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Use of a Radially Polarized Beam for Ultra-low Energy Threshold for Cancer Photothermal Therapy with Gold Nanorods [1] Hong Kang 1 * , Jingliang Li 1 , Baohua Jia 1 , Dru Morrish 1 and Min Gu 1 * Presenter 1. Centre for Micro-Photonics , Swinburne University of Technology, VIC, Austraila, 3122 We demonstrate that the use of a radially polarized beam is more efficient for both imaging and therapy for cancer cells labelled with gold nanorods, compared with that of a linearly polarized beam. OCIS codes (180.0180) Microscopy, (170.0170) Medical optics and biotechnology 1. Introduction Advances in the development of nanosized materials have created new opportunities and applications in biomedical and biological fields. Amongst these nanomaterials, metallic nanoparticles have attracted more interest owing to their flexible synthesis, enhanced collective dipole oscillation (surface plasmon resonance) and their strong luminescence emission when they are exposed to laser irradiation [1-3]. The luminescence property makes them a novel contrast agent for diagnosing and detecting specific cells. Furthermore, it enables plasmonic photothermal therapy (PPTT) in which the photon energy is converted into heat to induce hyperthermia in adjoining cells. Due to its localised and minimum invasive feature, PPTT has become a promising method for cancer treatment. Distinguished from spherically shaped particles, nanorods exhibit two absorption bands due to the surface plasmon resonance as shown in Fig. 1. The absorption peak located in the shorter wavelength region can be attributed to the oscillation along the transverse axis, which experiences relatively insignificant change with the aspect ratio (length divided by width) of nanorods. The other much stronger absorption peak resulting from the longitudinal oscillation is highly sensitive to the size variation and shifts to a longer wavelength as the aspect ratio increases. The absorption spectra for the longitudinal mode can be tuned to the near infrared (NIR) region, which is highly desirable for in vivo applications as NIR excitation experiences less absorption, scattering and a larger penetration depth [4-6]. Fig. 1 illustrates the absorption bands of the gold nanorods used in this paper. The transverse and longitudinal absorption peaks at wavelengths 517 nm and 780 nm, respectively, can be clearly identified from Fig. 1. Due to the surface plasmon property, gold nanorods are most efficiently excited by light polarized parallel to their longitudinal axis. However, when cells are labelled with gold nanorods, the nanorods are randomly dispersed and orientated. This makes the excitation of the nanorods by a linearly polarized beam less efficiency because only a small portion of the randomly oriented nanorods aligning with the incident polarization direction can be excited. Higher incident laser power is required to compensate for the less efficient nanorod excitation. Too high an incident power can kill healthy cells outright. To overcome this challenge, a highly localized focal spot polarized in all three directions is highly desired. By utilizing such a radially polarized beam, nanorods in every orientation can be more efficiently excited, which subsequently reduces the required laser power and leads to an ultra-low energy threshold for cancer photothermal therapy. In this paper, a radially polarized beam is employed for this purpose. [1]. Published at the Conference of Novel Techniques in Microscopy, Vancouver Canada, April 30, 2009, @2009 Optical Society of America
Fig. 1. The transverse and longitudinal surface plasmon absorption spectra of gold nanorods. The solid arrows indicate the direction of the surface plasmon resonance modes within the nanorod. The dashed arrows indicate the transverse and longitudinal absorption peaks at wavelength 517 nm and 780 nm, respectively. As shown in Fig. 2, a radially polarized beam possesses even electric field aligning in a radial direction in free space. Once focused by an objective with high numerical aperture (NA), a strong longitudinal component is produced due to the depolarization effect [7,8]. As a result, a light field polarized in all directions is generated composing these three orthogonal electric fields. Thus by using such a tightly focused radially polarized beam in PPTT nanorods of arbitrary orientation can be efficiently excited. Therefore, the required energy fluence for imaging and treatment can be greatly reduced to a level below the medical standard for clinical applications. Fig. 2. Experimental intensity distribution of a radially polarized beam. The arrows indicate the polarization direction of the light. 2. Experiments The radially polarized beam was generated by a radial polarization converter (Arcoptix S.A.). Fig. 2 shows the experimental intensity distribution of a typical hollow radial beam in free space. To examine the polarization profile, an analyser is placed rotated to three orientations within the beam. Fig. 3 shows the obtained intensity patterns at the three different directions. The two high-intensity 113
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