Daniel A. Fletcher

Near-field infrared imaging with a microfabricated solid immersion lens

We report imaging in the infrared with a microfabricated solid immersion lens. The integrated 15-μm-diameter lens and cantilever are fabricated from single-crystal silicon and scanned in contact with a sample to obtain an image. We demonstrate a focused spot size of λ/5 and an effective numerical aperture of 2.5 with λ=9.3 μm light. The total power transmitted through the lens is a factor of 103 greater than through a metal aperture giving the same spatial resolution. Two 1.0 μm holes in a metal film separated by 3.0 μm are imaged with the solid immersion lens in transmission and shown to be resolved.

Microfabricated silicon solid immersion lens

We present the microfabrication of a solid immersion lens from silicon for scanning near-field optical microscopy. The solid immersion lens (SIL) achieves spatial resolution better than the diffraction limit in air without the losses associated with tapered optical fibers. A 15-/spl mu/m-diameter SIL is formed by reflowing photoresist in acetone vapor and transferring the shape into single-crystal Si with reactive ion etching. The lens is integrated onto a cantilever for scanning, and a tip is fabricated opposite the lens to localize lens-sample contact. Using the Si SIL, we show that microfabricrated lenses have greater optical transparency and less aberration than conventional lenses by focusing a plane wave of 633-nm light to a spot close to a wavelength in diameter. Microlenses made from absorbing materials can be used when the lens thickness Is comparable to the penetration depth of the light. Tolerance to errors in curvature and thickness is improved in micromachined lenses, because spherical aberrations decrease with lens diameter. We demonstrate scanning near-field optical microscopy with the Si SIL and achieve spatial resolution below the diffraction limit in air by resolving 200-nm lines with 633-nm light.

Pulsed liquid microjet for microsurgery

The precision of soft tissue dissection with pulsed lasers in liquid media is typically limited by collateral damage from vapor bubbles created during energy deposition. We present an alternative technique for creating incisions using a pulsed liquid microjet driven by an electric discharge-induced vapor bubble generated inside a micronozzle. We use this technique to create a pulsed jet 30 μm in diameter with a peak velocity of 90 m/s and total ejected volume on the order of 100 pl. Incision tests on a polyacrylamide gel simulating soft tissue show that the width of the cut is comparable to the diameter of the micronozzle and that collateral damage is significantly less than that produced by a vapor bubble not confined by the nozzle.

Focusing in microlenses close to a wavelength in diameter.

Light focused from air into a spherical microlens is affected by diffraction at the lens surface as its diameter approaches the wavelength of light. Through an extension of Mie theory, we show that a converging wave that is incident upon a Si microlens with a diameter less than approximately 4lambda creates a spot as much as 25% smaller than predicted with vector diffraction theory. Si microlenses only a wavelength in diameter are shown to be virtually insensitive to variations in the maximum illumination angle, and changes in index of refraction are not found to cause the proportional changes in spot size that would be expected from vector diffraction theory.

Thermal microscopy with a microfabricated solid immersion lens

The spatial resolution of infrared thermometry is limited by diffraction to dimensions close to the wavelength of the collected infrared radiation, typically 5 μm at room temperatures. Thermal properties variations, temperature gradients, and defects with dimensions smaller than the diffraction limit are inaccessible to far-field infrared thermometry. This work demonstrates a near-field method for improving the spatial resolution of infrared thermometry based on a solid immersion lens (SIL). The SIL is microfabricated from silicon and integrated with a cantilever that is scanned over the sample surface. Infrared radiation collected by the SIL is measured in a conventional infrared microscope, and we show that the SIL improves the edge response of the thermal microscope by a factor of four. This imaging approach is able to resolve differences in the radiance from a uniformly heated, patterned structure with feature sizes below the diffraction limit in air.

Micromachined silicon nitride solid immersion lens

We present a fabrication method for silicon nitride solid immersion lenses (SILs) integrated with atomic force microscope (AFM) cantilevers. We demonstrate a scanning optical microscope based on the microfabricated SIL that operates in reflection and transmission modes at a wavelength of /spl lambda/ = 400 nm. In this microscope, light is focused to a spot in a high refractive index SIL held close to the sample. The minimum spot size of a SIL-based microscope, which determines the transverse optical resolution, is /spl lambda//(2n) where n is the refractive index of the SIL. This is smaller than the minimum spot size of /spl lambda//2 in air. The SIL, therefore, makes possible optical resolution better than the diffraction limit in air. The full-width at half-maximum (FWHM) spot size of the SIL-based microscope is measured to be /spl sim/133 nm in transmission mode, which is /spl sim/1.98 times better than the spot size measured without the SIL (264 nm). This improvement factor is close to the refractive index of the silicon nitride SIL (n = 1.96).

Refraction contrast imaging with a scanning microlens

We demonstrate subwavelength spatial resolution with a scanning microlens operating in collection mode with a large-area detector. Optical contrast is created by refraction of off-axis light rays at angles larger than the maximum collection angle. With a microfabricated silicon microlens 10 μm in diameter, we measure spatial resolution due to refraction contrast of λ/4.3 at a wavelength of λ=10.7 μm. A model based on ray tracing is developed to explain our result, and we show that lens diameter and index of refraction limit resolution for large emission and collection angles.

Microfabricated solid immersion lens with metal aperture

We demonstrate spatial resolution better than /spl lambda//10 in the infrared with a transmittance of 10/sup -3/ using a microfabricated solid immersion lens and metal aperture.

Pulsed liquid microjet for intravascular injection

Occlusions of the retinal veins and arteries are associated with common diseases such as hypertension and arteriosclerosis and usually cause severe and irreversible loss of vision. Treatments for these vascular diseases have been unsatisfactory to date in part because of the difficulty of delivering thrombolytic drugs locally within the eye. In this article we describe a pulsed liquid microjet for minimally invasive intra-vascular drug delivery. The microjet is driven by a vapor bubble following an explosive evaporation of saline, produced by a microsecond-long electric discharge in front of the 25 micrometers electrode inside the micronozzle. Expansion of the transient vapor bubble produces a water jet with a diameter equal to the diameter of the nozzle, and with a velocity and duration that are controlled by the pulse energy. We found that fluid could be injected through the wall of a 60-micrometers -diameter artery in choriallantoic membrane using a 15-micrometers diameter liquid jet traveling at more than 60 m/s. Histological analysis of these arteries showed that the width of the perforation is limited to the diameter of the micronozzle, and the penetration depth of the jet is controlled by the discharge energy. The pulsed liquid microjet offers a promising technique for precise and needle-free intravascular delivery of thrombolytic drugs for localized treatment of retinal vascular occlusions.

Micromachined silicon nitride solid immersion lenses

We present a method for fabricating silicon nitride solid immersion lenses (SIL) integrated with atomic force microscope cantilevers. We demonstrate that a 200 nm line/space transmission grating (400 nm period) may be resolved using the SIL with /spl lambda/=633 nm illumination. Based on the refractive index (/spl sim/1.9 for PECVD silicon nitride) and geometry of the SIL, the effective numerical aperture is calculated to be /spl sim/1.8.