![](bg4.png)
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
th
(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.
Nonlinear Trapping and Interfering Modes in a Quasi-One-Dimensional Microcavity Laser
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