A visible semiconductor laser-induced fluorescence detection system for ultratrace chemical analysis of nanoliter-to-picoliter samples is optimized and characterized in detail. With low-power semiconductor lasers emitting at 675 and 639 nm for excitation, the best limit of detection is 8000 molecules for a model fluorescent compound (1,1',3,3,3',3'-hexamethylindotricarbocyanine iodide) in an ethanol solution. Fused silica capillaries with inner diameters between 250 and 11 microm were employed as picoliter volume flow cells, which permits the use of the system in miniaturized separation techniques such as packed and open tubular liquid chromatography and capillary electrophoresis.
GaAs/GaAlAs heterostructure lasers with a monolithic confocal unstable resonator were demonstrated. The curved mirrors satisfying the confocal condition were fabricated by etching. Close to threshold, the lasers operate in a single lateral mode with a nearly collimated output beam. A single-lobe far-field intensity distribution as narrow as 1.90 full width at half maximum was measured.Laser cavities of the unstable resonator (UR) type have been shown to exhibit many advantages when they are combined with a lasing medium characterized by a high-single-pass gain.'-The most prominent of these advantages is the potential for increasing the mode volume of a laser oscillator while maintaining high discrimination between transverse modes. As a result, the cavity is relatively insensitive to inhomogeneities in the lasing medium. These properties make UR geometries attractive for semiconductor lasers, 5 since they may provide an optimal solution to the task of increasing the lateral dimension of the laser and avoiding multilateral mode operation and filamentation.UR semiconductor lasers were demonstrated previously.6 -1 0 However, in all previous work the fabricated UR semiconductor lasers emitted a diverging optical beam. This is a disadvantage in applications in which the coupling of the laser output to optical devices with low numerical aperture is considered. The purpose of this Letter is twofold:(1) to demonstrate the operation of a semiconductor laser with a monolithic confocal UR that emits a nearly collimated output beam" and (2) to show that the fabrication of semiconductor lasers with etched curved mirrors permits the design of lasers with integrated cavities that fit specific optical output requirements. The confocal UR of positive branch is shown in Fig. 1. The cavity is composed of a large concave mirror M, and a small convex mirror M 2 with radii of curvature R, and R 2 , respectively, satisfying the confocality condition R, + R 2 = 2L, with L the cavity length. The geometrical magnification m is given by the ratio of the mirror lateral sizes a,/a 2 . The output beam is that portion of radiation reflected by Ml that misses striking M 2 (Fig. 1). A remarkable feature of this cavity is that its fundamental mode is a collimated two-section beam (or a collimated annular beam in the three-dimensional case). Figure 2 shows the geometry of the confocal UR semiconductor laser. GaAs/GaAlAs double heterostructures were grown by molecular-beam epitaxy, and conventional photolithographic methods were used to define the desired pattern for mirror etching and metallization. Apart from the slight complication of the need to provide three separate electrical contacts, the fabrication is similar to that of previously reported UR semiconductor lasers. 7 -1 0 The cavity dimensions are 2al = 150 gtm, 2a 2 = 30 ,m, R, = 400 gim, R2 = -80 gm, and L = 160 gim, leading to a geometrical magnification of m = 5, and an equivalent Fresnel number Neq = 350. We note that, unlike in the case of nonsemiconductor UR lasers, in which the c...
In this study the potential of using visible semiconductor lasers as the excitation source for laser-induced fluorescence spectroscopy (LIF) of nano-liter to pico-liter samples has been evaluated. The incentive to work with LIF in the upper visible and near infrared wavelength region is the possibility to design a system that is not limited by any background fluorescence, e.g. from a capillary flow-cell or the liquid eluent [1]. The system described here was mainly designed for applications to chemical ultra-trace analysis when combined with miniaturized separation techniques such as capillary liquid chromatography and capillary electrophoresis. A concentration limit of detection in the lower pico-molar range (6.3 - 58 · 10-12 mol/L) was obtained for a model fluorescent compound, HITCI (1,1’,3,3,3',3'-hexamethyl-indotricarbocyanine iodide). The smallest illuminated volume was 0.7·10-12 L using an 11 μm inner diameter fused silica capillary flow-cell. These figures compares well with the best obtained using conventional gas-lasers for LIF [2,3]. An important advantage with the semiconductor LIF system, except from that it is inexpensive, is that the compactness and the high performance of the components makes it highly suitable for portability.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.