Subscriber access provided by Rasmuson/BioSciences Library | University of Alaska Fairbanks
Article
Ultra-broadband Nano-spectroscopy with a Laser-driven Plasma Source Martin Wagner, Devon S. Jakob, Steve Horne, Henry Mittel, Sergey Osechinskiy, Cassandra Phillips, Gilbert C Walker, Chanmin Su, and Xiaoji G. Xu ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b01484 • Publication Date (Web): 05 Feb 2018 Downloaded from http://pubs.acs.org on February 6, 2018
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
ACS Photonics is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Photonics
Ultra-broadband Nano-spectroscopy with a Laser-driven Plasma Source Martin Wagner1*, Devon S. Jakob2, Steve Horne3, Henry Mittel1, Sergey Osechinskiy1, Cassandra Phillips4, Gilbert C. Walker,4 Chanmin Su1 and Xiaoji G. Xu2*
1
2
Department of Chemistry, Lehigh University, 6 E Packer Ave., Bethlehem, PA, 18015 3
4
Bruker Nano, 112 Robin Hill Road, Santa Barbara, CA, 93117
Energetiq Technology Inc., 7 Constitution Way, Woburn, MA 01801
Department of Chemistry, University of Toronto, Toronto, Ontario, M5S 3H6, Canada *Corresponding Email: Xiaoji G. Xu:
[email protected] Martin Wagner:
[email protected] Abstract: Scattering-type scanning near-field optical microscopy (s-SNOM) enables infrared spectroscopy at 10-20 nm spatial resolution through elastic light scattering. Coupled with an infrared light source, s-SNOM characterizes chemical compositions or probes nanoscale photonic phenomena on length scales two orders of magnitude below the diffraction limit. However, widespread use of s-SNOM as an analytical standard tool has been restrained to a large extent by the lack of a bright and affordable broadband light source. Here we present a turnkey thermal emitter based on a laser-driven plasma that offers incoherent radiation of a broader bandwidth (>1000 cm-1) and ~40-fold higher brilliance than previous blackbody radiators in addition to a compact size and at a fraction of the cost of alternative coherent laser systems or synchrotrons. We demonstrate a nearly one order of magnitude increase in signal-to-noise in near-field spectra compared to existing incoherent emitters, which allows probing of not only inorganic materials and polaritonic systems, but also various commonly-used polymers despite their weak near-field optical response. The latter important representative of soft matter was previously inaccessible 1 ACS Paragon Plus Environment
ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 25
by table-top thermal radiators. s-SNOM combined with the laser-driven plasma shall provide a widely accessible platform for infrared nano-spectroscopy. TOC graphic
Keywords: near-field microscopy; s-SNOM; infrared spectroscopy; nano-FTIR; polymer identification; laser-driven plasma source Fourier-transform infrared (FTIR) spectroscopy is a standard analytical technique. The diffraction limit, however, prevents optical techniques such as FTIR spectroscopy from reaching a spatial resolution smaller than one half of the wavelength.1 Many modern microscopy techniques have been developed to bypass the far-field diffraction limit.2-3 Infrared scattering-type scanning near-field optical microscopy (sSNOM) is one of the most popular spectroscopic techniques that routinely achieves sub-20 nm spatial resolution in the important infrared fingerprint region.4-5 s-SNOM has been widely used for imaging plasmons and polaritons,6-14 identifying chemical compositions of polymers or proteins, including in wet environments,15-17 tracking phase transitions,18-20 and mapping the electric fields of nanostructures.14, 21-23 Besides offering high spatial resolution, s-SNOM represents a sensitive, reliable method for nondestructive, label-free surface characterization that is unmatched by traditional spectroscopy and in many cases surpasses the capabilities of contemporary super-resolution microscopy.2 The technique of s-SNOM relies on elastic scattering of photons by a sharp metallic probe oscillating above the sample of interest in an atomic force microscope (AFM). The presence of an optical resonance in the sample, as well as the distance between the tip and sample, affect the efficiency at which the tip scatters the light. Near-field responses are obtained by analyzing the backscattered radiation via lock-in demodulation.4-5, 24 The scattered light is typically interferometrically homodyned with a reference field from the same light source to optically amplify the signal. When a broadband radiation source is used, by scanning the optical path of the reference beam, an asymmetric interferogram is obtained and
2 ACS Paragon Plus Environment
Page 3 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Photonics
subsequently processed by Fourier transformation to obtain infrared absorption spectra of the sample through a technique known as nano-FTIR.25-26 The most common infrared sources for s-SNOM are narrowband, tunable continuous wave quantum cascade lasers or CO2 lasers. They provide the necessary irradiance for the weak scattering process inherent to s-SNOM but can usually be tuned only over a limited spectral range of ~100 cm-1, reducing their applicability for a wide range of studies. Broadband infrared sources are more attractive for spectroscopy applications. One such source based on difference frequency generation (DFG) between table-top femtosecond laser pulses provides coherent broadband radiation of several hundred wavenumbers bandwidth with a tunable central frequency and has been applied successfully in sSNOM.26-28 However, the bandwidth of the IR pulse is unable to cover the entire infrared fingerprint region without switching its central frequency by adjusting the phase matching condition of the nonlinear crystal. Merging of the individual spectra from different central frequencies is then required.28 This is complicated by the fact that s-SNOM always demands a near-field reference spectrum on a known material (e.g. Si or Au) taken under the same experimental conditions, which may involve jumping between the different spatial locations of sample of interest and reference area for the same DFG setting. An additional drawback of DFG based systems is their cost and complexity since it involves a nonlinear mixing stage and femtosecond lasers whose temporal pulse overlap needs to stay constant over time within the nonlinear crystal. Another interesting light source is a synchrotron, which provides coherent broadband infrared radiation of high irradiance, ideal to enable chemical identification in s-SNOM. However, synchrotrons are only available at specific facilities in the world, hence access is limited.29-30 Heated ceramic resistor elements (Globars) as thermal emitters are an alternative and have found widespread use in far-field FTIR spectrometers. This incoherent light source is affordable and compact, but its spectral irradiance is extremely weak, around 2-3 orders of magnitude lower than that of synchrotrons. The low-intensity spectral density and incoherent nature of the blackbody radiator severely limits its application with sSNOM.31 An electrically driven plasma discharge in a noble gas under high pressure represents another incoherent thermal emitter that has been coupled to s-SNOM recently.32 However, despite a much higher brightness than a Globar, performance is still limited in terms of spectral coverage and signal-to-noise. To the best of our knowledge, apart from large synchrotron user facilities, only DFG based laser systems enable s-SNOM with nano-FTIR to investigate a wide range of materials including polymers with their often weak vibrational resonances. In contrast to these coherent lasers and synchrotrons, previous experiments with incoherent thermal emitters, i.e. Globars and electrically maintained plasma sources, were restricted exclusively to strong phonon and polaritonic resonances,31-33 while they failed to access 3 ACS Paragon Plus Environment
ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 25
the important class of polymers and soft matter. In addition, despite the ultra-broad far-field emission from these table-top blackbody radiators, the usable near-field spectra were surprisingly narrowband: they cover only the range
