Near-Field Second Harmonic Microscopy of Thin Ferroelectric Films
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ABSTRACT Near-field second harmonic microscopy is ideally suited for studies of local nonlinearity and poling of ferroelectric materials at the microscopic level. Its main advantages in comparison with other scanning probe techniques are the possibility of fast time-resolved measurements, and substantially smaller perturbation of the sample under investigation caused by the optical probe. We report second harmonic imaging of the surface of thin BaTiO 3 films obtained in a near-field microscopy setup using a Ti:sapphire laser system consisting of an oscillator and a regenerative amplifier operating at 810 nm. Optical resolution on the order of 80 nm has been achieved. INTRODUCTION Thin ferroelectric films form the basis of a new thin film technology for data storage [1]. They are widely used in nonvolatile ferroelectric random-access memory (NVFRAM) and dynamic random-access memory (DRAM) devices. The ultimate performance (density and speed of operation) of these devices depends on the spatiotemporal properties of individual ferroelectric domains. A better understanding of these properties requires experiments performed with nanometer spatial and sub-nanosecond temporal resolution. Near-field optical microscopy may be the technique of choice for such experiments. Optical second harmonic generation (SHG) is widely used to characterize the average structure and crystal orientation of ferroelectric films [2]. Nearfield second harmonic microscopy is the logical extension of these far-field techniques. Its main advantages in comparison with other scanning probe techniques, such as the recently developed piezoresponse atomic force microscopy [3], are the possibility of fast time-resolved measurements, and substantially smaller perturbation of the sample under investigation caused by the optical probe. Many attempts have been made recently [4,5] to implement different near-field optical techniques for SH imaging. Unfortunately, even the best resolution reported so far, 140 nm [5] obtained with a widely adopted aperture near-field probe [6], falls short of the desired range and can hardly be regarded as subwavelength resolution. The main experimental problem in near-field SH imaging is low optical signal, since SHG is generally a weak effect, and it is extremely difficult to detect using an aperture fiber probe with a typical throughput of less than 10-4 for a 100 nm aperture. An apertureless approach to near-field microscopy is usually based on laser-illuminated sharp metal tips. It was successfully used in linear near-field imaging [7]. Very recently it has been applied to two-photon luminescence microscopy [8]. It is based either on local field perturbation
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Mat. Res. Soc. Symp. Proc. Vol. 596 ©2000 Materials Research Society
by or on local field enhancement in the vicinity of the metal tip. In the of latter casethe thetip, metal tipthe provides a local excitation source for the spectroscopic response the sample under investigation. Unfortunately, this approach can not be directly applied to SH microscopy, since u
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