Abstract:
A device for emission of light is made including an emitting structure including an active part and a micro-cavity, delimited by mirrors and containing the active part, and a laser diode designed for pumping the emitting structure. The emitting structure is fixed to the laser diode. The device is particularly applicable to the detection of gas.

Description:
TECHNICAL DOMAIN 
     This invention relates to a micro-cavity device for emission of light and a process for making such a device, including an emitting structure photo-pumped by a laser diode. 
     More particularly, the invention relates to a micro-cavity device for emission of light, in the medium infrared range from 2 μm to 6 μm, and a process for manufacturing such a device. 
     The invention is particularly applicable to:
         detection of gas, more particularly in the automobile industry field (measurement of the exhaust gas concentration) and in the climatology field (measurement of carbon dioxide in the atmosphere) and   automatic sort of plastics that also absorb in the infrared at different wavelengths depending on their composition.       

     STATE OF PRIOR ART 
     A micro-cavity device for emission of light in the medium infrared range usually comprises an active part made of CdHgTe that is inserted in a micro-cavity. The micro-cavity is delimited by two Bragg mirrors added on to the active part. 
     The micro-cavity is very short, on the order of a few half wavelengths, and determines the wavelength of the emitted light. 
     In this type of light emission device, excitation of the emitting structure must be as intense as possible. Two excitation techniques are known for obtaining excitation power densities exceeding 100 W/cm 2 . 
     The first of these techniques consists of photo-pumping the emitting structure of the device by a laser emitting a light beam with a very weak divergence. Further information about this subject is given in the following document: 
     C. Roux et al., Appl. Phys. Lett., Vol. 75, No. 12, 1999, pp. 1661–1663. 
     However, the laser installation is relatively large and expensive compared with the emitting structure. 
     The second known technique consists of photo-pumping this emitting structure by a laser diode emitting a large divergence light beam that is focused by an appropriate optical installation. Further information about this subject is given in a technical bulletin by the Nanolase company entitled: “Technology and Applications”. The light beam emitted by the laser diode has to be focused because the power density in this beam reduces very quickly as a function of the distance. 
     This focusing is achieved by inserting appropriate optical means between the pumping diode and the emitting structure (collection and focusing lens) that must be aligned with respect to the diode and the emitting structure. 
     However, this alignment considerably increases the difficulty of manufacturing and the cost of the device. 
     Refer also to the following document: 
     E. Hadji et al., Optics Lett., Vol. 25, No. 10, 2000, pp. 725–727. 
     The device disclosed in this document also requires a precise alignment of an emitting structure, a laser diode and a lens between them. 
     PRESENTATION OF THE INVENTION 
     This invention is intended to overcome the disadvantages described above. 
     The invention proposes a micro-cavity device for light emission, in which the emitting structure is pumped by a laser diode without focusing light emitted by this diode, while keeping an efficiency equivalent to the efficiency of the device disclosed in the Nanolase company bulletin mentioned above. 
     Furthermore, the invention provides a means of obtaining a very compact light emission device within the range from 2 micrometers to 6 micrometers. Specifically, the purpose of this invention is to provide a light emission device, this device comprising:
         an emitting structure comprising an active part and a micro-cavity, delimited by first and second mirrors and containing the active part, and   a laser diode designed for optical pumping of the emitting structure through the first mirror of this structure, wherein the emitting structure is fixed to the laser diode. According to one preferred embodiment of the device according to the invention, the active part is able to emit infrared radiation with a wavelength within the interval of from 2 μm to 6 μm.       

     According to a first particular embodiment of the device according to the invention, the emitting structure also comprises a plate that is transparent to light generated by the laser diode and that is fixed to this laser diode. 
     According to a second particular embodiment, the laser diode comprises a port and the emitting structure is formed on this port. 
     According to a third particular embodiment, the emitting structure also comprises a plate that is transparent to light generated by the laser diode and that acts as a port for this diode. 
     This invention also relates to a process for manufacturing the device according to the invention in which the emitting structure is formed, and this emitting structure is fixed to the laser diode. 
     According to one particular embodiment of the process according to the invention, the active part is formed on a substrate and the first mirror is formed on this active part, this first mirror is fixed onto the transparent plate, the substrate is eliminated, the second mirror is formed on the active part and the plate is fixed to the laser diode. According to a second particular embodiment, the laser diode comprises a port and the active part is formed on a substrate and the first mirror is formed on this active part, the first mirror is fixed on the port of the laser diode, the substrate is eliminated and the second mirror is formed. According to a third particular embodiment, the laser diode comprises a port and the active part is formed on a substrate and the first mirror is formed on this active part, this first mirror is fixed on the transparent plate, the substrate is eliminated, the second mirror is formed on the active part, the port is eliminated and the port is replaced by the transparent plate. 
     Preferably, the emitting structure is fixed to the laser diode using a glue that is transparent to light emitted by this laser diode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       This invention will be better understood after reading the description of example embodiments given below, for information purposes only and in no way limitatively, with reference to the appended drawings, wherein:
           FIGS. 1A to 1E  diagrammatically show the steps in a particular embodiment of the process according to the invention,     FIG. 2  is a diagrammatic view of a particular embodiment of the device according to the invention,     FIGS. 3A and 3B  diagrammatically show the steps in another particular embodiment of the process according to the invention, and     FIG. 4  is a diagrammatic view of another particular embodiment of the device according to the invention.       

     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description applies to an example of a process according to the invention with reference to  FIGS. 1A to 1E . 
     This process can be implemented by forming an emitting structure on a substrate  2 , for example made of CdZnTe ( FIG. 1A ), capable of generating infrared radiation when it is optically pumped. 
     For example, the thickness of substrate  2  is 750 μm. 
     The emitting structure is formed by forming an active part  4  made of CdHgTe on the substrate  2 , by epitaxy. In this example, this active part is a stack comprising a layer  6  of HgTe, a layer  8  of Cd 0.7 Hg 0.3 Te, a layer  10  of Cd 0.3 Hg 0.7 Te and a layer  12  of Cd 0.7 Hg 0.3 Te, in sequence. 
     A Bragg mirror  14  ( FIG. 1B ) called a “background mirror” is then formed on this layer  12 , for which the reflectivity (for radiation generated by the active part) is very high and is equal for example to 99%. 
     This background mirror  14  is then glued on a silica plate  16  ( FIG. 1C ), with a thickness for example equal to 300 μm, using a layer of glue  18  transparent to the light intended for optical pumping of the emitting structure. 
     The next step is to eliminate the substrate  2 . The starting point is to remove 600 μm of the substrate by mechanical polishing. The remaining 150 μm are then chemically etched ( FIG. 1D ). This is achieved using chemical etching that is selective towards HgTe. This etching is done by a mix of acids, and stops on layer  6 . 
     The next step is to form another Bragg mirror  20  called an “output mirror” on the layer  6 , by an appropriate deposit ( FIG. 1E ). 
     For example, the reflectivity of this mirror  20  is 60% for radiation emitted by the active part  4 , for which the wavelength is equal for example to 3.3 μm. 
     Note that the reflectivity of the background mirror  14  is very low for optical pumping light for which the wavelength is equal to 1.06 μm, for example. 
     The emitting structure  22  thus obtained comprising a micro-cavity delimited by mirrors  14  and  20  is then glued on the casing  24  ( FIG. 2 ) of the laser diode  26  intended for optical pumping of this structure. This is done using a layer  28  of a glue that is transparent to light generated by this laser diode. 
       FIG. 2  also shows electrodes  30  that are included in the laser diode  26  and the emission area  32  of this diode and the port  34  of this diode, which is between this area  32  and the emitting structure  22 . 
     For example, the distance between the emission area  32  and the emitting structure  22  may be equal to 1 mm. 
     The silica plate  16  is eliminated, so that a laser diode with a lower pumping power can be used without changing the power density, and so that the efficiency of the device can be significantly increased. More precisely, the step in which the bottom mirror  14  is glued (described with-reference to  FIG. 1C ) is directly performed onto the port  36  of a laser pumping diode  38 , as shown diagrammatically in  FIG. 3A . 
     This is done using a glue layer  40  transparent to light emitted by this diode, for which the emission area  42  is also seen. 
     Then, after protecting the laser diode  38 , the step for mechanical polishing of the substrate  2  is applied followed by the selective chemical etching step to eliminate the rest of this substrate, and the output mirror  20  deposition step ( FIG. 3B ) is then performed as described above. 
     The procedure for protecting the laser pumping diode  38  is as follows: this diode  38  is protected by a mask during the step to deposit the material making up the mirror. For example, this mask is made of aluminium. 
     Instead of forming the emitting structure on the port, the casing  44  ( FIG. 4 ) of the laser pumping diode  38  can be cut out in order to remove the original port and replace this port by a silica plate  46  with the same dimensions as this original port and supporting the emitting structure, this structure firstly being formed and then glued on this plate  46  as was seen above with reference to  FIGS. 1A to 1E . 
     This process makes it possible to eliminate a step in manufacturing of the device and also to improve the external efficiency of this device by a factor of 10. 
     For information purposes only, and in no way limitatively, the laser diode  38  may be of the type marketed by the Thorlabs Inc. company, with reference HL 7851G.