GB2473491A

GB2473491A - THz Radiation Medium

Info

Publication number: GB2473491A
Application number: GB0916081A
Authority: GB
Inventors: Mauro Pereira
Original assignee: Sheffield Hallam University
Current assignee: Sheffield Hallam University
Priority date: 2009-09-15
Filing date: 2009-09-15
Publication date: 2011-03-16
Also published as: GB0916081D0

Abstract

Semiconductor media 100 and methods for generating or absorbing THz frequency electromagnetic radiation comprises a conduction band 110 and a valence band for charge carriers. A one of the conduction band and valence band has a substantially flat state. The other of the conduction band and valence band has a plurality of sub-band states 114 at different energies. The energy separation between the sub-band states corresponds to electromagnetic radiation in the THz frequency range of 0.3 THz to 100 THz for a charge carrier which transitions between the sub-band states. The charge carriers (electrons 122) may be pumped between the valence band 120 and the conduction band 110 using optical or infrared radiation. The semiconductor media may comprise materials having a continuous band heterostructure. AlAs0.08Sb0.92/InGaAsSb/AlAsSb0.92may be used in the barrier and wells 112 used to form the media for generating or absorbing THz radiation.

Description

THz Radiation Medium The present invention relates to a medium with an electromagnetic radiation property, and in particular to a gain medium which can generate radiation or an absorption medium which can absorb electromagnetic radiation in the THz to mid infrared frequency range.

Gain media are often used to generate and emit electromagnetic radiation after pumping a charge carrier, or other active element, into an excited state from which the element decays into a lower energy state with a concomitant emission of electromagnetic radiation. Gain media are commonly used in lasing devices, but it is not necessary for a gain medium to actually lase in order to be useful. In some applications, it is simply enough for the gain medium to be able to emit radiation at a preferred frequency, or range of frequencies, or to have some amplification, without generating stimulated emission.

Absorption media can be used to absorb electromagnetic radiation at certain frequencies and are useful in detection applications.

There are a large number of applications which can take advantage of THz frequency radiation, such as medical applications, security screening, materials science and high speed wireless communications. However, generating intensive, directional THz radiation is difficult, as THz waves are too long for direct optical techniques, but too short for electronic devices. THz radiation covers the frequency range from approximately 300GHz to 1OTHz (equivalent to wavelengths of approximately 1nim to 30 m) and sits between the far infrared and microwave parts of the electromagnetic spectrum.

Various approaches to generating THz radiation exist, but have pros and cons. Direct methods of THz generation include quantum cascade lasers, gas lasers and Shottky-Gunn-diodes. The first two of these generate high power. However, quantum cascade lasers require cryogenics, have low tunability and provide no access to the lower-frequency part of the THz spectrum. Gas lasers have no tunability and are an intricate technology.

Schottky-Gunn-Diodes have high power at low frequencies but very low power in useful ranges and limited tunability. Indirect methods include continuous wave (CW) photomixers and femtosecond lasers. CW photomixers have high spectral resolution and coverage and are tuneable. However, they are an indirect method and provide comparatively low power. Femtosecond lasers have spectral and temporal information and a broad band output, but again are an indirect method and have lower resolution than CW photomixers.

A current candidate to overcome the technological difficulties for the THz market are quantum cascaded lasers (qcls). They are unipolar devices based on inter-sub-band transitions that are not limited by the band gap. In such materials, the emitting transition is between two different states of the same band. They do not suffer from the intrinsic limitations of other THz solid state devices.

However, THz qcls are not yet at the same commercial level as mid-JR devices. The current constraints to the development of THz sources arise from the difficulties in obtaining efficient THz mode confinement of the radiation and a sufficient level of population inversion.

A contributory factor to the population inversion in solid state devices is broadening of the charge carrier states. The main source of broadening is electron-impurity scattering, which is difficult to avoid due to the need to dope the injector regions of conventional qcl designs. The doping provides the electrons that are accelerated by the electric field in a typical cascaded laser scheme.

The difficulty in achieving inter sub-band population inversion in THz qcls prevents continuous wave (CW) operation at room temperature. This is because the energy difference between the levels (ha -10-20 meV) is not much larger than the low temperature sub-band broadening (6E -4 meV) and so electrons can tunnel from the injector to either the upper or the lower energy level states and so the upper energy level states are not populated more than the lower energy level states. This problem is exacerbated at higher electronic temperatures and as the broadening of the energy level increases.

It is therefore desirable to be able to provide a gain medium which can generate THz radiation in a simple and efficient way, and preferably at or near room temperature.

The invention aims to provide a semiconductor material having inter-sub-band transitions in the THz range. Such a material can then be optically pumped with a conventional visible or near JR laser diode and without detrimental cross absorption and without an excessive energy level broadening at room temperature. Using such a material and pumping scheme, steady state population inversion can be achieved and efficient THz lasing can be obtained.

The present invention does so by providing a solid state gain medium structured to produce flat conduction or valence band states for charge carriers and sub-band states which are separated in energy so that a charge carrier transitioning between the sub-band states can emit TI-Iz frequency radiation.

Such a material can also be used as a detector of electromagnetic radiation in the THz frequency range.

According to a first aspect of the invention, there is provided a semiconductor medium which can generate or absorb THz frequency electromagnetic radiation, the medium having a conduction band and a valence band for charge carriers, wherein a one of the conduction band and valence band has a substantially flat state, and the other of the conduction band and valence band has a plurality of sub-band states at different energies, wherein the energy separation between the sub-band states corresponds to electromagnetic radiation in the THz frequency range emittable or absorbable by a charge carrier which transitions between the sub-band states.

The medium can be a gain medium and can be used to provide at least some amplification of electromagnetic radiation.

The medium can be an absorption medium and can be used to provide at least some detection of electromagnetic radiation.

As one of the valence and conduction band has a substantially flat state this reduces absorption losses and helps to maximise the recombination channel for charge carriers transitioning between the inter-sub-band states and hence emitting or absorbing radiation in the THZ frequency range.

A conduction or valence band can be considered substantially flat when the top of the valence bands of the different materials forming the semiconductor have substantially the same energy, i.e. line up, or when the bottom of the conduction bands of the different materials have substantially the same energy, i.e. line up. This can be contrasted with semiconductor materials in which the energies of the top of the valence band or bottom of the conduction band do not line up and which result in straddling, straddled or broken gap structures.

It will be appreciated that in practice perfectly flat conduction or valence bands may not be achievable. For example material fluctuations, due to alloy fluctuations, may make the tops of the valence bands or the bottoms of the conduction bands in the different materials at slightly different energies. However, as long as the fluctuation is so large that, e.g., the confinement potential for the holes leads to bound states with an energy difference of the order of the difference in energy between the conduction band levels, then there will be no significant cross absorption. Hence, the typical fluctuation in a quaternary semiconductor material grown by MBE of even up to about 10% will not influence the results.

The electromagnetic radiation can be in the frequency range of approximately O.3THz to 100THz, 0.3THz to 1OTHz, 0.3THz to 1THz, preferably approximately 1THz to 100THz and more preferably approximately lThz to lOTHz.

The charge carriers can be electrons or holes.

The conduction band can have the substantially flat state and the valence band can have the plurality of sub-band states.

The valence band can have the substantially flat state and the conduction band can have the plurality of sub-band states. -5..

The gain medium can have a further excited state into which a charge carrier can be excited prior to decaying into a one of the sub-band states. The excited state can be a quasi-delocalised state for a charge carrier.

The sub-band states can be at least partially localised charge carrier states. The sub-band states can be defined by the structure and/or composition of the semi-conductor material.

the sub-band states can be charge carrier states of a quantum well. Other types of mechanisms can be used to define or produce the sub-band states, such as multiple quantum wells or super lattice structures.

The gain medium can have a plurality of quantum wells. For example, the gain medium can have at least 10 or 100 or 1000 quantum wells. Preferably between about 10 to 100 quantum wells are provided. Increasing the number of quantum wells will increase the efficiency and output power of the gain medium.

The energy separation between the conduction and valence band can correspond to electromagnetic radiation in generally the optical and/or infrared range. For example the energy separation between the conduction and valence band can be within the range of approximately 100meV to 500meV, for example approximately 300meV and which can be pumped, for example, by a mid infrared quantum cascade laser or a InGaSb/InAlGaSb interband quantum well laser as described by G.R. Nash, et. a! in App!. Phys. Lett., 91, 131119 (2007).

A further aspect of the invention provides a device for generating THz frequency electromagnetic radiation, comprising a gain medium according to the preceding aspect of the invention and a source of pumping radiation configured to direct pumping radiation into the gain medium. The source of pumping radiation can be a source of optical and/or infrared radiation.

A further aspect of the invention provides a laser including a device according to the preceding aspect of the invention.

According to a further aspect of the invention, there is provided a method for generating THz frequency electromagnetic radiation in a semiconductor material, comprising pumping charge carriers from a valence band to a conduction band, wherein a one of the valence band and conduction band has a substantially flat state; and charge carriers transitioning between sub-band states of the other of the valence band and conduction band and emitting electromagnetic radiation in the THz frequency range.

The charge carriers can be pumped from the valence band directly to the conduction band.

The charge carriers can be pumped from the valence band to the conduction band indirectly via a further excited charge carrier state. The further excited charge carrier state can be a quasi-delocalised charge carrier state.

The method can further comprise charge carriers transitioning between sub-band states of the valence band. The transitioning charge carrier can transition before the pumped charge carrier decays. The transitioning charge carrier can be an electron. The electron can combine with a hole state in a lower energy sub-band state. The pumped charge carrier can be an electron which leaves the hole state.

Charge carriers can be pumped using optical andlor infrared electromagnetic radiation.

Embodiments of the invention will now be described in detail, by way of example only, and with reference to the accompanying drawings, in which: Figure 1 shows a schematic energy level diagram of a solid state medium according to a first embodiment of the invention; Figure 2 shows a schematic energy level diagram of a solid state medium according to a second embodiment of the invention; and Figure 3 shows a schematic energy level diagram of a solid state medium according to a third embodiment of the invention; Like items in the different figures share common reference numerals unless indicated otherwise.

The invention generally employs semiconductor materials having a continuous band heterostructure in which inter-sub-band transitions can generate or absorb electromagnetic radiation in the THz range. The semiconductor material can be used as a gain medium or an absorption medium depending on the application (amplification or detection of radiation) and will be described below as being used as a gain medium. However, the invention is not limited to a gain medium only.

When used as a gain medium, a pumping scheme can be used that eliminates one of the major sources of energy level broadening that does not allow THZ inter-sub-band emitters to operate at room temperature. A stable visible or mid-IR laser can be used. For example, electrons can be pumped from the continuum valence bands to the conduction sub-bands allowing efficient inversion of the population of the sub-band medium at room temperature. Inter sub-band decay of the electrons results in emission of radiation in the THz range. Appropriate design of the semiconductor materials can be used to maximize the THz recombination rate and minimize inter-band recombination loses. Cascaded designs are also possible.

Isolated well designs are possible as well as cascaded-laser structure designs based on an optical pumping carrier-injection scheme.

With reference to Figure 1 there is shown a schematic energy level diagram 100 for a first embodiment of the invention. The energy level diagram illustrates the energy levels for the various states of the charge carriers of a semiconductor material having a composition and structure designed to produce the desired charge carrier states and energy levels. The gain medium has a conduction band 110 which includes a quantum well 112 which has a plurality, in this example three, of non-degenerate sub-band states. The highest energy state is state 114. The quantum well gives rise to a plurality of at least partially localised sub-band states instead of a single conduction band state.

The gain medium also has a valence band which has a substantially flat state as the tops of [ the valence bands of the materials comprising the semiconductor have generally the same energy level. The composition and structure of the semiconductor material is selected to provide an energy separation between the valence and conduction band states which allows pumping from an optical source. Further, the quantum well is designed so that the separation between the sub-band states will correspond to emitted radiation in the THz range.

A pumping source (not shown) is used to pump charge carriers out of the valence band. A suitable pumping source, for practical applications, would have the properties of being compact, reliable and being operable at room temperature and powered by a lightweight power source. A suitable pumping source would be a stable visible or mid-infrared laser, for example, a mid infrared quantum cascade laser or a lnGaSb/InAlGaSb interband quantum well laser as described by G.R. Nash, et. at in Appl. Phys. Lett., 91, 131119 (2007). An JR pumping source can be used, but it will be appreciated that the pumping source to use will depend on the inter-band energy separation. The pumping source pumps 122 electrons from the continuum valence band state 120 to the conduction sub-bands, e.g. 114, so as to efficiently invert the inter-sub-band medium even at room temperature. As each excited electron decays 124 into a lower energy inter-sub-band state, e.g. 115, electromagnetic radiation 126 is emitted in the THz range, owing to the energy spacing of the inter-sub-band states.

As described above, in previous approached to generating THz radiation, strong dephasing mechanisms lead to significant broadening of the inter-sub-band states which is of similar order to their inter sub-band transition energy separation, creating a significant obstacle to population inversion and lasing or even any degree of amplification. However, the invention has a generally flat valence band, i.e. a generally continuous band state, thereby reducing absorption losses and maximizing the THz re-combination channel. This approach can eliminate the need for doping and hence reduce the main source of dephasing that leads to inter-sub-band state broadening in other inter-sub-band devices.

Figure 2 shows a schematic energy level diagram 200 for a second embodiment of the invention. The energy level diagram illustrates the energy levels for the various states of the charge carriers of a semiconductor material having a composition and structure designed to produce the desired charge carrier states and energy levels. The gain medium has a conduction band 210 which includes a quantum well 212 which has a plurality of non-degenerate sub-band states, in this example three. The highest energy state is state 214. The quantum well gives rise to a plurality of at least partially localised sub-band states instead of a single conduction band state. A flrther excited charge carrier state 216 is also available. For example, excited state 216 can be a quasi-delocalised electron state.

The gain medium also has a valence band 220 which has a substantially flat state. The composition and structure of the semiconductor material is selected to provide an energy separation between the valence 220 and conduction band states which allows pumping from an optical source. Further, the quantum well 214 is designed so that the separation between the sub-band states will correspond to emitted radiation in the THz range.

Similarly to as described above, a pumping source (not shown) is used to pump charge carriers out of the valence band. The pumping source pumps 222 electrons from the continuum valence band state 220 to the excited state, 216. From the excited state 216, the electron can decay 224 to a state 214 of the quantum well by emitting a phonon. As in any laser, non-radiative coupling, such as phonon emission, should be avoided, and that is the main limitation which the semiconductor structure can be designed to minimize. Interband recombination is reduced by design by creating the flat valence band. The holes are delocalised and the transition dipole moment between states 214 and 215 is much larger than between 214 and 220. As each excited electron decays 226 into a lower energy inter-sub-band state, e.g. 215, electromagnetic radiation 228 is emitted in the THz range, owing to the energy spacing of the inter-sub-band states.

Suitable semiconductor materials for the barrier and well of the embodiments illustrated in Figures 1 and 2 are, for example, A1As0o8Sbo92/InGaAsSb/AlAso.osSbo92 where the InGaAsSb alloy is determined by (InAs1Sb)i(GaSb), with z = 0.35 and y = 0.09.

Examples of semiconductor materials having a continuous valence band, i.e. showing minimum confinement, are described in Physical Review B54, Rl 1078 (1996), the disclosure of which is incorporated herein by reference for all purposes. Examples of semiconductor materials having a continuous conduction band are described in Applied Physics Letters 65, 572 (1999), the disclosure of which is incorporated herein by reference for all purposes. However, in the systems described in these documents are inter-band devices rather than the inter-sub-band devices of the present invention.

Figure 3 shows a schematic energy level diagram 300 for a third embodiment of the invention. The energy level diagram illustrates the energy levels for the various states of the charge carriers of a semiconductor material having a composition and structure designed to produce the desired charge carrier states and energy levels. The gain medium has a valence band 310 which includes a quantum well 312 which has a plurality of non-degenerate sub-band states, in this example three. The highest energy state is state 316.

The quantum well gives rise to a plurality of at least partially localised sub-band states instead of a single valence band state.

The gain medium also has a conduction band 320 which has a substantially flat state. The composition and structure of the semiconductor material is selected to provide an energy separation between the valence and conduction band states which allows pumping from an optical source. Further, the quantum well 312 is designed so that the separation between the sub-band states will correspond to emitted radiation in the THz range.

In the absence of external excitation, the flat conduction band 320 has no electrons and the valence bands, 314, 315, 316 are filed with electrons. A pumping source (not shown) is used to pump charge carriers out of the valence band 310 and into the conduction band 320 as generally described above. For example, the pumping source pumps 322 an electron from a one of the valence sub-band-states, e.g. 315, to the continuum conduction band state 320 and leaving a hole in level 315. An electron from level 316 decays 324 into lower energy inter-sub-band state 315 filing the hole state and electromagnetic radiation 326 is emitted in the THz frequency range, owing to the energy spacing of the adjacent inter-sub-band states. On a longer time scale, the originally excited electron can then decay 323 from the conduction band 320 to fill the hole left in state 316 and the system returns to equilibrium. Another input of pumping energy starts the whole process again and another THz photon is generated. The inter-sub-band medium can be efficiently inverted even at room temperature, because the broadening of the levels in the barrier due to temperature increase will not affect the absorption of the inter-band photon.

Hence, the use of flat valence or conduction bands to help generate a population inversion in inter-sub-band states separated by energies corresponding to THz frequencies allows a source of THz radiation to be provided at room temperature using a simple pumping source. Such a gain medium can be of benefit in simply acting as a source of THz frequency radiation or can be used in a laser device as the lasing medium if desired. For example, an optically pumped quantum cascade laser could be provided using the structures described above. It will be appreciated that either the efficiency of the gain medium can be increased by using larger numbers of quantum wells. For example a gain medium might have of order 100 quantum wells and may provide an output power of around a few tens of mW to about 100mW at steady state.

It will be appreciated that the invention is not limited to the specific structure and materials described herein and that various modifications and changes can be made to the specific embodiments described herein. The present invention can be applied to a broad range of suitable semi-conductor materials and a broad range of configurations of conduction and valence band states and localised states therein. For example the localised states giving rise to the inter-sub-band states can be realised using structures other than a quantum well, such as superlattices.

Claims

CLAIMS: 1. A semiconductor medium which can generate or absorb THz frequency electromagnetic radiation, the medium having a conduction band and a valence band for charge carriers, wherein a one of the conduction band and valence band has a substantially flat state, and the other of the conduction band and valence band has a plurality of sub-band states at different energies, wherein the energy separation between the sub-band states corresponds to electromagnetic radiation in the THz frequency range emittable or absorbable by a charge carrier which transitions between the sub-band states.
2. A medium as claimed in claim 1, wherein the conduction band has a substantially flat state and the valence band has the plurality of sub-band states.
3. A medium as claimed in claim 1, wherein the valence band has a substantially flat state and the conduction band has the plurality of sub-band states.
4. A medium as claimed in claim 1, and having a further excited state into which a charge carrier can be excited prior to decaying into a one of the sub-band states.
5. A medium as claimed in any preceding claim, wherein the sub-band states are states of a quantum well.6. A medium as claimed in claim 5, wherein the medium has a plurality of quantum wells.5. A medium as claimed in any preceding claim, wherein the energy separation between the conduction and valence band corresponds to electromagnetic radiation in generally the optical or infrared range.
6. A device for generating THz frequency electromagnetic radiation, comprising: a medium as claimed in any preceding claim; and a source of pumping radiation configured to direct pumping radiation into the medium.
7. A laser including a device as claimed in claim 6.
8. A method for generating THz frequency electromagnetic radiation in a semiconductor material, comprising: pumping a charge carrier from a valence band to a conduction band, wherein a one of the valence band and conduction band has a substantially flat state; and a charge carrier transitioning between sub-band states of the other of the valence band and conduction band and emitting electromagnetic radiation in the THz frequency range.
9. A method as claimed in claim 8, wherein the charge carrier is pumped from the valence band directly to the conduction band.
10. A method as claimed in claim 8, wherein the charge carrier is pumped from the valence band to the conduction band indirectly via a further excited charge carrier state.
11. A method as claimed in claim 8, further comprising: the charge carrier transitioning between sub-band states of the valence band.
12. A method as claimed in any of claims 8 to 11, wherein charge carriers are pumped using optical electromagnetic radiation.
13. A medium which can generate THz frequency electromagnetic radiation substantially as hereinbefore described.
14. A method for generating THz frequency electromagnetic radiation in a semiconductor material substantially as hereinbefore described.