Nonlinear Trapping and Interfering Modes

in a Quasi-One-Dimensional Microcavity Laser

Maciej Pieczarka

1,*

, Christian Schneider

, Sven Höfling

2,3

and Grzegorz Sęk

Laboratory for Optical Spectroscopy of Nanostructures, Department of Experimental Physics, Faculty of Fundamental

Problems of Technology, Wrocław University of Science and Technology, W. Wyspianskiego 27, 50-370 Wrocław, Poland

Technische Physik, Physikalisches Institut and Wilhelm Conrad Röntgen-Research Center for Complex Material Systems,

Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany

SUPA, School of Physics and Astronomy, University of St. Andrews, St. Andrews, KY 16 9SS, U.K.

Keywords: Nonlinear Optics, Microcavities, VCSEL, Exciton-polaritons.

Abstract: Experimental studies of the emission from one-dimensional microcavity laser structure under nonresonant

optical excitation are presented. The one-dimensional laser was prepared by electron-beam lithography and

reactive ion etching from a planar microcavity sample. Below the lasing threshold, the system was in the

strong coupling regime, where the emission exhibits the common exciton-polariton far-field dispersion.

Above the threshold, the system switched to the weak coupling regime and the photon lasing was observed.

Interestingly, under higher pumping powers above the threshold, a strong blueshift of the lasing mode was

observed, with localisation of the far-field emission at finite wavevectors. The near-field images showed

interference fringes corresponding to the interference of propagating modes in k-space. This is interpreted in

terms of self-interfering modes confined between the pumping spot and the edges of the 1D microlaser.

1 INTRODUCTION

Light confinement in the sub-micrometre scale has

led to the realisation of novel nonlinear sources of

coherent light and paved the way for studies of

quantum electrodynamics in the semiconductor

material platform (Reithmaier et al. 2004). Since

reaching the strong coupling between the exciton and

photon (quasiparticles called exciton-polaritons) in

planar semiconductor cavities (Weisbuch et al. 1992),

there is a vast interest in nonlinear phenomena in

quantum bosonic fluids of light and matter realized in

a solid state system (Byrnes et al. 2014; Carusotto &

Ciuti 2013).

Due to the excellent development of etching

techniques in semiconductor technology, there is a

possibility to harness the photonic confinement into

any desired pattern. On the other hand, spatial

shaping of the nonresonant optical excitation can

create an effective potential for exciton-polaritons

(Schneider et al. 2017) or create nonlinear coupled

structures in photon lasers (Pal et al. 2017). In view

of the abovementioned approaches, one can combine

the etching techniques with strong nonresonant

excitation to provide custom photonic confinement

for the lasing modes.

In this paper, we present investigations of a one-

dimensional microcavity laser with nonresonant

optical excitation. Under strong pumping, the system

reaches the weak coupling regime and photon lasing

is observed at finite wavevectors in the far-field

spectrum. The emission is strongly blueshifted with

respect to the bare cavity mode resonance at low

pumping power densities. Power dependent

measurements in near and far field spectra give

insight into the observed phenomena, which are

interpreted in terms of nonlinear confinement of the

modes outside the pump region.

2 EXPERIMENTAL DETAILS

2.1 Optical Setup

The experiments were performed on the setup

depicted in Fig. 1. Excitation was provided with the

mode-locked femtosecond pulsed Ti: Sapphire laser

(parameters are indicated in the figure), tuned around

242

Pieczarka, M., Schneider, C., Höﬂing, S. and S˛ek, G.

Nonlinear Trapping and Interfering Modes in a Quasi-One-Dimensional Microcavity Laser.

DOI: 10.5220/0006637302420246

In Proceedings of the 6th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2018), pages 242-246

ISBN: 978-989-758-286-8

800 nm for efficient absorption in the GaAs material.

The laser beam was focused with a high numerical

aperture microscope objective (NA = 0.42) to a

diffraction limited Gaussian spot of diameter around

3 µm on the sample. The laser power is controlled

with the optical power meter placed after the beam

splitter. The investigated sample was kept in the

continuous-flow liquid-helium cryostat in order to

cool it down to the cryogenic temperature of 5 K.

Luminescence from the sample is collected with

the same objective and imaged with a set of

achromatic lenses onto the entrance slit of a

monochromator. A two-lens setup is used to

efficiently switch between near-field and far-field

images (one of the lenses is placed on a kinematic

mount). Cuts of the sample emission images are

analysed with the half-meter focal-length

monochromator coupled to a 2D InGaAs-based near-

infrared CCD camera. To conveniently adjust the

direction of the cuts done by the monochromator slit

a Dove prism on a rotational mount is placed in the

optical path, which enables rotation of the image with

respect to the detection system.

Figure 1: Scheme of the experimental setup.

2.2 Sample Details

We investigated a microcavity sample, which was

used in the previous work (Pieczarka et al. 2017). The

planar microcavity is grown in molecular beam

epitaxy technique (MBE) on a semi-insulation GaAs

substrate. It consists of two GaAs/AlGaAs distributed

Bragg reflectors (16 bottom and 12 top layer pairs

respectively), where the active region consists of two

stacks of four In

0.28

0.72

As 7 nm thick quantum wells

(QWs) placed at the antinodes of the confined

fundamental photon mode (in the growth direction).

The sample is characterized by the normal mode

splitting of the exciton and photon modes, with Rabi

splitting of 7.5 meV, where the theoretical exciton

resonance appears around 1.2605 eV. The cavity

mode is characterized by a quality factor exceeding

1000 (measured spectrally far from the exciton-

polariton resonance).

To obtain high-quality one-dimensional micro-

wires, a piece of the sample was etched in the post-

processing. Microwires of the length of tens to

hundreds of micrometres and widths of few micro-

metres were created via electron beam lithography

and etched deeply into the structure using electron-

cyclotron-resonance reactive-ion-etching. Due to the

optimized etching technique resulting in smooth and

steep sidewalls of extremely low roughness, no

detrimental influence on the cavity quality factor was

observed (Fischer et al. 2014). In this work, we focus

on a microwire of 75 µm length and width of 8 µm,

which is presented in the inset of Fig. 2.

The planar sample was characterized previously

with the high impact of local fluctuations of photonic

and excitonic disorder, which influences the exciton-

polariton fluid flow in the strong coupling regime

(Pieczarka et al. 2015). To check the level of disorder

in the investigated microwire, we excited the sample

with a defocused laser spot (with an additional lens in

the laser path) with low power density, very far from

the lasing threshold. Photoluminescence (PL) along

the wire was cut with the monochromator slit and

analysed spectrally. One can see in Fig. 2 that the PL

is fragmented, as the low-density polaritons are

confined in the local potential minima. However, the

amplitude of the local energy fluctuations is around

1 meV only, being much smaller from the disorder in

the planar sample observed previously.

Figure 2: Emission spectrum at wide spot excitation below

lasing threshold along the investigated microwire. Top view

white-light microscope image of the investigated

microlaser is in the top inset.

3 RESULTS

Excitation power dependent measurements of the

Nonlinear Trapping and Interfering Modes in a Quasi-One-Dimensional Microcavity Laser

243

polariton and photon luminescence from the

microwire were performed in the wide range of pump

powers. Results of the data analysis are presented in

Fig. 3. In the integrated intensity power curve in Fig.

3a, one can observe a distinct nonlinear threshold

= 13 mW) with a simultaneous drop in the

linewidth and blueshift of the emission resonance,

Figs. 3b and 3c. This is an indication of the transition

to a coherent lasing state, as the spectrally narrow

lasing resonance is located around the bare cavity

mode, Fig. 3c.

Moreover, in the range of low pumping power

density below the threshold, the emission is located at

energies lower than the bare cavity mode, Fig. 3b and

the far-field dispersion corresponds to the lower

exciton-polariton branch (LPB), see Fig. 4a. The

emission is fragmented, which indicates the influence

of disorder, as presented in Fig. 2. At lasing threshold,

the emission is located around the bare cavity mode,

Fig. 4b, although the far-field dispersion is rather flat,

indicating a localised mode in real space, perhaps at a

local defect near the pump region. The observation

of the intensity dependence threshold, linewidth

narrowing and blueshift of the emission from LBP

to the cavity mode resonance are signatures of the

Figure 3: Power dependent analysis. (a) Integrated

intensity, (b) linewidth of the emission and (c) energy of the

lasing resonance.

Figure 4: Normalised far-field emission spectra below (a)

and slightly above the lasing threshold (b). LPB is depicted

(solid line) together with bare cavity photon dispersion

(dashed line) and QW exciton resonance (dotted line).

transition from strong to weak coupling regime and

occurrence of the photon lasing (Tempel et al. 2012).

The most important observation has been made at

pumping levels exceeding the threshold power. The

emission energy shows a continuous blueshift with

the pumping power up to P = 6P

, see Fig. 3c. This

kind of spectral behaviour is expected rather for a

polariton laser than for weak coupling photon lasing

(Bajoni et al. 2008). The far-field characteristics in

this power range show a flat and blueshifted

dispersion around the theoretical cavity mode, Fig.

5a. Further increase of the pumping level leads to the

pinning of the emission energy around the bare QW

exciton energy at 1.261 eV, Fig. 3. indicates the

maximum spectral gain at this energy, which

amplifies the lasing mode. Nevertheless, the far-field

dispersion showed a very distinct change, where

lasing at a well-defined wavevector is observed, Fig.

5b. This is a result of two counter-propagating

coherent wave packets along the microwire.

Figure 5: Normalised far-field emission spectra at higher

pumping powers. Flat and blueshifted dispersion (a) and

well-defined finite wavevector lasing (b).

PHOTOPTICS 2018 - 6th International Conference on Photonics, Optics and Laser Technology

244

In order to verify this observation, we performed a

near-field imaging along the one-dimensional

microcavity at the highest pumping level. The result

is shown in Fig 6a. At the pump spot position

(x = 0 µm) the strongest emission is observed with

signatures of the occupation of higher energy

confined modes in the perpendicular direction, see

Fig. 6b. Moreover, the spatially extended emission is

detected, being spread around 20 µm in both

directions from the pump spot along the microwire at

the main lasing energy, Fig. 6a. This feature is

characterised by distinct interference fringes at both

sides of the near-field image. Interestingly, the fringe

spacing is equidistant and is equal to d = 2.2 µm. This

spacing corresponds almost exactly to the

interference of wave-packets travelling with the

emission wavevector observed in the far-field

luminescence k = π/d ≈ 1.43 µm

-1

. It is worth noting

that the equidistant spacing cannot be caused by the

irregular defect pattern, see Fig. 2.

The observed features can be interpreted in terms

of self-interference of coherent photon waves

travelling along the one-dimensional laser cavity.

Photons are emitted locally within the pump spot and

propagate in positive and negative directions in the

wire. Further, they are reflected from the cavity ends,

providing the counter propagating mode. This mode

Figure 6: Normalised near-field emission spectra at high

pumping power P = 23P

(a) (dashed lines indicate

equidistant interference maxima). The colour scale is

saturated at 0.5 to enhance the visibility of fringes. Far-field

spectrum in the perpendicular direction to the microwire at

the pumping spot (b). (c) Schematic picture of the

interpretation of the described phenomena. The red dot

indicates the pump spot, local blueshift as a red peak and

black arrows indicate travelling waves along the wire.

can be once again enhanced with the remnant gain in

the pump region. This mechanism provides

amplification of the propagating waves, similarly to

one-dimensional polariton condensates (Wertz et al.

2012). The scheme of the proposed interpretation is

presented in Fig. 6c.

Propagation outside of the pump spot is possible

by a local change in the refractive index due to the

excess carriers generated in the GaAs material (Henry

et al. 1981), which creates an effective potential

gradient. This change of the refractive index is

evidenced in the far field spectrum measured at the

pump spot in the direction perpendicular to the wire,

see fig. 6b. One can observe a series of confined

transversal modes following the strongly blueshifted

cavity dispersion curve. The local blueshift at k = 0 is

as large as 3 meV and is definitely not caused by any

strong coupling phenomena, like polariton-polariton

nonlinear interactions. The observed blueshift of the

emission occurs in the weak coupling regime, so it is

solely caused by the nonlinear refractive index

change in the microcavity. This effective potential

hill provides an additional spatial confinement of the

modes propagating in both directions between the

spot and the edges of the microwire.

4 CONCLUSIONS

To conclude, we investigated lasing properties of a

quasi-one-dimensional microcavity laser. We

observed continuous blueshift of the emission with

the increase of the pumping power below and above

the photon lasing threshold, although the system

entered the weak coupling regime. Well defined,

oblique angle lasing was observed, which is described

as self-interference of confined modes between the

blueshifted pump spot and the microwire edges.

Further investigation will be conducted to verify the

proposed interpretation, especially time-resolved

measurements of the near-field and far-field patterns,

which can give insight into the dynamics of the

observed propagating laser modes and its

amplification.

ACKNOWLEDGEMENTS

Authors would like to acknowledge useful

discussions with Elena Ostrovskaya. This work is

supported by National Science Centre, grant

PRELUDIUM 2016/23/N/ST3/01350.

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245

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