symmetric and antisymmetric band-edge modes exist in distributed feedback surface-emitting semiconductor lasers, with the dominant difference being the radiation loss. Devices generally operate on the low-loss antisymmetric modes, although the power extraction efficiency is low. Here we develop graded photonic heterostructures, which localize the symmetric mode in the device centre and confine the antisymmetric modes close to the laser facet. This modal spatial separation is combined with absorbing boundaries to increase the antisymmetric mode loss, and force device operation on the symmetric mode, with elevated radiation efficiency. Application of this concept to terahertz quantum cascade lasers leads to record-high peakpower surface emission ( > 100 mW) and differential efficiencies (230 mW A − 1 ), together with low-divergence, single-lobed emission patterns, and is also applicable to continuous-wave operation. such flexible tuning of the radiation loss using graded photonic heterostructures, with only a minimal influence on threshold current, is highly desirable for optimizing secondorder distributed feedback lasers.

We demonstrate the possibility to efficiently split the near-field heat flux exchanged between graphene nano-disks by tuning their doping. This result paves the way for the developement of an active control of propagation directions for heat fluxes exchanged in near-field throughout integrated nanostructures networks.The control of electric currents in solids is at the origin of modern computer technology which has revolutionized our daily life. Until the 2000s no thermal counterpart had been developed to control the flow of heat at the nanoscale in a similar manner. In 2006, a step forward in this direction has been done by Li et al.[1] when they introduced the first concept of a thermal transistor for controlling heat fluxes carried by phonons through solid segments. Later, several prototypes of phononic thermal logic gates [2] as well as thermal memories [3] were developed in order to process information by means of the heat fluxes carried by phonons [4]. Besides, different solid-state thermal diodes were conceived [5-9] allowing for rectifying these fluxes in asymmetric solid segments.Very recently, there has been a fast growing interest in developing active functionalties to manage heat transfers by radiation rather than by conduction between contactless solids. Since 2010 several radiative thermal rectifiers [10][11][12][13][14][15][16][17][18][19] have been proposed using spectrally selective nanostructures and phase-changes materials. In 2014 and 2015 a radiative analog of a transistor was suggested theoretically which allows for switching, modulating and amplifying the heat flux exchanged both in the nearfield [21] or far-field regime [22]. Furthermore, a concept of a radiative thermal memory was introduced working in the far-field [23] and the near-field regime [24]. Such devices open the way for new perspectives concerning the development of contactless thermal circuits intended for an active thermal management with photons instead of electrons or phonons. Finally, the thermal diode concept based on phase-change materials has already been tested, experimentally, in the far-field regime [20] in 2014. A review of these recent developments can be found in Ref. [25].In this Letter, we introduce the concept of a heat flux splitter which allows us to tune the direction of propagation of heat flux exchanged in the near-field. To demonstrate the operating modes of this heat splitter we consider a set of three graphene nano-disks in mutual interaction. We show that the heat flux exchanged between these nano-disks cannot only be splitted equally in two predefined directions as for a 50:50 beam splitter, which is trivial, but it can also be oriented in mostly one predefined direction acting like a 99:1 beam splitter by an appropriate tuning of the Fermi levels in the graphene nano-disks. Since this tuning can be achieved by electrical gating, for instance, we thus demonstrate, in particular, that the direction of propagation of the radiative heat flow can be dynamically controlled in nano-architectures by electr...