Systems and methods using phonon mediated intersubband laser

Details Systems and methods using phonon mediated intersubband laser Systems and methods using phonon mediated intersubband laser

2002-05-21

2004-12-07

Wong, Don (Department: 2828)

Coherent light generators

Particular active media

Semiconductor

C372S033000

Reexamination Certificate

active

06829269

ABSTRACT:

BACKGROUND OF THE INVENTION
The physics of low dimensional electronic systems, for example, up to two dimensions, has been an active area of research for the past 25 years. Broad interest in these systems has risen because of a wealth of fundamental scientific discoveries, which have been turned into technological applications. Several extremely successful optoelectronic and photonic devices such as the quantum well diode laser, quantum well detector and quantum well light emitting diodes (LED) are important examples of the research. The feasibility of fabricating such quantum heterostructures has been in part due to the advancement of Molecular Beam Epitaxy (MBE) technology, which enables control of semiconductor layer thickness formation to one atomic monolayer accuracy (~2.56 Å in GaAs).
Quantum cascade lasers (QCL) since their first experimental demonstration in 1994 have exemplified remarkable progress and have since then, instilled immense amount of research activity in that area. The QCLs have been successfully operated in the 5-24 &mgr;m range of the electromagnetic spectrum. However, there still remains the problem of extending the operating frequency of the cascade lasers into the terahertz (THz) range which is extremely challenging due to the highly efficient non-radiative electron-electron (e-e) scattering and large waveguide losses. As a first step towards the development of such an intersubband THz laser, THz electroluminescence from cascade structures has been observed.
Dual frequency lasing from cascade devices have been reported earlier using various schemes such as, tuning the oscillator strength of the radiative transition, using superlattice structures capable of emitting photons at two frequencies, and more recently using heterogeneous cascade structures. These have been in the mid infrared range of the spectrum.
Intersubband transition energies can vary anywhere between a few milli-electron volt (meV) to a few hundred meV depending on the choice of the material system, making them the most suitable candidates for developing devices all the way from near to far infrared (THz). The first experimental evidence of intersubband absorption in quantum wells was demonstrated by West and Eglash in a GaAs/AlGaAs material system. Following this, interest in intersubband transitions grew extensively both for developing detectors as well as emitters, for example, there are detectors referred to as Quantum Well Infrared Photodetector (QWIPs). The interest in emitters grew slowly but steadily until the first demonstration of the quantum cascade laser in 1994. This was the first evidence of electromagnetic emission based on intersubband transitions. Lasers for either extending the wavelength or for better operational characteristics have received a large degree of interest.
There still remains a need to provide emissions in the terahertz frequency range and for producing infrared radiation using compact coherent sources.
SUMMARY OF THE INVENTION
The present invention is directed to the development of compact, coherent sources emitting in the terahertz frequency region using interface phonons. In accordance with a preferred embodiment, a semiconductor heterostructure light emitting device includes a quantum cascade structure having at least an upper lasing level and a lower lasing level. The system uses heterostructure interface phonon bands to depopulate the lower lasing level of at least a three level semiconductor device. The device includes multiple coupled quantum well modules. In alternate preferred embodiments, the device includes quantum dot layers and/or, quantum wire structures, and/or mini-bands in a superlattice, for example, GaAs/AlGaAs superlattice. The phonons in the device improve efficiency, decrease the threshold current and result in system temperatures that are as high as room temperature. The semiconductor device provides emission of terahertz radiation.
In a preferred embodiment the semiconductor device provides at least one emission in the terahertz radiation region and in a far infrared region of the electromagnetic spectrum. The emissions include a first emission having a first energy level and a second emission having a second energy level.
In an alternate preferred embodiment, the semiconductor device includes quantum cascade emitters embedded in a structure having photonic crystals. The semiconductor device can include at least one plasma reflector to confine device emissions.
In preferred embodiments, the threshold current of the device ranges between 0.001 amps/cm
2
and 100,000 amps/cm
2
. The device is portable and preferably weighs less than 5 pounds. The emitter weighs approximately less than one ounce and the support structure weighs less than 5 pounds.
The terahertz (THz) frequency range (1-10 THz or 30-300 &mgr;m wavelength or preferably wavelengths between 60-300 &mgr;m) has potential applications in spectroscopy, astronomy, biological imaging, exo-atmospheric radar systems, and free space communication. THz spectroscopic techniques may accurately identify chemical and biological substances agents.
In preferred embodiments, quantum cascade THz emitters are fabricated to provide emissions at 4.2 THz (17.5 meV) and 2.9 THz (12 meV) with Full Width at Half Maximum (FWHM) of 2.01 meV and 0.75 meV from two structures, respectively, at a temperature of T=10 K. Further, the corresponding applied biases agree well with the theoretically predicted values. Also dual wavelength emission is observed from one preferred embodiment.
In a preferred embodiment, the structures can be grown by solid source MBE such as, for example, in a RIBER 32 system. Post-growth fabrication includes photolithography, electron-beam deposition and rapid thermal annealing. Ni/Ge/Au gratings are deposited on the surface of the structure to couple the radiation out of the structure. These gratings also act as the top contact for the device.
The characterizations can be performed using a Bruker IFS 66 V FTIR spectrometer in the step scan mode. A Si bolometer may be used to detect the signal. The structures are placed in a variable temperature cryostat capable of going down to liquid He temperatures. Emission spectra are taken at different temperatures and bias voltages.
Intersubband absorption measurements are performed at room temperature on a multiple quantum well structure using the Brewster angle geometry. A Lorentzian fit to the absorption data indicates a full width at half maximum of 1.59 meV, which gives a lifetime of 0.83 ps. This is close to the phonon mediated lifetimes calculated theoretically for such a structure (0.75 ps).
In another preferred embodiment, resonator geometries are used, like a stadium shaped resonator or an elliptical resonator, to increase the emission.
In a preferred embodiment, absorption due to interface phonons is observed in two different structures at theoretically predicted frequencies. This result necessitates consideration of the effects of confinement on the phonon modes in layered heterostructures.
In a preferred embodiment, at THz frequencies, the dominant nonradiative mechanism is the electron-electron scattering at low temperatures. This rate is found to be proportional to the upper subband population and can become as high as 10 ps
−1
for realistic subband densities. A five-fold increase in the peak gain with lower injection current is observed using a preferred embodiment design. Wavefunction engineering and phonon wavefunction engineering are employed to enhance device performance in preferred embodiments.
In preferred embodiments, terahertz intersubband emission from GaAs/AlGaAs quantum cascade structures employing interface phonons for depopulation is observed. Emission is observed at 12.0 meV (2.9 THz) and 17.5 meV (4.2 THz) with full width at half maximum of 0.7 meV and 1.6 meV, respectively, at a temperature T=10 K from two different structures. The structures consisted of 40 periods of the quantum cascade module and rely on spatially diagonal (interwell) transition for the terahertz emission.

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