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Strain-modulated high-quality ZnO cavity modes on different crystalorientations

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IOP Publishing Journal Title

Journal XX (XXXX) XXXXXX https://doi.org/XXXX/XXXX

xxxx-xxxx/xx/xxxxxx 1 © xxxx IOP Publishing Ltd

Strain-modulated High-quality ZnO Cavity Modes

on Different Crystal Orientations

Qiushuo Chen1,2, Yiyao Peng2, Fangtao Li2, Wenda Ma2, Ming-hua Zhuge3,

Wenqiang Wu2, Junlu Sun2, Xiaohong Yang1,*, Junfeng Lu4,* and Caofeng Pan2,5,6,7*

1 College of Physics and Electronic Engineering, Chongqing Normal University, Chongqing 401331,

People’s Republic of China 2 CAS Center for Excellence in Nanoscience, Beijing Key Laboratory of Micro-nano Energy and

Sensor, Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing

100083, People’s Republic of China 3 State Key Laboratory of Modern Optical Instrumentation, College of Optical Science and Engineering,

Zhejiang University, Hangzhou 310027, People’s Republic of China 4 College of Science, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, People’s

Republic of China 5 College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen, 518060, People’s

Republic of China 6 Center on Nanoenergy Research, School of Physical Science and Technology, Guangxi University,

Nanning, Guangxi, 530004, People’s Republic of China 7 School of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing 100049,

People’s Republic of China

E-mail: [email protected]

Received xxxxxx

Accepted for publication xxxxxx

Published xxxxxx

Abstract

Dynamically regulated the coherent light emission offers a significant impact on improving

white light generation, optical communication, on-chip photonic integration, and sensing. Here,

we have demonstrated two mechanisms of strain-induced dynamic regulation of ZnO lasing

modes through an individual ZnO microbelt and microrod prepared by vapor-phase transport

method. And systematically explained the dependence on externally applied strain and crystal

orientation. Compared with the reduced size of resonant cavity played a major role in the

microbelt, the resonant wavelength variation of the microrod under tensile stress is affected by

the change in both the cavity size and the refractive index, which tend to antagonize in the

direction of movement. It shows that the refractive index can be effectively regulated only when

the stress is in the same direction along the c-axis. The results on the linear relationship between

the resonance wavelength variation and applied strain imply the capacity of the devices to

detect tiny stresses due to the ultra-narrow line width of the cavity mode with a high-quality

factor of ~ 104. It not only has a positive influence in the field of the modulated coherent light

source, but also provides a feasible strategy for implementing color-resolved non-contact strain

sensors.

Keywords: ZnO, mode regulation, high-quality, strain, crystal orientation

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1. Introduction

Since the first semiconductor laser was invented in 1962, it

has been rapidly developed and gradually applied in scientific

research [1-3], national defense, industrial manufacturing and

other fields. Subsequent wavelength tunability has been one

of the important themes of laser performance optimization due

to its demonstrated potential for white light generation [4],

optical communication [5], on-chip photonic integration [6],

and sensing [7, 8]. To date, the traditional ways to implement

wavelength-tunable lasers are to adjust the bandgap [9-11] and

design the cavity [12-15], while it lacks the ability to

reversibly modulate due to the limitations in mechanism and

technology. By applying stress, the resonant wavelength of the

cavity modes can be dynamically modulated and has been

demonstrated in Ⅱ-Ⅵ semiconductors, Ⅲ-Ⅴ semiconductors,

and inorganic halide perovskites, but the mechanism of the

variation is still controversial and further proof of comparison

is indispensable. According to the laser principle, the resonant wavelength

of the corresponding mode is determined mainly by two

factors: cavity size and relative refractive index, which means

that cannot be explained by the photoluminescence spectrum

shift caused by the bandgap change under stress [16, 17]. In

general, the researchers control the number of modes in the

gain region by reducing the cavity size to increase the free

spectral range, and rarely use it to regulate the resonant

wavelength of lasing mode because the lasing mode cannot be

dynamically modulated in the predesigned resonant cavity

[18, 19]. On the other hand, K.Vedam et al. reported that the

dominant factor of crystal refractive index change caused by

external mechanical strain in wurtzite structure semiconductor

materials such as ZnO and CdS can be attributed to the

polarization of the medium, which produces a directional

dipole moment resulted to the change in the dielectric constant

of the active cavity, as early as 1969 [20]. Recently, dynamic

regulation of the lasing mode through the variable refractive

index induced by piezoelectric polarization effect has been

widely applied to various asymmetric center semiconductor

active cavity, such as ZnO [21, 22], CdS [23], GaN [24],

perovskite [25], and so on. However, the above discussion is

obtained under the permission of ignoring cavity changes, the

proportion of both applied stress is still unclear. Therefore, it

is essential to prove the exact influence of stress on the

refractive index and cavity size through further experimental

means.

Here, combined with an induvial ZnO microbelt and

microrod prepared by vapor-phase transport method, we have

demonstrated two mechanisms of stress-induced dynamic

regulation of ZnO lasing modes. While the shrinkage of the

cavity plays a major role in the microbelt, the redshift of the

resonant wavelength in ZnO microrod under tensile stress is

affected by both the reduced cavity and the increased relative

refractive index, which tend to antagonize in the direction of

movement. It reflects that the spatial relationship between the

applied strain and the crystal orientation is the predominant

factor in determining mode regulation. Moreover, it is

promising as a high-resolution sensor for detecting tiny

stresses based on a linear relationship between the variational

resonant wavelength and applied strain. It could be a positive

influence in the field of modulated coherent light source and

color-resolved stress sensor.

2. Experimental

2.1 Material Preparation and Characterization

High-quality ZnO microbelts and microrods were

synthesized by vapor-phase transport method [26-29]. Briefly,

a mixture of ZnO and graphite power with mass ratio of 1:1

was used as a reaction source, which was placed at the bottom

of a single-pass quartz tube to ensure the stability of reaction

process. A clean silicon wafer was placed near the nozzle as a

substrate. Then, it was placed into the center of a tubular

furnace to keep a reaction temperature of 1130℃ for 30 min.

Subsequently, the as-prepared ZnO microrods were obtained

on the substrate while microbelts were obtained on the inner

wall of the quartz tube. Finally, an individual ZnO microbelt

and microrod was mechanically transferred to the flexible PET

and fixed with epoxy resin glue, a mixture of epoxy resin and

hardener with mass ratio of 2:1. The morphology and structure

of as-synthesized ZnO was characterized by a hot-field-

emission SEM (Quanta 450) and HRTEM (Tecnai G2 F20 S-

TWIN YMP) with a working voltage at 200 kV.

2.2 Optical Measurement

A highly integrated microsystem was used to characterize

the lasing performance of ZnO microbelts and microrods. A

femtosecond pulsed laser (repetition rate 1KHz, pulse

duration 190fs) was employed as the excitation source for the

wavelength of 355nm. The spontaneous and stimulated

emission were collected and analyzed by a charge-coupled

device (CCD) detector and an optical multichannel analyzer

(Andor, SR-500i-D1-R, 1800 g/mm grating) equipped with a

confocal μ-PL system (Zeiss M1). The microsystem was also

used to collect the Photoluminescence (PL) Spectroscopy as

long as the grating was set to 600 g/mm. Raman spectra were

recorded by laser confocal micro-Raman system (LabRAM

HR Evolution) with an excitation wavelength of 532 nm. The

applied strain was provided through bending the flexible PET

with a manual displacement stage.

3. Results and discussion

Figure 1a shows an atomic structure model of wurtzite ZnO

in which the Zn2+ cation and the adjacent O2- anion form a

cation-centered tetrahedron. The centers of the anions and

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cations coincide with each other at the normal state, while the

relative displacement of the center of the anion and the cation

generates a dipole moment by apply the c-axis stress.

Moreover, potential distribution in the direction of the stress

is generated macroscopically [30,31]. As shown in Figure 1b,

the numerical calculation of the ZnO microbelt and microrod

subjected to long-axis strain indicate that the piezoelectric

potential [32] is related to the crystal orientation [0001], which

is critical for subsequent experiments. Therefore, we have

prepared the ZnO microbelts and microrods with different c-

axis orientations by vapor-phase transport method. Figure 1c

and 1g show scanning electron microscope (SEM) images of

an individual ZnO microbelt and microrod with both lengths

of approximately several hundred micrometers. It can be seen

that ZnO has a uniform and smooth surface regardless of its

morphology. Then more direct evidence is needed to

demonstrate the crystal structure and orientation [0001], as it

is critical for estimating the piezoelectric effect caused by the

external mechanical strain in this experiment.

Figure.1 Piezoelectric effect, Morphology, structure and optical properties of ZnO microbelts and microrods. (a) Atomic

structure model of wurtzite structure ZnO crystal without strain and applied strain. (b) Numerical calculation results of

piezoelectric potential distribution in the ZnO microbelt and microrod subjected to long-axis strain, where the Z-axis represents

the crystal orientation [0001]. SEM images marked with the crystal orientation [0001] of an individual ZnO (c) microbelt and

(g) microrod. HRTEM image along the crystal orientation [0001] of an individual ZnO (d) microbelt and (h) microrod. The

corresponding SAED pattern of the microbelt (e) and microrod (i) showing the [0001] zone axis. Photoluminescence spectra

of an individual ZnO (f) microbelt and (j) microrod.

As shown in Figure S1, an individual microbelt was

selected and sliced along the short axis direction using

Focused Ion beam (FIB) technology. In contrast, the microrod

was sliced along the long axis. Subsequent, the sliced samples

were characterized by High-Resolution Transmission Electron

Microscope (HRTEM) and the results are shown in Figure 1d-

e and 1h-i. Based on the corresponding selected-area electron

diffraction(SAED) pattern and HRTEM images, it can be

known that the samples possess a wurtzite-type hexagonal

single crystal structure, which is consistent with the typical

structure of ZnO. Moreover, the crystal orientation [0001] of

ZnO is easily inferred and has been marked with the green

dotted arrow in Figure 1c and 1g. The difference between

microbelts and microrods is that the growth direction of the

former is perpendicular to [0001], while that of the latter is

parallel to the [0001] direction. This naturally formed cavity

structure provides a good platform for studying the mode shift

induced by cavity size or refractive index changes. As an


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excellent optical gain medium, ZnO provides a natural

configuration with good crystalline and regular geometry,

which ensures the light could propagate circularly and total

reflect at the ZnO/air interface through the cavity while

maintaining low energy loss. Therefore, a strong intrinsic near

band edge (NBE) emission peak near 390nm was observed in

Figure 1f and 1j, which can afford sufficient optical gain for

laser generation.

Figure.2 Effect of tensile strain and polarization angle on Raman peaks. Raman peaks evolution under tensile strain derived

from an individual (a) ZnO microbelt and (c) ZnO microrod. The intensity variation of vibration mode E2H at 438 cm-1 under

different polarization angles derived from an individual (b) ZnO microbelt and (d) ZnO microrod. (e) Photon frequency of E2H

mode as a function of tensile strain value. The inset shows the relationship between atomic vibration mode and c-axis. (f)

Diagram of polarization light test which the coaxial stress direction is set to 0°.


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In order to apply uniaxial stress, an individual ZnO

microbelt and microrod were selected and fixed on the PET

substrate with epoxy resin glue at both ends as schematically

shown in Figure S2. Stress can be applied parallelly to the

crystal orientation [0001] of the ZnO microrod by bending the

substrate through a manual displacement stage, whereas the

microbelt applies stress perpendicularly to the c-axis. It is also

essential to take into account that the tiny focal distance

movement caused by the strain can be processed by refocusing

the incident light to ensure the measured data coming from the

same spatial position. Subsequently, the Raman spectra under

different tensile strains are discussed in detail to confirm that

the strain does act on the microcavities and affect its intrinsic

physical properties. As shown in Figure 2a, the Raman spectra

of an individual ZnO microbelt without strain shows several

typical ZnO Raman peaks centered at 438.7, 409.2, 377.9,

332.8, and 99.1cm-1, which represent the E2H, E1TO, A1TO, 2nd-

order, and E2L phonon modes, respectively. In comparison, the

peak position of the microrod is basically consistent with the

former, except for the E1TO and 2nd-order phonon modes with

a difference within 5cm-1, which is acceptable. Here, we

mainly focus on the E2H phonon modes originated from the

oxygen atom motion [33, 34] perpendicular to the crystal

orientation [0001] as shown in the inset Figure 2e. Along with

the gradual increase of tensile strain up to 0.66%, a significant

downward shift of the E2H phonon mode was observed on the

microrod, while no obvious variation occurs to the microbelt.

Figure 2f presents the evolution of the E2H phonon mode. Such

a result can be ascribed to the tensile strain along the crystal

orientation [0001] will weaken the interaction between the

bonds and reduce the atomic vibration frequency from the E2H,

resulting in a downward shift of the Raman peak. As for the

microbelt, even considering the dependence of the transverse

strain and the axial strain in an elastic medium, the maximum

strain after conversion by Poisson’s ratio (~0.35) is still

extremely small and is not enough to cause the obvious

movement of the Raman peak, which is consistent with the

experimental phenomenon. So far, based on the uniformity of

material growth and device fabrication, we conclude that

uniaxial stress is effectively applied to the micro cavity

regardless of the morphology. In addition, a method for

estimating the crystal orientation by using the intensity

variation of E2H Raman peak dependence of polarization

angles is discussed in detail. A polarizer is placed in the optical

path to control the polarization angle of the incident light as

shown in Figure 2f. When the polarization angle is parallel to

the long axis of the microbelt or microrod, it is defined as 0

degree so that the polarizer can be adjusted with a step of 20

degrees to obtain a polar diagram about the E2H Raman peak

intensity variation. As shown in Figure 2b and 2d, the E2H

intensity of the microbelt reaches a maximum near 0 degree,

and 90 degrees for the microrod, respectively. This result can

be attributed to the enhanced interaction of polarization and

the E2H vibration direction, where the long axis of the

microbelt is parallel to the E2H vibration but perpendicular to

the microrod. Therefore, it can be utilized to estimate the ZnO

c-axis by the intensity variation of Raman peak which is

nondestructive.

To confirm the optical properties of ZnO microcavity, the

lasing spectrum was measured through a confocal microscope

equipped with a data collection system including the camera

and a spectrograph with charge-coupled device (CCD)

detector coupled with a focused femtosecond pulsed laser (~

190 fs) operating at 355 nm as the excitation source, as shown

in Figure 3a. Figure 3b shows the laser spectrum derived from

an individual ZnO microbelt under different pumping power

at room temperature. When the power is lower near 2.0 mW,

a weak and broad spontaneous emission centered at around

393 nm can be observed, and the full width at half maximum

is about 11.98nm. Multiple discrete peaks are generated in the

spontaneous emission region when the pump power is

increased to 2.3 mW. As the pump power further increases and

exceeds 2.6 mW above the threshold, the peak intensity rises

sharply, and the full width at half maxima (FWHM) is

drastically reduced to a minimum of 0.06nm obtained by

Lorentz fitting. According to the formula Q=λ/Δλ, the Q factor

can be estimated to about 6400, where λ and Δλ are the

wavelength of the peak and FWHM, respectively. Figure 2c

shows more clearly the integrated PL intensity and FWHM as

a function of pumping power, and a significant inflection point

around the threshold 2.6 mW can be observed, which is

consistent with previous analysis. The Figure S4 presents a

dark field optical image of the stimulated emission excited by

a focused laser, which shows the resonant process from a

typical F-P cavity provided by a ZnO microbelt. Similarly, an

individual ZnO microrod are also explored in detail as shown

in Figures 2d and 2e, which possesses better laser quality with

the threshold of 7.1 mW and the minimum FWHW of 0.03 nm

according to an obvious process of stimulated amplification

radiation, and the Q quality factor of the microrod can also be

obtained as ≈11200. Subsequently, the lasing mode was

carefully calculated to confirm the resonant process, because

the smaller size and bright laser spot caused by the higher

threshold block the possibility of judgment through the dark

field image. It is essential to emphasize that Only TE

polarization is taken into account because the corresponding

TM polarized emission is much weaker and can be ignored.

According to the plane wave model for the hexagonal WGM

cavity [35], the equation of the model number N can be

obtained as

N =3√3𝑛𝐷

2𝜆−

6

𝜋tan−1(𝑛√3𝑛2 − 4)

Where D is the diameter of the cavity. Moreover, the

relationship between the refractive index n of TE mode and

the corresponding resonant wavelength λ can be further

described by the Sellmeier’s dispersion function


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n(λ) = (1 +2.48885𝜆2

𝜆2 − 102.302+

0.215𝜆2

𝜆2 − 372.602

+0.2550𝜆2

𝜆2 − 18502)

12⁄

Figure S5 shows the mapping and spectrum of lasing

emission at 7.7 mW of pumping power, it also reveals the

correspondence between the resonance wavelength and the

mode numbers which are plotted as red solid dots. The

resonant wavelength of mode numbers 60-63 calculated by the

theoretical model matches the experimental value very well in

Table 1. Therefore, it can be confirmed that the resonant

process comes from the WGM cavity.

Figure.3 Lasing characteristics of ZnO mircobelts and microrods. (a) Schematic diagram of the entire optical path and test

system. Lasing spectra derived from an individual (b) ZnO microbelt and (d) ZnO microrod under different pumping power.

Lasing intensity (black) and FWHM (red) as a function of pumping power derived from an individual (c) ZnO microbelt and

(e) ZnO microrod.


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Because of the excellent optical properties for this

microcavity with, uniaxial stress can be applied to an

individual ZnO microbelt or microrod for investigating the

ultra-high resolution of the mode variation-based sensor, as

shown in Figure 4a and 4b. By controlling the manual

displacement stage, the lasing peak variation under different

strains can be observed obviously, accompanied the

appearance and disappearance of the lasing modes.

Figure.4 Dynamic regulating of lasing mode in ZnO microcavity by strain. Lasing spectra under different strain derived

from an individual (a) ZnO microbelt and (b) ZnO microrod. (c) The mode-shift of an individual ZnO microbelt (black) and

ZnO microrod (red) as a function of strain. The change rate of resonant cavity length L (black) and lasing peak wavelength 𝜆

(red) under different strain derived from an individual (d) ZnO microbelt and (f) ZnO microrod. Gain spectra and resonance

wavelength positions at the normal (black solid line) and tensile (red dotted line) states derived from an individual (e) ZnO

microbelt and (g) ZnO microrod. Inset: the corresponding principle diagram.


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With increasing tensile strain, the lasing peak of the microbelt

is significantly blue-shifted while that of the microrod is

moving towards the long-wave direction, and the approximate

linear relationship between the peak wavelength variation and

applied strain is shown in Figure 4c. Relying on such a linear

relationship, we can not only dynamically regulate the lasing

mode through a corresponding strain but also detect the

extremely tiny stresses according to the shift of the resonance

wavelength as a strain sensor. Moreover, a sensor that reflects

the stress distribution by the color mapping is expected to be

realized, based on the high color purity of laser and the

correspondence of color and wavelengths. Refer to the idea of

Rayleigh Criterion in optics, we propose a formula to quantify

the sensor color-resolving ability (R) [24], which is defined as

R = |𝜆2 − 𝜆1 Δ𝐻⁄ | where λ1 and λ2 are the resonant

wavelengths before and after the applied stress, and ΔH is the

part where the two lasing peaks overlap each other. It is a

critical case when R-value is equal to 1, which means the two

lasing peaks can be distinguished by color. Once the R-value

is less than 1, the overlap between the peaks will be greater

than the distance of the peaks which no longer being able to

distinguish. Therefore, the minimum strain resolution for the

strain sensor can be estimated as 0.09% for the microbelt and

0.07% for the microrod because of the ultra-narrow line width

of the cavity modes.

Furthermore, the mechanism of mode variation is carefully

discussed. It is necessary to clarify that the movement

mechanism of the lasing peak is completely different from the

PL peak movement derived from the stress-induced bandgap

change, which has been discussed in detail in our previous

work [36]. According to the FP-type modes formula λ =

2𝑛𝑒𝑓𝑓𝐿 𝑁⁄ [37], it can be inferred that the rate of the

wavelength variation Δλ 𝜆⁄ is completely derived from the

rate of the resonant cavity length variation Δ𝐿 𝐿⁄ and the rate

of the relative refractive variation Δ𝑛𝑒𝑓𝑓 𝑛𝑒𝑓𝑓⁄ under fixed

mode numbers N. Thereof, the change of the resonant cavity

can be estimated by the Poisson effect induce the inward

shrinkage of the microbelt cavity under the tensile stress

shown in the inset of figure 4e, which will result in the cavity

length becoming decrease and the resonant wavelength blue-

shifting. The results calculated by Poisson’s ratio (~0.35) and

the wavelength changes for comparison are plotted in figure

4b which shows a consistent variation, indicating that the main

reason for the resonance wavelength shift is derived from the

cavity. As for the reason why the relative refractive index

variation is not obvious enough, it will be discussed in more

detail in the microrod case. Along the similar approach, the

change in the cavity of a microrod is also calculated and

plotted against the wavelength variation in figure 4f which

shows a completely different situation from the former. When

the tensile stress is applied to a microrod, the cavity should

shrink inwardly similar to the microbelt shown in the inset of

figure 4g which will result in the resonant wavelength blue-

shifting. However, the experimental phenomenon shows a

significant red-shifting indicating that the relative refractive

index is a major factor rather than the cavity length in the

microrod case. Since the excited position exposed to a stable

atmosphere can eliminate the external refractive index change,

it can be further inferred that the change in the ZnO internal

refractive index caused by strain-induced piezoelectric

polarization effect is the main reason. ZnO possessing a

wurtzite crystal structure has obvious anisotropy between the

c-axis direction and perpendicular to the c-axis direction. In

the absence of external stress, the cationic and anionic charge

centers coincide with each other, while the charge center of

both will be relatively displaced and produce a dipole moment

under the tensile external stress applied along the c-axis,

thereby creating the potential redistribution and increase the

refractive index. Therefore, it can be explained why the

influence of relative refractive index variation on the lasing

mode is so diverse in both cavities. The microbelt cavity

suffered a weak influence on the relative refractive index due

to the tensile stress being perpendicular to the c-axis, resulting

in the lasing mode variation dominated by the cavity length as

shown in the inset of figure 4e. While the tensile stress is

coaxial with the c-axis in the microrod, the red-shift caused by

the increase of the relative refractive index far exceeds the

blue-shift caused by the shrinkage of the cavity, resulting in

red-shift of the lasing mode. The above detailed calculation is

shown in the Supporting information, Table S1. To analyze

the appearance and disappearance of the modes, the schematic

diagram of the gain spectrum and resonance wavelength

position under normal and tensile states are shown in figure 4e

and 4g. In the normal case, the gain spectrum marked with the

black solid Gaussian line is divided into two parts by the

threshold value (black horizontal dashed line), and only the

modes above the threshold can be amplified and selected. As

the tensile stress is applied, the movement of the gain

spectrum marked with black arrow may cause some modes to

fall below the threshold and disappear, and some modes rise

above the threshold and appear.

4. Conclusions

In summary, we have demonstrated two mechanisms of

strain-induced dynamic regulation of ZnO lasing modes by

preparing high quality factor microbelt and microrod cavities.

Compared with the cavity length change caused by the

Poisson effect, the relative refractive index variation derived

from the piezoelectric polarization effect exhibits crystal

orientation dependence. Only the tensile strain applied along

c-axis, the increase of refractive index is significant enough to

compensate for the blue-shift originate from the cavity

shrinkage, which promotes the lasing mode to move to long

wavelengths. Based on the linear relationship between the

resonance wavelength variation and applied stress, we can not

only dynamically modulate the lasing mode but also promise


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to detect tiny stresses as a high resolution sensor. The

experimental minimum in strain resolution for the strain

sensor reaches up to 0.07%, which is the latest detection limit

of the lasing mode variation-based sensor induced by external

applied strain. Our research not only has a positive influence

on constructing mode-adjustable coherent light source, but

also provides strategies for obtaining high-resolution strain

sensors.

Acknowledgements

The authors thank the support of national key R & D project

from Minister of Science and Technology, China

(2016YFA0202703), National Natural Science Foundation of

China (No. 61805015, 51622205, 61675027, 51432005,

61505010 and 51502018), Beijing City Committee of science

and technology (Z171100002017019 and

Z181100004418004), Natural Science Foundation of Beijing

Municipality (4181004, 4182080, 4184110, 2184131 and

Z180011), and the University of Chinese Academy of

Sciences.

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