Ultra-sensitive surface absorption spectroscopy using sub-wavelength diameter optical fibers

Abstract

The guided modes of sub-wavelength diameter air-clad optical fibers exhibit a pronounced evanescent field. The absorption of particles on the fiber surface is therefore readily detected via the fiber transmission. We show that the resulting absorption for a given surface coverage can be orders of magnitude higher than for conventional surface spectroscopy. As a demonstration, we present measurements on sub-monolayers of 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA) molecules at ambient conditions, revealing the agglomeration dynamics on a second to minutes timescale.PACS numbers: 78.66. Qn, 39.30.+w, 68.43.Jk, 78.66.Jg During the last twenty years, numerous optical tools for surface and interface analysis have been developed [1]. The selective sensitivity to surface effects is often obtained by carrying out spectroscopy with evanescent waves (EW), created by total internal reflection of light at the interface. This is straightforwardly realized by exciting waveguide modes in unclad optical fibers [2,3]. If the EW is resonant with the transition frequency of particles (atoms, molecules, quantum dots, etc.) in the surrounding medium, one can use both the particles' fluorescence [4] or the peak attenuation of the waveguide mode [2,5] to infer the concentration of particles at the interface. Moreover, the line shapes allow to spectroscopically retrieve detailed physical information about the nature and strength of the particle-surface interaction.Fiber-based evanescent wave spectroscopy (EWS) is used in various sensors [6]. The robustness, reliability, and ease of use of an all-fiber-based sensor technology is advantageous for in situ sensing in a remote or isolated location or in a harsh environment, e.g., in industrial applications or environmental studies. Furthermore, such sensors also profit from the multiplexing and miniaturization potential inherent to fiber technology. When measuring a volumetric concentration of particles in the surrounding medium, these sensors yield however a reduced sensitivity compared to conventional free-beam absorption: a significant fraction of the light propagates inside the waveguide and therefore does not interact with the particles of interest. This problem can partially be overcome by increasing the power fraction in the EW through proper choice of the fiber mode or geometry [7,8,9]. Yet, even in the ultimate case of 100 % EW, the sensitivity will not exceed that of free-beam absorption techniques.In this letter, we demonstrate that the situation can be dramatically different when employing fiber-based EWS for the spectroscopic study of surface coverages instead of volumetric concentrations: The ultimate sensitivity of fiber-based surface absorption spectroscopy (SAS) is shown to strongly depend on the fiber diameter and to exceed free-beam SAS by several orders of magnitude in the case of sub-wavelength diameter fibers. Fiber-based surface absorption spectroscopy (SAS) has already been used for a number of applications, e.g., in bio-sensors [10]. However, to our kno...

Nanophotonics of optical fibers

“…These fibers are referred to as microfibers. Optical microfibers are proposed as building blocks for micro-optical devices having applications in photonics, physics, chemistry, and biology [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]. Important advantages of microfibers are the potential for compact assembly in three dimensions, possibility of strong coupling to the environment and/or localization of radiation, and low transmission loss.…”

Optical Nanofibers

“…This flame brushing method allows us to reliably produce ONFs with controllable taper geometries and uniform waists. With our setup the waist can vary in length from 1 to 100 mm, and we can achieve radii as small as 150 nanometers Bilodeau et al (1988); Birks and Li (1992); Warken et al (2007); Warken (2007); Garcia-Fernandez et al (2011). Rather than sweep the flame back and forth over the fiber, we keep the flame stationary while sweeping the motors; this action reduces air currents, which could lead to nonuniformities on the fiber waist.…”

Optical microfibers and nanofibers

“…In 2007, based on nanoparticleinduced Rayleigh-Gans scattering in a waveguiding MNF, Wang et al calculated the possibility of detection single nanoparticles adsorbed on the surface of an MNF, and showed that, by optimizing the wavelength of the probing light and the diameter of the MNF, nanoparticle-induced scattering intensity can reach detectable level with possibilities for single-molecule detection [26]. In 2007, relying on absorption of molecules adsorbed on the surface of a 500-nm-diameter MNF, Warken et al reported an ultrasensitive molecular sensor that was possible to detect sub-monolayers of 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA) molecules at ambient conditions [27]. In 2011, by integrating a 900-nm-diameter MNF into a 125-μm-wide microfluidic channel, Zhang et al demonstrated a MNF sensor for chemical and biological applications ( Figure 16A-E) [32].…”

“…(2) Strong evanescent field Strong evanescent field offers strong near-field interaction between the MNF and its surroundings, making the MNF highly favorable for optical sensing [21][22][23][24][25][26][27][28][29][30][31][32] and evanescent coupling between the MNF and other waveguides (e.g., a semiconductor [33,34], metal [35,36] nanowire or planar waveguide [37]) or a substrate [30,35,38,39]. Based on the high-efficiency evanescent coupling, a variety of optical components or devices (e.g., loop and knots resonators [40][41][42][43][44][45][46][47][48][49][50][51], lasers [52][53][54][55][56][57][58], and sensors [21][22][23][24][25][26][27]…”

“…Based on the high-efficiency evanescent coupling, a variety of optical components or devices (e.g., loop and knots resonators [40][41][42][43][44][45][46][47][48][49][50][51], lasers [52][53][54][55][56][57][58], and sensors [21][22][23][24][25][26][27][28][29][30][31][32]) have been demonstrated. Meanwhile, since the high fractional evanescent filed is tightly confined around the MNF, it can produce steep field gradient which provides large optical gradient force for manipulating cold atoms [19,[59][60][61][62][63] or micro-/nano-particles [64,65], as well as large or manageable waveguide dispersion under single mode condition [11].…”