A fundamental limit on the size of objects we are able to "see" is imposed by the wavelength of the light we are using. Light in the visible range (~500 nm) is "diffraction-limited" on length scales of about 1 micron, meaning that it is impossible to clearly resolve objects smaller than this using conventional optical techniques. This is unfortunate, because many interesting optical semiconductor processes occur on length scales 10-100 times smaller than this.
To circumvent this diffraction limit and obtain true nanometer-scale spatial resolution, a near-field scanning optical microscope (NSOM) scans a small 100 nm aperture positioned very close (a fraction of a wavelength) to the surface of interest. This aperture couples to the high spatial-frequency (evanescent) modes of light that decay exponentially from the surface and that cannot be seen with traditional optical methods.
In practice, the aperture (which is a tapered single-mode optical fiber tip) is scanned across the surface of interest using piezoelectric transducers while maintaining a small (~25 nm) tip-sample separation. This method allows one to simultaneously obtain topographic and light intensity images, as below.
We have developed a time-resolved low-temperature near-field scanning optical microscope to study spin phenomena in semiconductor heterostructures with high spatial (sub micron) and temporal (100 fs) resolution. Specifically, this technique has been applied to digital magnetic heterostructures (DMH) to study the effect of laterally patterned defects on spin evolution. Polarization analysis of collected luminescence yields complimentary images of luminescence intensity and polarization, providing information about both the spatial distribution of recombining carriers and their spin. By combining the above NSOM techniques with time-resloved techniques that involve the use of femtosecond pulsed lasers, we can achieve simultaneously high temporal and spatial resolution and directly obtain spatio-temporal carrier spin dynamics.
Near-Field Optical-Fiber Tips
Tips for the near-field microscope are made using single-mode optical fiber which is heated locally by a carbon-dioxide laser and then pulled, forming a tapered 50-100 nm aperture through which light may be collected and then analyzed. The entire tip (except for the aperture itself) is coated with a few angstroms of metal to reduce light losses.
The high spatial resolution of a near-field microscope is particularly useful in studying systems in which carriers are localized by a confinement potential in all three dimensions, as is the case in semiconductor quantum dots. Such confinement on nanometer length scales results in the quantization of energy levels and should lead to narrow luminescence lines emitted from individual quantum dots. Conventional optics is not satisfactory as it probes a rather large number of dots (~106) which, when combined with a non-uniform distribution of dot sizes, results in the observation of a broad photoluminescence peak. In the near-field, a small number of dots can be probed, allowing the direct observation of fine structure in the spectra. By collecting near-field spectra over a two-dimensional area above the heterostructure, spatial maps of the luminescence intensity can be obtained at specific energies. By doing so, a direct correlation between the spectral and spatial distribution of the quantum dots can be obtained and then used to identify both the position and energy levels of the quantum dots.
Low Temperature NSOM
We have designed a low temperature near field scanning optical microscope for measurements in variable magnetic fields. The microscope is constructed as a top-loading insert for a magneto optical cryostat and allows us to perform measurements in a wide range of temperatures (T=5-300 K) and magnetic fields (B=0-7 T).
In our design of the microscope, samples are mounted on a kinematically supported sample stage whose position can be roughly adjusted by a mechanical feedthrough. Two piezoelectric transducers perform fine scanning in the X-Y plane (12 µm at 5 K) and maintain constant separation between the sample and near-field tip via shear-force feedback.
