Jacopo Frigerio

Vladylav Vakarin

Papichaya Chaisakul

Andrea Ballabio

Daniel Chrastina

Xavier Le Roux

Laurent Vivien

Giovanni Isella

Delphine Marris-Morini

Electro-absorption and electro-refraction in Ge/SiGe coupled quantum wells

Keywords: Germanium, quantum wells, silicon photonics, modulators

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTRODUCTION

In the last decade, optical devices based on QCSE in Ge/SiGe multiple quantum wells (MQW) grown on Si [START_REF] Kuo | Strong quantum confined Stark effect in germanium quantum wells[END_REF] have undergone an impressive development. QCSE causes a red-shift of the absorption spectrum and a significative reduction of the excitonic absorption peak [START_REF] Miller | Bandedge Electro-absorption in Quantum Well Structures: The Quantum Confined Stark Effect[END_REF] , features that can be exploited to realize compact and efficient intensity modulators. The optical properties of Ge/SiGe MQW have been widely studied, and modulators working at 1490 nm [START_REF] Chaisakul | Quantum-confined Stark effect measurments in Ge/SiGe quantum wells structures[END_REF][START_REF] Chaisakul | Polarization dependence of quantum confined Stark effect in Ge/SiGe quantum well planar waveguide[END_REF][START_REF] Ren | Selective epitaxial growth of Ge/Si 0.15 Ge 0.85 quantum wells on silicon substrate using reduced pressure chemical vapor deposition[END_REF][START_REF] Ren | Ge/SiGe quantum well waveguide modulator monolithically integrated with SOI waveguides[END_REF] and at 1550 nm [START_REF] Kuo | Quantum-Confined Stark Effect in Ge/SiGe Quantum Wells on Si for Optical Modulators[END_REF][START_REF] Dumas | Ge/SiGe quantum confined Stark effect electro-absorption modulation with low voltage swing at λ = 1550 nm[END_REF] , as well as high-speed operation [START_REF] Chaisakul | 23GHz Ge/SiGe multiple quantum wells electro-absorption modulator[END_REF] have been demonstrated. The most interesting feature of intensity modulators based on QCSE, thanks to their efficiency, is the possibility to reach power consumption levels as low as 10 fJ/bit [START_REF] Chaisakul | 23GHz Ge/SiGe multiple quantum wells electro-absorption modulator[END_REF] , thus meeting the aggressive requirements for on-chip optical interconnects [START_REF] Miller | Energy consumption in optical modulators for interconnects[END_REF] . Composition and thicknesses of Ge/SiGe MQW can be finely tailored in order to engineer the material bandgap and to consequently address a very wide range of modulation wavelengths. This approach has been exploited to demonstrate intensity modulation at 1300 nm by increasing the compressive strain in the Ge layers in Ge/Si 0.35 Ge 0.65 grown on a Si 0.3 Ge 0.7 buffer [START_REF] Lever | Modulation of the absorption coefficient at 1.3 μm in Ge/SiGe multiple quantum well heterostructures on silicon[END_REF][START_REF] Rouifed | Quantum-confined stark effect at 1.3 μm in Ge/Si 0.35 Ge 0.65 quantum well structure[END_REF] or simply by reducing the well thickness in Ge/Si 0.15 Ge 0.85 MQW [START_REF] Chaisakul | O-band quantumconfined Stark effect optical modulator from Ge/Si 0.15 Ge 0.85 quantum wells by well thickness tuning[END_REF] . All the aforementioned modulators works by exploiting QCSE of the first excitonic transition (cΓ1-HH1), nevertheless, also the second excitonic transition (cΓ1-LH1) was investigated [START_REF] Chaisakul | Strong quantum-confined Stark effect from light hole related direct-gap transitions in Ge quantum wells[END_REF] . Very recently, an integrated optical interconnection made by a Ge/SiGe MQW modulator and photodetector connected through a low-loss SiGe waveguide was demonstrated [START_REF] Chaisakul | Integrated germanium optical interconnects on silicon substrates[END_REF] . Remarkably, all the devices were monolithically integrated on silicon by a single epitaxial growth, paving the way for the integration of Ge/SiGe MQW devices with passive optical circuitry. All the aforementioned modulators work by exploiting the electro-absorption based on QCSE. In contrast, much less efforts have been made in order to investigate the electro-refractive effect in Ge/SiGe MQW. QCSE causes strong variations in the absorption spectrum, leading to a significant change in the refractive index as stated by Kramers-Kronig relations. Recently an effective refractive index variation up to 1.3x10 -3 at -8V bias, with a figure merit V π L π of 0.46 V cm was reported for Ge/Si 0.15 Ge 0.85 MQW [START_REF] Frigerio | Electro-refractive effect in Ge/SiGe multiple quantum wells[END_REF] . It has been theoretically predicted that in symmetrically coupled Ge/SiGe MQW (CQW) the electro-refractive (ER) effect should be even stronger, thanks to the tendency of electron and hole envelope functions to be confined in different QWs under bias [START_REF] Iseri | Analysis of electrorefractive index change in Ge/SiGe coupled quantum well for low-voltage silicon-based optical modulators[END_REF] . Symmetrically and asymmetrically CQW exhibit a peculiar physical behavior due to the coupling of the wavefunctions through the barrier [START_REF] Frigerio | Giant electro-optic effect in Ge/SiGe coupled quantum wells[END_REF] . For this reason, they were widely investigated in III-V semiconductors for many purposes including non-linear optics [START_REF] Sirtori | Giant, triply resonant third order non linear susceptibility in coupled quantum wells[END_REF] , photodetection [START_REF] Chiang | A voltage-tunable multicolor triple-coupled InGaAs/GaAs/AlGaAs quantum-well infrared photodetector for 8-12 μm detection[END_REF] and optical modulation [START_REF] Debbar | Coupled GaAs/AlGaAs quantum-well electroabsorption modulators for low-electric-field optical modulation[END_REF] . In this work, electro-absorption (EA) and ER have been investigated in Ge/Si 0.15 Ge 0.85 CQWs by means of optical transmission measurements.

SAMPLE GROWTH

The CQW heterostructure was grown by Low Energy Plasma enhanced chemical vapor deposition (LEPECVD) [START_REF] Isella | Low-energy plasmaenhanced chemical vapor deposition for strained Si and Ge heterostructures and devices[END_REF] on a 100 mm n-Si(001) substrate with a resistivity of 1-10 Ω cm. Before the heteroepitaxial growth, the substrate was dipped in an aqueous hydrofluoric acid solution for 30 seconds to remove the native oxide. The first part of the structure consists of a Si 1-y Ge y graded buffer, with a total thickness of 13 μm, where the Ge concentration y was linearly raised from 0% to 90% with a grading rate of 7%/μm. The growth rate was 5-10 nm/s, while the substrate temperature was linearly decreased from 740°C to 525°C. The graded buffer was then capped with a 2 μm thick p-doped (5x10 18 cm -3 ) Si 0.1 Ge 0.9 layer to form a fully relaxed virtual substrate (VS) and the p-type contact of the p-i-n structure embedding the CQWs. The threading dislocation density was 6 x 10 6 cm 2 as measured by chemical defect etching [START_REF] Marchionna | Defect imaging of SiGe strain relaxed buffers grown by LEPECVD[END_REF] . The MQW consists of seven repetitions of the following period (10 nm Ge well + 3 nm Si 0.15 Ge 0.85 inner barrier + 10 nm Ge well + 37 nm Si 0.15 Ge 0.85 outer barrier). Individual layer thicknesses and compositions were designed to realize a strainsymmetrized structure. The outer barrier is thick enough to avoid coupling between adjacent periods. This part of the structure was grown at rate of 1 nm/s and the temperature was lowered at 475°C to avoid the inter-diffusion at the barrier/well interface. Finally a 200 nm phosphorous doped (1 x 10 19 cm -3 ) Si 0.1 Ge 0.9 n-type contact layer was deposited. A cross section of the structure is shown in Fig. 1a. Layer compositions and strain states were measured by highresolution x-ray diffraction (HR-XRD) by using a PANalytical X'Pert PRO MRD diffractometer. Out-of-plane and inplane lattice parameters, were measured (relative to the Si reflection) for the VS peak and the superlattice satellites. Ge content and strain were then obtained using the known lattice parameters for relaxed SiGe alloys [START_REF] Dismukes | Lattice Parameter and Density in Germanium-Silicon Alloys[END_REF] and interpolated elastic constants of Si and Ge [START_REF] Wortman | Young's Modulus, Shear Modulus, and Poisson's Ratio in Silicon and Germanium[END_REF] . The final composition of the VS was found to be 90% (with a residual in-plane strain of 0.12%). The in-plane lattice parameter of the MQW stack is the same as that of the VS, meaning that the MQW stack is coherently matched to the VS. In Fig. 1b the omega-2theta scan (measured with respect to the 004 Si reflection) is reported. The reciprocal space maps are reported in Fig. 1(a andb).

DEVICE FABRICATION AND MEASUREMENT SET-UP

In order to investigate the EA and ER effects in Ge/SiGe CQWs, 64 μm long, 100 μm wide planar waveguides have been fabricated. The waveguides were patterned by optical litography and then dry etched to the p-doped Si0.1Ge0.9 layer. The sidewall roughness of the etched mesa was smoothened by hydrogen peroxide (H2O2) solution. 100 nm of silicon dioxide were deposited as passivation layer on the left and right walls of the waveguide by plasma-enhanced chemical vapor deposition (PECVD). For n and p contacts 10 nm of Ti and 300 nm of Au was evaporated and lifted off. The I-V characteristic of the device is shown in Fig. 2. The measurements have been performed at room temperature with a spectral resolution of 0.1 nm. A tunable laser emitting light from 1250 to 1450 nm with a power of 1 mW has been used. Light from the laser has been butt coupled into the planar waveguide using a taper-lensed fiber, which has been positioned to inject light in the waveguide region not covered by the top metal contact to reduce optical losses. An objective has been used to couple the output light into a photodetector. A schematic of the measurement set-up is shown in Fig. 3.

ELECTRO-ABSORPTION

The absorption spectra of the device at different electric field obtained from optical transmission measurements are reported in Fig. 4 for light with TE polarization. The baseline of each curve has been shifted of 200 cm -1 to better highlight the spectral features. In a standard uncoupled quantum well, cΓ1-HH1 and cΓ2-HH2 transitions would be separated by a few tens of meV , but in the case of a CQW, the cΓ1 and cΓ2 states represents the "bonding" (zero-node wavefunction) and "anti-bonding" (one-node wavefunction) forming due to QW coupling and are separated only by ≈ 10 meV. A few meV energy also separates the HH1 and HH2 states. When a small electric field is applied to the structure, the bonding and antibonding states become more localized in two different wells. A detailed explanation of the evolution of the absorption spectrum as a function of the electric field can be found in 18. Figure 4: Absorption spectra at different electric field.

ELECTRO-REFRACTION

The effective index variation was experimentally characterized by the shift of the Fabry-Perot (FP) fringes in the absorption spectra of the planar waveguide as a function of the applied bias. FP fringes experience a spectral shift when a voltage is applied to the device. In a cavity, the resonance condition determines the wavelength of the maxima (λ p ) in the transmission spectra according to the equation (1):

2𝑛 𝑒𝑓𝑓 (𝜆)𝐿 = 𝑝𝜆 𝑝 ( 1
)
where L is the device length, n eff is the effective index of the guided mode and p is an integer number representing the order of the transmission peak. Taking into account the dispersion properties of the medium, a spectral shift Δλ (see fig. 5) is related to the effective index variation by:

∆𝑛 𝑒𝑓𝑓 (𝜆) = 𝛥𝜆 𝜆 𝑛 𝑔 (𝜆) (2)
Finally the group index n g (λ) can be experimentally deduced from the longitudinal mode spacing Δλ p by using:

𝑛 𝑔 (𝜆 𝑝 ) = 𝜆 𝑝 2 2𝐿𝛥𝜆 𝑝 (3)
The effective index variation ∆n eff was thus obtained from Eq. 2 using the measured values of Δλ and n g (λ), and is reported in Fig. 6 for different wavelengths from 1420 to 1440 nm. Strikingly, the refractive index variation shows a local maximum at an electric field of 40 kV/cm (1.5 V) for all the considered wavelengths. In order to evaluate the viability of the device as a phase shifter, the device length necessary to achieve a p phase shift L π was calculated as:

𝐿 𝜋 = 𝜆 2∆𝑛 𝑒𝑓𝑓 (4)
For λ = 1420 nm, where the electro-refractive effect is stronger, Lπ is 300 μm for an electric field of 40 kV/cm (1.5 V bias) with a VπLπ figure of merit of 0.046 V cm according to eq. 4. This value is one order of magnitude lower with respect to uncoupled Ge/SiGe QWs [START_REF] Frigerio | Electro-refractive effect in Ge/SiGe multiple quantum wells[END_REF] and competitive with respect to pure Si [START_REF] Green | Ultra-compact, low RF power, 10 Gb/s silicon Mach-Zehnder modulator[END_REF] .

CONCLUSIONS

To summarize, we have investigated the electro-absorption and electro-refraction in Ge/SiGe symmetrically coupled quantum wells. The evolution of the absorption spectra of the device as a function of the applied electric field is very different with respect to standard quantum wells thanks to the coupling, leading to an enhanced electro-refractive effect with effective index variation higher than 2x10 -3 under moderate electric field. In order to exploit the potentiality of Ge/SiGe CQW as compact, high speed and low power consumption optical modulators, the active region will have to be integrated into an interferometric structure such as a Mach-Zehnder interferometer. The integration of QW active regions with low-loss SiGe waveguides on top of graded buffer is a promising approach, as sharp bends and Mach-Zehnder interferometers [START_REF] Vakarin | Sharp bends and Mach-Zehnder interferometer based on Ge-rich-SiGe waveguides on SiGe graded buffer[END_REF] as well as the integration of a passive SiGe waveguide with an electro-absorption modulator and a photodetector [START_REF] Chaisakul | Integrated germanium optical interconnects on silicon substrates[END_REF] were demonstrated recently.

Figure 1 :

 1 Figure 1: Schematic of the sample with detailed growth steps (a). Reciprocal space maps with respect to the 224 Si reflection (b) and to the 004 Si reflection.

Figure 2 :

 2 Figure 2: Dark current density as a function of voltage of the processed device.

Figure 3 :

 3 Figure 3: Schematic of the experimental set-up used for the transmission measurements.

Figure 5 :

 5 Figure 5: Shift of the Fabry-Perot fringes as a function of the electric field.

Figure 6 :

 6 Figure 6: Effective index variation as a function of the applied electric field for different wavelengths.

ACKNOWLEDGEMENT

The CARIPLO foundation is acknowledged for financial support through the grant EIDOS 2011-038. The fabrication of the device was performed at the nano-center CTU-IEF-Minerve, which is partially funded by the 'Conseil General de l'Essonne'. This work was partly supported by the French RENATECH network. D.M-M. acknowledges support by the Institut Universitaire de France. Marie Curie International Outgoing Fellowships through grant agreement PIOF-GA-2013-629292 MIDEX is partly acknowledged.This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement N°639107-INsPIRE).


Valérie Boussard
email: valerie.boussard@u--paris10.fr

Keywords: Organisation, Profession, Savoirs, Management. SUMMARY Organization, Profession, Knowledge, Management

À partir d'une mise en lumière des liens entre l'univers académique, l'univers du conseil et celui des organisations clientes de ces derniers, cet article s'interroge sur la place et l'influence croissante des consultants dans le monde économique, administratif et même associatif. Il propose de comprendre ces dernières comme le résultat d'une dynamique qui amène chacun des acteurs de ces univers à se comporter en « professionnel », c'est--à--dire à asseoir ses pratiques sur la maîtrise de savoirs, techniques, outils spécialisés et appropriés. Ceux--ci leur permettent de prétendre légitimement intervenir sur la résolution des problèmes organisationnels. C'est en tant qu'experts de ces savoirs que les consultants se présentent. Ils répondent ainsi, tout en la nourrissant, à l'exigence d'une figure professionnelle institutionnalisée par le monde académique et partagée par leurs clients. Cette figure est celle du « manager », réputé seul capable de faire advenir les principes gestionnaires de Maîtrise, Performance et Rationalité.

Rationality" since the second half of the 19th century. The use of consultants as experts in these principles has enjoyed continuous growth in France since the end of the Second World War, as evidenced by the growth in the number of consultancy firms and the volume of their activity [START_REF] Villette | Sociologie du conseil en management[END_REF]Berrebbi--Hoffmann, 2002) as well as the proliferation of the services on offer and the diversity of their profile [START_REF] Henry | Le conseil, un espace professionnel autonome ?[END_REF]. Moreover, the diversification of their interventions and their clientele alike seems to show their increasing influence in spheres in which management was not previously a norm. This is true of the public sector as well as the voluntary sector which have both implemented the widespread use of consultants in order to (re)organise their activities. How then can this ubiquity of consultants advocating this management rationale be explained? Following on from the work of J. K. [START_REF] Galbraith | Le nouvel état industriel[END_REF], is their influence to be understood from the standpoint of their ability as experts, thus making them the last avatars of technocracy? Or, should we not instead adopt a Weberian theory and explain this "consultocracy" (De Saint--Martin, 2000) as a function that occurred simultaneously with the historical emergence of a "management mindset", in the same way as bureaucracy went hand in hand with the spirit of capitalism [START_REF] Weber | Économie et société[END_REF]? As an aside to these two suppositions, this article will seek to explain the growing role of consultants, based on what they generate and on the more general development of a professional "management" space. Indeed, it is within this configuration [START_REF] Elias | La société des individus[END_REF] that consultants take their position, in close interdependence with other actors: the academics who institutionalise management knowledge on the one hand and on the other hand the executives, managers, unit leaders or directors who operationalise this knowledge in order to maintain their executive, supervisory or managerial roles. The analysis of the interdependence between these actors brings the internal dynamic of this space up to date. This existence of consultants and their influence can thus only be understood through these interdependent networks, which are themselves enshrined in the much wider professional dynamic of management space. The consultants construct the systems through which the knowledge necessary for the legitimate exercise of management activities in organisations plays out. This pursuit of legitimacy by the different professional groups involved in this space acts as a catalyst both for the renewal of management systems by those deemed as the experts i.e. the consultants, and for their ever broader and more specialised application, thanks to their help. This article, following on from more general works on the distribution of management form (Boussard, 2008) will focus on the French example, examining in particular the relationships between organisations and the worlds of consultancy and academia.

THE WORK OF EXPERTS IN MANAGEMENT

If management space can be described as a world characterised by a set of activities linked to managing, then the latter can be divided heuristically into two subspaces. On one side are the consultants and on the other their customers who are responsible for the management within an organisation, The former offer their expertise to the latter to help them choose from the different solutions that are open to them and then assist with their implementation. The above modelling of the work of consultants however conceals an underlying and essential feature of their activity, namely the conception of management processes and their commercial implementation. When they are not with the customer, consultants further develop their solutions, closely monitor rival and emerging techniques, devise new models and tools and construct material and discursive presentations which allow them to transfer this technical knowledge into a commercial proposal. As such, the activity of consultants is a "knowledge industry" (Kipping and Engwall, 2002). It develops and sells knowledge of organisations and their management. However, consultants are ostensibly not the only actors who play a part in the production of management knowledge. The producers of management knowledge Since the beginning of the 20th century engineers have specialised in offering advice to companies, both in the United States and France. These engineers can be defined as scholars concerned with the implementation of scientific methods and mindsets into business, like Taylor in the United States or H. Fayol in France. Following the Second World War firms began to develop based on the anglo--saxon model of consultant networks. Since 1980 however the prevailing model has been the multinational model of consultancies, which has standardised their methods and knowledge with the aim of gaining market share (Ramirez, 2005). The market is dominated by the "big four" firms, which are nevertheless able to exist side by side with much smaller firms, some of which are reliant on the knowledge of one person who has carved themselves a place in the market as an expert of a particular technique or approach [START_REF] Villette | Sociologie du conseil en management[END_REF].

Managerial knowledge of management processes

When viewed from a material or symbolic viewpoint, the analysis of the different processes which exist may seem to suggest a substantial heterogeneity in managerial methods. What though do management control systems, an ERP system, customer segmentation and an ISO certification have in common with one another? When looking at these processes in greater details in situation, it can be observed that they all follow a common logic built around the pursuit of an ideal for an organisation, which can be divided into three principles (Boussard, 2008). The first defines the purpose of the management, in this case maintaining the control of the organisation (Control). The second sets a goal for the organisation, namely to enable its efficient functioning (Performance). The third determines its methods, namely the implementation of a methodical and rational approach to problems (Rationality). In this way, Control, Performance and Rationality form the management "logos" 1 . Through these principles, this logos proposes the basis, objectives and methods of every management system. The logos, which can be traced back to the formation of the first management practices at the beginning of the 19th Century (industrial accounting) can be found in the most recent approaches from Knowledge Management to project--based organisation. It maintains that the act of managing is to incorporate the operation of organisations into this foundational framework and above all to control an organisation with the aim of delivering performance via a methodical, calculated and scientific approach. This management logos can be seen very explicitly in expert publications (encyclopaedias and management manuals, practical guides and methods written by consultants, consulting or training propositions). However it can also be found implicitly within management systems in discourses embedded into physical tools. The technical and discourse agendas can thus not be separated as every technical element is also a "statement" [START_REF] Foucault | L'archéologie du savoir[END_REF]. The term system enables this interconnection of discursive and technical levels to be understood [START_REF] Foucault | Dits et écrits[END_REF]. Better than a "tool" (Berry, 1983) or "instrument" [START_REF] Moisdon | Du mode d'existence des outils de gestion[END_REF]Lascoumes, Le Galès, 2004), it furthermore emphasises the existence of heterogenous elements and their different levels : management systems are combinations of knowledge, tools, materials, places, actors, discourses and symbols which are intertwined and cohesive. They thus form part of a management logos, which they simultaneously help to found and perpetuate.

The production of management systems: competition, segmentation, performativity

The production of management systems comes across paradoxically: it is at once marked by high homogeneity as per the management logos, and yet is simultaneously fragmented into a multitude of different forms. Management is in fact divided into several functions (finance, production, marketing etc.), within which the management logos is applied in a specific way (management control systems, production management systems, market analysis models, etc,). Furthermore, each function is itself the basis for competition between systems (for example the competition 1 The logos is here understood to be both an organising principle and discourse justifying this principle.

ABC and BSC), and even more so for passing trends (systems succeeding and replacing one another). Seen from this perspective of differentiation and competition, the systems seem to constitute a market in which it can be assumed that its suppliers respond to customer demand while lacking ever more effective tools. However, the propositions offered by economic sociology, notably following the work of M. [START_REF] Granovetter | Le marché autrement[END_REF], urge that the market be considered not as an adjustment between a supply and a demand but as the result of "mediations". This implies viewing management systems as "specific" productions executed by middlemen. Notably, this is the analysis that E. Abrahamsson and G. Fairchild (1999) make of the "quality circle" system. They show that the developers of this system initially formulated the difficulties experienced by American companies in the 1980s as a problem of productivity caused by the arrival of Japan into the economic arena. They thus presented the adoption of quality circles, based on the Japanese model, as the solution to the problem, however ultimately they only helped to further identify the issue. THE DYNAMIC OF MANAGEMENT SPACE The experts thus produce the systems and recommend them to their clients. To understand management space the second subspace; that of the customer, must also be understood. The customers are the executives, managers, unit leaders or directors responsible for managing an organisation and to this end, they use management systems. They themselves were trained in the management logos, sometimes even in the same places as the consultants who they pay to assist them. Within their own organisation, these customers are themselves experts or future experts in the management of their specific domain.

A professional space?

In contrast with other specialist activities (such as medicine or law), the customer is himself an actor in the field in which he is seeking knowledge from his advisors. The customer is a management "professional", in the same way that the adviser is. Management space is however not a professional space in the functionalist sense of the term (Merton, 1957;Wilensk 1964). The labour market in this space is indeed not closed by a legal monopoly on the exercise of this activity. Even though the knowledge in this field has been institutionalised and an academic management format has existed since the 1920s and even though there are professional organisations in existence which represent the managers or the executives of this space, at no moment can these professionals ever create a monopoly on the exercise of this activity. Ultimately, these qualification mechanisms call for the existence of a closure process of the professional market, however from a neo--weberian perspective of professionalisation, such as that proposed by M. [START_REF] Larson | The Rise of Professionalism. A Sociological Analysis[END_REF], E. [START_REF] Freidson | Profession of Medicine[END_REF][START_REF] Freidson | Professional Powers, a Study of Institutionalization of Formal Know--ledge[END_REF]. Management space can therefore be analysed as a world in which complex closure processes of professional market segments by different "professional groups" are prevalent.

Closure by knowledge

According to E. [START_REF] Freidson | Professional Powers, a Study of Institutionalization of Formal Know--ledge[END_REF]A. Abbott (1988), this multitudinous competition supports professional knowledge. If recognised, this knowledge enables a group to legitimate its occupancy of a professional territory. In fact, this knowledge offers definitions of problems experienced by customers and permits the group to resolve them, to the detriment of those claiming to be be specialists in the field. The recognition process of this knowledge, which is a fundamental element, involves the construction of a diagnosis, inference methods and adapted actions. By defining the problems, this knowledge also provides adapted solutions and by virtue of the practical knowledge that they possess, makes the group members indispensable in their realisation.

FORMS OF INTERDEPENDENCIES BETWEEN ACTORS IN MANAGEMENT SPACE

The homogeneity constructed around the management logos evokes the process of "imitation" in social groups as brought to light by G. Simmel (2004). This process is always completed by "differentiation" in which the members of a group, initially formed by reciprocal imitation, seek to distinguish themselves from each other. This fragmentation of management space into groups formed around individual systems only makes sense within the framework of the more global homogeneity of the management logos. Rivalry and coherence are here not contradictory but consistent. Management space is ultimately a differentiated and hierarchical professional space set against a backdrop of the legitimacy of the proposed systems, which is also what ensures the space's closure to the outside as well as its different internal closures. This dynamic promotes the production of systems, their institutionalisation (recognition as academic knowledge) and their employment (practical knowledge). The experts, namely both consultants and academics alike, play a central role in this dynamic. As we have already shown, it is these actors who create the systems by interweaving academic knowledge and practices. Ultimately however they position themselves as the experts of experts: their propositions to customers only carry value as they are aimed at customers who themselves act in this management space and thus share the same dynamic. Moreover, their role is also to position themselves as experts in management knowledge in order to ensure the occupation of a territory of activity, a legitimate status and a symbolic position and identity. To be a true "Manager" The managers trained in this way see their contribution in a normative context where management virtues are legitimate [START_REF] Meyer | Institutionalized organizations : Formal structure as myth and ceremony[END_REF]. To be a true manager, one must assimilate with the figure idealised in the discourse. In this framework the systems become indispensable accessories for "playing" the role of manager. As they embody and verbalise the managerial logos, they allow the existence of a manager. Practical knowledge is indispensable to the professional in order to maintain his position. In the same way that a doctor is legitimate thanks to the therapies that he offers, the manager does not maintain his role without techniques. The historical works of B.G Carruthers and W.N Espeland (1991) on the distribution of the double-entry method thus show that the adoption of this method by a businessman allowed him to garner a reputation for seriousness and honesty. In an institutional environment which makes rationality sacrosanct, managers look for a symbolic effect by adopting the systems which embody this rationality. Moreover, the manager is permanently acting in a situation of uncertainty and their success is never assured. The measure of performance is a thorny issue as the correlation between the different variables of action and the result is never clear. Yet, in situations of radical uncertainty, the only rational behaviour is to imitate the other actors in order to avoid risks and to limit negative sanctions [START_REF] Keynes | Théorie générale de l'emploi, de l'intérêt et de la monnaie[END_REF]. The adoption of a management system (like the others have), where possible the most renowned in terms of efficiency, enables accusations of negligence in cases of failure to be avoided [START_REF] Meyer | Institutionalized organizations : Formal structure as myth and ceremony[END_REF]. This situation leads to what DiMaggio and Powell (1983) have called "mimetic isomorphism". Managers select management solutions that have been adopted by similar organisations by "opportunistic use" [START_REF] Segrestin | Les chantiers du manager[END_REF]. The adoption of ISO standards, ERP systems or "project mode" work are good examples of these. However in order to defend a territory, it is also necessary that each segment continues to prove itself by proposing conceptions of problems and their respective solutions in a seemingly much more adapted way than those in other segments. It is at this point the offers of systems are received from the experts. On one hand, the offer of training which offers to "help them to develop their technical and managerial competencies based on a range of tools and pedagogies specially adapted for the professionals of the field". On the other hand, the offer of a consultancy service offering assistance with the implementation of management systems.

Circulation between management subspaces

In addition, it should be recalled that experts and customers alike were trained in the same management logos and their differences are most often played out on the reputation of the diplomas that they receive. The most prestigious firms as well as the best rated companies and units recruit from the most esteemed training courses. In other words, experts and customers alike belong to networks which link them with one another. In this way, it is futile to oppose the subspaces between experts on one side and customers on the other, as we have been doing up to now. This divide conceals the existence of these networks, the unique feature of which is more than a mere nexus of relations. In fact these networks are cognitive spaces in which the members are linked by a set of expertises and common conventions. These conventions are the result of theoretical and practical knowledge which is distributed by management systems. These are themselves integrated into the network as technical objects within the socio--technical framework (Callon, 1988): they enable the mobilisation of the actors around a common and shared translation of a problem and corresponding solution. They arise at any given moment as the professional method of excellence. It is within these networks that definitions of professional activity and the tiering of segments are circulated by way of these systems. The systems, whether learned at university, through continuous training or through consultancy, are the keys to access to these networks. And, at the same time, they are defined by the network itself: as soon as a member of these networks changes position, function or role, he carries these definitions with him and and surrounds himself with the necessary systems to support the role. The management norms spread by consultants are therefore to be considered as the result of an interdependence between different professional groups, structured in networks. The force of management reasoning is hence neither a consequence of the "power" of consultants, nor that of an inevitable rationalisation of the modern world in which they act unconsciously. It is located at an intermediate and widespread position, which is a dynamic that pushes the actors of this space, whatever they may be, to identify as professionals by using theoretical and practical knowledge appropriate to the management logos. As stated by this executive in continuous training: "You have to learn and put new tools into place in order gain professionalism and continue to grow our business". The consultant is just behind the university door: he prepares the systems necessary to meet this demand for "professionalism". BIBLIOGRAPHIE Abbott A., The System of the Professions. An Essay on the Division of Expert Labour, Chicago, University Press, 1988. Abrahamson E., Fairchild G., Management fashion : Lifecycles, triggers, and collective learning process, Administrative Science Quarterly, 1999, 44, p. 708--740. Berrebi--Hoffmann I., Les multinationales du conseil, Sociologies pratiques, 2002, no 6, p. 47.69. Berry M., Une technologie invisible ? L'impact des instruments de gestion sur l'évolu--tion des systèmes humains, CRG, École polytechnique, 1983. Boussard V., Sociologie de la gestion. Les faiseurs de performance, Paris, Belin, 2008. Boussard V., Maugeri S., Introduction, in Du politique dans les organisations, Paris, L'Harmattan, 2003, p. 25--61. Bucher R., Strauss A., Professions in process, American Journal of Sociology, 1961, 66 (4), p. 325--334, trad. franç. dans A. Strauss, La trame de la négocia--tion, Sociologie qualitative et interactionnisme, Paris, L'Harmattan, 1992. Callon M. (sous dir.), La science et ses réseaux, Paris, La Découverte, 1988. Carruthers B. G., Espeland W. N., Accounting for rationality : Double--entry bookkeeping and the rhetoric of economic rationality, American Journal of Sociology, 1991, vol. 97, no 1, p. 31--69. Chandler H. (1977), La main visible des managers, Paris, Éd. d'Organisation, trad. franç., 1988. Chessel M. E., Pavis F., Le technocrate, le patron et le professeur. Une histoire de l'enseignement supérieur de gestion, Paris, Belin, 2001.