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Li, Diao; Jussila, Henri; Karvonen, Lasse; Ye, Guojun; Lipsanen, Harri; Chen, Xianhui; Sun,
Zhipei
Polarization and thickness dependent absorption properties of black phosphorus
Published in:
Scientific Reports
DOI:
10.1038/srep15899
Published: 30/10/2015
Document Version
Publisher's PDF, also known as Version of record
Published under the following license:
CC BY
Please cite the original version:
Li, D., Jussila, H., Karvonen, L., Ye, G., Lipsanen, H., Chen, X., & Sun, Z. (2015). Polarization and thickness
dependent absorption properties of black phosphorus: New saturable absorber for ultrafast pulse generation.
Scientific Reports, 5, 1-9. [15899]. https://doi.org/10.1038/srep15899
1
SCIENTIFIC RepoRts | 5:15899 | DOI: 10.1038/srep15899
www.nature.com/scientificreports
Polarization and Thickness
Dependent Absorption Properties
of Black Phosphorus: New
Saturable Absorber for Ultrafast
Pulse Generation
Diao Li
1,2,*
, Henri Jussila
1,*
, Lasse Karvonen
1
, Guojun Ye
3,4
, Harri Lipsanen
1
, Xianhui Chen
3,4,5
& Zhipei Sun
1
Black phosphorus (BP) has recently been rediscovered as a new and interesting two-dimensional
material due to its unique electronic and optical properties. Here, we study the linear and nonlinear
optical properties of BP akes. We observe that both the linear and nonlinear optical properties
are anisotropic and can be tuned by the lm thickness in BP, completely dierent from other
typical two-dimensional layered materials (e.g., graphene and the most studied transition metal
dichalcogenides). We then use the nonlinear optical properties of BP for ultrafast (pulse duration
down to ~786 fs in mode-locking) and large-energy (pulse energy up to >18 nJ in Q-switching) pulse
generation in ber lasers at the near-infrared telecommunication band ~1.5 µm. We observe that
the output of our BP based pulsed lasers is linearly polarized (with a degree-of-polarization ~98%
in mode-locking, >99% in Q-switching, respectively) due to the anisotropic optical property of BP.
Our results underscore the relatively large optical nonlinearity of BP with unique polarization and
thickness dependence, and its potential for polarized optical pulse generation, paving the way to BP
based nonlinear and ultrafast photonic applications (e.g., ultrafast all-optical polarization switches/
modulators, frequency converters etc.).
Pulsed laser sources are used in a variety of applications
1–3
, ranging from basic research to telecom-
munications, medicine, and industrial material processing
1–3
. e most-widely used pulsed lasers uti-
lize a Q-switching method or a mode-locking technique
1–3
, in which a typical nonlinear optical device,
called saturable absorber (SA), turns the continuous wave output of the laser into a periodic train of
optical pulses. e SA technology is currently dominated by semiconductor saturable absorber mir-
rors (SESAMs)
1–3
. However, they typically have limited bandwidth and require complex fabrication and
packaging
1
. Recently, carbon nanotubes (CNTs)
4,5
and graphene
6,7
have been demonstrated for SAs with
superior performances
8–11
, such as broad operation bandwidth
12–14
, fast recovery times
15–18
, low satura-
tion intensity
4–18
, cost-eective and easy fabrication
4–19
. Nevertheless, SAs based on these materials still
suer from drawbacks. For example: when operating at a particular wavelength, CNTs which are not
Hefei National Laboratory for Physical
*
R
A
P
OPEN
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SCIENTIFIC RepoRts | 5:15899 | DOI: 10.1038/srep15899
in resonance cannot be used, and thereby give relatively large insertion losses
6–8,19
; On the other hand,
mono-layer graphene typically has rather weak absorption (~2.3%
20,21
), not suitable for various lasers
(e.g., ber lasers), which typically need relatively larger modulation depth
8–10
; Layered transition metal
dichalcogenides (TMDs) (e.g., MoS
2
22
, WS
2
23
, and MoSe
2
24,25
) have also been demonstrated for SAs, but
with limited performance for current lasers typically operating at the near-infrared and mid-infrared
range, due to their comparatively large bandgap near or in the visible region
26
(~1.8 eV for MoS
2
, ~2.1 eV
for WS
2
, ~1.7 eV for WSe
2
27
).
Black phosphorus (BP), a layered material consisting of only phosphorus atoms, has recently been
rediscovered for various applications in electronics and optoelectronics
28–54
(such as transistors, solar
cells, and photodetectors). In contrast to graphene and TMDs, BP has its own unique properties
28–54
. For
example, its direct electronic band gap can be tuned from ~0.3 to ~2 eV (corresponding to the wave-
length range from ~4 to ~0.6 µ m), depending on the lm thickness
28–54
. is is particularly interesting for
photonics, as it can oer a broadly tuneable bandgap with number of layers for the near and mid-infrared
photonics and optoelectronics, and thus bridge the present gap between the zero bandgap graphene and
the relatively large bandgap TMDs
33
.
However, thus far, intensive research eorts on BP have mainly focused on its electronic proper-
ties (e.g., transistor performance) and linear optical response (e.g., photo-detector performance). In this
paper, we investigate the thickness and polarization dependent linear and nonlinear optical properties
of BP thin lms, which are integrated into ber devices, the most commonly-used format for optical
telecommunication. Our results show that both linear and nonlinear absorption properties are strongly
thickness/polarization dependent, completely dierent from other typical two-dimensional layer materi-
als (e.g., graphene
8–11
and the most studied TMDs
22–25
). We also demonstrate the use of nonlinear optical
property of BP for ultrafast (pulse duration down to ~786 fs in mode-locking) and large-energy (pulse
energy up to > 18 nJ in Q-switching) pulse generation in ber lasers at the near-infrared telecommunica-
tion band ~1.55 µ m. Intriguingly, we observe that the output polarization state of our pulsed ber lasers
is linear (with a degree-of-polarization ~98% in mode-locking, ~99% in Q-switching) due to the unique
anisotropic absorption property of BP. ese results open the avenue to BP based nonlinear and ultrafast
photonic applications (e.g., ultrafast optical switches/modulators, frequency converters etc.).
Results and Discussion
Atomic Force Microscopy and Raman spectroscopy. BP thin lms are produced by microme-
chanical cleavage of a bulk BP crystal, and then transferred to optical ber ends (details in Methods).
e thicknesses of the transferred lms on ber ends are measured by Atomic Force Microscopy (AFM).
Figure1a,b show AFM image taken from a typical BP lm and its line prole along the dashed white
line. e circular ber cladding can be resolved from the image, and the location corresponding to the
ber core (marked with the green circle) of a standard single mode ber (Corning SMF-28, with a core
diameter of ~10 microns) is drawn schematically in Fig.1a. e thickness of the transferred BP lm is
estimated to be ~25 nm at the location corresponding to the ber core (Fig.1b). Typically, the thickness
of transferred BP lms ranges between ~20 nm and ~1 µ m, depending on the micromechanical cleavage
process. To verify that the transferred material is BP, we perform polarization-resolved Raman scattering
measurements. Raman spectrum of a BP crystal is depicted in Fig.1c. ree peaks located at the wave-
numbers of 363 cm
−1
, 441 cm
−1
and 469 cm
−1
can be observed from the Raman spectrum, and attributed
to A
g
1
, B
2g
and A
g
2
vibration modes of BP crystal lattice, respectively. is agrees well with previously
published results on BP lms
30,31,55
. e Raman peak intensity is also strongly dependent on excitation
light polarization (Supplementary Fig. 2) due to its highly anisotropic optical responses
31,45,46
, and this
has been noted to oer a unique method for determining the crystal orientation of BP lms
31,45,46
.
Thickness and polarization dependent linear optical absorption. We characterize the linear
absorption properties of BP lms transferred to the optical ber ends. e linear transmittance results
Figure 1. (a) AFM image of transferred black phosphorus lm on the ber end. (b) Line prole along the
dashed white line (marked in (a)). e thickness of BP lm is ~25 nm at the ber core (marked with a green
circle in (a)). (c) Raman spectrum of a typical BP lm.
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SCIENTIFIC RepoRts | 5:15899 | DOI: 10.1038/srep15899
acquired at 642 nm (~1.93 eV, Fig.2a) and 520 nm (~2.38 eV, Fig.2b) show that the transmittance of BP
thin lms decreases with the lm thickness. Note that transmittance includes the contribution from light
absorption and reection. As shown in Fig.2a,b, the transmittance T agrees well with the t (solid lines)
using the Beer-Lambert law (i.e.,
=∼ (−α× )T expd
where α is the absorption coecient and d is the
lm thickness) with the tted values of α
642
nm
= ~5.7 µ m
−1
, α
520
nm
= ~10 µ m
−1
. ese values are compa-
rable to what previously measured and predicted
36
. anks to the availability of our polarization-tuneable
continuous-wave light source at 1.55 µ m (~0.8 eV), we measure the transmittance change of our BP lms
as a function of incident light polarization angle at this wavelength (i.e., 1.55 µ m). e results from 25 nm
and 1100 nm thick BP lms are given in Fig 2c. It appears that the input light polarization direction
strongly aects absorption of the BP lm (and thus the transmittance). For instance, we observe that the
transmittance of the 1100 nm thick BP lm can increase by a factor of > 9 (from 3.6% to 33.2%) when
Figure 2. Linear and nonlinear optical properties of BP lms: Transmittance of BP lms as a function of
thickness at the wavelengths of 642 nm (a) and 520 nm (b). (c) Polarization dependent transmittance for 25
nm and 1100 nm thick BP lms. e polarization directions corresponding to the maximum and minimum
transmittance are linked with the zigzag and armchair axes of BP thin lms. (d) Transmittance of BP lms
as a function of lm thickness at the wavelength of 1550 nm with two orthogonal polarized light directions.
(e) Fluence dependent transmittance of the 1100 nm thick BP lm measured with ultrafast pulses at two
orthogonal polarized light directions. (f) Relative transmittance change measured from 25 nm, 350 nm and
1100 nm thick BP lms as a function of input pulse uence. e input polarization direction is along the
armchair direction of the BP lms.
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4
SCIENTIFIC RepoRts | 5:15899 | DOI: 10.1038/srep15899
the input polarization direction is altered. It clearly shows the absorption anisotropy of BP
36,37,51,54
. e
polarization directions corresponding to the maximum and minimum transmittance are assigned along
the zigzag and armchair directions of BP thin lms
36,37
, respectively. erefore, such an anisotropic
absorption property can be employed to determine the crystal orientation of BP lms, similarly to the
Raman approach
31,45,46
. Worth noting that this property can be utilized directly for various
polarization-based photonic applications (e.g., polarizers).
We nd that the polarization dependent transmittance change is signicantly larger in thicker sam-
ples, which agrees with the recent theoretical simulation
36,37
. For example, the transmittance change
(~29.6%) of the 1100 nm thick sample is > 6 times larger than the result (~4.8%) of the 25 nm sample
(Fig.2c). Detailed transmittance of samples with variable thicknesses at two orthogonal polarized light
directions (Fig. 2d) further conrms that the polarization-introduced transmittance change, which is
linked to the selection rules associated with symmetries of the anisotropic material
36,37,54
, is thickness
dependent. At this wavelength (i.e., 1.55 µ m, Fig.2d), we also observe that the lm thickness dependent
transmittance matches well with a bi-exponential decay t which contains two dierent absorption coef-
cients, in contrast to the single exponential decay t of using the Beer-Lambert law at the wavelengths
of 642 nm and 520 nm (Fig.2a,b). As depicted in Fig.2d, the transmittance decreases rst rapidly until
the thickness of ~80 nm. Aer that, the transmittance decreases slowly. e thickness-dependent band-
gap (E
g
) change of BP has been predicted to follow a power law (e.g.,
≈+.
.
.
E
03eV
g
17eV
n
073
, in which n
is the number of layers)
36,37,49
. Hence, the change in bandgap attributable to the increasing lm thickness
can be deduced to be extremely small and will not aect the absorption signicantly (compared to the
0.8 eV (~1.55 µ m) photon energy used in this experiment), when the sample is thicker than 10 nm.
However, it has been calculated that sub-bands close to the bandgap signicantly change with the thick-
ness
36,37,43
. erefore, we assign the rapid decrease in transmittance (when the ake thickness is < 80 nm)
mainly to evolution of sub-band energy states
36,37,43
in BP with the lm thickness. We believe the
Beer-Lambert law dominates the thickness-dependent transmittance change for thicker samples
(> ~80 nm), similarly to what we observed for the relatively large photon energy transmittance measure-
ment experiments (642 nm in Fig.2a, and 520 nm in Fig.2b).
Thickness and polarization dependent nonlinear optical absorption. e nonlinear absorption
measurement results are illustrated in Fig.2e,f. In our measurement setup (Supplementary Fig. 4), we
placed a polarization controller before the BP lms to adjust the polarization direction of the input
ultrafast pulses. Figure2e depicts the nonlinear absorption measurement results of an 1100-nm thick
BP lm with two orthogonal polarization directions. A clear increase in the transmittance with the
increased pump uence can be observed in the 1100 nm thick BP sample and is attributed to saturable
absorption
43,53
. e polarization dependent nonlinear optical performance dierence is also observed
in Fig. 2e, which is of great interest for various photonic applications, e.g., tuning operation states in
ultrafast lasers
56
, switching optical pulses with their polarization directions, and ultrafast vector soliton
generations.
Figure2f shows the relative transmittance change (∆ T/T
0
, where ∆ T and T
0
are transmittance change
and the transmittance at the minimum input power, respectively) for three BP lms with the polarization
state corresponding to the maximum absorbance (i.e., the armchair-polarized input). Nonlinear saturable
absorption is clearly observed in all samples and occurs when the uence reaches to ~100 µ J/cm
2
. We
also note that the thicker sample has ~8-time larger relative transmittance change than the thinner one.
is shows that the nonlinear property of BP can be adjusted by the thickness (i.e., number of layers).
Such property can be utilized for pulse generation in dierent laser formats (e.g., ber and semiconduc-
tor lasers), in which nonlinear saturable absorbers with dierent parameters are needed
8–11
.
To estimate the saturation uence and modulation depth from the nonlinear absorption curves, we
use a simplied uence dependent absorption formula to t the measurement results (descripted in
Supplementary Information). e tted curves match decently with the measurement results and are
plotted with solid lines in Fig.2e,f. e obtained saturation uence from all the samples varies in the
range of 2000 µ J/cm
2
and is, therefore, around an order of magnitude larger than that typically measured
with SAs fabricated from CNTs or graphene
7–11,16
. On the other hand, the transmittance change obtained
from the measured curves is observed to be larger than 1% (Fig.2e). However, the modulation depth
obtained from the ts typically ranges between 50% and 90%. If true, this observation is promising as
the tted modulation depths are extremely large. However, we note that the tted modulation depth is
probably unrealistically high and most likely relate to the fact that the nonlinear absorption measurement
should be continued to larger uence range which is currently unavailable in our setup. In our nonlinear
absorption measurement setup (Supplementary Fig. 4), the available maximum uence is ~450 µ J/cm
2
.
Q-switched high-energy pulse generation. We use our BP integrated ber device to build a pulsed
ber laser working at the main telecommunication window of 1.55 µ m. Fiber laser is selected in our
experiments, as it can oer simple and compact design, ecient heat dissipation, and high-quality pulse
generation
57,58
. e layout of our designed ber laser is schematized in Fig.3a. A ~1-m Erbium-doped
ber (EDF) is utilized as the gain medium, which is pumped by a 980 nm laser diode (LD) via a wave-
length division multiplexer (WDM). A polarization-independent isolator (ISO) is placed aer the gain