Laser diodes with layer of graphene

According to an example of the present disclosure a semiconductor laser diode includes a layer of graphene between an active laser region and a semiconductor substrate structure. The semiconductor laser diode may further include a first pair of electrodes to apply a potential difference across the active laser region and a second pair of electrodes to apply a potential difference across the layer of graphene.

BACKGROUND

A laser diode includes a semiconductor laser with an active laser region. The active laser region may be electrically pumped by applying a potential difference across the active laser region. One known method of modulating the output power of a laser diode is to vary the injection current or bias voltage that is used to pump the active laser region.

DETAILED DESCRIPTION

A laser diode includes an active laser region. A voltage may be applied across the active laser region in order to inject a current and pump the active laser region. This causes stimulated emission of coherent light. The output power of the laser diode may be modulated by varying the injection current or bias voltage that is used to pump the active laser region.

However, in the above method the modulation bandwidth is proportional to laser output power. Therefore, this approach requires a very large injection current for high-speed modulation and can cause high power consumption, overheating etc. Furthermore, varying the injection current changes the lasing wavelength which leads to chirp (different laser wavelength at high and low power points). This may limit channel spacing when using modulated lasers in WDM applications and risks cross talk between channels. Another approach, instead of modulating the output laser power directly, is to use a separate optical modulator, such as a semiconductor modulator, downstream of the laser.

The present disclosure proposes including a layer of graphene in the semiconductor diode in order to modulate the output laser power. For example, the layer of graphene may be between the active laser region and a semiconductor substrate structure. The semiconductor laser diode may further include a first pair of electrodes to apply a potential difference across the active laser region and a second pair of electrodes to apply a potential difference across the layer of graphene.

In this way the optical absorption of the graphene layer and thus absorption in the optical cavity of the laser diode can be varied by varying the potential difference applied to the layer of graphene. Even very thin layers of graphene have excellent optical absorption characteristics. Therefore, a thin layer of graphene may be integrated directly into the laser diode structure, rather than using a separate optical modulator. The optical characteristics of graphene are very responsive to changes in the voltage and so this structure may provide quick modulation of the output laser power. Further, as this approach does not change the frequency of the light, it may modulate the laser output without causing chirp.

FIG. 1shows a semiconductor laser diode according to one example of the present disclosure. The semiconductor laser diode includes a semiconductor laser structure20including an active laser region mounted on a semiconductor substrate structure10, and a layer of graphene30between the semiconductor substrate structure10and the semiconductor laser structure20.

The laser diode includes an optical cavity, or laser cavity, which is a region through which the laser light is reflected back and forth or circulates around. For example, the light may be reflected back and forth by reflectors at either end of the optical cavity in the case of a straight laser structure, or may travel continuously around the cavity in the case of a ring structure laser etc.FIG. 1is a cross section of a straight laser structure in which the optical cavity extends out of the page. The laser cavity includes the active laser region and light is amplified as it passes through the active laser region. The graphene layer may form part of the optical cavity of the laser, i.e. an optical mode of the laser cavity may overlap with the graphene layer.

As the optical cavity of the laser diode overlaps both the active laser region and the graphene, by varying a voltage bias applied to the graphene layer, the output of the laser diode may be modulated even when a bias voltage applied to the active laser region is kept constant.

The semiconductor substrate structure10may include a waveguide. An optical mode of the waveguide may overlap with the active laser region and the graphene layer. Thus laser light may be coupled from the laser cavity into a waveguide of the semiconductor substrate10. In one example, the waveguide15is formed by a core region of the semiconductor substrate. The core region15may have a higher refractive index than regions on either side of the core. The regions on either side of the core may for example be an air interface, another type of semiconductor or an oxide. In the example shown inFIG. 1, the regions14on either side of the core are an oxide. Thus the semiconductor substrate structure10in the example ofFIG. 1is a semiconductor on oxide structure (SOI).

The SOI10may include at least a semiconductor layer and an oxide layer. The oxide layer may be a buried oxide layer, meaning the oxide layer is surrounded by the semiconductor. In the illustrated example the SOI includes a buried oxide layer12, between a semiconductor substrate11and a further semiconductor layer13. Oxide regions14are inserted within the semiconductor layer13on either side of a core15which forms a semiconductor waveguide.

In general the semiconductor substrate structure10provides a waveguide through which light from the active laser region can be directed out of the laser diode. The semiconductor substrate structure10may include any suitable semiconductor, for example Silicon, Germanium, Gallium Arsenide etc.

The active laser region is a region which is a source of the optical gain within the laser. The active laser region comprises semiconductor materials. In some examples the semiconductor laser structure20, including the active laser region, is a III-V structure. A III-V structure is a laser structure formed from doped or un-doped semiconductor materials taken from group III to group V of the periodic table. In other examples, some or all of the semiconductors for the laser structure may be taken from elsewhere on the periodic table, for instance group II-VI compound semiconductors.

In some examples, the semiconductor laser structure20including the active laser region is a heterostructure. A heterostructure is a structure including a plurality of semiconductor layers. At least some of the layers may have different materials, or the same material but different doping to the adjacent layers.

In the illustrated example, shown inFIG. 1, the heterostructure20includes an intrinsic semiconductor layer22between a positively doped semiconductor layer21and a negatively doped semiconductor layer23. In one example, the layer21is a P-InP or positively doped Indium Phosphate layer and the layer23is a negatively N-InP or negatively doped Indium Phosphate layer. Examples of other possible materials for the semiconductor layers include, but are not limited to P-AlGaAs, N-(Al)GaAs, P-SiGe and N-SiGe. In other examples, the polarity of the doping of the first doped semiconductor layer21and second doped semiconductor layer23may be reversed.

In general, the junctions between the different layers of the heterostructure are referred to as heterojunctions. The active laser region may be formed by a heterojunction or a plurality of heterojunctions. In the example ofFIG. 1, the intrinsic semiconductor layer22acts as a quantum layer, for instance a quantum well, multiple quantum well, quantum dash or quantum dot layer. The first di-electric layer31is between the upper face30A of the graphene layer and the second doped semiconductor layer23. The first di-electric layer in the arrangement ofFIG. 1, thus insulates the second doped semiconductor layer23from the graphene layer30. The second di-electric layer32is between the bottom face30B of the graphene layer and an upper semiconductor layer13of the SOI structure. The second di-electric layer32between the graphene layer30and the semiconductor layer13effectively forms a capacitor between the third electrode N2and the fourth electrode P2as schematically shown in the circuit diagram70to the right ofFIG. 1. This enables a bias voltage to be applied across the graphene layer30so as to adjust the optical absorption of the graphene layer.

In the example ofFIG. 1the active laser region has a lateral extent, or width W, which may be approximately equal to a width or lateral extent of the quantum well22or the first doped semiconductor21. The graphene layer meanwhile has a lateral extent, or width G, which is greater than the width of the active laser region. This helps to ensure that any laser light from the active laser region has to pass through the graphene layer30before entering the SOI10.

The graphene layer30, the graphene layer may extend through the optical mode of the laser diode. In one example the graphene layer extends through the center of the optical mode. In one example active laser region is adjacent a waveguide region15of the SOI and the waveguide is separated from the active laser region by the graphene layer30. In one example, the graphene layer30is continuous so there is no gap through which the laser light can enter the waveguide15without passing through the graphene.

In one example, the graphene layer30extends continuously in a lateral direction from at least a first side of the laser structure20(left inFIG. 1) to a second side of the laser structure (right inFIG. 1). The third P2and fourth N2electrodes are both on the first side of the laser structure20. Meanwhile, a third pair of electrodes including fifth electrode N3and sixth electrode P3are on a second side of the laser structure. Although not explicitly shown inFIG. 1, the fifth electrode N3may connected to the same voltage source60and be held at the same potential as the third electrode N2. The sixth electrode P3may connected to the same voltage source60and be held at the same potential as the fourth electrode P2. In this way the potential difference may be applied relatively evenly between the first and second faces30A,30B of the graphene layer along its width G. A seventh electrode N4may be located on a part of the second doped semiconductor layer23which extends outwardly to the right side of the heterostructure20. Although not explicitly shown inFIG. 1the first electrode P1and the seventh electrode N4may be connected to the voltage source50so as to apply the pump voltage to the active laser region.

FIG. 2is another example of a laser diode which is similar toFIG. 1and in which like reference numerals denote like parts. The difference betweenFIG. 1andFIG. 2is that in the example ofFIG. 2, the first electrode pair and second electrode pair share a common electrode N. Thus, instead of having two separate electrodes N1, N2, inFIG. 2a single electrode N acts as an electrode for both the laser pump50and the voltage bias60to be applied to the graphene layer30. There may be two such single electrodes acting as common electrodes of the laser pump and graphene bias, one common electrode N on a first side of the active laser region (left inFIG. 2) and another common electrode N-R on a right side of the active laser region (right inFIG. 2).

Compared toFIG. 1, the arrangement ofFIG. 2is more compact. It may also have lower resistance. On the other hand, the arrangement ofFIG. 1with separate electrodes for the laser pump and graphene bias, allows more independence in the control of laser pump and the modulation of light via the graphene bias.

FIG. 3is a perspective diagram showing the arrangement ofFIG. 2in three dimensions. In the example ofFIG. 3, the laser diode has a straight design. A length of the laser diode L extends in the direction which is into the paper inFIG. 2. In the example ofFIG. 3, the graphene layer30extends throughout the length of the laser diode.

FIG. 4is a perspective diagram of another example arrangement based onFIG. 2in three dimensions. The example ofFIG. 4is the same asFIG. 3, except that the graphene layer30extends just a part of the way through the length of the laser diode and not the whole length. However, it still extends continuously in a lateral direction across the entire width W of the active laser region. It also extends continuously from one side to another of an optical mode through which light may travel from the active laser region to a waveguide15of the SOI structure.

Although not shown, the arrangement ofFIGS. 1 and 2, and alsoFIGS. 5 to 7described below, will have a similar three dimensional arrangement to that shown inFIG. 3 or 4. Further,FIGS. 3 and 4are just two examples and the teaching of the present disclosure may be applied to other shapes of laser diode.

FIG. 5is similar toFIG. 2, and like reference numerals denote like parts. However, the second electrode N and corresponding fourth electrode N-R on the other side of the active region, are in direct contact with second doped semiconductor layer23, but not in direct contact with the graphene layer30. Meanwhile, the third electrode P2and corresponding fifth electrode P3on the right side of the active laser region are in contact with the graphene layer30. The result is to provide an effective capacitive arrangement between the second and third electrode (and fourth and fifth electrode) as shown in the circuit diagram71to the right of the Figure.

FIG. 6is similar toFIG. 2, except that the second electrode N and the corresponding fourth electrode N-R are in contact with the second doped semiconductor layer23, but not with the graphene layer30. Further, the graphene layer30is in electrically conductive contact with an upper semiconductor layer13of the SOI. Thus, there is one insulating di-electric layer31inFIG. 6, rather than two insulating di-electric layers sandwiching the graphene layer30as inFIG. 2. The result is to provide an effective capacitive arrangement between the second electrode N and third electrode P2(and fourth electrode N-R and fifth electrode P3) as shown in the circuit diagram72to the right ofFIG. 6.

FIG. 7is similar toFIG. 6with one insulating di-electric layer, except that the insulating di-electric layer32is between the graphene layer30and the upper semiconductor layer13of the SOI, rather than between the second semiconductor layer23of the heterostructure and the graphene layer. Thus, in this example the second semiconductor layer23is in electrically conductive contact with the graphene layer. The result is to provide an effective capacitive arrangement between the second and third electrode (and fourth and fifth electrode) as shown in the circuit diagram73to the right of the Figure.

FIG. 8is a perspective diagram showing another example in which the laser diode has a ring structure, rather than a straight Fabry-Perot structure. The basic structure and principle of operation is similar and like reference numerals denote like parts to the previous diagrams. As can be seen in the diagram, the first electrode P1is on top of the ring structure, the second electrode N is inside the ring structure and the third electrode P2extends is outside of the ring structure. The electrical connections to the layers may be the same as inFIG. 2. E.g. the first electrode P1contacts the first doped semiconductor layer21which is part of the ring, the second electrode N contacts the second doped semiconductor layer23which is inside the ring and the graphene layer which extends through the ring, and the third electrode P2contacts a semiconductor layer13of the SOI on which the ring shaped heterostructure is mounted. The arrangements ofFIGS. 1 and 5-7may also be implemented in a ring structure.

Laser light may exit the laser diode through a semiconductor layer of the semiconductor substrate to an external waveguide100as shown inFIG. 8.FIG. 9is a cross sectional diagram showing a cross section ofFIG. 8along the line A-A. Like reference numerals indicate like parts as inFIG. 8.

The above described example laser diodes may be manufactured by any appropriate process. For example, by transferring the graphene layer30to the structure with the active laser region20or the semiconductor substrate structure10and then wafer bonding them together, or by depositing or growing graphene and the layers in the semiconductor laser structure20on semiconductor substrate structure10in a sequential manner.

FIGS. 10-12are graphs showing the simulated optical absorption and light current characteristics of example laser diodes having the configuration shown inFIG. 2.

FIG. 10shows the optical absorption by the graphene layer in dB/μm as a function of waveguide height according to a computer simulation. The simulation assumes a 5 nm thick SiO2layer as the di-electric layer31. The results are shown for a plurality of example semiconductor diodes having waveguide widths of 1.00 μm, 1.25 μm, 1.50 μm and 1.75 μm respectively. The dimensions in width and height of the waveguide portion of the laser diode determine the optical mode profile and its overlap with the graphene. The optical absorption can be maximized when the overlap is strongest.

FIG. 11shows the simulated optical absorption by the graphene layer in dB/μm as a function of bias voltage applied to the graphene layer. As forFIG. 10, the results are shown for a plurality of example semiconductor diodes having a plurality of different waveguide widths. As can be seen in theFIG. 11the absorption is very responsive to variation in bias voltage and is minimized when the bias is low, e.g. below 2V. In this example, the absorption increases sharply between 2 and 4V. Thus the output from the laser diode can be effectively modulated by changing the bias voltage applied to the graphene layer between 2V and 4V. While a positive voltage bias is shown inFIG. 11, similar results may be obtained by applying a similar magnitude of negative voltage bias to the graphene layer.

FIG. 12shows the simulated light-current (LI) characteristic for a 150 μm long laser diode having a waveguide width of 1.75 μm. Specifically, it shows the output laser power as a function of injection current when the bias voltage applied to the graphene layer is 2V, 3V and 4V respectively. It can be seen that at each injection current level, the output power could be modulated by changing the bias voltage applied to the graphene layer. This data was obtained by combining the graphene absorption loss from the simulation ofFIG. 11with in a simulation for a laser diode without the graphene layer. The simulation assumed a relatively large intrinsic cavity loss (without graphene absorption) of 20 cm−1. For laser diodes with lower intrinsic cavity loss the graphene absorption effect would be even greater in relative terms as it would be a more dominant factor in the total cavity loss. There is little or no chirp associated with the optical modulation by the graphene layer as the injection current and thus wavelength of the laser may remain the same. Furthermore, as the optical absorption in the graphene layer occurs inside the laser cavity, the power consumption may be much lower than for designs which use an external optical modulator outside of the laser diode.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the blocks of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or blocks are mutually exclusive.