|
Alexey
Belyanin
Announcements and news
Graduate
course for Spring 2009: Optical semiconductor devices |
Quantum-Cascade Laser
Research
We are trying to develop a new
approach in physics and design of nonlinear optical devices based on fully
resonant interaction of light with electrons confined in quantum wells and
monolithic integration of nonlinear elements and pump sources. Our aim is to
combine the tunability and flexibility of nonlinear optical sources with
advantages that only semiconductor injection lasers can offer: compactness,
high efficiency, injection pumping scheme, low cost, and possibility of monolithic
integration with electronic chips. This work is a collaboration with groups led
by Federico Capasso (Harvard) and Claire Gmachl (
A standard setup for nonlinear optical generation includes a passive nonlinear crystal, in which the optical pump beams from one or several high-power external lasers are mixed to generate a nonlinear signal at sum-frequency, difference-frequency, or second and higher harmonics of the fundamental pump frequencies. To avoid absorption, a nonlinear crystal should be transparent for all beams participating in the nonlinear mixing process. This means that all frequencies should be far from any resonances in the nonlinear medium. Non-resonant nonlinear optical susceptibilities are usually quite small. For example, the second-order nonlinear susceptibilities c(2) in standard nonlinear crystals are of the order of a few pm/V. Then, to produce an appreciable power, one needs to use long nonlinear crystals and high-power external pumping lasers. The resulting nonlinear optical setup is bulky and expensive (typically $200K).
The lasers that
we design and fabricate are injection-pumped quantum-cascade lasers, in which
the laser field serves as an optical pump for the resonant, nonlinear optical interaction in the very same
active region. In other words, the optical pump source and the nonlinear
optical medium are integrated in a single, injection-pumped device. Such a
design takes full advantage of giant resonant optical nonlinearities of coupled
quantum wells (QWs) that have been observed since late 1980s. Indeed, since the
optical pump is generated inside the nonlinear medium, resonant absorption is
not a problem anymore. Therefore, we can have all fields participating in the
nonlinear interaction to be resonant with corresponding intersubband
transitions. As a result, the second-order nonlinear susceptibility c(2)
is enhanced by orders of magnitude, up to hundreds nm/V.
Nonlinear processes involved include sum-frequency and second-harmonic generation, difference frequency generation, Raman lasing, parametric down-conversion, and even lasing without inversion. An example of laser design for second-harmonic generation is illustrated in Figs. 1,2.
The spectra of both fundamental laser radiation and the nonlinear signal are shown in Fig. 3. The key figure of merit is an efficiency of transformation of the fundamental pump power into the nonlinear signal power, which is measured as the ratio of the nonlinear power to the fundamental laser power squared. The highest efficiency we achieved so far is 35 mW/W2 with the highest nonlinear power of 2 mW. An improvement in both power and efficiency by at least a factor of 10 is feasible for second harmonic generation. With other nonlinear processes even higher power is expected. For example, we have already demonstrated resonant Raman generation with about 16 mW of the nonlinear power; see below. This is enough for the majority of mid/far-infrared applications in spectroscopy, remote sensing, wireless communications etc.
Raman Injection Laser
[8]
In a general
scheme of the stimulated Raman scattering sketched in Fig. 4a, the incident
photon of energy 
is converted
into a Stokes photon of energy 
. The underlying physical mechanism is the scattering of the incident
fundamental radiation by a certain eigenmode of oscillations in a material,
which is self-consistently excited by the parametric interaction between the
incident and the scattered light. The wavelength shift between the fundamental
and the Raman emission is determined by a resonance frequency of the internal
oscillations and can be due to vibrational (phonon), rotational, plasmon or
electronic resonances.
(c) (b)
Figure 4. (a) A schematic of Raman scattering of laser light (blue
arrow) into a lower-frequency Stokes wave (red arrow). (b) Calculated Raman
gain spectrum when the detuning D is much larger
than linewidth. (c) Stokes gain spectrum calculated for the structure shown in
Fig. 5, with the detuning D equal to 15 meV.
The arrow indicates the position of the two-photon resonance 
.
Stimulated
Raman scattering has been observed in all kinds of media: gases, liquids,
solids, and plasmas. Until now, a distinctive feature of all Raman amplifiers
has been the necessity of an external optical pump. Also, existing solid-state
Raman sources universally employ scattering off a vibrational (phonon)
resonance in a crystal or fiber. A powerful fundamental laser radiation is
usually required to offset a small value of the Raman gain (several 10-9cm/W).
(b) (a)
Figure 5 (a) Active
region of a resonant Raman laser that integrates fundamental laser cascade
In
recent work [8] we demonstrated the first injection-pumped
Raman laser, where the fundamental and the Raman radiations are both generated
by intersubband electronic transitions in the very same active region of a
quantum cascade laser (QCL). The stimulated scattering and lasing are due to
the excitation of coherent electronic polarization on the mid-infrared
intersubband transition 2-1. Its frequency 
defines
the Raman shift; it has nothing to do with phonons and can vary in a very broad
range. It could be also efficiently tuned by a voltage bias since the
transition 2-1 is diagonal in real space.
One
period of the Raman laser structure reported in Ref. [8] is shown in Fig. 5(a). Laser light at 6.7 mm generated on the transition 6-5 serves as a
resonant optical pump for lasing at the Stokes wavelength of 9 mm, which is detuned by 12 meV from the
transition 3-2. Resonant absorption of the pump at the transition 1-3 is
overcome by amplification in the pump laser section at the transition 6-5. The
triply resonant nature of the process makes Raman scattering very efficient:
peak fundamental power is only 40 mW at the threshold for Raman lasing (Fig.
5b), which implies very large gain coefficient of the order of 2x10-5
cm/W per period (6x 10-4 cm/W for the whole stack of 30 stages) and
high efficiency of the nonlinear interaction. The nonlinear
conversion efficiency around 30% has been measured.
Recent publications:
[1] N. Owschimikow, C. Gmachl, A. Belyanin, et al. Phys. Rev. Lett. 90, 043902 (2003).
[2] C.
Gmachl, A. Belyanin, D.L. Sivco et al., IEEE J. Quantum Electron. 39(11), 1345 (2003).
[3]
O. Malis, A. Belyanin, C. Gmachl, et al., Appl. Phys. Lett.
84, 2721 (2004).
[4]
A.
Belyanin, C. Bentley, F. Capasso et al., Phys. Rev. A,
64, 013814 (2001).
[5] C. Gmachl,
N. Owschimikow, A. Belyanin, et al., Appl. Phys.
Lett. 84,
2751 (2004).
[6]
O. Malis, A. Belyanin, D.L. Sivco, J. Chen, A.M. Sergent, C.
Gmachl, and A.Y. Cho, Electron. Lett., 40, 1586
(2004).
[7] T. Mosely, A.
Belyanin, C. Gmachl, D. L. Sivco, M. L. Peabody, and A. Y. Cho, Optics Express, 12, 2972 (2004).
[8] M.
Troccoli, A. Belyanin, F. Capasso et al., Nature,
433, 845 (2005).