a
a
b
c
b
a,*
a,* a Organic Semiconductor Centre, SUPA, School of Physics and Astronomy, University of St. Andrews, North Haugh, St. Andrews, Fife KY16 9SS, United Kingdom b
WestCHEM, Department of Pure and Applied Chemistry, University of Strathclyde,
Thomas Graham Building, 295 Cathedral Street, Glasgow G1 1XL, United Kingdom c
Centre for Materials Science, Faculty of Science and Technology, University of
Central Lancashire, Preston, PR1 2HE, United Kingdom
Abstract
We report amplified spontaneous emission (ASE) and optically pumped deep-blue-emitting distributed feedback (DFB) lasers based on a star-shaped oligofluorene truxene molecule. A low ASE threshold of 2.1 kW/cm
2
at 439 nm was achieved. The material exhibits a high net gain of 38 cm
-1 and also low optical loss coefficient of 3.5 cm
-1
. Second-order DFB lasers show tuning of the emission wavelength from 422 to 473 nm, and a minimum threshold density of 515 W/cm
2
.
This is the broadest tuning range (51 nm) reported for organic deep-blue/blue lasing materials.
1

1.
Introduction
Over the past decade, the development of organic semiconductors has attracted a great deal of interest [1-4]. They have become very attractive materials for a range of photonic devices including light-emitting diodes [5], photovoltaic cells [6], organic field effect transistors [7] and lasers [4]. The solution processing of these materials promises low-cost optoelectronic devices and simplified fabrication of wavelength-scale structures. This is particularly relevant for organic semiconductor lasers which have been demonstrated in a wide range of novel resonator structures allowing low-threshold optical pumping by lasers or even LEDs [8, 9]. One of the most interesting properties of organic semiconductor gain materials is that they have broad emission spectra and a selection of materials can cover the whole visible spectrum [10-15].
While many of the advances in organic semiconductor lasers have used conjugated polymers as the gain medium, conjugated oligomers offer some advantages,
Oligomers are monodisperse and characterized by a well-defined and uniform molecular structure, and can have superior chemical purity [16]. However, oligomers are prone to aggregation which can quench light emission [17, 18]. This issue can addressed by attaching the oligomers to the truxene
(10,15-Dihydro5H -diindeno[1,2a ;1’,2’c ]fluorene), which is a polycyclic aromatic system with C
3
symmetry [19], leading to a promising new class of oligomeric organic laser material [20, 21]. Here, we report a systematic study of the photophysics, gain and lasing behaviour of the T3 oligomer, a truxene core with three fluorene units on each arm. We have found that the material T3 has high photoluminescence quantum yield (PLQY) and low ASE threshold. Solid-state DFB lasers have also been fabricated with T3. In addition to low threshold we demonstrate a very broad tunability for this deep-blue to blue emitter.
2

2.
Experimental
The chemical structure of the truxene oligofluorene molecule studied (T3) is shown in
Fig. 1a. The molecule consists of a central truxene core with three fluorene arms attached. Each arm consists of three 9, 9-dihexylflurorene units. Films of 80 – 130 nm thickness were made by spin-coating from 20 mg/ml toluene solutions on quartz substrates. The film thicknesses were measured with a Veeco Dektak 150 surface profiler. Absorption spectra were measured with a Cary 300 UV-Vis spectrophotometer and PL spectra were measured with a Jobin Yvon FluoroMax2 fluorometer. PLQY measurements of thin films (of approximately 130 nm thickness) were made using the method of Greenham et al [22], with the 325 nm output of a
He-Cd laser for excitation. The photoluminescence lifetime was measured with a streak camera in the spectral window of 400 nm to 500 nm.
To assess the potential of T3 as an optical gain medium we first measured amplified spontaneous emission in the films under pulsed optical excitation.
The excitation light in the ASE measurements was generated by a Nd:YAG laser pumped optical parametric oscillator (OPO) with 4 ns output pulses at a repetition rate of 20 Hz. The pump wavelength was 375 nm which is close to the absorption maxima of T3.
Calibrated neutral density filters were inserted in the beam path to adjust the pump intensity incident on the samples. A cylindrical lens was used to shape the excitation beam into a narrow stripe with a width of approximately 200 μm on the surface of the film. The length of the pump stripe was in the range 0.6 to 2 mm, controlled by an adjustable aperture, between 2 mm and 0 mm. The edge emission from the films was collected and recorded with a fibre-coupled grating spectrograph and a charge coupled device (CCD) detector. During all the measurements, the film was located in a vacuum chamber with a pressure of approximately 10
-5
mbar. The net gain per unit length, g , was studied using the variable stripe length method [23, 24]. For gain measurements, the output intensity was recorded as a function of excitation length, for a range of different pump intensities. In theory, when ASE occurs the output intensity
3

from one end of the pump stripe is given by:
I (

)

AI g pump
(

)
( e g (

) l 
1 ) , (1) where l is the length of the pump stripe. To measure the waveguide loss coefficient,
α
, the pumped stripe on the film was moved away from the edge of the sample without changing its length. The pump stripe was fixed in the measurements to be 4.5 mm ×
200 μm. The emission from the edge should then be given by:
I

I
0 e
(
  x )
, (2) where x is the distance between the end of the pump stripe and the sample edge.
The optical constants of the thin films of T3 truxene were investigated by variable angle spectroscopic ellipsometry using a J. A. Woollam Co. Inc. M2000-DI ellipsometer. Eight samples of different thickness (30 nm – 50 nm) were prepared by spin coating from a T3 solution (toluene, 10 mg/ml) onto fused silica substrates.
Polarised light reflection data were collected for angles of incidence between 45° and
75° in intervals of 5° and in the wavelength range of 190 nm to 1700 nm.
Transmission measurements at normal incidence were also measured and a general oscillator Cauchy model was fitted to the combined reflection and transmission data.
T3 films were deposited onto corrugated fused silica substrates to form distributed feedback lasers. These provide laser feedback at a wavelength determined by the
Bragg condition, m

Bragg

2 n eff

, where

Bragg
is the wavelength of the light,
Λ
is the period of the resonator structure, n eff
is the effective refractive index of the waveguide and m is an integer that represents the order of the diffraction. The gratings used in our T3 truxene lasers are designed for second order ( m = 2) DFB laser operation. They had periods
Λ of 270 nm, 280 nm (feedback in one-dimension, 1D) and 301 nm (square-array, two-dimensional feedback, 2D) and depths of 30 nm to 50 nm with fill factors of ≈ 50%, shown from atomic force microscope (AFM) images.
By spin-coating T3 toluene solutions (25 mg/ml or 30 mg/ml) onto the corrugated
4

silica substrates at speeds in the range of 800 – 1500 rpm, the thickness of the films was varied between 100 and 160 nm, and hence, the effective refractive index of the waveguide could be changed to support different laser output wavelengths. The T3 truxene lasers were optically pumped with 355 nm pulses at a repetition rate of 500
Hz from a frequency-tripled Nd: YVO
4
laser. The pump beam was incident onto the samples at an angle of 20° to the surface normal and was focused to a 1.1 mm diameter spot (measured with a Coherent Inc. Beam Profiler at the position of the T3 films), which resulted in the excitation area of 0.0095 cm
2
.
3.
Results and Discussions
Fig. 1b shows the absorption, PL excited at 325 nm, and ASE spectra of T3 truxene.
The absorption has a maximum at 367 nm and the main emission peaks (excitation wavelength 325 nm) are at 414 nm (0-0 vibronic transition), and 438 nm (0-1 vibronic transition), with a shoulder near 461nm (0-2 vibronic transition). The solid-state
PLQY of T3 was measured to be 86% ± 10%, a value much higher than other typical organic blue emitting materials [25].
When pumped at high excitation densities, we observed ASE from the edge of the film, with a line-narrowed spectrum (solid line in Figure 1b) peaked at 439 nm, close to the 0–1 vibronic peak of the PL emission. This is consistent with a quasi-four-level vibronic system allowing low threshold operation [26]. Figure 2 shows the full-width at half-maximum (FWHM) linewidth of the emission spectrum, and relative output power, as a function of the pump intensity. We find that the FWHM dropped dramatically from 47 nm to 4 nm when pumping above a threshold intensity of 2.1 kW / cm
2
. This threshold intensity is substantially lower than the corresponding value of 58 kW / cm
2
reported for polyfluorene [27]., The reduction in linewidth occurs because the net gain is maximal near the 0-1 peak of the PL emission spectrum and thus the spectrum exhibits gain narrowing as the pump intensity increases.
5

Figure 3a shows the ASE output intensity at 439 nm as a function of pump stripe length, at four different pump intensities. The data were fitted to Equation 1, and the net gain coefficient was found to be as high as g = 38 cm

1
for a pump power density of 58 kW / cm
2
. At higher pump energies, the gain coefficient no longer increased with the increasing pump intensity. This suggests gain saturation in the oligomer waveguide, when the light travelling in the waveguide becomes sufficiently intense that it depletes a substantial fraction of the excited population [24]. The results of optical loss measurements are shown in Figure 3b. In the semi-log graph, the slope of the lines corresponds to a loss coefficient of 3.5 cm
-1
, a relatively low value compared to other fluorene-based organic emitters [28].
The measured ellipsometry data were fitted with both isotropic and uniaxial Cauchy models. The isotropic model yielded an excellent fit with a mean squared error
(MSE) value of 0.8879, indicating that the T3 molecules in the film are arranged in random directions. The uniaxial fit to the data gave a high agreement of the ordinary and extraordinary refractive indices, confirming that the T3 films are isotropic. These results show that there is a random arrangement of the molecules, which is also consistent with the low waveguide losses within the films. The refractive index of T3 at the peak ASE wavelength of 439 nm is found to be n eff
= 1.77 from the ellipsometric measurements. With this value known, the effective refractive index of the guided fundamental TE
0
mode can be then calculated for the air-T3-silica waveguide structure. Taking the refractive index of fused silica, n silica
= 1.46 and assuming a film thickness of 150 nm, we calculate n eff
= 1.59. The Bragg wavelength could now be determined from the Bragg condition m

Bragg

2 n eff

with a certain period of the grating for second order surface emitting laser.
We next measured the laser characteristics of surface-emitting T3 DFB lasers for a range of film thicknesses. Figure 4 shows how the threshold of the lasers varied with
6

operating wavelength. A minimum laser threshold of 515 W/cm 2 was obtained close to the ASE wavelength at 430 nm with the 1D 270 nm period grating (inset Figure 5).
Towards shorter wavelengths self-absorption increases the optical loss and hence lasing threshold. For longer wavelengths, the threshold increases due to the decrease of the wavelength dependent emission cross-section. Above the pump threshold, the emission beam from the 1D T3 laser is a fan-shaped divergent stripe and parallel to the orientation of the grating grooves.
In the case of the 2D laser, the emission forms a cross shape as expected for a two-dimensional resonator pumped far above threshold
[4]. Figure 5 shows a series of lasing spectra from the 1D and 2D T3 DFB lasers. We observe a very broad wavelength tuning, spanning the range of 422 – 473 nm (2.62 –
2.94 eV in energy). The tuning range of 51 nm (0.32 eV) compares very favourably with not only blue lasing materials such as the tuning range of 39 nm (0.24 eV) reported for polyfluorene [12] and 33 nm (0.23 eV) for spirobifluorene [11], but also red emitters such as Dow Red F with a tuning range of 0.21 eV [10]. For some T3 lasers with films thicker than 100 nm we observe two lasing modes at the same time.
These are separated clearly in wavelength and are identified as lasing in the TE
0
and
TM
0
modes, with a blue shift of TM mode against TE mode because of its lower effective refractive index.
4.
Conclusion
In summary, we have presented a detailed investigation of the photophysical characteristics of the truxene T3 oligomer and of organic thin-film semiconductor lasers based on T3. The emission of this material spans the wavelength range from
390 nm to 520 nm. The solid-state PLQY was as high as 86% ± 10%. The thin film slab waveguide ASE measurement showed a net gain of up to 38 cm
-1
and a low waveguide loss coefficient of 3.5 cm
-1
. A low ASE threshold of 10.6 μJ/cm
2
was obtained (at 439 nm). The material was found to form isotropic films by spin-coating, which contributes to the low waveguide losses of the thin films. A series of blue-emitting second-order DFB lasers based on this oligofluorene truxene molecule
7

have been demonstrated. A minimum T3 lasing threshold of 515 W/cm 2 has been demonstrated, which is slightly higher than the truxene oligomer T4 [21], possibly because the resonator structure needs further optimisation. T3 has very broad tunability across 51 nm (422 – 473 nm) in the deep blue/blue region of the spectrum, corresponding to an energy tuning range of 0.32 eV. Compared to the photophysical properties of the truxene oligomer T4, T3 has higher solid-state PLQY, lower amplified spontaneous emission threshold, higher net gain coefficient and much wider lasing tunability.
8

(b)
0.8
1.0
0.6
0.8
0.6
0.4
ASE
0.2
0.4
0.2
300 350 400 450 500
0.0
Wavelength (nm)
Figure 1: (a) Chemical structure of the T3 oligofluorene truxene
(b) Absorption (dotted line) and photoluminescence (dashed line) spectra of T3 truxene, along with the ASE spectrum (solid line)
9

16
14
12
10
800
600
400
200
8
6
0
0 10 20 30 40 50 60
Pump Energy Intensity (

J/cm
2
)
4
0 10 20 30 40 50 60
Pump Energy Intensity (

J/cm
2
)
Figure 2: The dependence of the FWHM of the emission spectra on the pump energy density; (Inset) The total emission intensity from the edge of a T3 truxene waveguide versus pump energy density
10

10000
pump@92kW/cm
2
g=37.6
pump@58kW/cm
2
g=37.7
pump@15kW/cm
2
g=22.5
1000
100
10
0.06 0.08 0.10 0.12 0.14 0.16
Excitation Length (cm)
(a)
1000
100
0.0
0.1
0.2
0.3
0.4
Distance From Edge (cm)
0.5
(b)
Figure 3: (a) emission intensity as a function of excitation length at four different pump intensities above ASE threshold and (b) the ASE intensity emitted from the edge of the sample as a function of the distance between the pump stripe and the edge
11

of the sample
1000
900
800
1.0
0.8
0.6
0.4
0.2
0.0
200 700
600
400 600
Wavelength (nm)
800
500
420 430 440 450 460 470
Wavelength (nm)
Figure 4: Laser threshold power densities for the T3 DFB lasers. The inset shows the lowest threshold at 430 nm.
12

Figure 5: Laser emission spectra for different grating periods of 2 nd order T3 DFB lasers with the PL spectra (dotted line). Lasing from 422 – 427 nm are from 280 nm period 1D grating, 429 nm – 443 nm are from 270 nm period 1D grating and 445 to
473 nm are from 301 nm period 2D grating.
13

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