Converging thermal lenses, and optical systems, kits, and methods for formation and use thereof

A converging thermal lens is transiently formed by directing a shaped pulsed light beam having at least a first wavelength to a thermo-optic material, whereby the thermo-optic material absorbs the light beam and experiences local heating in response thereto. The heating induces a refractive index profile in the thermo-optic material that temporarily forms the converging thermal lens. In some embodiments, the refractive index of the thermo-optic material has a negative temperature dependence, and the pulsed light beam is shaped to have an inverted light pattern with a maximum intensity in an outer region of the beam cross-section. Alternatively, in some embodiments, the refractive index of the thermo-optic material has a positive temperature dependence, and the pulsed light beam is shaped to have a radially-varying light pattern with a maximum intensity in a central region of the beam cross-section.

FIELD

The present disclosure relates generally to focusing of electromagnetic radiation, and more particularly, to formation and use of transient, converging thermal lenses for focusing.

BACKGROUND

In imaging applications, localization techniques and stimulated emission have been used to overcome the diffraction limit by exploiting chemical properties of fluorophores. However, in these methods, light is not physically focused beyond the diffraction limit. The discovery of metamaterials has enabled the design of super-lenses, which are capable of sub-diffraction imaging by enhancing the evanescent field that is otherwise be lost in typical configuration. In addition, microspheres placed in the vicinity of an object have been shown to provide sub-wavelength resolution by curving the incident and reflected rays. However, these modalities require precise placement of external optical elements in close proximity to sample, which can present challenges depending on the type of sample to be imaged.

SUMMARY

Embodiments of the disclosed subject matter provide focusing of light by using a thermal lens transiently formed in a thermo-optic material by absorption of a radially-varying light pattern. The absorption of the light pattern can result in localized heating that generates a semi-parabolic refractive index profile within the thermo-optic material, which refractive index profile has the effect of a converging lens. In some embodiments, the thermal lens can be directly formed within a sample being imaged. Alternatively, in some embodiments, the thermal lens is formed within a thermo-optic material placed before a sample to be imaged. In some embodiments, the transient thermal lens can be combined with an optical system (e.g., microscope) and used to achieve a physical focus that would otherwise be beyond the diffraction limit of the optical system. For example, an optical system employing the disclosed thermal lens can be used for super-resolution imaging by scanning the focused spot across the sample or thermo-optic material.

In a representative embodiment, an optical system comprises a light source, a beam-shaping optical assembly, a thermo-optic material, and a transient converging thermal lens. The light source can provide a focus-activation beam of light having at least a first wavelength. The beam-shaping optical assembly can comprise one or more optical components disposed in or along an optical path to a sample. The beam-shaping optical assembly can form the focus-activation beam from the first light source to have a radially-varying light pattern. The thermo-optic material can be disposed in the optical path before a target portion of a sample and can absorb light at the first wavelength. A temperature of the thermo-optic material can increase in response to the absorption. The transient converging thermal lens can be formed in the thermo-optic material by a heating-induced refractive index profile generated by the absorption of the focus-activation beam with the radially-varying light pattern.

In another representative embodiment, a method can comprise forming a transient converging thermal lens. The forming the thermal lens can comprise generating a first beam of light having at least a first wavelength, shaping the first beam to have a radially-varying light pattern, and directing the shaped first beam to a thermo-optic material. The thermo-optic material can absorb light at the first wavelength, and a temperature of the thermo-optic material can increase in response to said absorption. The thermal lens can be formed in the thermo-optic material by a heating-induced refractive index profile generated by the absorption of the shaped first beam with the radially-varying light pattern.

In some embodiments, a temperature dependence of refractive index of the thermo-optic material can be negative, and the radially-varying light pattern can be an inverted light pattern having a minimum or zero intensity within a central region and a maximum intensity within an outer region surrounding the central region. Alternatively, a temperature dependence of refractive index of the thermo-optic material can be positive, and the radially-varying light pattern can have a maximum intensity within a central region and a minimum or zero intensity within an outer region surrounding the central region.

In another representative embodiment, a kit for improving resolution of an optical microscope can comprise a focus-activation light source, a beam-shaping optical assembly, and a thermal lensing member. The optical microscope can have an objective lens and a probe beam light source. The focus-activation light source can be constructed to provide a beam of light having at least a first wavelength that is different from a wavelength emitted by the probe beam light source. The beam-shaping optical assembly can comprise one or more optical components to be disposed in or along an optical path to a sample. The beam-shaping optical assembly can be constructed to form the beam from the focus-activation light source to have a radially-varying light pattern. The thermal lensing member can be constructed to be disposed in the optical path before the sample. The thermal lensing member can also be being constructed to absorb light at the first wavelength such that a temperature of the thermal lensing member increases in response to said absorption. The thermal lensing member can absorb light at the first wavelength different than light at the wavelength emitted by the probe beam light source.

In some embodiments, a temperature dependence of refractive index of the thermal lensing member can be negative, and the radially-varying light pattern can be an inverted light pattern having a minimum or zero intensity within a central region and a maximum intensity within an outer region surrounding the central region. Alternatively, the temperature dependence of refractive index of the thermal lensing member can be positive, and the radially-varying light pattern can have a maximum intensity within a central region and a minimum or zero intensity within an outer region surrounding the central region.

Any of the various innovations of this disclosure can be used in combination or separately. This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. The foregoing and other objects, features, and advantages of the disclosed technology will become more apparent from the following detailed description, which proceeds with reference to the accompanying figures.

DETAILED DESCRIPTION

General Considerations

For purposes of this description, certain aspects, advantages, and novel features of the embodiments of this disclosure are described herein. The disclosed methods and systems should not be construed as being limiting in any way. Instead, the present disclosure is directed toward all novel and nonobvious features and aspects of the various disclosed embodiments, alone and in various combinations and sub-combinations with one another. The methods and systems are not limited to any specific aspect or feature or combination thereof, nor do the disclosed embodiments require that any one or more specific advantages be present, or problems be solved. The technologies from any embodiment or example can be combined with the technologies described in any one or more of the other embodiments or examples. In view of the many possible embodiments to which the principles of the disclosed technology may be applied, it should be recognized that the illustrated embodiments are exemplary only and should not be taken as limiting the scope of the disclosed technology.

The disclosure of numerical ranges should be understood as referring to each discrete point within the range, inclusive of endpoints, unless otherwise noted. Unless otherwise indicated, all numbers expressing quantities of components, molecular weights, percentages, temperatures, times, and so forth, as used in the specification or claims are to be understood as being modified by the term “about.” Accordingly, unless otherwise implicitly or explicitly indicated, or unless the context is properly understood by a person of ordinary skill in the art to have a more definitive construction, the numerical parameters set forth are approximations that may depend on the desired properties sought and/or limits of detection under standard test conditions/methods, as known to those of ordinary skill in the art. When directly and explicitly distinguishing embodiments from discussed prior art, the embodiment numbers are not approximates unless the word “about” is recited. Whenever “substantially,” “approximately,” “about,” or similar language is explicitly used in combination with a specific value, variations up to and including 10% of that value are intended, unless explicitly stated otherwise.

Directions and other relative references may be used to facilitate discussion of the drawings and principles herein, but are not intended to be limiting. For example, certain terms may be used such as “inner,” “outer,”, “upper,” “lower,” “top,” “bottom,” “interior,” “exterior,” “left,” right,” “front,” “back,” “rear,” and the like. Such terms are used, where applicable, to provide some clarity of description when dealing with relative relationships, particularly with respect to the illustrated embodiments. Such terms are not, however, intended to imply absolute relationships, positions, and/or orientations. For example, with respect to an object, an “upper” part can become a “lower” part simply by turning the object over. Nevertheless, it is still the same part and the object remains the same.

As used herein, “comprising” means “including,” and the singular forms “a” or “an” or “the” include plural references unless the context clearly dictates otherwise. The term “or” refers to a single element of stated alternative elements or a combination of two or more elements, unless the context clearly indicates otherwise.

Although there are alternatives for various components, parameters, operating conditions, etc. set forth herein, that does not mean that those alternatives are necessarily equivalent and/or perform equally well. Nor does it mean that the alternatives are listed in a preferred order, unless stated otherwise. Unless stated otherwise, any of the groups defined below can be substituted or unsubstituted.

Unless explained otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this disclosure belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present disclosure, suitable methods and materials are described below. The materials, methods, and examples are illustrative only and not intended to be limiting. Features of the presently disclosed subject matter will be apparent from the following detailed description and the appended claims.

Overview of Terms

The following explanations of specific terms and abbreviations are provided to facilitate the description of various aspects of the disclosed subject matter and to guide those of ordinary skill in the art in the practice of the disclosed subject matter.

Light or optical: Referring to the visible light portion of the electromagnetic spectrum as well as wavelengths bordering the visible light portion that are typically used for imaging, e.g., having wavelengths within a range of 300 nm to 15 μm.

Radially-varying pattern: A light pattern that varies across a width of the light beam, either in a continuous or discontinuous manner. In some embodiments, the radially-varying pattern may be substantially annular with a peak-intensity in an outer ring and a minimum or zero intensity in a central region.

Inverted light pattern: A light pattern that has a profile (e.g., location of peaks and troughs) that is opposite to a desired profile for the refractive index to be generated in the thermo-optic material.

Thermo-optic material: A material that changes refractive index in response to temperature. The relationship between temperature of the thermo-optic material may be positive (e.g., increased temperature generates an increase in refractive index) or negative (e.g., increased temperature generates a decrease in refractive index). While most materials are considered thermo-optic to some extent, embodiments of the disclosed subject matter align the light absorption characteristics of the material (e.g., light wavelength where absorption may be especially high) with an incident light pattern to achieve localized heating that forms a refractive index profile for focusing.

Introduction

In embodiments of the disclosed subject matter, a converging thermal lens can be transiently formed by directing a light pulse to a material. The light pulse is shaped to have a radially-varying intensity pattern. Absorption of the light pulse by the material generates localized heating therein, corresponding to the pattern of the incident light. Due to the material's thermo-optic coefficient, the radially-varying temperature profile resulting from the localized heating induces a radially-varying refractive index profile in the material that can act like a focusing lens, e.g., a converging thermal lens. Thermal diffusion within the material over time flattens the temperature profile, and thereby the refractive index profile, such that the focusing effect of the thermal lens is only available for a short period (e.g., on the order of milliseconds or less). The converging thermal lens is thus transient, although subsequent light pulses to the material can periodically reform the thermal lens.

In some embodiments, the transient converging thermal lens can be used in combination with an existing optical system, such as a microscope. The combination can be capable of focusing light to a spot size that is otherwise below the diffraction limit of the existing optical system. In some embodiments, one or more components for forming the transient thermal lens can be offered as a kit for modification of the existing optical system. The availability of sub-diffraction spot size offered by the disclosed thermal lens technique can be used to advantage in numerous applications, such as delivery of light to photoactive nanostructures, super-resolution imaging of a sample without requiring labels, and spectroscopic measurements at higher resolutions, as well as other applications.

Thermal Lens Formation

Referring initially toFIGS.1-2C, interaction of light with a material will initially be described. In particular, the material104is illuminated with a pulsed beam100of light, as shown inFIG.1. Pulse102of beam100may have a narrow intensity profile with respect to a width of the beam100, e.g., with respect to radial direction, r. As shown inFIG.2A, the intensity profile202can have a peak at a center of the beam100that decreases radially outward, for example, to form a substantially Gaussian curve. The pulse102can be at a wavelength that is absorbed by the material104, such that the pulse absorption causes localized heating in the material, which in turn generates a corresponding localized temperature rise in a pattern corresponding to the beam intensity profile202. For example, heating due to absorption of pulse102by material104can yield a maximum temperature increase at a location coinciding with the peak of beam intensity profile202. However, thermal diffusion of the heating from the localized regions to the surrounding material will cause the temperature to evolve toward equilibrium over time. Thus, the profiles204of temperature within the material104will progress from an initial profile204a, which may have a shape corresponding to the intensity profile202(e.g., substantially Gaussian) of the absorbed pulse, to a final profile204d, which may be substantially flat.

Due to the thermo-optic coefficient of the material104, the index of refraction of the material104will change in response to the temperature change. For many materials, the thermo-optic coefficient is negative, such that an increase in temperature leads to a decrease in refractive index, and vice versa. Accordingly, the initial temperature profile204agenerated in material104by the absorption of pulse102yields an initial refractive index profile206a. The profiles206of the refractive index will continue to evolve over time in correspondence with the evolution of the temperature profiles, for example, from initial profile206ato a final profile206d, which may be substantially flat. While the absorption of pulse102by material104may lead to formation of a transient refractive index profile, refractive index profiles206a-206chave a local minimum value at its center surrounded by higher refractive index values. Refractive index profiles206a-206cthus have the effect of defocusing or acting as a negative or diverging lens with respect to incident unabsorbed light, but are incapable of providing focusing.

Accordingly, to provide focusing (or acting as a converging lens) in some embodiments of the disclosed subject matter, the incident beam (also referred to herein as a focus activation beam) is shaped to have an inverted pattern with respect to a refractive index profile that provides focusing. For example, inFIGS.3A-3B, material104is illuminated with a pulsed beam300of light that has been shaped to have inverted light pattern306. For example, the inverted light pattern306can be a ring with an annular region308have peak intensity (or at least a higher intensity than central region310and the radially-outer region immediately surrounding region308) and a central region310surrounded by the annular region having a minimum or zero intensity (or at least a lower intensity than annular region308).

The shaping of the beam300to have the inverted light pattern can be performed by a beam shaping optical assembly, which can include any type of diffractive optical element, refractive optical element, and/or adaptive optics (AO), such as a spatial light modulator (SLM). Exemplary SLMs include, but are not limited to, deformable mirrors (DM), digital micromirror devices (DMDs), or a liquid crystal device, such as liquid crystal on silicon (LCoS) modulators. In some embodiments, the beam shaping optical assembly includes a phase plate, for example, a vortex phase plate. Other types of electro-optic devices or phase-changing elements to provide beam intensity shaping (whether in a transmissive configuration or a reflective configuration) are also possible according to one or more contemplated embodiments.

Absorption of the shaped light pulse302causes localized heating in material104that generates an initial temperature profile408a, which in turn degrades over time toward uniform temperature profile408ddue to thermal diffusion, as shown inFIG.4B. However, since the thermo-optic coefficient of the material is negative, the refractive index profiles414a-414cnow have local maxima at center416surrounded by lower refractive index values in region418. Refractive index profiles414a-414cthus have the effect of focusing with respect to incident unabsorbed light, in essence sculpting a transient converging lens with a diameter equal to the optical diffraction limit of beam300.

The focusing provided by the transient refractive index profiles414is the result of a thermal nonlinear effect that can be described mathematically by considering the temperature effect on the refractive index (e.g., the thermo-optic coefficient) given by:

n⁡(r,t)=n0+(d⁢nd⁢T)⁢T⁡(r,t)(1)
where T(r, t) is the temperature distribution in the material104. In some embodiments for operation on the millisecond time scale (e.g., due to operational limitations in pulse intensity and/or pulse width), the temperature distribution in the material104can be determined from the heat diffusion equation with a given set of initial and boundary conditions dictated by the absorbed light pattern306and the geometrical configuration. Alternatively, in some embodiments for operation on the microsecond time scale (e.g., with high power laser sources generating pulse), the setup may operate in the thermal confinement regime, where heat diffusion can be neglected and the temperature distribution will closely follow the light beam profile. Such a setup may be available using high-power (e.g., ˜1-10 mJ) laser sources that produce pulses of duration much shorter than the thermal relaxation time of material104(e.g., by an order of magnitude or more), for example, having a pulse width, tp, less than 500 ns. Once the refractive index profile is obtained, the focusing parameters can be calculated from the geometrical ray equation.
In some embodiments, the transient thermal lens can be used in combination with an existing optical system or assembly, for example, to improve the focusing performance thereof, as described in further detail below. In such embodiments, for a time-varying parabolic profile414aof refractive index, the focal length can be shown to follow a power-law scale F(t)˜t−1and approach the value of:

F⁡(∞)=k⁢π⁢n0⁢ω020.2⁢4⁢bPl·(dndT)(2)
where ω0is the beam waist of the physical lens of the optical system (which may be the diffraction limited radius), b is the absorption coefficient, l is the length of the absorbing material104, k is the thermal conductivity, and P is the laser power. Assuming the material104is thick enough to absorb all the light, the product of the absorption coefficient and the material length since can be eliminated since b˜l−1. Thus, the modulation of the refractive index can be understood as a transient microlens with radius ω0and focal length F(∞) sculpted within the focal region of the physical lens.

With respect to lateral (x-y) coordinates (e.g., parallel to the plane containing r inFIG.3A), the focal spot formed by the transient lens can be moved to different locations via scanning, using, for example, a scanning head or scanning principles similar to those currently employed in conventional microscopes. Alternatively or additionally, in some embodiments, the focal spot formed by the transient lens can be moved laterally by modulating the beam intensity profile306, and thereby the refractive index profile414of material104with the desired spatial distribution. For example, such modulation can be performed by the same optical elements responsible for beam shaping (e.g., SLM) or by separate optical elements. Such a configuration may allow simultaneous or sequential measurement of multiple points by the optical system without requiring lateral scanning, which can lead to faster multiplexed operation of the optical system as well as potentially wide-field operation.

With respect to axial or depth (z) scanning, the focal depth of the transient thermal lens can be adjusting the refractive index difference between the peak region416and the trough region418, for example, by adjusting the light intensity difference between the annular region308and the central region308of the inverted light pattern306. The principle behind axial location tuning is described with respect to the partial schematic500ofFIG.5, where a collimated beam502of radius ω0is transmitted through thermo-optic material504of thickness, l, that has a radially-decreasing refractive index centered at the optical axis. Let F represent the distance from material504to the point at which the focal point is obtained. Beam502propagating through the varying refractive index profile of material504will follow a trajectory given by:

1R=1n⁢(dndr)(3)
where r is the radial coordinate with respect to the optical axis. Assuming a parabolic refractive index variation of the form:

n=n0[1+δ(rω0)2](4)
and combining Eqns. 3 and 4, the radius of curvature can be obtained as:

R≈ω022⁢δ⁢r(5)
If it is assumed that l<<F, then, from geometrical considerations, the expression for the focal distance can be approximated as:

F≈ω02l·2⁢δ(7)
To estimate the value of δ, the heat profile can be obtained by:

n⁡(r,t)=n0+(dndT)⁢Δ⁢T⁡(r,t)(8)
and solving the heat diffusion equation for a singular heat source yields:

Δ⁢T⁡(r,t)≈0.0⁢6⁢b·Pπ⁢k·[ln(1+8⁢Dtω02)-16⁢Dtω02+8⁢Dt·r2ω02](9)
where D is the heat diffusion coefficient. By substituting the r dependent term of Eqn. 9 into Eqn. 8, and comparing the result to the expression of Eqn. 6, the value of δ can be extracted as:

δ=(dndt)⁢0.0⁢6⁢b·Pπ⁢⁢kn0·16⁢Dtω02+8⁢Dt(10)
By further combining Eqns. 7 and 10, a time-dependent expression for the focal length can be given as:

F⁡(t)=π⁢n0⁢k⁢ω02⁡(ω02+8⁢Dt)0.2⁢4⁢bPl⁡(dn/dT)·8⁢Dt(11)
Defining a constant time scale as

tc=ω024⁢D
and the focal length at steady-state as

F∞=k⁢π⁢n0⁢ω020.24⁢bPl⁡(dn/dT),
the expression in Eqn. 11 can be rearranged as:

F⁡(t)=F∞·[1+tc2⁢t](12)
where the t−1behavior is clearly evident. It should be noted that the above derivation applies to scenarios in which heat diffusion is significant. However, as also noted above, the use of high-power lasers to deliver pulses of sufficiently small duration (e.g., ≤500 ns) can enable operation within the thermal confinement regime, in which case thermal diffusion can be ignored and the temperature distribution will closely follow the light beam profile.

Moreover, the discussion above with respect toFIGS.3A-4Capplies to materials that have a temperature dependence of refractive index

(e.g.,dndT<0)
that is negative. For materials having a thermo-optic coefficient that is positive, an increase in temperature leads to an increase in refractive index, and vice versa. For such materials, the beam profile is altered to have a radially-varying profile that matches (rather than being inverted) the desired refractive index profile. For example, inFIGS.6A-6B, material604is illuminated with a pulsed beam600of light that has been shaped to have radially-varying light pattern606. For example, the radially-varying light pattern606can be substantially Gaussian, with a central region610having peak intensity (or at least a higher intensity than radially outer region608) and annular region610surrounding the central region having a minimum or zero intensity (or at least a lower intensity than central region610). As with the setup ofFIGS.3A-4C, the shaping can be performed by a beam shaping optical assembly, which can include any type of diffractive optical element, refractive optical element, and/or AO.

Absorption of the shaped light pulse602causes localized heating in material604that generates an initial temperature profile706a, which in turn degrades over time toward uniform temperature profile706ddue to thermal diffusion, as shown inFIG.7B. Since the thermo-optic coefficient of the material is positive, the refractive index profiles712a-712dhave a similar shape as the temperature profiles706a-706d, with local maxima at center714surrounded by lower refractive index values in region716. Refractive index profiles712a-712cthus have the effect of focusing with respect to incident unabsorbed light.

Methods for Formation and Use of Thermal Lenses

FIG.8illustrates an exemplary method800for forming a transient converging thermal lens and use thereof. The method800can begin at process block802, where light from a light source is directed to a material to transiently form the thermal lens therein based on thermo-optic effect. The light from the light source can have a wavelength that is readily absorbed by the material to cause heating thereof. In some embodiments, the wavelength of light is selected to be a wavelength preferentially absorbed by the material (e.g., a wavelength having the highest absorption as compared to other wavelengths of light). Alternatively, in some embodiments the wavelength of light is selected to be one where absorption by the material is substantial (e.g., greater than a minimum absorption for other wavelengths of light).

In some embodiments, the light from the light source is substantially monochromatic, for example, emitted from a laser or laser diode, or filtered from a polychromatic source. Alternatively, in some embodiments, the light from the light source is polychromatic and includes one or more wavelengths readily absorbed by the material. In process block802, the light from the light source may be in the form of a single pulse or a train of light pulses. Alternatively, in some embodiments, the light source may be configured to continuously emit light, and periodic illumination to allow thermal reset of the material can achieved by on-off control of the light source or actuation of a separate optical element in the beam path (e.g., a shutter).

The method800can proceed to process block804, where the light form the light source is shaped to have a radially-varying pattern prior to reaching the thermo-optic material. When the thermo-optic material has a temperature dependence of refractive index that is negative (as is the case with many materials), the radially-varying pattern can be an inverted pattern (e.g., having a minimum or void in a central region and a maximum in an annular region surrounding the central region). Alternatively, when the thermo-optic material has a temperature dependence of refractive index that is positive, the radially-varying pattern can have a maximum in the central region and a minimum or void in the annular region surrounding the central region. As noted above, the shaping of the beam can be accomplished using a beam-shaping optical assembly, which can include any type of diffractive element, refractive element, or AO, alone or in combination. In particular fabricated example, the beam shaping is performed by a vortex phase plate.

In some embodiments, the shaping of process block804can further include forming multiple patterns simultaneous in a one-dimensional (e.g., linear) or two-dimensions array, for example, to allow a corresponding array of thermal lenses to be simultaneously generated within the thermo-optic material (e.g., to allow multiplexed imaging). Alternatively or additionally, the shaping of process block804can include altering of beam propagation (e.g., beam steering) to be incident on a different part of the thermo-optic material, for example, to achieve scanning with respect to a sample.

In some embodiments, the generating of process block802and/or the shaping of process block804can further include altering an intensity of the light from the light source, for example, to change a focal length of the resulting thermal lens. For example, by increasing the peak intensity of the light beam, the difference between peak and trough of the thermal profile resulting from absorption of the light by the material can be increased. This, in turn, can increase the difference in refractive indices between the central and outer regions, thereby yield shorter focal lengths. Longer focal lengths can be obtained by decreasing the peak intensity of the light beam.

The method800can proceed to process block806, where the shaped light beam is directed onto the thermo-optic material to transiently form the converging thermal lens therein. In some embodiments, the light can be directly incident on the thermo-optic material (e.g., from beam-shaping optical assembly without an intervening functional optical assembly (as used here functional optical assembly excludes non-functional optical components, such as reflectors or dichroic mirrors that are designed to merely alter a beam path)). Alternatively, in some embodiments, the light can be incident on the thermo-optic material after passing through a functional optical assembly (e.g., through a focusing lens or microscope objective). In some embodiments, the directing of process block806can include altering the beam propagation to be incident on a different part of the thermo-optic material, for example, to achieve scanning with respect to a sample. For example, in some embodiments, the altering of the beam propagation can be performed using one or more pivoting mirrors within the optical beam path between the beam shaping device and the thermo-optic material.

The method800can proceed to decision block808, where it is determined if operation in the thermal confinement regime is possible. As noted above, thermal confinement refers to when heat diffusion can be neglected, such that the temperature distribution will closely follow the light beam profile, and thereby avoiding detailed calculations to determine refractive index profile based on diffusion. Generally, thermal confinement can occur when the pulse of light incident on the thermo-optic material has a pulse width much less than the thermal relaxation time of the material, which relaxation time can vary based on material composition, thermal conductivity, and other factors. For example, the pulse width for the light may be less than 500 ns in order to operate in the thermal confinement regime. However, with such short pulse lengths, high power can be used to obtain the necessary heating to generate the refractive index difference for focusing. Accordingly, thermal confinement may also require the use of high-power light sources (e.g., ≥1 mJ).

If the light source provides light of sufficiently high power and sufficiently short duration, the method800can proceed to process block810, where the thermal lens is used in the thermal confinement regime. In this configuration, the refractive index profile generated within the thermo-optic material closely follows the inversion of the beam intensity profile, which refractive index profile may persist for up to several microseconds, for example, 10 μs or less. Absorption of the light by the material initially generates an acoustic wave that causes fluctuation in the thermal profile, and thus the refractive index profile. Accordingly, use of the thermal lens can be delayed until after the acoustic effects subside, for example, at least 10 ns after the light pulse. Operation in the thermal confinement regime thus defines a first time window of between 10 ns and 10 μs after absorption where the refractive index profile in the thermo-optic material can be used as a converging thermal lens.

Alternatively, if the light source does not provide light of sufficiently high power or sufficiently short duration (or if operation at millisecond time scales is preferred, or for any other reason), the method800can proceed from decision block808to process block812, where the thermal lens is used in the thermal diffusion regime. In this configuration, the refractive index profile depends on the thermal diffusion in addition to the beam intensity profile, which refractive index profile may take on the order of milliseconds to evolve, for example, at least 1 ms. Thus, operation in the thermal diffusion regime defines a second time window of, for example, 1-10 ms after absorption where the refractive index profile in the thermo-optic material can be used as a converging thermal lens. Calculations or simulations may be used to determine the exact use timing and light characteristics (e.g., pulse width, intensity, profile shape) for a refractive index profile for the thermal lens, for example, taking into account the time evolution of the profile based on thermal diffusion.

In either process block810or process block812, the generated thermal lens can be used to provide focusing for imaging, illumination, detection, etc. For example, in some embodiments, the thermal lens can be used to enhance the focusing and/or resolution of an optical microscope (e.g., transmission, inverted, confocal, fluorescence, etc.) beyond its diffraction limit (e.g., super-resolution imaging). Alternatively, the thermal lens can be employed in any other optical application where a converging lens would be useful.

Although some of blocks802-812of method800have been described as being performed once, in some embodiments, multiple repetitions of a particular process block may be employed before proceeding to the next decision block or process block. In addition, although blocks802-812of method800have been separately illustrated and described, in some embodiments, process blocks may be combined and performed together (simultaneously or sequentially). Moreover, althoughFIG.8illustrates a particular order for blocks802-812, embodiments of the disclosed subject matter are not limited thereto. Indeed, in certain embodiments, the blocks may occur in a different order than illustrated or simultaneously with other blocks.

FIG.9depicts a generalized example of a suitable computing environment920in which the described innovations may be implemented, such as aspects of method800, controller1146, or function generator1438. The computing environment920is not intended to suggest any limitation as to scope of use or functionality, as the innovations may be implemented in diverse general-purpose or special-purpose computing systems. For example, the computing environment920can be any of a variety of computing devices (e.g., desktop computer, laptop computer, server computer, tablet computer, etc.). In some embodiments, the computing environment is an integral part of an optical imaging system. Alternatively, in some embodiments, the computing environment is a separate system connected to the optical imaging system, for example, by making operative electrical connections (e.g., wired or wireless) to the optical imaging system or components thereof.

With reference toFIG.9, the computing environment920includes one or more processing units930,935and memory940,945. InFIG.9, this basic configuration950is included within a dashed line. The processing units930,935execute computer-executable instructions. A processing unit can be a general-purpose central processing unit (CPU), processor in an application-specific integrated circuit (ASIC) or any other type of processor. In a multi-processing system, multiple processing units execute computer-executable instructions to increase processing power. For example,FIG.9shows a central processing unit930as well as a graphics processing unit or co-processing unit935. The tangible memory940,945may be volatile memory (e.g., registers, cache, RAM), non-volatile memory (e.g., ROM, EEPROM, flash memory, etc.), or some combination of the two, accessible by the processing unit(s). The memory940,945stores software925implementing one or more innovations described herein, in the form of computer-executable instructions suitable for execution by the processing unit(s).

A computing system may have additional features. For example, the computing environment920includes storage960, one or more input devices970, one or more output devices980, and one or more communication connections990. An interconnection mechanism (not shown) such as a bus, controller, or network interconnects the components of the computing environment920. Typically, operating system software (not shown) provides an operating environment for other software executing in the computing environment920, and coordinates activities of the components of the computing environment920.

The tangible storage960may be removable or non-removable, and includes magnetic disks, magnetic tapes or cassettes, CD-ROMs, DVDs, or any other medium which can be used to store information in a non-transitory way, and which can be accessed within the computing environment920. The storage960can store instructions for the software925implementing one or more innovations described herein.

The input device(s)970may be a touch input device such as a keyboard, mouse, pen, or trackball, a voice input device, a scanning device, or another device that provides input to the computing environment920. The output device(s)970may be a display, printer, speaker, CD-writer, or another device that provides output from computing environment920.

Furthermore, any of the software-based embodiments (comprising, for example, computer-executable instructions for causing a computer to perform any of the disclosed methods) can be uploaded, downloaded, or remotely accessed through a suitable communication means. Such suitable communication means include, for example, the Internet, the World Wide Web, an intranet, software applications, cable (including fiber optic cable), magnetic communications, electromagnetic communications (including RF, microwave, and infrared communications), electronic communications, or other such communication means. In any of the above described examples and embodiments, provision of a request (e.g., data request), indication (e.g., data signal), instruction (e.g., control signal), or any other communication between systems, components, devices, etc. can be by generation and transmission of an appropriate electrical signal by wired or wireless connections.

Optical Systems Employing Transient Thermal Lenses

FIG.10Aillustrates a setup1000, which can be adapted as a kit for modification of an existing optical system, such as a microscope. Setup1000can include a focus-activation light source1002(e.g., a polychromatic source or a substantially monochromatic source, such as a laser), a beam-shaping optical assembly1008, and a library1016of different thermal lensing members1012a-1012d. In some embodiments, the focus-activation light source1002can emit a pulsed light beam1004comprised of a train of individual light pulses1006. As with the previously described examples, light pulse1006is shaped by a beam-shaping optical assembly1008to have a radially-varying intensity profile (e.g., inverted profile or otherwise depending on the thermo-optic characteristics of the thermal lensing member), thereby forming a shaped output beam1010that is directed to the thermal lensing member1012a. Absorption of a pulse1006by a thermal lensing member1012acan generate heating that forms a temperature profile, which in turn induces a refractive index profile forming the transient thermal lens.

The thermal lensing member1012acan be a thermo-optic material selected or designed to maximize, or at least enhance, thermo-optic interaction with light beam1010to form the transient thermal lens. For example, the thermal lensing member1012acan be custom-engineered to strongly absorb at the wavelength(s) emitted by the light source1002, and/or the wavelength(s) of light emitted by the light source1002can be chosen to be one strongly absorbed by the thermal member1012a. In some embodiments, the absorption profile for the thermal lensing member1012acan have a strong peak at the wavelength of the focus-activation beam1010, such that the beam is fully absorbed within the thickness of the thermal lensing member1012aand the thermal effect is maximized. Moreover, the thermal lensing member1012acan be selected or designed to minimize, or at least reduce, absorption at another wavelength(s), e.g., so as to be substantially transparent for imaging or other use of the thermal lens. In some embodiments, the absorption profile for the thermal lensing member1012acan have minimal absorption at wavelengths of probe light or detection light in the optical system in which the setup1000is installed, such that the optical focusing effect can be achieved with minimal insertion loss. In some embodiments, the thermal lensing member1012acan be formed of a material or materials having a high temperature dependence of refractive index (e.g., thermo-optic coefficient) so as to minimize, or at least reduce, the energy required for focus-activation beam1010. In some embodiments, the thermal lensing member1012acan be formed of a material that has a low heat conductivity so as to minimize heat diffusion and thereby obtain time stable focusing patterns.

Since the wavelength at which the thermal lens is used may change depending upon the application (e.g., imaging with a probe beam only versus imaging of fluorescence excited by a probe beam), the library1016can include alternative thermal lensing members1012b-1012dwith different optical characteristics, such as different spectral absorption profiles and/or different non-absorption wavelengths (e.g., transparent windows). Accordingly, a user can select between the different lensing members1012a-1012dto customize the optical system for probing/imaging of a particular sample. Thermal lensing members1012a-1012dcan comprise glass, polymer, ceramic, a container with liquid solution, or any combination thereof. For example, in some embodiments, the thermal lensing member comprises a thin planar member (e.g., parallelepiped shaped, for example, like a coverslip) formed of glass or plastic with one or more additives therein or thereon to enhance absorption of light. For example, the additives can include absorbing particles (e.g., dye), plasmonic particles, or both. In some embodiments, the distribution of additive within the thermal lensing member can be substantially homogenous throughout or at least in areas in which beam1010may be incident thereon.

When configured as a kit, the components of setup1000(or setup1020described below) can be provided to a user for independent installation into the beam path of an existing optical system (e.g., microscope), for example, in a manner similar to that illustrated inFIGS.11A-11B. In some embodiments, the components of the kit can be individually provided for respective installation within the optical system, e.g., with focus activation light source1002separate from beam-shaping optical assembly1008and library1016. Alternatively, in some embodiments, the focus activation light source1002and the beam-shaping optical assembly1008can be provided in a common housing for simultaneously installation within the optical system. Instead of configuring as a kit, in some embodiments, setup1000or setup1020can be integrated directly into the manufacture of a new optical system.

In some embodiments, the separate thermal lensing member can omitted. For example,FIG.10Billustrates an alternative setup1020that is substantially similar to the setup1000ofFIG.10A, but eliminates the separate thermal lensing member1012ain favor of forming the thermal lens in a portion1022aof a sample1022. The sample portion1022acan be disposed along an optical path prior to a target portion1022b, which corresponds to a desired location of the focal spot formed by the thermal lens. In such setup1020, the wavelength of the light from focus-activation light source is selected for increased or maximum absorption by the sample. The configuration of setup1020advantageously allows focusing to be achieved without a separate component near the sample; however, it may also complicate thermal lens formation since the light absorption and/or thermo-optic properties of the sample may be less than ideal and may not otherwise be capable of engineering or customization.

Moreover, the use of thermal lensing member1012aof setup1000has the added benefit of isolating the sample from heating, whereas the setup1020would generate heating within the sample1022. Such heating may be problematical for samples, such as live cells or other biological samples, which are easily damaged by elevated temperatures. A common metric to assess tissue damage is the cumulative equivalent minutes at 43° (CEM43), which for temperatures higher than 43° is given by:
CEM43=Δt·0.543-T(13)
where Δt is the exposure time and T is the temperature reached by the sample. Damage thresholds vary between tissue types and species. In general, however, CEM43values between 0-20 minutes indicate minor and/or reversible effects. Setting CEM43=1 and considering a short exposure time that can be provided by a pulsed laser from nanosecond to femtosecond yields, for example, for a pulse of duration Δt=100 fsec, a temperature of up to 86° C. (359 K) is still under the threshold for damage.

Referring toFIGS.11A-11B, an exemplary installation of a thermal-lens-forming optical assembly into an existing microscope system1102is illustrated. In particular,FIG.11Aillustrates the original microscope setup1100, andFIG.11Billustrates the modified optical setup1120including thermal-lens-forming optical assembly1122. In the original microscope setup1100, a probe light source1108generates light1110that is directed via beam splitter or dichroic1106along optical path1118to a focusing lens or objective1104. Objective1104focuses the light1110to a diffraction limited focal spot1116within sample1114. The objective1104can also collect light emitted from the focal spot1116and direct it along optical path1118to an eyepiece or detector1112via dichroic1106.

To achieve tighter focusing of the probe beam (or to collect light from a narrower focal spot), the microscope setup1100is modified to insert thermal-lens-forming optical assembly1122within the optical path1118, as shown inFIG.11B. The thermal-lens-forming optical assembly1112has a focus-activation light source1128, a beam-shaping optical assembly1124, and a second beam splitter or dichroic1126. The focus-activation light source1128generates light1130that is shaped by a beam-shaping optical assembly1124to have the radially-varying light pattern, and the shaped beam1132is directed via beam splitter or dichroic1126along optical path1118to the objective1104. The light1130emitted by the focus-activation light source1128can be at a wavelength (or if multiple wavelengths, have at least one wavelength) that is different from wavelength(s) in the light1110emitted by the probe light source1108.

Objective1104also focuses the shaped beam1132, but the beam1132can be absorbed by a thermal lensing member1134, which is disposed in the optical path between the objective1104and the sample1114, before it reaches the sample. The absorption of the shaped beam1132(e.g., having a radially-varying light pattern1140) by thermal lensing member1134generates a temperature profile1142therein, which in turn induces a refractive index profile1144therein based on the thermo-optic properties of the thermal lensing member1134. Such refractive index profile1144forms the transient thermal lens that can be used to modify optical performance of the microscope. For example, when the probe beam1110passes through the thermal lensing member1134, it is focused to focal point1138, which is tighter than focal point1116previously achievable by the unmodified microscope system1102. Alternatively, in some embodiments, the thermal lensing member1134is eliminated such that the transient thermal lens is formed directly within the sample1114itself.

In some embodiments, a controller1146can optionally be provided, for example, to control operation of components of the microscope system1102and/or thermal-lens-forming optical assembly1122. For example, the controller1146can coordinate detection by detector1112to correspond with the time when the thermal lens is formed and effective (e.g., on the order of microseconds or milliseconds after pulse absorption, depending on the applicable operation window). Alternatively or additionally, controller1146can control operation of components of assembly1122(e.g., focus activation light source1128or beam-shaping optical assembly1124) to alter thermal lens formation, for example, to change focal spot location. For example, the controller1146can control the light source1128to alter emitted power in order to change focal depth within the sample1114and/or control the beam-shaping optical assembly1124to move location of the thermal lens within the thermal lensing member1134by shaping the beam. In some embodiments, the controller1146can communicate with a user via an input/output interface (e.g., GUI) to allow control of microscope, for example, to change location of imaging or probing within sample1114.

In the context of the microscope setup1120ofFIG.11B, the modulation of the refractive index1144can be understood as a transient microlens with radius ω0and focal length F(∞) sculpted within the focal region of the physical lens1104. The resulting spot size1138, ω(∞), of such system1120is dictated by the Abbe diffraction limit of the transient microlens, which is smaller than the diffraction limit of the physical optical system and is given by:

ω⁡(∞)≈λ⁢F⁡(∞)2⁢ω0(14)
The final spot size ω(∞) depends on the specific material and system parameters. For example, if a power 0.1 W is used, the expected improvement of the focal size can be about ω(∞)≈0.4ω0.

In order to go beyond the focusing limits of currently available high-index immersion objectives, the heat profile1142radius needs to be scaled down to ω0<1 μm, which can pose two technical challenges. First, light absorption would need to occur within the Rayleigh range of the focused vortex (<2 μm). Hence, a combination of wavelength of the focus-activation light1130and material of the thermal lensing member1134that results in high light absorption may be used. Second, when such a small spatial scale is considered, the stability of the heat profile can become difficult to maintain because of heat diffusion. However, pulsed laser sources of high intensity, and potentially samples with low heat conductivity, can counter these issues by operating in the regime of negligible heat diffusion (e.g., thermal confinement). In some embodiments, it is also possible to use shorter wavelengths for the focus-activation light1130and longer wavelengths for the probe beam1110, which may allow more efficient heat generation using, for example, plasmonic nanostructures. However, such shorter wavelengths may be more susceptible to scattering. Moreover, the efficiency and availability of materials that can operate using higher wavelengths as probes for photochemical effects or fluorescence microscopy may be limited.

Fabricated Examples and Experimental Results

A setup similar to that illustrated inFIG.11Bwas fabricated and used to investigate optical properties of the system. When the probe beam1110passes through the sculpted microlens (either collimated or focused), it will converge to a tighter focal point1138than what is allowed by the optical system1102. For example,FIG.12Ashows the diffraction limited focal spot size obtained by the optical system1102alone, andFIG.12Bshows the focal spot size obtained by the modified optical system employing the transient thermal lens. The smaller focal point ofFIG.12Bcan be used as a scanning probe for imaging at superior resolution without any labeling process or material insertion, providing high flexibility of the optical configuration.

The setup was further used to image a USAF resolution chart placed behind the thermal lensing member (e.g., an absorbing chamber). The chart was placed at a plane where the thermally-induced focal point was obtained. A two-dimensional motorized scanning stage was used for lateral scanning, and a SCMOS camera was placed behind the object to collect the transmitted light. The intensity of the final image for each pixel was determined by the integration over a selected area of the camera. The image obtained without use of the transient thermal lens (FIG.13Aand curve1302inFIG.13C) resulted in a lower spatial resolution than the one obtained by utilizing the tighter focal point achieved through the thermal effect (FIG.13Band curve1304inFIG.13C).

FIG.14illustrates a fabricated experimental setup1400employing the transient thermal lens. A monochromatic beam1404of wavelength 1064 nm from laser1402was expanded by a set of lenses (lens1410having a focal length of 50 mm, and lens1412having a focal length of 150 mm). A beam shutter1406was disposed along the beam path from laser1402and was controlled by a function generator1438to determine the duration of an exposure. The beam1404was directed via mirrors1408,1414through a vortex phase plate1440to produce shaped beam1442, which was subsequently directed via dichroic1416to spherical lens1420, having focal length of 200 mm, and focused by lens1420to produce a ring-shaped light pattern. At the same time, a probe beam1428of wavelength 780 nm from laser1426was directed via mirrors1430along the optical beam path to thermal lensing member1422. For convenience, the probe beam1428was directed through the same limiting aperture1418as a plane wave; however, it is also possible to focus it to the exact location of the ring-shaped focus-activation beam. The probe beam1428was introduced using dichroic mirror1416, and lenses1432(having focal length of 100 mm) and spherical lens1420as a telescope.

The thermal lensing member1422was designed to strongly absorb at 1064 nm but negligibly absorb at 780 nm. The thermal lensing member1422was produced by mixing 1% of NIR absorbing dye in water and placing it in a thin glass chamber. The heat profile generated by the absorption of the shaped light beam1442resulted in a tight focus of just after the absorbing material in a plane indicated by dashed line1424. The transient focusing plane was imaged using a 4-f imaging system (not shown) and a camera1436triggered by the function generator1438. A bandpass filter1434was used to block any residual light from laser1402not absorbed by the thermal lensing member1422. For imaging applications, a sample of interest was placed just behind the thermal lensing member1422at the plane indicated by dashed line1424, e.g., where the focal point is formed and scanned.

FIG.15Ashows the experimental focus spot size at three different distances from the vortex location after the thermal lensing member1422absorbed a total energy of 1 mJ. The confinement of the focal point is formed as the beam propagates away from the heated region. Accordingly, the signal to background (SBR) value increases as the tighter focal point is formed, as shown inFIG.15B.FIG.15Cshows the focal spot size as a function of increasing energy absorption of the focus-activation beam1442(and thus heating). InFIG.15C, the spot size has been normalized to the diffraction limit of the optical system (dashed line). The experimental data initially follow the expected power law of ˜t−1as shown by the fit (red line), but slightly deviates from the expected value at ω(∞). This deviation arises from assumptions made to enable the analytical solution of the heat equation and obtain the closed-form expression in the equations above, such as an infinite medium model and a parabolic heat profile.

To estimate the heat profile and the corresponding refractive index map in the experimental setup ofFIG.14, a heat diffusion simulation of the process was performed using COMSOL Multiphysics software. The exterior boundaries of the structure were kept at room temperature (T=293.15 K). Material properties, such as thermal conductivity and density, were taken from the built-in parameters of the software, under the assumption that the thermal lensing member had properties similar to water. The ring profile thickness was set to 30 μm since the absorptivity of the material is ˜30 L/g·cm. Thus, after 30 μm the intensity drops to e−1of the initial intensity. The total power absorbed by the ring structure was set to 0.075 W, in agreement with the value of 0.1 W used in the experiments and considering the relative absorption in the 30 μm scale. From the heat distribution, the refractive index map can be calculated using pre-measured values of water refractive index at different temperatures. The heat profile across the heated region and the refractive index map are shown inFIGS.16A-16B, respectively. The threshold damage temperature of 86° C. for a biological sample is reached after absorption of more than 0.3 mJ; however, as such levels, a nearly optimal focus can still be achieved.

Conclusion

Although some of the embodiments described above refer to “imaging,” the production of an actual image is not strictly necessary. Indeed, the mentions of “imaging” are intended to include the acquisition of data where an image may not be produced. Accordingly, the use of the term “imaging” herein should not be understood as limiting.

Although particular optical components and configuration have been illustrated in the figures and discussed in detail herein, embodiments of the disclosed subject matter are not limited thereto. Indeed, one of ordinary skill in the art will readily appreciate that different optical components or configurations can be selected and/or optical components added to provide the same effect. In practical implementations, embodiments may include additional optical components or other variations beyond those illustrated, for example, additional reflecting elements to manipulate the beam path to fit a particular microscope geometry. Accordingly, embodiments of the disclosed subject matter are not limited to the particular optical configurations specifically illustrated and described herein.

Any of the features illustrated or described with respect toFIGS.1-16Bcan be combined with any other features illustrated or described with respect toFIGS.1-16Bto provide systems, methods, devices, and embodiments not otherwise illustrated or specifically described herein. All features described herein are independent of one another and, except where structurally impossible, can be used in combination with any other feature described herein.

In view of the many possible embodiments to which the principles of the disclosed technology may be applied, it should be recognized that the illustrated embodiments are only preferred examples and should not be taken as limiting the scope of the disclosed technology. Rather, the scope is defined by the following claims. We therefore claim all that comes within the scope and spirit of these claims.