height of non-binary optical element 125 does not change continuously, but changes step-wise. Non-binary optical element 125 comprises a hexagonal, rather than square, pattern.

Figure 3F shows a non-binary optical element 130 which comprises a plurality of concentric circular or ring like optical features 131 wherein the thickness of each ring increases sequentially from the outer ring to the inner ring. In this way, the non-binary optical element 130 is akin to a Fresnel lens. The non-binary optical element 130 has ring zones of alternating phase with radii of:

where f is the focal length of the lens. The non-binary optical element 130 can trap and/or cool atoms in a vacuum and optically tweeze dielectric particles in a liquid, provided the radii of the optical features 131 of the non-binary optical element 130 are designed using the wavelength of the incident radiation in the liquid (λ/η, where n is the refractive index of the liquid).

Inverse patterns, where the areas of maximum and minimum height are inversed, are equally effective at generating diffracted beams and creating areas of

52700552-1 -IROBERTSON spatial overlap in which chemical entities can be trapped and or cooled as the noninverse patterns.

Figure 4A shows an incident beam 205 of radiation incident on a 1D diffraction grating 210 being diffracted into positive and negative first order diffraction beams 215a, 215b. The incident 205 and diffracted 215a, 215b beams of radiation will spatially overlap and create a region in which atoms can be trapped and/or cooled. As the incident beam 205 of radiation and the diffracted beams 215a, 215b of radiation all lie in a plane, the atoms will only be trapped in the plane defined by the incident 205 and diffracted 215a, 215b beams. Trapped atoms will therefore be able to move parallel to the diffraction grating 210 and perpendicular to all three beams 205, 215a, 215b. A series of such optical elements 210 in a row will therefore be able to trap atoms in a 1D channel when used with a magnetic quadrupole field with an axis along the direction of the diffraction grating 210.

Figure 4B shows a beam 225 of radiation incident on a 2D diffraction grating 230 being diffracted into positive and negative first order diffraction beams 235a, 235b, 235c, 235d. The incident 225 and diffracted beams 235a, 235b, 235c, 235d will spatially overlap and create a region 240 in which atoms can be trapped and/or cooled. A single optical element 230 and a single incident beam 225 of radiation can be used to generate a large area of spatial overlap 240 for trapping and/or cooling atoms.

Figure 4C shows a beam 245 of radiation incident on an optical element 247 comprising three 1D diffraction gratings 250a, 250b, 250c which are arranged such that they meet at a centre point. Incident light 245 that is incident on the optical element 247 at a centre point where the diffraction patterns 250a, 250b, 250c meet will be diffracted by all three diffraction patterns 250a, 250b, 250c and create a spatial overlap region 260 where atoms can be trapped and/or cooled. The power of the diffracted beams is controlled by derating the diffraction patterns 250a, 250b, 250c by coating the diffraction patterns 250a, 250b 250c with aluminium. The reflectivity of aluminium is such that the reflectivity of each of the diffraction patterns 250a, 250b, 250c produce first order diffraction beams with approximately a third of the energy and/or light of the incident light 245. As the spatial overlap 260 is limited to the centre point where the three diffraction patterns 250a, 250b, 250c meet, the spatial overlap region 260 is small, which limits the number of atoms which can be trapped and cooled with the optical element 247. However, a particularly high degree of cooling may be achieved with this arrangement.

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Although various examples have been provided above, it will be appreciated that the present invention is not limited to these specific examples but is instead defined by the claims. For example, although various examples of non-binary optical element are described above, it will be appreciated that the invention is not limited to these and that other non-binary optical elements, e.g. having curved or rounded optical features may be used. In addition, although various dimensions and arrangements of optical features are described, it will be appreciated that these may differ depending on the wavelength of the incident radiation and/or the desired optical effect. Furthermore, whilst light, e.g. visible light, could be used as the radiation, it will be appreciated that radiation having other wavelengths could be used, e.g. infra-red, ultra violet, terahertz waves, and/or the like. In addition, although the optical elements are described as being constructed of silicon, it will be appreciated that other substrate materials may be used. Furthermore, although the described examples relate to optical features which protrude from an optical element and have a height, it will be appreciated that analogous or comparable optical features, for example optical features embedded within a transparent optical element, may be used which will have the same or comparable effect, and that such optical features may be more clearly described by a depth and/or thickness profile, such as an optical depth and/or thickness profile, rather than a height profile.

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Claims

CLAIMS:

1. An optical element for use in an optical trap, diffraction element or holographic element, the optical element comprising optical features, wherein each optical feature has a profile, and each profile comprises one or more heights, thicknesses or depths between a maximum height, thickness or depth and a minimum height, thickness or depth.

2. The optical element of claim 1, wherein the optical features are at least one of rounded, chamfered, bevelled, sinusoidal, triangular, pyramidal, smooth, non-square, curved, stepped, sawtooth, discontinuous, non-binary, analogue and/or multi-levelled.

3. The optical element of claims 1 or 2, wherein the optical element is planar, curved, and/or sheet or plate like

4. The optical element of any of claims 1 to 3, wherein optical features are arranged such that the optical element is a diffraction grating, hologram or comprises concentric rings.

5. The optical element of any of claims 1 to 4, wherein each profile is at least one of linear, rounded, chamfered, bevelled, sinusoidal, triangular, pyramidal, smooth, nonsquare, curved, radiused, stepped, sawtooth, discontinuous, non-binary, analogue and/or multi-levelled.

6. The optical element of any of claims 1 to 5, comprising a first side and a second side, wherein both the first and second sides comprise optical features.

7. The optical element of any of claims 1 to 6, further comprising a hole.

8. The optical element of any of claims 1 to 7, wherein the optical element comprises a component in a differentially-pumped vacuum system.

9. A method of trapping and/or cooling chemical entities, comprising:

a beam of radiation incident on the optical element of any of claims 1 to 8,

52700552-1 -IROBERTSON the beam of incident radiation being diffracted into at least one diffracted beam, such that the incident beam and the at least one diffracted beam spatially overlap; wherein an optical lattice is created by the spatial overlap of the incident beam and the at least one diffracted beam; and the chemical entities are trapped and/or cooled by the optical lattice.

10. The method of claim 9, further comprising applying a magnetic field to the region of spatial overlap of the incident beam and at least one diffracted beam.

11. The method of claims 9 or 10, wherein the at least one diffracted beams are all first order diffraction beams.

12. The method of any of claims 9 to 11, wherein the optical features are configured such that the incident beam is not back-reflected and/or there is no zero-order diffraction beam.

13. A magneto-optical trap comprising the optical element of any of claims 1 to 8, a light source and a magnetic field generator.

14. A measurement tool, such as an atomic clock, comprising the optical element of any of claims 1 to 8, or the magneto-optical trap of claim 13.

15. A method of manufacturing the optical element of any of claims 1 to 8.

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