Characterization of power induced heating and damage in fiber optic probes for near-field scanning optical microscopy

Abstract

Tip-induced sample heating in near-field scanning optical microscopy ͑NSOM͒ is studied for fiber optic probes fabricated using the chemical etching technique. To characterize sample heating from etched NSOM probes, the spectra of a thermochromic polymer sample are measured as a function of probe output power, as was previously reported for pulled NSOM probes. The results reveal that sample heating increases rapidly to ϳ55-60°C as output powers reach ϳ50 nW. At higher output powers, the sample heating remains approximately constant up to the maximum power studied of ϳ450 nW. The sample heating profiles measured for etched NSOM probes are consistent with those previously measured for NSOM probes fabricated using the pulling method. At high powers, both pulled and etched NSOM probes fail as the aluminum coating is damaged. For probes fabricated in our laboratory we find failure occurring at input powers of 3.4± 1.7 and 20.7± 6.9 mW for pulled and etched probes, respectively. The larger half-cone angle for etched probes ͑ϳ15°for etched and ϳ6°for pulled probes͒ enables more light delivery and also apparently leads to a different failure mechanism. For pulled NSOM probes, high resolution images of NSOM probes as power is increased reveal the development of stress fractures in the coating at a taper diameter of ϳ6 m. These stress fractures, arising from the differential heating expansion of the dielectric and the metal coating, eventually lead to coating removal and probe failure. For etched tips, the absence of clear stress fractures and the pooled morphology of the damaged aluminum coating following failure suggest that thermal damage may cause coating failure, although other mechanisms cannot be ruled out.

Near-Field Scanning Optical Microscopy: A New Tool for Exploring Structure and Function in Biology

“…9.5. Interestingly, similar sample temperature profiles were measured with NSOM probes fabricated with the chemical etching technique [ 13]. Despite being much more efficient than probes fabricated using the heating method, sample heating with output power remains unchanged.…”

“…Studies attempting to measure temperatures directly at the NSOM aperture yielded results that ranged from less than 40°C to 390°C [25]. Recently, these measurements were extended using a thermochromic polymer to directly characterize the heating experienced by samples below an NSOM aperture [13,21].…”

“…9.6c. This loss of aluminum from around the taper region leads to the loss of the NSOM aperture and a dramatic increase in light throughput that is indicative of NSOM probe failure at high powers [13].…”

Optically induced forces in scanning probe microscopy

“…Thermal effects can impact the measurement in a number of ways, for example probe elongation or shortening [26,[28][29][30][31][32][33][34], widening of the probe aperture [31], expansion of a metal coating past the dielectric core of the probe [32], changes in the metal work function and thus the capacitance present between the tip and sample [35], and of course in extreme cases thermally induced damage [36].…”

Analysis of nanopatterning through near field effects with femtosecond and nanosecond lasers on semiconducting and metallic targets

“…18 Note that compared with far field laser ablations, the generation of nanopatterns with NSOM probes cannot be achieved through simply increasing the input laser energy. NSOM probes can be easily damaged ͑i.e., melting and exfoliation of the metal coating of the NSOM probes͒ [20][21][22] under moderate to high laser fluence. The damage threshold of NSOM probes can be similar or even smaller than the required energy level for generating patterns on targets with NSOM probes.…”