Not Applicable.
Not Applicable.
1. Field of Invention
The present invention relates generally to optics and, more particularly, solid immersion lenses for focusing collimated light in the near-field region.
2. Description of the Background
In modern optical data storage systems, data is stored on an optical storage medium in the form of marks carried on a surface of the optical medium. The data may be accessed by focusing a laser beam onto the data surface of the optical medium and analyzing the light reflected by the marks. Storage density of the system may be increased by reducing the size of the beam (called the xe2x80x9cspotxe2x80x9d) focused on the data surface. In addition to optical data storage applications, reduction of spot size is beneficial for photolithography and microscopy applications as well. For example, in photolithography, smaller spot sizes allow for the exposure of finer features in photoresist.
The diffraction-limited spot diameter obtained from classical scalar diffraction theory is provided by:                               d          FWHM                =                  λ                      2            ⁢            NA                                              (        1        )            
where d is measured at the full width half maximum (FWHM), xcex is the wavelength of the light, and NA is the numerical aperture. The numerical aperture of a lens system, such as the lens system illustrated in FIG. 1, is an indication of the focusing power and may be approximated as:
NA≈nmedium2sin xcex8xe2x80x83xe2x80x83(2)
where the definition of the variables of equation 2 are provided with reference to FIG. 1. The numerical aperture of any lens system cannot exceed the value of the refractive index of the lens at the focal plane. Lenses are typically characterized by the value of the numerical aperture in air. For example, with reference to FIG. 1, if xcex8 is 30xc2x0, and because nmediun2≈1(air), then NAair=0.5, and the diffraction limited spot size dFWHM≈xcex. In optical data storage systems, as discussed hereinbefore, the size of a recorded bit, and hence the aerial density, is proportional to the spot size. From equation 1, it is evident that one way of reducing the diffraction limited spot size is to increase the numerical aperture.
One known lens system used in applications where reduced spot size is critical, such as optical data storage systems, involves using an objective lens 10 in conjunction with a solid immersion lens (SIL) 12, as illustrated in FIG. 2. Using the SIL 12 allows for the increase of the refractive index at the focal plane f of the objective lens 10. In FIG. 2, the surface 14 of the SIL 20 is hemispherical. Light from the objective lens 10 is incident normal to the upper surface 14 at all points, and no refraction at the upper surface 14 occurs. Therefore xcex8, which is determined by the objective lens 10, will be unchanged and the refractive index of the media at the focal plane f is increased. Instead, the numerical aperture of the system of FIG. 1 is:
NA=nSILsin xcex8=nSILNAair.xe2x80x83xe2x80x83(3)
It is apparent from equation 1 that by using the SIL 12, the diffraction limited spot size is reduced by a factor of nSIL. The optical spot may be evanescently coupled to an optical data storage medium with minor expansion provided that the medium is within the near-field region of the bottom surface 16 of the SIL 12, i.e., very close, typically within a fraction of a wavelength, or a few nanometers depending on the wavelength. The evanescent coupling effectively allows the small spot size to be xe2x80x9ccopiedxe2x80x9d across the gap from the bottom surface 16 of the SIL 12 to the media.
Another known type of lens system using an SIL 12, referred to as the xe2x80x9csuper SILxe2x80x9d or xe2x80x9cSSILxe2x80x9d, is shown in FIG. 3. For the lens system of FIG. 3, the surface 14 of the SSIL 12 is spherical. In addition, the focal plane f of the objective lens 10 is below the lower surface 16 of the SSIL 12. The SSIL 12 does some additional focusing of the light from the objective lens 10 and, when the incident angle of the light from the objective lens 10 on the SSIL 12 is 90xc2x0, xcex8xe2x80x2 is also 90xc2x0. Therefore, sin xcex8xe2x80x2=1, and the numerical aperture of the system is:
NA=nSIL.xe2x80x83xe2x80x83(4)
One restriction of the SSIL arrangement of FIG. 3 is that the numerical aperture of the objective lens 10 must be 1/nSIL for maximum performance.
Additionally, to improve the off-axis performance or other aberrations caused by a hemispherical SIL, the lens system of FIG. 3 may use an aspheric SIL. A lens system using an aspheric SIL in conjunction with an objective lens to improve off-axis performance, however, may sacrifice spot size.
In all three of these cases, however, the objective lens 10 is separated from the SIL 12 by a spacing. In most near-field applications, the dimensions of the spacings are critical, and consequently must be accurate to within a fraction of a wavelength. Otherwise, if the focal plane deviates slightly from the designed location, the performance of the lens system is severely degraded. In addition, where the objective lens 10 and the SIL 12 are mechanically aligned, their alignment may shift, thereby possibly destroying the precise alignment.
Accordingly, there exists a need in the prior art for a lens system which yield a reduced spot size yet does not require precise mechanical alignment of the objective lens and the SIL. There further exists a need for such a lens system to be adaptable to modem near-field applications, such as optical data storage, photolithography, and microscopy.
The present invention is directed to a lens for focusing collimated light. According to one embodiment, the lens includes a single, optically transmissive material having an aspherical focusing surface and a second surface, such that collimated light incident on the aspherical focusing surface is focused in a near-field region of the second surface.
According to another embodiment, the present invention is directed to a lens for focusing collimated light, including a first focusing portion having a first refractive index, wherein the first focusing portion includes a focusing surface and a second surface, and a second focusing portion having an aspherical focusing surface and a second surface, wherein the aspherical focusing surface of the second focusing portion is connected to the second surface of the first focusing portion, wherein the second focusing portion has a second refractive index which is not equal to the first refractive index, such that collimated light incident on the focusing surface of the first focusing portion is focused in a near-field region of the second surface of the second focusing portion.
According to another embodiment, the present invention is directed to a lens for focusing collimated light, including a first focusing portion having a first refractive index, wherein the first focusing surface includes a focusing surface and a second surface, a second focusing portion having first and second surfaces, wherein the first surface of the second focusing portion is connected to the second surface of the first focusing portion, wherein the second surface of the second focusing portion defines a cavity, and wherein the second focusing portion has a second refractive index which is not equal to the first refractive index, and a third optically transmissive portion disposed in the cavity defined by the second surface of the second focusing portion, wherein the third optically transmissive portion has a high refractive index relative to a wavelength of the collimated light.
The present invention provides an advantage over prior art lens systems for focusing collimated light in the near-field region in that it provides the focusing power of a solid immersion lens while obviating the need to employ a separate and distinct objective lens. Concomitantly, the present invention obviates the need to precisely orient the spacing between a separate and distinct objective lens and a solid immersion lens. In addition, the lenses of the present invention may be incorporated in, for example, optical data storage, photolithography, and microscopy systems, as well as in two-dimensional waveguide structures. These and other benefits of the present invention will be apparent from the detailed description of the invention hereinbelow.