Patent Application
20120091916
The invention relates to cascade semiconductor radiation (light) sources. Hereinafter, the term “light” refers to any sort of electromagnetic radiation of any wavelength, whether visible or not.
Multi-wavelength Quantum Cascade Lasers (QCLs) have recently attracted quite some attention [1-7] in the QCL community. For instance, in U.S. Pat. No. 6,278,134 Capasso (Ref. 1) describes a bi-directional semiconductor light source that provides emission in response to either a positive or negative bias voltage. With an asymmetric injector region in the cascade structure, the device will emit at a first wavelength under a negative bias and a second wavelength under a positive bias. This asymmetric injector must be designed to work as an injector in both bias directions, necessitating design compromises that complicate performance optimization.
An objective of the present invention is to provide a cascade semiconductor light source that assumes a high performance for at least two different wavelengths that are selected by the bias polarity.
A further objective of the present invention is to provide a cascade semiconductor light source structure which allows optimizing the emission characteristics for at least two different wavelengths without the dual-use injector region of Ref. 1.
An embodiment of the present invention relates to a cascade semiconductor light source comprising:
at least one cascade of a first type and a first contact region coupled to said first cascade, the first contact region being capable of injecting carriers into the first cascade; and at least one cascade of a second type and a second contact region coupled to said second cascade, the second contact region being capable of injecting carriers into the second cascade;
wherein the application of a first polarity voltage to said light source results in any cascade of the first type to be in an active mode and any cascade of the second type to be in an inactive mode;
wherein the application of a second, opposite polarity voltage to said light source results in any cascade of the first type to be in an inactive mode and any cascade of the second type to be in an active mode;
wherein any cascade of the first type is adapted to emit light at a first wave-length in its active mode and to passively conduct electrical current in its inactive mode; and
wherein any cascade of the second type is adapted to emit light at a second wavelength in its active mode and to passively conduct electrical current in its inactive mode.
A cascade may be or comprise a region containing multiple layers of different semiconductors (e.g. multi heterostructure) designed so that when biased a certain way, a higher-energy state (upper laser state) becomes populated with charge carriers, an intersubband transition can take place in which the charge carriers make a transition from the upper laser state to a state with lower energy (lower laser state), resulting in the emission of light, and the lower laser state is depopulated of the charge carriers, that are then transferred into the next (“neighbor”) cascade.
A contact region may be or comprise a region supplying or removing electrons to or from their adjacent cascades.
According to a preferred embodiment a transfer region may be disposed between a first block comprised of cascades of the first type connected to each other in series and a second block comprised of cascades of the second type. When the device is positively biased, the first type of cascade is active and when the device is negatively biased, the second type of cascade is active. A transfer region may be a region that removes the electrons from the last cascade of the block of active cascades and supplies it to the first cascade of the adjacent inactive block in a way to reduce the resistance of the inactive block. The transfer region is adapted to conduct electrical current in any polarity.
There may be more than 1 block of each type of cascade and each block may be comprised of any number of cascades including one.
In order to provide efficient radiation the cascades of the first and/or second types are preferably adapted to emit light via intersubband transitions of electrons during their active mode.
An advantage of the invention compared to Ref. 1 is that the first type of cascade and the second type of cascade may be independently optimized as both cascade regions including their respective injectors are distinct from the other type of cascade with its injector. As such, for example, the emitting wavelengths of the first and second cascades may be individually engineered.
Further, due to the independent optimization of the cascades even room-temperature operation may be achieved for both wavelengths.
According to a preferred embodiment, the transfer region may be adapted to boost the conductivity of the cascades of the first and/or second type during their inactive mode. As such, the voltage drop over the region that is in inactive mode, and thus the generation of heat may be reduced.
The transfer region is preferably adapted to boost the conductivity of the cascades of the first and/or second types by transferring carriers into the quasicontinuum of states largely derived from the materials' F-point of the conduction band of the first and/or second cascade types.
Alternatively or additionally, the transfer region may be adapted to boost the conductivity of the cascades of the first and/or second types by transferring carriers into indirect X- and/or L-valleys of the conduction band of the first and/or second cascade types.
The first and second types of cascades may provide that the first and second wavelengths differ from one another.
Alternatively, the first and second types of cascades may be configured to emit light at the same wavelength.
Cascades of the first and second types may each include alternating barrier and quantum well layers in order to increase the efficiency.
The barrier layers of cascades of the first type preferably differ from the barrier layers of cascades of the second type in order to allow individual optimization. Further, the quantum well layers of cascades of the first type may differ from the quantum well layers of cascades of the second type.
Preferably, the barrier and quantum well layers are undoped.
A further embodiment of the present invention relates to a spectroscopy system comprising:
a detector for detecting radiation and for providing a detection signal; an evaluation unit connected to the detector and configured to evaluate the detection signal; and
a cascade semiconductor light source having:
at least one cascade of a first type and a first contact region coupled to said first cascade, the first contact region being capable of injecting carriers into the first cascade; and
at least one cascade of a second type and a second contact region coupled to said second cascade, the second contact region being capable of injecting carriers into the second cascade;
wherein the application of a first polarity voltage to said light source results in any cascade of the first type to be in an active mode and any cascade of the second type to be in an inactive mode;
wherein the application of a second, opposite polarity voltage to said light source results in any cascade of the first type to be in an inactive mode and any cascade of the second type to be in an active mode;
wherein any cascade of the first type is adapted to emit light at a first wavelength in its active mode and to passively conduct electrical current in its inactive mode; and
wherein any cascade of the second type is adapted to emit light at a second wavelength in its active mode and to passively conduct electrical current in its inactive mode.
As discussed above, the wavelength of the light (radiation) emitted by the cascade semiconductor light source may be changed by inverting the voltage polarity. As such, a spectroscopy system comprising such a light source allows detecting several gases and/or several isotopes in a quasi-simultaneous fashion by inverting the voltage polarity, only.
In order that the manner in which the above-recited and other advantages of the invention are obtained will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended figures and tables. Understanding that these figures and tables depict only typical embodiments of the invention and are therefore not to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail by the use of the accompanying drawings in which
The preferred embodiments of the present invention will be best understood by reference to the drawings, wherein identical or comparable parts are designated by the same reference signs throughout.
It will be readily understood that the present invention, as generally described herein, could vary in a wide range. Thus, the following more detailed description of the exemplary embodiments of the present invention, is not intended to limit the scope of the invention, as claimed, but is merely representative of presently preferred embodiments of the invention.
The light source 5 comprises a first cascade 11 and a first contact region 12 which is coupled to the first cascade 11. The first contact region 12 injects carriers into the first cascade 11 if a positive bias voltage is applied to the light source 5.
The light source 5 further comprises a second cascade 21 and a second contact region 22 coupled to the second cascade 21. The second contact region 22 injects carriers into the second cascade 21 if a negative bias voltage is applied to the light source 5.
The first cascade 11 and the second cascade 21 are separated by a transfer region 30.
When a positive voltage is applied to light source 5, the first cascade 11 is in an active mode and the second cascade 21 is an inactive mode. Then, only the first cascade 11 will emit radiation at a first wavelength λ1. The transfer region 30 extracts charge carriers from the first cascade 11 and boosts the conductivity of the second cascade 21, for instance by transferring carriers into the quasi-continuum of a F-point of the conduction band of the second cascade 21 or by transferring carriers into indirect X- and/or L-valleys of the conduction band of the second cascade 21.
When a negative voltage is applied to light source 5, as shown in
In the exemplary embodiment of
λA=c/υA
via intersubband transitions of electrons.
A second emitter zone B passively conducts electrical current at this bias.
A transfer region referred to as “transfer zone” in
For a negative electrical bias (right contact is negative,
λB=c/υB,
and the first emitter zone A passively conducts electrical current. The functionality of the transfer region (transfer zone) (i.e. to boost the conductivity over the passive zone) is the same for both polarities.
The evaluation unit 210 is connected to the light source 5 and to the detector 205. The evaluation unit 210 controls the emission of light λ by light source 5 and evaluates the detection signal S. The evaluation unit 210 may invert the polarity of voltage U applied to light source 5 in order to detect gases 300 and/or isotopes 300 in a simultaneous or quasi-simultaneous fashion.
1. F. Capasso et al, “Bi-directional unipolar semiconductor light source”, Patent U.S. Pat. No. 6,278,134 B1 (2001).
2. V. Berger, “Unipolar Multiple-Wavelength Laser”, patent U.S. Pat. No. 6,091,751 (2000).
3. F. Capasso et. al, “Article comprising a dual-wavelength quantum cascade photon source”, patent U.S. Pat. No. 6,144,681 (2000).
4. F. Capasso et. al, “Multiple wavelength quantum cascade light source”, patent U.S. Pat. No. 6,148,012 (2000).
5. F. Capasso et. al, “Engineering the gain/loss profile of intersubband optical devices having heterogeneous cascades”, patent U.S. Pat. No. 6,728,282 (2004).
6. F. Capasso et. al, “Broadband cascade light emitters”, patent U.S. Pat. No. 7,0100,10 (2006)
7. C. Gmachl, A. Tredicucci, D. L. Sivco, A. L. Hutchinson, F. Capasso, and A. Y. Cho, “Bidirectional Semiconductor Laser”, Science 286, 749 (1999).
