APPLIED PHYSICS LETTERS 101, 191106 (2012)

Ultrafast nonlinear optical responses of bismuth doped silicon-richsilica filmsKenji Imakita,1,a) Yuya Tsuchihashi,1 Ryo Naruiwa,1 Minoru Fujii,1 Hong-Tao Sun,2Jianrong Qiu,3 and Shinji Hayashi1

1Department of Electrical and Electronic Engineering, Faculty of Engineering, Kobe University,Rokkodai, Nada, Kobe 657-8501, Japan
2Division of Materials Science and Engineering, Faculty of Engineering, Hokkaido University, Kita 13,Nishi 8, Kita-ku, Sapporo 060-8628, Japan
3
Institute of Optical Communication Materials, South China University of Technology,Guangzhou 510640, China(Received 22 September 2012; accepted 22 October 2012; published online 7 November 2012)
Nonlinear optical responses of bismuth (Bi) doped silicon-rich silicon dioxide (Si-rich SiO2 ) films
were studied by a z-scan and an optical Kerr gate method under femtosecond excitation around
800 nm. It was found that the Bi-doping enhances the nonlinear optical response of Si-rich SiO2
films by several orders of magnitudes. The nonlinear refractive index was of the order of1011 cm2 =W and the response time was shorter than our time resolution of 100 fs. The nonlinear
refractive index was independent of the wavelength in the range from 750 to 835 nm, suggestingC 2012 American Institute ofthat virtual transitions are involved in the nonlinear optical processes. V
Physics. [http://dx.doi.org/10.1063/1.4766269]

During past decades, silicon-rich silicon dioxide (Si-rich
SiO2 ) has attracted great attention.1,2 This material, consisting of silicon quantum dots (Si-QDs) embedded in silicon
dioxide (SiO2 ), shows much larger nonlinear opticalresponse in the near infrared region than that of bulk-silicon(bulk-Si), owing to their quantum and surface effects.3,4 The
response time of the optical nonlinearity is less than 100 fs,
which is short enough for a future high-bit-rate optical telecommunication system.5 Furthermore, Si-rich SiO2 is highly
compatible with conventional complementary metal oxide
semiconductor (CMOS) technologies, enabling low-costmass-productions of nonlinear photonic devices which canbe hybridized with conventional electronic devices.5
Impurity-doping in Si-QDs is an important approach tofurther enhance the nonlinear optical response of Si-rich
SiO2 . A localization of an electron or a hole at the impurity
site can cause breakdown of the momentum conservation
rules in Si-QDs, and enhances the oscillator strength of theinterband transition.6,7 Consequently, the nonlinear opticalresponse of Si-QDs (and hence that of Si-rich SiO2 ) isexpected to be enhanced. In fact in our previous work, wefound that the nonlinear optical response of Si-rich SiO2 canbe enhanced several times by phosphorus (P)- and/or boron(B)-doping.8–11 The response time was found to be shorter
than our time resolution of 100 fs. Thus, impurity doping isexpected to be a promising tool in designing a Si-QDs-basedmaterial toward the potential application in nonlinear
photonics.
In this work, we focus on bismuth (Bi) as an impurity
atom. Bi is known to be a n-type impurity for bulk-Si andpossesses the largest ionization energy and the smallest Bohrradius among group-V donors.12 We demonstrate that Bidoping dramatically enhances the nonlinear optical responsea)

Author to whom correspondence should be addressed. Electronic mail:imakita@eedept.kobe-u.ac.jp.

0003-6951/2012/101(19)/191106/4/$30.00

of Si-rich SiO2 and that the nonlinear optical response isfaster than 100 fs. The results suggest that Bi-doping is much
more effective than P- or B-doping for the enhancement of
the nonlinear optical response in Si-rich SiO2 .
Bi-doped Si-rich SiO2 was prepared by a co-sputtering
method.13 Si, SiO2 , and Bi-doped SiO2 ½SiO2 : Bi2 O3 ¼ 50 :50ðmol%Þ were simultaneously sputter-deposited in Ar gas
on a quartz substrate (350 nm in thickness) by using a multitarget sputtering apparatus. The concentrations of Si, O, andBi were controlled by changing the sputtering rates of eachgun. After the deposition, the films were annealed in a N2
gas atmosphere for 30 min at temperatures from 950 to1250  C. The linear refractive index in the near infrared
region, measured by spectroscopic ellipsometry, stronglydepends on the excess Si concentration and ranges from 1.45
to 1.63 in the present samples. The nonlinear opticalresponses were evaluated by a z-scan and an optical Kerrgate (OKG) methods.11 For both measurements, the excitation source was a mode-locked Ti:sapphire femtosecond
laser with the pulse width of about 100 fs and the repetition
frequency of 82 MHz. The wavelength (k) is changed from750 to 835 nm. In the z-scan measurement, the beam waist
and diffraction length determined by a knife edge method
were 15 lm and 1 mm, respectively. In an OKG measurement, the laser beam was divided into a pump and a probe
beams with 10:1 intensity ratio by a beam splitter. The probe
beam passed through a time-delay device. The polarizationof the probe beam was set to 45 with respect to that of thepump beam by a half wave plate. The pump and probe beams
were focused onto the samples by a lens with 80-mm focallength. The incident angle difference between the pump andthe probe beams is less than 10 . The beam waist and the diffraction length of these beams were 25 lm and 1.8 mm,respectively. Samples were placed between a half wave plate
and a polarizer in a cross-Nicol configuration. The probe
beam was detected by a photodiode connected to a lock-in

101, 191106-1C 2012 American Institute of Physics
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Imakita et al.

amplifier. The linear absorption spectra were measured byconventional spectrophotometer. The contribution of Fresnel
reflection loss was eliminated by using experimentallyobtained reflectance spectra.
Figure 1(a) demonstrates a typical result of a closedaperture z-scan measurement. The excess Si concentration isfixed at 13.6 mol. %, and the Bi2 O3 concentration is changedfrom 0 to 1.15 mol. %. Note that the signal of the undoped
sample is below our detection limit. We can see the signal
increase with increasing the Bi2 O3 concentration, suggestingthat the nonlinear optical response of Si-rich SiO2 is significantly enhanced by Bi-doping. The Bi-doped samples show
symmetric signals of a valley-to-peak structure. This meansthat the nonlinear optical response is mainly refractive andthe contribution of two photon absorption is negligible. Infact, open-aperture z-scan signals of these samples werebelow our detection limit. The solid curves represent resultsof fittings by a common model used for the analysis of a zscan measurement.14 The results are well-fitted by the modeland thus the nonlinear refractive index (n2 ) can be obtainedwith high accuracy. Figure 1(b) plots n2 as a function of theBi2 O3 concentration. The data for the Bi2 O3 concentrationof 0 mol. % are obtained for the samples 20 times thicker
than others. The excess Si concentration is fixed at 13.6 mol.
% for the Bi-doped samples, and changed from 4.8 to
20.2 mol. % for the undoped samples. It is clear that n2
increases with increasing Bi2 O3 concentration. With reference to the undoped samples, the enhancement factor of n2by Bi-doping reaches a maximum of 54, indicating that Bidoping can effectively enhance the nonlinear opticalresponse of Si-rich SiO2 . For all the samples, no signal wasdetected in the open-aperture z-scan measurements. In theFIG. 1. (a) Bi2 O3 concentration dependence of a closed-aperture Z-scan
spectra for Bi-doped Si-rich SiO2 films. (b)Bi2 O3 concentration dependenceof n2 estimated from the Z-scan signals. The data for the Bi2 O3 concentration of 0 mol. % are the same as those in our previous work on Si-rich
SiO2 .15Appl. Phys. Lett. 101, 191106 (2012)

present experimental setup, this indicates that the two photonabsorption coefficient (b) is smaller than 2  108 cm=W.
The nonlinear figure of merit (n2 =bk) is thus larger than 7for the samples with the highest Bi2 O3 concentration.
In order to obtain the information on the time responseof the nonlinear optical response, we performed OKG measurements. Figure 2 represents the results. The excess Si concentration is fixed to 13.6 mol. %. The signal of the undoped
sample is below our detection limit. On the other hand, we
can see a signal around zero time delay for the sample with
the Bi2 O3 concentration of 1.15 mol. %. The solid curve is a
result of fitting by the Lorentzian function. The signal issymmetric about the zero time delay indicating that theresponse time is comparable to or faster than our laser pulse
width (100 fs). The inset of Fig. 2 shows a result obtained
for a carbon disulfide (CS2 ) reference sample. The response
curve within a few picoseconds is consistent with previous
reports.16 The magnitudes of n2 of our samples can be estimated by comparing their OKG signal amplitude with that ofCS2 using the following equation:
ns2 ¼ ðIs =Ir Þ0:5 nr2 ðLr =Ls Þ;(1)where the subscripts s and r represent the sample and the reference, I and L represent the absolute magnitude of OKG
signals and the effective sample thickness, respectively.17Since the CS2 sample is rather thick (1 cm), the diffraction
length of the pump beam (1.8 mm) was used for Lr , assuming
a collinear approximation.18 By using 1:5  1014 cm2 =W as
the n2 of CS2 ,19 the n2 of Bi-doped Si-rich silica film wasfound to be 2:5  1011 cm2 =W. This value is consistent
with the value obtained from the z-scan measurement,
implying that the optical nonlinearity observed in the z-scanand the OKG measurements arises from the same origin. The
ultrafast response in the femtosecond order suggests that virtual, rather than real, transitions are responsible for the optical nonlinearity of Bi-doped Si-rich SiO2 . This is important

FIG. 2. The result of OKG measurements for Bi-doped and undoped Si-rich
SiO2 films. The inset shows the result for a CS2 reference sample.

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191106-3

Imakita et al.

because virtual-transition-related nonlinear optical processes
show the fastest response among all kinds of nonlinear optical processes. In addition, the virtual processes ensure thesmall wavelength dependence of the optical nonlinearity,implying that this material has potential applications in awide range of wavelengths, including the whole telecommunication windows.
Figure 3 shows the wavelength dependence of n2 . The
wavelength ranges from 750 to 835 nm. The data of theundoped sample are the same as those obtained in our previous report.15 For all the samples, we can see very weak
wavelength dependences. For example, when the Bi2 O3 concentration is 1.15 mol. %, the wavelength dependence is onlywithin 10% fluctuations. This small wavelength dependence
indicates that the optical nonlinearity arises from virtualtransition-related processes. Thus, the large enhancement of
n2 by Bi-doping can be expected even in telecommunication
wavelengths.
In order to discuss the origin of the nonlinear opticalresponse, we investigated n2 as a function of a wide range ofsample preparation parameters. Figure 4(a) shows excess Si
concentration dependences of n2 for the samples annealed atdifferent temperatures. The Bi2 O3 concentration is fixed at
0.7 mol. %. With increasing the excess Si concentration, n2increases, while it does not depend strongly on the annealing
temperature. When the excess Si concentration was zero, thez-scan signals were below our detection limit. Figure 4(b)shows Bi2 O3 concentration dependences of n2 for the samples annealed at different temperatures. The excess Si concentration is fixed at 13.6 mol. %. With increasing the Bi2 O3
concentration, n2 increases systematically. For all the annealing temperatures, the signals of the undoped samples werebelow our detection limit. Based on these results, we canconclude that both excess Si and doped Bi play an importantrole in enhancing the nonlinear optical response of Bi-doped
Si-rich SiO2 films. Probably, Bi atoms modify the electronic
configuration of Si-QDs and cause the enhancement of theoptical nonlinearity.

FIG. 3. Wavelength dependence of n2 of Bi-doped Si-rich SiO2 estimatedby Z-scan measurements. Bi2 O3 concentration is changed from 0 to
1.15 mol. %. The data for the Bi2 O3 concentration of 0 mol. % are the sameas those in our previous work on Si-rich SiO2 .15

Appl. Phys. Lett. 101, 191106 (2012)

FIG. 4. (a) Excess Si concentration dependence and (b) Bi2 O3 concentration
dependence of n2 for the samples annealed at different temperatures.

It should be mentioned here that there are two other possibilities for the enhancement of the optical nonlinearity. The
first one is Bi2 O3 in the SiO2 matrix glass. If Bi2 O3 is doped
into the SiO2 matrix glass, it can increase the linear refractive index of the matrix glass and consequently enhances thenonlinear optical response, according to the Miller’s rule.20However, this possibility can be ruled out by considering thefact that n2 of Bi2 O3 doped glass is 2 orders of magnitude
smaller than that found in the present work, despite the muchhigher Bi2 O3 concentration (>50 mol. %).21 The second possibility is the formation of Bi metal nano particles (Bi-nps).
If Si acts as a reducing agent for Bi2 O3 , Bi-nps can beformed in our samples, and as a result, the nonlinear opticalresponse can be enhanced. Indeed, Bi-nps doped glasses
have been reported to show strong nonlinear optical response
under femtosecond excitation at 800 nm.22 However, in oursamples, no Bi-nps were found by transmission electron
microscope (TEM) observation. Probably, as discussed byPark et al.,23 high temperature annealing (>900  C) is notfavorable for the formation of the Bi-nps in amorphous oxide
glass because of the low melting temperature of Bi metal(271  C).
In fact, our samples show much different absorption
spectra from those of Bi2 O3 or Bi-nps doped glasses. Figure5 shows the Bi2 O3 concentration dependence of the absorption spectra. We can see that the absorption is strongly
enhanced by Bi-doping in the visible region. The absorption
coefficient in the visible region for the most heavily doped
sample is of the order of 103 cm1 , which is two orders ofmagnitudes larger than that of a Bi2 O3 doped glass.24 The
absorption spectra have no distinct peaks in the ultravioletregion. This is significantly different from that of Bi-nps,
which show a distinct absorption peak around 240 nm arisingfrom the surface plasmon resonance.23 The enhancement ofDownloaded 03 Jan 2013 to 133.30.106.15. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

191106-4

Imakita et al.

Appl. Phys. Lett. 101, 191106 (2012)technical support on the measurement of spectroscopic
ellipsometry. This work was partly supported by KAKENHI
Grant No. 23310077.

1FIG. 5. (a) Bi2 O3 concentration dependence of absorption spectra ofBi-doped Si-rich SiO2 . The inset shows the expansion around 800 nm.

the absorption by Bi-doping is also considered to originate
from the modification of the electronic configuration of SiQDs caused by Bi-doping.
It is interesting to note here that there is no clear correlation between the optical nonlinearity and the linear absorption in the present work. The inset of Fig. 5 shows theabsorption spectra around 800 nm. The absorbance of Bidoped Si-rich SiO2 decreases with increasing the wavelength. On the other hand, as shown in Fig. 3, the n2 does notdepend on the wavelength. This contradiction is a clear evidence for the involvement of virtual transitions in the nonlinear optical processes of this material.
In conclusion, we have succeeded in observing a majorenhancement of ultrafast nonlinear optical response of Sirich SiO2 by Bi-doping. The enhancement factor of n2 by Bidoping reaches 54. The response time was shorter than ourtime resolution of 100 fs. n2 was shown to be independent of
the wavelength in the range from 750 to 835 nm. Theseresults indicate that Bi-doping is a promising approach indesigning a Si-QDs-based material toward the application in
nonlinear photonics.
The authors thank Mr. Y. Miwa for the valuable discussion on the sample preparation and Dr. S. Tomita for his

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