Nonlinear optical properties of methyl red under CW irradiation
Yu Zheng
a
, Qing Ye
a,
n
, Chen Wang
a
, Jin Wang
a
, Zhichao Deng
a
, Jianchun Mei
b
,
Wenyuan Zhou
a
, Chunping Zhang
a
, Jianguo Tian
a
a
The Key Laboratory of Weak-Light Nonlinear Photonics, Ministry of Education, School of Physics and TEDA Applied Physics School, Nankai University, Tianjin
300071, China
b
Advanced Technology Institute, Nankai University, Tianjin 300071, China
article info
Article history:
Received 31 March 2015
Received in revised form
4 June 2015
Accepted 30 June 2015
Available online 7 July 2015
Keywords:
Optical nonlinearities in organic materials
Nonlinear optics
Z-scan
abstract
Organic materials have wide potential application in nonlinear optical devices. The nonlinear optical
(NLO) properties of methyl red (MR) doped polymethyl methacrylate (MR-PMMA) are investigated under
CW laser irradiation at 473 nm, 532 nm and 632.8 nm, respectively. By combining Kramers–Kronig (K–K)
relation and CW Z-scan technique, the effective refractive index n
2
and the change of refractive index
nΔ
are obtained under different scanning speed at 473 nm and 532 nm.
nΔ
is positive at 473 nm, while
nΔ
is
negative at 532 nm. The experimental result is consistent with that of K–K relation. With the scanning
speed decreasing, the NLO properties of MR-PMMA are enhanced. With different laser powers at
632.8 nm, MR-PMMA has only nonlinear absorption rather than nonlinear refraction. Meanwhile, the
sample is investigated under pulse laser irradiation at 532 nm. Through the comparison of results of CW
Z-scan and pulse Z-scan, the influence of the cumulative thermal effect on NLO properties of material is
investigated. The results indicate that, under CW irradiation near the absorption peak wavelength, the
cumulative thermal effect has great influence to the NLO properties of MR-PMMA.
& 2015 Elsevier Ltd. All rights reserved.
1. Introduction
The organic photochromic materials have the advantage that
the optical properties will change under the irradiation of light
with a certain wavelength instead of external electric field and
magnetic field. Hence the materials can be used to process optical
signal [1]. Under CW irradiation, organic photochromic materials
have the wide potential application in optoelectronic field. For
example, ethyl red doped polymer film has potential application in
optical switching [2], and disperse red doped polymethyl metha-
crylate (PMMA) constitutes a demonstrator of an optically con-
trolled integrated optical device [3].
Methyl red (MR) is an azobenzene organic photochromic
material [4]. The structure character of azobenzene is double-bond
N¼ N between the two phenyls [5]. Because the structure has
active photochemical property, azobenzene can generate trans and
cis isomerization easily by light irradiation, which is so-called
photoisomerization. Some azobenzene studies have shown that
the trans-cis configuration changes are an effective mechanism for
creating large optical nonlinearities [6], which benefits application
development. However under CW irradiation, the cumulative
thermal effect has powerful influence on NLO properties of
materials [6]. Hence, the influence of the cumulative thermal
effect on NLO properties of materials is worthy of further inves-
tigations for potential applications. In this paper, we use CW
Z-scan to investigate the influence at 473 nm and 532 nm under
different scanning speed and at 632.8 nm with different laser
powers. Through the comparison of results of CW Z-scan and pulse
Z-scan at 532 nm, the order of magnitudes of the influence is
investigated.
2. Experiment
The sample, MR-doped PMMA thin film, is prepared as follows:
MR and PMMA were dissolved in cyclohexanone to form a uniform
solution. Subsequently, the solution was stirred well for two hours
using a magnetic stirrer. The mixed solution was dried into the
viscous solution at room temperature. The viscous solution was
spread by spin-coating on a glass slide. Last step, the sample was
dried in an oven for 10 h. In the sample, MR is dominant to occur
nonlinear effect, and the nonlinear effect of PMMA is negligible.
The steady state and excited state absorption spectrum of
sample (sample A) were measured. The experimental setup is
shown in Fig. 1. Light source is a white-light source. The focal
Contents lists available at ScienceDirect
journal homepage: www.elsevier.com/locate/optlastec
Optics & Laser Technology
http://dx.doi.org/10.1016/j.optlastec.2015.06.031
0030-3992/& 2015 Elsevier Ltd. All rights reserved.
n
Corresponding author.
E-mail addresses: yeqing@nankai.edu.cn (Q. Ye), jjtian@nankai.edu.cn (J. Tian).
Optics & Laser Technology 75 (2015) 132–137