Subscriber access provided by UNIVERSITY OF SASKATCHEWAN LIBRARY
Article
Stimulated Anti-Stokes Raman emission generated by gold nanorod coated optical resonators Soheil Soltani, Vinh M. Diep, Rene Zeto, and Andrea M. Armani ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00296 • Publication Date (Web): 27 Jul 2018 Downloaded from http://pubs.acs.org on July 29, 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.
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 27 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
Stimulated Anti-Stokes Raman emission generated by gold nanorod coated optical resonators Soheil Soltani1 , Vinh M Diep2, Rene Zeto, Andrea M Armani*,1,2 1
Ming Hsieh Department of Electrical Engineering, University of Southern California, Los
Angeles, California, United States 90089 2
Mork Family Department of Chemical Engineering and Materials Science, University of
Southern California, Los Angeles, California, United States 90089
* Corresponding Author:
[email protected] KEYWORDS Plasmonics, nanomaterial, nonlinear optics
ABSTRACT: Plasmonic nanomaterials and nanostructured substrates have made a significant impact in sensing and imaging due to their ability to improve optical field confinement at interfaces. This improved confinement increases the optical field intensity, enabling numerous nonlinear effects to be revealed. However, one challenge is effectively coupling light into and out of the plasmonic nanomaterial. One approach is to directly integrate the plasmonic nanomaterials onto the surface of light emitting optical devices. In this work, the highly nonlinear complex of organic small molecule functionalized gold nanorods is coated on the surface of optical resonators. The evanescent tail of the optical field circulating inside the resonator directly interacts with the nanoparticle complex, creating a hybridized plasmonwhispering gallery mode. Due to the strong field localization by the nanorods and the large
ACS Paragon Plus Environment
1
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 27
circulating power within the resonator, Stimulated Anti-Stokes Raman Scattering (SARS) is generated with only 14mW of input power, which is a 4-fold reduction as compared to previous work.
INTRODUCTION
Plasmonic nanomaterials and nanostructured substrates are able to tightly confine the optical field near the surface of the metal structure. Using this resonance phenomenon, researchers have developed high performance biosensors1, cloaking devices2,3, and optical wires4 as well as studied the fundamental photophysics of the system. However, even when off-resonance, a surface plasmon-polariton (SPP) wave can be generated if the particle is located on a dielectric surface. The SPP provides some optical field enhancement at the metal nanoparticle-dielectric interface. While many SPP-based devices have been made, previous work investigating the nonlinear optical behavior has been limited due to the intrinsic difficulties surrounding coupling high intensity light sources into SPPs.
One promising approach for studying SPPs and SPP-enabled phenomena is based on combining metal nanoparticles with optical resonators. High quality factor (Q) optical resonators act as optical amplifiers, enabling nonlinear behaviors to be excited with low threshold powers. For example, in past work using whispering gallery mode optical resonators, frequency comb formation5,6, harmonic generation7-9, and Stimulated Raman Scattering (SRS)10-12 have been observed with only a few mW’s of input power. One particularly interesting case is when the resonant cavity is fabricated from silica, a material that has inversion symmetry. Therefore, the
ACS Paragon Plus Environment
2
Page 3 of 27 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
lowest order nonlinear behaviors observed in Silica devices are from third order nonlinear effects, including Stimulated Raman scattering (SRS)13,14 and Stimulated Anti-Stokes Raman scattering (SARS) 15,16.
Stimulated Raman Scattering is generated in whispering gallery mode microcavities by the interaction between the circulating optical field and optical phonons. The lifetime of the optical field, and thus the overall strength of this interaction, is determined by the cavity Q. Because phonons have flat dispersion, SRS does not require phase matching. In contrast, Stimulated AntiStokes Raman Scattering needs phase matching, and its amplitude depends on the interaction between the pump and Stokes Raman signals. As a result, the SARS signal is usually weaker than other third order nonlinearities such as Kerr or Raman. There have been efforts to enhance SARS emission by using high Raman gain materials17,18 or by seeding the Raman emission with a second laser 19,20, but these methods are limited by the available wavelength ranges for the seed laser and add to the overall complexity of the system. An alternative strategy is to combine whispering gallery mode devices with SPP nanomaterials.21 While the overall Q factors achieved in plasmon-based devices are lower, the ability to focus the optical field is improved.22,23 In a recent work, combining silica spherical resonators with Polyethylene Glycol (PEG)functionalized gold nanorods (GNR) resulted in enhancement of Optical Parametric Oscillations (OPO), which, in turn improved the threshold and span of frequency combs.24 This strategy combined several effects: the optical field amplification of the optical resonator with the plasmon-polariton optical field focusing on a high gain nanoparticle complex.
Here, we show that coating silica spherical resonators with PEGylated GNRs improves SARS generation through pump and SRS field enhancement (Figure 1a). We have also modeled the
ACS Paragon Plus Environment
3
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 27
field intensity enhancement at 1550 nm and have shown that, although the PEGylated GNRs are off-plasmonic resonance, the field is locally enhanced by plasmon-whispering gallery mode hybridization.
Figure 1. (a) Artistic rendering of a gold nanorod coated resonator coupled to a fiber taper. (b) Energy diagram for Stokes and Anti Stokes Raman generation process.
THEORY
A. Stimulated Anti-Stokes Raman Scattering
SARS is a third order nonlinear process during which electrons from vibrational states are excited to a virtual state and eventually return to the ground state. For SARS emission to start, a single wavelength pump laser generates Stimulated Raman emission, which redistributes the vibrational state population. Next, the excited electrons react with the pump laser to generate Stimulated Anti-Stokes Raman scattering (Figure 1b). As a result, the Stokes and Anti-Stokes photons form a symmetric pair around the pump line.
The nonlinear polarization for Anti-Stokes emission is defined by the general nonlinear polarization as:
i (2 k p − kS ) z
PAS = χ (3) E p2 ES* e
(1)
where χ(3), Ep , and Es are third order nonlinear susceptibility, pump, and Stokes Raman field amplitudes, respectively. Also, kp and ks are the propagation constants for the pump and the Stokes Raman waves, respectively. Unlike SRS, which does not need phase matching, SARS
ACS Paragon Plus Environment
4
Page 5 of 27 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
requires ∆k = 2 k p − k S − k AS to be zero for efficient generation of SARS 25,26. Therefore, experimental realization of SARS has to overcome both the inefficiency of the fundamental process as well as meet the phase matching requirement.
B. Gold Nanoparticle Enhancement
Surface plasmons enhance the electromagnetic field intensity by storing a portion of the energy in the free surface electrons of the metallic layer. Therefore, if the plasmonic structure is located on the surface of an optical cavity, it will enhance the nonlinear processes by increasing the intensity near the metal-dielectric interface. One main drawback for using plasmonic particles is the high plasmon loss, which deteriorates the quality factor of the resonator containing the plasmonic particles.
To address this issue, one strategy is to place the particles close to the surface of the cavity where they can interact with the evanescent tail of the optical mode, which extends beyond the cavity surface. This strategy ensures that the main optical mode is not affected, as the particle will only disturb the mode marginally. Also, the concentration of the plasmonic particles that are the main source of loss should be optimized.27-29
In addition, it is worth noting that, for the case of sub-wavelength metallic nanoparticles (a
