Tunable nanowire nonlinear optical probe

Martijn Tesselaar
Developments in nanotechnology have recently expanded to include the search for nanometer scale optical elements. This search has already yielded such building blocks as nanometer sized light-emitting diodes, lasers, photo detectors, and waveguides. A new step forward is the discovery of a nanometer sized, tunable coherent visible light source, described in a recent article in Nature by Nakayama et al. This light source consists of a tiny needle made from a nonlinear material that will start to emit light by means of a nonlinear process when irradiated by a laser beam.

The light source needle consists of Potassium Niobate (KNbO₃) that has a large second-order susceptibility (χ⁽²⁾). The material is prepared for this experiment in the form of nanowires by means of self-assembly, where a modified crystal growth process is stopped shortly after induction (when the first seed crystals appear) to obtain very small crystals. These naturally take the form of nanowires that ranged in width from 40 to 400 nm and in length from 1 to 20 mm, depending on when the growth was stopped. The nanowires were shown to be single-crystalline with an orthorhombic crystal lattice by means of x-ray powder diffraction measurements.

The nanowires have further been shown to exhibit nonlinearity by their response to high intensity laser light. When subjected to the light from a single laser, emitting femtosecond pulses, the nanowires start to emit light at twice the frequency-in this case, blue light is generated-in a process called second harmonic generation. When subjected to two laser beams with two different input frequencies, the nanowires generate light at the sum of the two frequencies, called sum frequency generation. The effect of these nonlinear processes are used to produce light at a frequency different from the frequency of the incident laser beam, making it easily distinguishable in subsequently demonstrated applications.

When a nanowire is trapped in an optical tweezers instrument, which traps small particles in a focused laser beam, the nanowire generates frequency-doubled light internally as a side effect. Since the nanowire also functions as a waveguide the generated light is emitted from the ends of the wire. A remarkable feature of the experiment is that the nanowire spontaneously orients itself to the optical axis of the trap/pump laser, thereby accomplishing alignment without the need for manual orientation of the crystal. Most optical tweezers use a laser beam with a wavelength of about 1000 nm because of the transparency of most biological specimens at this wavelength. Consequently, the light emitted from both ends of the nanowire by means of SHG with this setup is around 500 nm in wavelength.

One possible application of this trapped nanowire arrangement is in optical imaging systems, where it could be used to dramatically increase the image's spatial resolution. Since the nanowire's end is, in effect, a nanometer scale illumination aperture, it can be used to perform transmission microscopy with a very high resolution. To do this, a sample is scanned through the beam emanating from one end of the nanowire while a detector registers the amount of transmitted light. This 'nanowire scanning microscopy' can be used to image objects with sufficient resolution to identify nanometer-sized features.

Another application of the trapped nanowire arrangement is pinpoint excited fluorescence, which could be used in fluorescence microscopy. For this application the loose end of the trapped nanowire is brought in contact with a fluorescent dye, which causes the dye to emit radiation in turn by means of fluorescence. Again, because of the extremely small size of the light source, use of this technique should result in a considerable increase in resolution.

One remaining problem with the trapped nanowire arrangement is that the optical tweezers do not hold the nanowire absolutely still. The nanowire is presumed to move laterally because of thermal fluctuations of the optical potential by distances of about 10 nm, while displacement in the longitudinal direction is presumed to be even larger.