Patent Publication Number: US-2023140206-A1

Title: Photonic crystal surface-emitting laser and optical system

Description:
FIELD 
     The subject matter relates to optical detection, and more particularly, to a photonic crystal surface-emitting laser and an optical system using the photonic crystal surface-emitting laser. 
     BACKGROUND 
     A Photonic crystal surface-emitting laser (PCSEL) has advantages of excellent beam quality, small size, low energy consumption, easy integration, and high reliability and can be widely used in optical systems such as systems for three-dimensional detection, consumer electronic devices, automotive LIDARS, smart devices, and medical examination devices. 
     A main structure of an existing PCSEL includes an epitaxial substrate with a thickness of at least 100 microns, for maintaining and increasing strength of a chip in the PCSEL. During application of high electric currents, an active light-emitting layer will generate a lot of heat, the heat is transferred by the epitaxial substrate and a packaging material of the PCSEL. However, the epitaxial substrate increases the length of the heat-dissipation path, efficiency of heat dissipation becomes low, an optical output power of the PCSEL is decreased, and a service life is shortened. The existing PCSEL has a light-emitting area, and an electrode wiring area around the light emitting area must be reserved. An area of the electrode wiring area is several times an area of the light-emitting area, this is not optimal, and a cost of the packaging material is increased. Parasitic capacitance and inductance of the electrode wiring area is common, reducing a response speed of the PCSEL and making high-frequency operation of the PCSEL problematic. In the existing PCSEL, a density of electric current applied to the active light-emitting layer tends to be uneven due to different diffusion speeds of electric currents in various directions. 
     Therefore, the existing PCSEL needs to be improved. 
     SUMMARY 
     A first aspect of the present disclosure provides a photonic crystal surface-emitting laser, including: 
     a light emitting module, including:
         a photonic crystal layer,   an active light emitting layer on a side of the photonic crystal layer,   a first electrode on a side of the active light emitting layer facing away from the photonic crystal layer, and   a second electrode at least partially on the side of the active light emitting layer facing away from the photonic crystal layer; and       

     a driving module in electrical contact with surfaces of the first electrode and the second electrode facing away from the photonic crystal layer, wherein the driving module is configured to output driving signals to the first electrode and the second electrode to drive the active light emitting layer to generate photons, the photons are incident into the active light emitting layer to generate a laser light through oscillation on Bragg diffraction. 
     A second aspect of the present disclosure provides an optical system comprising the above photonic crystal surface-emitting laser and a control device, the control device is electrically connected with the photonic crystal surface-emitting laser and configured to output driving signals to the photonic crystal surface-emitting laser to drive the photonic crystal surface-emitting laser to generate a laser light. 
     In the photonic crystal surface-emitting laser, the driving module is integrated with the light emitting module, improving a switching speed of the photonic crystal surface-emitting laser. Since there is no substrate for the light emitting module, the first electrode and the second electrode of the light-emitting module are directly bonded to the driving module instead of being separated by a substrate. This improves heat dissipation from the light emitting module, and thus the light emitting power and service life of the photonic crystal surface-emitting laser are improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a block diagram of an optical system according to an embodiment of the present disclosure. 
         FIG.  2    is a schematic plan view of a light-emitting module of the optical system of  FIG.  1   . 
         FIG.  3    is a cross-sectional view along line III-III of  FIG.  2   . 
         FIG.  4    is a cross-sectional view along line IV-IV of  FIG.  2   . 
         FIG.  5    is a schematic plan view of a photonic crystal surface-emitting laser according to another embodiment of the present disclosure. 
         FIG.  6    is a schematic plan view of a driving module of the optical system of  FIG.  1   . 
         FIG.  7    is a cross-sectional view of the light-emitting module and the driving module taken along line VII-VII of  FIG.  6   . 
         FIG.  8    is a graph depicting the relationship between the operating temperature and the driving current of different photonic crystal surface-emitting lasers at an ambient temperature of 360K. 
         FIG.  9    is a graph depicting the relationship between the operating temperature and the driving current of a photonic crystal surface-emitting laser at different ambient temperatures in a first comparative example. 
         FIG.  10    is a graph depicting the relationship between the operating temperature and the driving current of a photonic crystal surface-emitting laser at different ambient temperatures in a second comparative example. 
         FIG.  11    is a graph depicting the relationship between the operating temperature and the driving current of this embodiment of a photonic crystal surface-emitting laser at different ambient temperatures. 
     
    
    
     DETAILED DESCRIPTION 
     Implementations of the disclosure will now be described, by way of embodiments only, with reference to the drawings. The disclosure is illustrative only, and changes may be made in the detail within the principles of the present disclosure. It will be appreciated that the embodiments may be modified within the scope of the claims. 
     Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. The technical terms used herein are to provide a thorough understanding of the embodiments described herein, but are not to be considered as limiting the scope of the embodiments. The term “comprising,” when utilized, means “including, but not necessarily limited to”; it specifically indicates open-ended inclusion or membership in the so-described combination, group, series, and the like. 
     Referring to  FIG.  1   , an optical system  100  of an embodiment of the present disclosure includes a photonic crystal surface-emitting laser  10 . The optical system  100  can be a face recognition sensor, a laser radar, etc., which can be applied to various electronic devices such as smart phones, augmented reality (AR) glasses, and virtual reality (VR) glasses, and can also be applied to automobiles, household, medical equipment, or unmanned vehicles, for use in smart chemical plants or automated warehouses. 
     The optical system  100  further includes a control device (not shown), the control device is electrically connected with the photonic crystal surface-emitting laser  10 . When the photonic crystal surface-emitting laser  10  is applied in the above various optical systems  100 , it is configured to emit a laser light according to driving signals output by the control device, so that the optical system  100  can implements such functions as interactive display, safety recognition, indoor environment sensing, or proximity sensing. In this embodiment, the control device may be a chip, a chip group, a control motherboard, etc. 
     The photonic crystal surface-emitting laser  10  includes a light emitting module  2  and a driving module  3  electrically connected to each other. The driving module  3  is configured to apply a driving signal (driving voltage or driving current) to the light emitting module  2 , and the light emitting module  2  is configured to emit a laser light according to the driving signal (driving voltage or driving current). 
     Referring to  FIGS.  2  and  3   , the light emitting module  2  includes a photonic crystal layer  21 , an active light emitting layer  22 , a first cladding layer  23 , and a Bragg mirror  24 , these are sequentially stacked. 
     The photonic crystal layer  21  includes an ohmic contact layer  211  and a stacked second cladding layer  212 , the second cladding layer  212  is located between the ohmic contact layer  211  and the active light emitting layer  22 . In this embodiment, the ohmic contact layer  211  is made of P-type gallium arsenide, and the second cladding layer  212  is made of P-type aluminum gallium arsenide. In other embodiments, the ohmic contact layer  211  may also be made of indium phosphide or indium gallium arsenide. The photonic crystal layer  21  defines a plurality of through holes  213  which are spaced from each other. Each through hole  213  penetrates through the ohmic contact layer  211  and the second cladding layer  212 . 
     The active light emitting layer  22  includes a plurality of quantum well active light emitting layers  221  and a plurality of energy barrier layers  222 . The plurality of quantum well active light emitting layers  221  and the plurality of energy barrier layers  222  are stacked in alternating fashion. That is, the quantum well active light emitting layer  221  and the energy barrier layer  222  are arranged alternately. In this embodiment, the active light-emitting layer  22  includes three quantum well active light-emitting layers  221  individually sandwiched between four energy barrier layers  222 . A material of each quantum well active light-emitting layer  221  is indium gallium arsenide, and a material of each energy barrier layer  222  is gallium arsenide, which is configured to emit a laser light in a wavelength range from 905 to 1550 nm. In other embodiments, the quantum well active light emitting layers  221  may also be made of aluminum gallium indium arsenide or indium gallium arsenide, and the energy barrier layers  222  may also be made of aluminum gallium arsenide or aluminum gallium indium arsenide. 
     The active light emitting layer  22  is configured to generate photons under driving of the driving signal. The photons generated by the active light emitting layer  22  propagate everywhere, and the photons propagating into the photonic crystal layer  21  repeatedly oscillate in the photonic crystal layer  21  until the light emitting module  2  reaches a state of balance between gain and loss and a laser light is generated. 
     A material of the first cladding layer  23  may be N-type aluminum gallium arsenide. The first cladding layer  23  and the second cladding layer  212  cooperate to lock in the photons emitted from the active light emitting layer  22 , reducing the propagation of photons toward the Bragg mirror  24 . In this embodiment, the material of the first cladding layer  23  is aluminum gallium arsenide. In other embodiments, materials of the first cladding layer  23  and the second cladding layer  212  may also be indium aluminum arsenide, indium phosphide, or gallium phosphorus arsenide. 
     The Bragg mirror  24  includes a plurality of first refractive layers  241  and a plurality of second refractive layers  242  stacked in an alternating fashion. The first refractive layers  241  have the same refractive index, the second refractive layers  242  have the same refractive index, and the refractive indices of one first refractive layer  241  and one second refractive layer  242  are different. The Bragg mirror  24  is configured to reflect the received photons toward the photonic crystal layer  21  to recover photons escaping from the first coating layer  23 . This reduces light loss and improves light emitting power of the photonic crystal surface-emitting laser  10 . In this embodiment, the Bragg mirror  24  includes three first refractive layers  241  and three second refractive layers  242 , these are arranged in an alternating fashion. 
     Referring to  FIGS.  2  and  3   , the light emitting module  2  further includes a transparent conductive layer  25 , a light-transmitting substrate  26 , and a thermally conductive adhesive layer  27 . 
     The thermally conductive adhesive layer  27  is located between the transparent conductive layer  25  and the light-transmitting substrate  26 , the thermally conductive adhesive layer  27  bonds and fixes together the transparent conductive layer  25  and the light transmitting substrate  26  and guides heat generated during operation of the light emitting module  2 . The transparent conductive layer  25  is on a side of the photonic crystal layer  21  facing away from the Bragg mirror  24  and fills the through holes  213  which are spaced and distributed in the photonic crystal layer  21 . The light-transmitting substrate  26  is on the side of the transparent conductive layer  25  facing away from the Bragg mirror  24 . 
     In this embodiment, the transparent conductive layer  25  is indium tin oxide (ITO). The light-transmitting substrate  26  may be made of a light transmitting material such as sapphire, gallium arsenide (GaAs), gallium nitride (GaN), silicon (Si), silicon carbide (SiC), indium phosphide (InP). The laser light generated from the photonic crystal layer  21  is emitted from a surface of the light-transmitting substrate  26  facing away from the Bragg mirror  24 . 
     The light emitting module  2  further includes a first electrode  28  and a second electrode  29  electrically isolated from each other. The first electrode  28  and the second electrode  29  are configured to receive the driving signal. The first electrode  28  and the second electrode  29  are made of metal, such as titanium (Ti), germanium (Ge), nickel (Ni), gold (Au), platinum (Pt), and alloys thereof. In this embodiment, the first electrode  28  is an N-type electrode, and the second electrode  29  is a P-type electrode. 
     The first electrode  28  is on the side of the Bragg mirror  24  facing away from the light-transmitting substrate  26  and is in direct contact with the Bragg mirror  24 . The second electrode  29  includes a first conductive portion  291  and a second conductive portion  292 . 
     The first conductive portion  291  is on the side of the Bragg mirror  24  facing away from the light-transmitting substrate  26 . A surface of the first conductive portion  291  facing away from the light-transmitting substrate  26  is flush with a surface of the first electrode  28  facing away from the light transmitting substrate  26 . That is, the first conductive portion  291  is coplanar with the first electrode  28 . 
     The first conductive portion  291  encloses a first accommodation space S 1  having a notch S 0 . The first electrode  28  includes a third conductive portion  281 , a fourth conductive portion  282 , and an extension portion  283  connecting the third conductive portion  281  and the fourth conductive portion  282 . The third conductive portion  281  is in the first accommodation space S 1 , the fourth conductive portion  282  is located outside the first accommodation space S 1 , and the extension portion  283  extends from the third conductive portion  281  to the fourth conductive portion  282  through the notch S 0 . The third conductive portion  281  is spaced apart from the first conductive portion  291  to be electrically isolated from each other. 
     Referring to  FIGS.  3  and  4   , the second conductive portion  292  extends from the first conductive portion  291  towards the light-transmitting substrate  26 . The second conductive portion  292  is a cylindrical structure in which a hollow space S 2  is formed. The active light emitting layer  22 , the first cladding layer  23 , and the Bragg mirror  24  are in the hollow space S 2 . The thermally conductive adhesive layer  27  covers an end face of the second conductive portion  292  near the light-transmitting substrate  26 , a part of an outer surface of the second conductive portion  292  (which is the surface of the second conductive portion  292  facing away from the hollow space S 2 ), and the transparent conductive layer  25 . One end of the second conductive portion  292  near the light-transmitting substrate  26  is in contact with the transparent conductive layer  25 . 
     In this embodiment, the light emitting module  2  further includes an insulating layer  20 . The insulating layer  20  may be silicon nitride (SiN x ), silicon dioxide (SiO 2 ), or polymethyl methacrylate (PMMA). The insulating layer  20  is in the hollow space S 2  formed on the second conductive part  292  and is partially attached to an inner wall  2921  of the second conductive part  292 . The insulating layer  20  is also located between the second conductive part  292  and the photonic crystal layer  21 , the active light emitting layer  22 , the first coating layer  23 , and the Bragg mirror  24 . 
     A portion of the insulating layer  20  which is not attached to the inner wall  2921  of the second conductive portion  292  is gapped from the inner wall  2921 , and the transparent conductive layer  25  infills the gap to make electrical contact with the second conductive portion  292 . 
     The insulating layer  20  further extends to the side of the Bragg mirror  24  facing away from the light-transmitting substrate  26  and is located between the first electrode  28  and the first conductive portion  291 , so that the first electrode  28  and the first conductive portion  291  are spaced apart and electrically isolated from each other. 
     When driving signals of respectively different magnitudes are applied to the first electrode  28  and the second electrode  29 , the driving current moves from the side of the photonic crystal layer  21  near the light-transmitting substrate  26 . The active light emitting layer  22  generates photons under driving of the driving current. When the photons generated by the active light emitting layer  22  propagate to the photonic crystal layer  21 , they oscillate repeatedly in the photonic crystal layer  21  until the light emitting module  2  reaches the state of balance between gain and loss and a laser light is generated, and the laser light is emitted from the side of the transparent substrate  26  facing away from the Bragg mirror  24 . 
     In the above process, the first cladding layer  23 , the second cladding layer  212 , and the Bragg mirror  24  all prevent the emission of photons from the side facing away from the light-transmitting substrate  26 , which improves use efficiency, thereby improve the light emitting power of the light emitting module  2 . 
     The third conductive portion  281 , the fourth conductive portion  282 , and the extension portion  283  are all rectangular as seen in  FIG.  2   . The first conductive portion  291  is a rectangular frame adapted to a shape of the third conductive portion  281  and having the notch S 0 . The accommodation space S 1  is filled with the insulating layer  20  to electrically insulate the third conductive portion  281  from the first conductive portion  291 . 
     In other embodiments, the third conductive portion  281  and the first conductive portion  291  may have other shapes. For example, as shown in  FIG.  5   , the third conductive portion  281  may be circular, and the first conductive portion  291  may be a circular ring adapted to the shape of the third conductive portion  281 , and having the notch S 0 . 
     The shape of the third conductive portion  281  as seen in  FIG.  2    is the same as a shape of the orthographic projection of the third conductive portion  281  onto the active light emitting layer  22 . The third conductive portion  281  is opposite to the photonic crystal layer  21 , and an orthographic projection of the photonic crystal layer  21  onto the active light emitting layer  22  is within an orthographic projection of the third conductive portion  281  onto the active light emitting layer  22 . That is, the orthographic projection of the third conductive portion  281  onto the active light emitting layer  22  overlaps with the orthographic projection of the photonic crystal layer  21  onto the active light emitting layer  22 , and an orthographic projection area of the third conductive portion  281  onto the active light emitting layer  22  is equal to or greater than an orthographic projection area of the photonic crystal layer  21  onto the active light emitting layer  22 . 
     The through holes  213  spaced and distributed in the photonic crystal layer  21  create a large impedance at their individual locations but the impedance at other locations is small. A difference in impedance across all the locations of the photonic crystal layer  21  may cause the driving current to diffuse unevenly in a horizontal direction, as shown in  FIG.  3    or  FIG.  4   . In this embodiment, when the first electrode  28  receives the driving signal, the location and shape of the third conductive portion  281  of the first electrode  28  promotes a uniform diffusion of the driving current in the horizontal direction. 
     Referring to  FIGS.  6  and  7   , the driving module  3  is a transistor. In this embodiment, the driving module  3  is a transistor with a high electron mobility (HEMT), such as gallium nitride transistor. 
     The driving module  3  includes a substrate  31 , a buffer layer  32 , a channel layer  33 , and an electrode layer  34  which are sequentially stacked. 
     A material of the substrate  31  is sapphire, silicon, silicon oxide, silicon carbide, or diamond. The substrate  31  is configured to support the buffer layer  32 , the channel layer  33 , and the electrode layer  34  during processing. The buffer layer  22  is on one surface of the substrate  31  and may be made of gallium nitride or aluminum nitride. 
     The channel layer  33  includes a P-type gallium nitride layer  331 , an aluminum gallium nitride barrier layer  332 , and a non-doped gallium nitride channel  333  which are sequentially stacked. 
     A material of the electrode layer  34  may be titanium (Ti), aluminum (Al), nickel (Ni), gold (Au), or palladium (Pd). The electrode layer  34  includes a gate electrode G, a source electrode S, a drain electrode D, and a connecting electrode P. The gate electrode G, the source electrode S, the drain electrode D, and the connecting electrode P are spaced and insulated from each other. In this embodiment, the driving module  3  further includes an insulating material layer  35 , which is located between the gate electrode G, the source electrode S, the drain electrode D, and the connecting electrode P to create a separation between the gate electrode G, the source electrode S, the drain electrode D, and the connecting electrode P. 
     The P-type gallium nitride layer  331  is in direct contact with the gate electrode G, and the P-type gallium nitride layer  331  is insulated from the source electrode S, the drain electrode D, and the connecting electrode P because of the insulating material layer  35 . The aluminum gallium nitride energy barrier layer  332  and the non-doped gallium nitride channel  333  are in contact with the source electrode S and the drain electrode D, respectively. 
     The drain electrode D is in contact with the first electrode  28  to apply a negative voltage to the first electrode  28 . The connecting electrode P is in contact with the second electrode  29  to apply a positive voltage to the second electrode  29 . The positive voltage and the negative voltage are the above-mentioned driving signals. The driving signals cause a voltage difference between the first electrode  28  and the second electrode  29 . A current loop (that is, a driving current) is formed in the light emitting module  2  to emit a laser light. The non-doped gallium nitride channel  333  serves as the main conducting semiconductor channel, and the arrangement of the P-type gallium nitride layer  331  raises the height of the energy barrier of the aluminum gallium nitride energy barrier layer  332  above the Fermi energy level. 
     The drain electrode D and the first electrode  28 , and the connecting electrode P and the second electrode  29  are fixed by metal bonding. The bonding method for metals is, for example, a face-to-face bonding technique for gold to gold. 
     In other embodiments, the driving module  12  does not include the connecting electrode P, and the second electrode  29  would be in direct contact with the gate electrode G. The gate electrode G would provide driving signals for the second electrode  29 . That is, the voltage on the gate electrode G would be the driving signals of the second electrode  29 . Compared with the manner of electrically connecting the second electrode  29  through the connecting electrode P, the step of forming the connecting electrode P is omitted in the above arrangement. 
     When the voltage on the gate electrode G reaches the turn-on voltage of the driving module  3 , the channel layer  33  is turned on, the source electrode S is electrically connected to the drain electrode D, and the drain electrode D applies a negative voltage (driving signal) to the first electrode  28 . The connecting electrode P receives a positive voltage (driving signal) and applies the positive voltage to the second electrode  29 . The light emitting module  2  emits a laser light under the driving of the driving signal. 
     In the photonic crystal surface-emitting laser  10 , the driving module  3  is integrated with the light emitting module  2 , the driving module  3  is a gallium nitride transistor with a high electron mobility (in some embodiments, the electron mobility can be more than 2000 cm 2 /V·s), which improves switching speed of the photonic crystal surface-emitting laser  10 . The light emitting module  2  generates heat during operation. The light emitting module  2  does not include a substrate as the first electrode  28  and the second electrode  29  of the light-emitting module  2  are directly bonded to the electrode layer  34  of the driving module  3  instead of being separated by a substrate, this enables rapid and powerful heat dissipation from the light emitting module  2 , and thus the light emitting power and service life of the photonic crystal surface-emitting laser  10  are improved. 
     Heat dissipation of the photonic crystal surface-emitting lasers in the examples and comparative examples is described below. 
       FIG.  8    is a graph depicting the relationship between the operating temperature and the driving current of different photonic crystal surface-emitting lasers at an ambient temperature of 360K. In  FIG.  8   , curve X represents the photonic crystal surface-emitting laser in a first comparative example, curve Y represents the photonic crystal surface-emitting laser in a second comparative example, and curve Z represents the photonic crystal surface-emitting laser in an embodiment of the present disclosure. As can be seen from  FIG.  8   , under the same ambient temperature, when the driving current is equal, the operating temperature corresponding to the curve Z is the smallest, that is, the operating temperature of the photonic crystal surface-emitting laser in the present disclosure is the lowest and the heat dissipation is the most effective. 
       FIG.  9    is a graph depicting the relationship between the operating temperature and the driving current of the photonic crystal surface-emitting laser at different ambient temperatures in the first comparative example. In  FIG.  9   , curves X 1 , X 2 , X 3 , and X 4  are curves of the operating temperature with the driving current at the ambient temperatures Tc of 300K, 320K, 340 k, and 360K, respectively. 
       FIG.  10    is a graph depicting the relationship between the operating temperature and the driving current of a photonic crystal surface-emitting laser at different ambient temperatures in the second comparative example. In  FIG.  10   , curves Y 1 , Y 2 , Y 3 , and Y 4  are curves of the operating temperature with the driving current at the ambient temperatures Tc of 300K, 320K, 340 k, and 360K, respectively. 
       FIG.  11    is a graph depicting the relationship between the operating temperature and the driving current of a photonic crystal surface-emitting laser at different ambient temperatures in an embodiment of the present disclosure. In  FIG.  11   , curves Z 1 , Z 2 , Z 3 , and Z 4  are curves of the operating temperature with the driving current at the ambient temperatures Tc of 300K, 320K, 340 k, and 360K, respectively. 
     At any particular ambient temperature, when the driving currents are equal, the operating temperature corresponding to the curve Z (Z 1 , Z 2 , Z 3  and Z 4 ) is the least, that is, the operating temperature of the photonic crystal surface-emitting laser in an embodiment of the present disclosure is the lowest and the heat dissipation is the most effective. 
     The driving module  3  is integrated with the light emitting module  2 , and there is no substrate, which is also advantageous to miniaturization of the structure. Since the first electrode  28  and the second electrode  29  are coplanar, an area of perforations to allow electrical connections is not needed, which reduces the area of the light emitting module  2  and avoids parasitic capacitances and inductances inherent in a perforated area. 
     While the present disclosure has been described with reference to particular embodiments, the description is illustrative of the disclosure and is not to be construed as limiting the disclosure. Therefore, those of ordinary skill in the art can make various modifications to the embodiments without departing from the scope of the disclosure as defined by the appended claims.