We report high-speed optical modulation in a resonant tunneling relaxation oscillator consisting of a resonant tunneling diode (RTD) integrated with a unipolar optical waveguide and incorporated in a package with a coplanar waveguide transmission line. When appropriately biased, the RTD can provide wide-bandwidth electrical gain. For wavelengths near the material band edge, small changes of the applied voltage give rise to large, high-speed electroabsorption modulation of the light. We have observed optical modulation at frequencies up to 14 GHz, associated with subharmonic injection locking of the RTD oscillation at the fundamental mode of the coplanar transmission line, as well as generation of 33 ps optical pulses due to relaxation oscillation

Boyd, A. R.

Figueiredo, J. M. L.

Ironside, C. N.

Leite, A. M. P.

McMeekin, S. G.

Stanley, C. R.

Sapientia

APPLIED PHYSICS LETTERS VOLUME 74, NUMBER 9 1 MARCH 1999Optical modulation in a resonant tunneling relaxation oscillatorJ. M. L. Figueiredo,a) C. R. Stanley, A. R. Boyd, and C. N. IronsideDepartment of Electronics and Electrical Engineering, University of Glasgow, Glasgow G12 8LT, UnitedKingdomS. G. McMeekinCardiff School of Engineering, University of Wales Cardiff, Cardiff NP2 1XH, United KingdomA. M. P. LeiteCentro de Fı´sica do Porto—ADFCUP, Universidade do Porto, Rua do Campo Alegre 687, 4169-007 Porto,Portugal~Received 16 July 1998; accepted for publication 22 December 1998!We report high-speed optical modulation in a resonant tunneling relaxation oscillator consisting ofa resonant tunneling diode ~RTD! integrated with a unipolar optical waveguide and incorporated ina package with a coplanar waveguide transmission line. When appropriately biased, the RTD canprovide wide-bandwidth electrical gain. For wavelengths near the material band edge, small changesof the applied voltage give rise to large, high-speed electroabsorption modulation of the light. Wehave observed optical modulation at frequencies up to 14 GHz, associated with subharmonicinjection locking of the RTD oscillation at the fundamental mode of the coplanar transmission line,as well as generation of 33 ps optical pulses due to relaxation oscillation. © 1999 AmericanInstitute of Physics. @S0003-6951~99!01809-4#Resonant tunneling diodes ~RTDs! have been widelystudied because of their potential application in high fre-quency signal generation,1 high speed signal processing atmicrowave frequencies,2 and in optoelectronics.3–6 Recentstudies on resonant tunneling relaxation oscillators7,8 haveshown generation of 30 ps electrical pulses at a repetitionrate of 1.1 GHz, and harmonic and subharmonic locking. Inan optical waveguide containing a double-barrier RTDimplemented in the AlGaAs material system, our group pre-viously reported electroabsorption modulation at 900 MHzwith a modulation depth of 7 dB.9 The modulation relies ona depletion region that is formed in the waveguide core; de-pending on the bias condition, a substantial part of the ter-minal voltage may be dropped across this region. From thecurrent–voltage (I – V) characteristic of the RTD opticalwaveguide modulator, it can be seen that small changes ofbias voltage close to the peak-to-valley transition region cancause large changes in the electric field distribution acrossthe depleted part of the waveguide core. The electric fieldshifts the absorption band edge to longer wavelengths via theFranz–Keldysh effect10 and therefore changes the transmis-sion characteristics of the waveguide. Compared to the con-ventional pn electroabsorption modulator, the advantage ofthe RTD modulator is that when dc biased close to the NDRregion, the device behaves as an optical waveguide electro-absorption modulator integrated with a wide bandwidth elec-trical amplifier. This letter reports results of high speed lightmodulation in a resonant tunneling relaxation oscillator con-figuration, consisting of a RTD optical waveguide modulatorintegrated with a coplanar waveguide transmission line.The RTD optical waveguide structure was grown by mo-a!Also with the Centro de Fı´sica do Porto—ADFCUP, Universidade doPorto, Rua do Campo Alegre 687, 4169-007 PORTO, Portugal. Electronicmail: jlfiguei@fc.up.pt1190003-6951/99/74(9)/1197/3/$15.00lecular beam epitaxy in a Varian Gen II system, on a semi-insulating GaAs substrate @Fig. 1~a!#. It consists of two 1.4nm thick AlAs barriers separated by a 7 nm wide GaAsquantum well, and two 500 nm thick moderately doped(Si:231016 cm23) GaAs spacer layers each side of thedouble barrier, which are surrounded by heavily doped(Si:231018 cm23) AlGaAs cladding layers, allowing lightconfinement in the direction parallel to the double barrierplane. A n1 GaAs cap layer was provided for formation ofAu–Ge–Ni ohmic contacts. The ridge waveguides ~2–6 mmwide! and the large-area mesas each side of the ridges wereFIG. 1. ~a! Schematic diagram of the wafer structure. ~b! The RTD opticalmodulator ~RTD-OM! configuration. ~c! Diagram of a packaged device andmeasurement setup.7 © 1999 American Institute of Physics1198 Appl. Phys. Lett., Vol. 74, No. 9, 1 March 1999 Figueiredo et al.fabricated by dry etching. Ohmic contacts ~100–400 mmlong! were deposited on top of the ridges and mesas. A SiO2layer was deposited, and access contact windows wereetched on the ridge and the mesa electrodes @Fig. 1~b!#, al-lowing contact to be made to high frequency bonding pads.After cleaving, the devices were die bonded on packagescontaining 50 V coplanar waveguide ~CPW! transmissionlines of different lengths. The device pads were connected tothe package CPW pads via gold wires; a SMA connector wasthen soldered to the package CPW line @Fig. 1~c!#. The dcand rf signals were applied through a 40 GHz bandwidth biastee. The dc I – V characteristics of the packaged devicesshow typical RTD behavior, with the peak current densitybeing approximately 13 kA cm22 ~800 mm2 active area! anda peak-to-valley current ratio around 1.5. Peak voltages werein the range 2.1–2.5 V and the valley voltage varied from 2.6to 3.2 V.The optical characterization employed light from aTi:sapphire laser, tunable in the wavelength region aroundthe absorption edge of the GaAs waveguide ~850–950 nm!.Light was coupled into the waveguide by a microscope ob-jective end-fire arrangement. To measure the change in theoptical absorption spectrum induced by the peak-to-valleytransition, a rf signal was injected to switch the RTD be-tween the extremes of the NDR region, and a photodetectorwas used to measure the transmitted light. The band edgeshift was found to be approximately 12 nm. The high-speedoptical response of the modulator was measured with astreak camera ~Hamamatsu C5680! with a minimum timeresolution of around 2 ps. A part of the injected rf signalpower was required to trigger the streak camera. Figure 1~c!schematically presents the experimental setup.Due to the highly nonlinear I – V characteristic in theNDR region, the RTD is able to generate many high-orderodd harmonics of the injected signal.8 We have studied op-tical modulation in a RTD optical waveguide relaxation os-cillator due to injection locking of the RTD to the fundamen-tal mode of a cavity formed by a transmission line. Opticalmodulation at 14 GHz was observed in a packaged devicewith a 5 mm long transmission line, dc biased in the NDRregion ~2.25 V!, when a rf signal of 0.4 V amplitude andfrequency around 1 GHz was injected. A packaged devicewith a 7 mm long transmission line, again dc biased in theNDR region ~2.4 V! showed optical modulation at 8 GHz foran injected signal of 0.6 V amplitude and frequency around 2GHz. The streak camera traces for the 14 and 8 GHz opticalresponses are presented in Fig. 2. In both cases, switchingoccurs between the extremes of the NDR region, producingsinusoidallike oscillating wave forms. At this stage the cavitycomprised only the modulator CPW and the package CPWterminated with the SMA connector. The left side of themodulator CPW is unterminated ~open circuit!, as indicatedin Fig. 1~c!. The SMA connector limits the apparatus band-width to 18 GHz. By inserting coaxial transmission linesbetween the packaged device and the bias tee, we also ob-served modulation at the coaxial transmission lines funda-mental mode frequency. Figure 3~a! shows a diagram of theequivalent electrical circuit. The modulation efficiency of theRTD optical modulator, characterized by the bandwidth-to-drive-voltage ratio which is defined as the ratio of the opera-tion bandwidth to the operating voltage for the 10 dB modu-lation depth, is higher than 33 GHz/V.When scanning the frequency of the injected signal withamplitude in the range 0.8–1.4 V, it was found that the pack-aged RTD optical modulator connected through a coaxialline 30 cm long, produced optical pulses with a full width athalf maximum ~FWHM! of approximately 33 ps, at frequen-FIG. 2. Streak camera traces of the modulator optical response associatedwith locking of the RTD oscillation to the fundamental mode of a packagewith two different transmission lines. ~a! Trace corresponding to 14 GHzoptical modulation for a packaged device with a CPW transmission line 5mm long. ~b! Trace of the 8 GHz modulator optical response for a packageddevice with a CPW transmission line 7 mm long.FIG. 3. ~a! Schematic diagram of the electrical circuit. ~b! Streak camerameasurement of the 33 ps pulse optical response, showing modulation up to18 dB.1199Appl. Phys. Lett., Vol. 74, No. 9, 1 March 1999 Figueiredo et al.cies also associated with the device operation as a relaxationoscillator.7 Figure 3~b! displays the streak camera trace of anoptical pulse with 33 ps FWHM. According to Ref. 7, thepulses are caused by the discharge of the RTD capacitancewhen the RTD switches from the second positive differentialresistance ~PDR! region to the first PDR region. On a longertime scale other streak camera results indicate a pulse repeti-tion rate around 166 MHz, further confirmed by analysis ofthe optical signal with a 2 GHz bandwidth photodetector.The oscillation wave forms were square wavelike. For a 15cm long coaxial line the pulse repetition rate was around 340MHz. The trace in Fig. 3~b! shows a maximum modulationdepth of 18 dB, which suggests that the RTD now switchesbetween two astable points well into the PDR regions. If wefollow the explanation of Ref. 7 and the inset in Fig. 3~b!,when starting at point A ~the upper dwell zone of the relax-ation oscillator! the collector is fully depleted and the electricfield is high, giving low transmission. A return pulse forcesthe trajectory down below the valley, turning up a few pslater, toward point B ~the lower dwell zone!. During switch-ing, the RTD capacitance first discharges, strongly decreas-ing the electric field across the depleted region, which givesrise to high transmission. The RTD capacitance starts to re-charge causing the electric field to rise to the astable valueassociated with the first dwell zone. Transmission at point Bis higher than at point A, because the collector region is nownot fully depleted. Switching from B to A depletes the entirecollector region, recharging the RTD capacitance and mak-ing the transmission drop again. This drop is not evident inFig. 3~b! because of the short time scale operation of thestreak camera required to resolve the 33 ps pulse.In conclusion, integrating a RTD with an optical wave-guide is a relatively easy way of combining a wide band-width electrical amplifier with an electroabsorption modula-tor, opening up the possibility of a variety of modes ofoperation. A RTD optical waveguide modulator in a relax-ation oscillator configuration has been demonstrated, provid-ing optical modulation up to 14 GHz when a 0.4 V amplituderf signal around 1 GHz is injected. Relaxation oscillator op-eration of the RTD produces optical pulses as short as 33 psand with a modulation depth up to 18 dB, for a range ofamplitudes and frequencies of the injected signal. At thisstage, development of the device concept appears to be apromising route toward a high-speed, low power, optoelec-tronic modulator, with operation extended to 1550 nm pos-sible by using the InAlGaAs quaternary system.J. M. L. Figueiredo acknowledges FCT-PRAXIS XXI-Portugal for his Ph.D. grant.1 E. R. Brown, J. R. Soderstrom, C. D. Parker, L. J. Mahoney, K. M.Molvar, and T. C. McGill, Appl. Phys. Lett. 58, 2291 ~1991!.2 E. Ozbay, D. M. Bloom, D. H. Chow, and J. N. Schulman, IEEE ElectronDevice Lett. 14, 400 ~1993!.3 S. C. Kan, S. Wu, S. Sanders, G. Griffel, and A. Yariv, J. Appl. Phys. 69,3384 ~1991!.4 A. F. Lann, E. Grumann, A. Gabai, J. E. Golub, and P. England, Appl.Phys. Lett. 62, 13 ~1993!.5 S. C. Kan, P. J. Harshman, K. Y. Lau, Y. Wang, and W. I. Wang, IEEEPhotonics Technol. Lett. 8, 641 ~1996!.6 T. S. Moise, Y.-C. Kao, C. L. Goldsmith, C. L. Schow, and J. C. Camp-bell, IEEE Photonics Technol. Lett. 9, 803 ~1997!.7 E. R. Brown, C. D. Parker, S. Verghese, M. W. Geis, and J. F. Harvey,Appl. Phys. Lett. 70, 2787 ~1997!.8 S. Verghese, C. D. Parker, and E. R. Brown, Appl. Phys. Lett. 72, 2550~1998!.9 S. G. Mc Meekin, M. R. S. Taylor, and C. N. Ironside, IEE Proc.: Opto-electron. 143, 12 ~1996!.10 S. G. McMeekin, M. R. S. Taylor, B. Vo¨gele, C. R. Stanley, and C. N.Ironside, Appl. Phys. Lett. 65, 1076 ~1994!.

Optical modulation in a resonant tunneling relaxation oscillator

Universidade do Algarve

APPLIED PHYSICS LETTERS VOLUME 74, NUMBER 9 1 MARCH 1999Optical modulation in a resonant tunneling relaxation oscillator
J. M. L. Figueiredo,a) C. R. Stanley, A. R. Boyd, and C. N. Ironside
Department of Electronics and Electrical Engineering, University of Glasgow, Glasgow G12 8LT, United
Kingdom
S. G. McMeekin
Cardiff School of Engineering, University of Wales Cardiff, Cardiff NP2 1XH, United Kingdom
A. M. P. Leite
Centro de Fı´sica do Porto—ADFCUP, Universidade do Porto, Rua do Campo Alegre 687, 4169-007 Porto,
Portugal
~Received 16 July 1998; accepted for publication 22 December 1998!
We report high-speed optical modulation in a resonant tunneling relaxation oscillator consisting of
a resonant tunneling diode ~RTD! integrated with a unipolar optical waveguide and incorporated in
a package with a coplanar waveguide transmission line. When appropriately biased, the RTD can
provide wide-bandwidth electrical gain. For wavelengths near the material band edge, small changes
of the applied voltage give rise to large, high-speed electroabsorption modulation of the light. We
have observed optical modulation at frequencies up to 14 GHz, associated with subharmonic
injection locking of the RTD oscillation at the fundamental mode of the coplanar transmission line,
as well as generation of 33 ps optical pulses due to relaxation oscillation. © 1999 American
Institute of Physics. @S0003-6951~99!01809-4#Resonant tunneling diodes ~RTDs! have been widely
studied because of their potential application in high fre-
quency signal generation,1 high speed signal processing at
microwave frequencies,2 and in optoelectronics.3–6 Recent
studies on resonant tunneling relaxation oscillators7,8 have
shown generation of 30 ps electrical pulses at a repetition
rate of 1.1 GHz, and harmonic and subharmonic locking. In
an optical waveguide containing a double-barrier RTD
implemented in the AlGaAs material system, our group pre-
viously reported electroabsorption modulation at 900 MHz
with a modulation depth of 7 dB.9 The modulation relies on
a depletion region that is formed in the waveguide core; de-
pending on the bias condition, a substantial part of the ter-
minal voltage may be dropped across this region. From the
current–voltage (I – V) characteristic of the RTD optical
waveguide modulator, it can be seen that small changes of
bias voltage close to the peak-to-valley transition region can
cause large changes in the electric field distribution across
the depleted part of the waveguide core. The electric field
shifts the absorption band edge to longer wavelengths via the
Franz–Keldysh effect10 and therefore changes the transmis-
sion characteristics of the waveguide. Compared to the con-
ventional pn electroabsorption modulator, the advantage of
the RTD modulator is that when dc biased close to the NDR
region, the device behaves as an optical waveguide electro-
absorption modulator integrated with a wide bandwidth elec-
trical amplifier. This letter reports results of high speed light
modulation in a resonant tunneling relaxation oscillator con-
figuration, consisting of a RTD optical waveguide modulator
integrated with a coplanar waveguide transmission line.
The RTD optical waveguide structure was grown by mo-
a!Also with the Centro de Fı´sica do Porto—ADFCUP, Universidade do
Porto, Rua do Campo Alegre 687, 4169-007 PORTO, Portugal. Electronic
mail: jlfiguei@fc.up.pt1190003-6951/99/74(9)/1197/3/$15.00lecular beam epitaxy in a Varian Gen II system, on a semi-
insulating GaAs substrate @Fig. 1~a!#. It consists of two 1.4
nm thick AlAs barriers separated by a 7 nm wide GaAs
quantum well, and two 500 nm thick moderately doped
(Si:231016 cm23) GaAs spacer layers each side of the
double barrier, which are surrounded by heavily doped
(Si:231018 cm23) AlGaAs cladding layers, allowing light
confinement in the direction parallel to the double barrier
plane. A n1 GaAs cap layer was provided for formation of
Au–Ge–Ni ohmic contacts. The ridge waveguides ~2–6 mm
wide! and the large-area mesas each side of the ridges were
FIG. 1. ~a! Schematic diagram of the wafer structure. ~b! The RTD optical
modulator ~RTD-OM! configuration. ~c! Diagram of a packaged device and
measurement setup.7 © 1999 American Institute of Physics
1198 Appl. Phys. Lett., Vol. 74, No. 9, 1 March 1999 Figueiredo et al.fabricated by dry etching. Ohmic contacts ~100–400 mm
long! were deposited on top of the ridges and mesas. A SiO2
layer was deposited, and access contact windows were
etched on the ridge and the mesa electrodes @Fig. 1~b!#, al-
lowing contact to be made to high frequency bonding pads.
After cleaving, the devices were die bonded on packages
containing 50 V coplanar waveguide ~CPW! transmission
lines of different lengths. The device pads were connected to
the package CPW pads via gold wires; a SMA connector was
then soldered to the package CPW line @Fig. 1~c!#. The dc
and rf signals were applied through a 40 GHz bandwidth bias
tee. The dc I – V characteristics of the packaged devices
show typical RTD behavior, with the peak current density
being approximately 13 kA cm22 ~800 mm2 active area! and
a peak-to-valley current ratio around 1.5. Peak voltages were
in the range 2.1–2.5 V and the valley voltage varied from 2.6
to 3.2 V.
The optical characterization employed light from a
Ti:sapphire laser, tunable in the wavelength region around
the absorption edge of the GaAs waveguide ~850–950 nm!.
Light was coupled into the waveguide by a microscope ob-
jective end-fire arrangement. To measure the change in the
optical absorption spectrum induced by the peak-to-valley
transition, a rf signal was injected to switch the RTD be-
tween the extremes of the NDR region, and a photodetector
was used to measure the transmitted light. The band edge
shift was found to be approximately 12 nm. The high-speed
optical response of the modulator was measured with a
streak camera ~Hamamatsu C5680! with a minimum time
resolution of around 2 ps. A part of the injected rf signal
power was required to trigger the streak camera. Figure 1~c!
schematically presents the experimental setup.
Due to the highly nonlinear I – V characteristic in the
NDR region, the RTD is able to generate many high-order
odd harmonics of the injected signal.8 We have studied op-
tical modulation in a RTD optical waveguide relaxation os-
cillator due to injection locking of the RTD to the fundamen-
tal mode of a cavity formed by a transmission line. Optical
modulation at 14 GHz was observed in a packaged device
with a 5 mm long transmission line, dc biased in the NDR
region ~2.25 V!, when a rf signal of 0.4 V amplitude and
frequency around 1 GHz was injected. A packaged device
with a 7 mm long transmission line, again dc biased in the
NDR region ~2.4 V! showed optical modulation at 8 GHz for
an injected signal of 0.6 V amplitude and frequency around 2
GHz. The streak camera traces for the 14 and 8 GHz optical
responses are presented in Fig. 2. In both cases, switching
occurs between the extremes of the NDR region, producing
sinusoidallike oscillating wave forms. At this stage the cavity
comprised only the modulator CPW and the package CPW
terminated with the SMA connector. The left side of the
modulator CPW is unterminated ~open circuit!, as indicated
in Fig. 1~c!. The SMA connector limits the apparatus band-
width to 18 GHz. By inserting coaxial transmission lines
between the packaged device and the bias tee, we also ob-
served modulation at the coaxial transmission lines funda-
mental mode frequency. Figure 3~a! shows a diagram of the
equivalent electrical circuit. The modulation efficiency of the
RTD optical modulator, characterized by the bandwidth-to-
drive-voltage ratio which is defined as the ratio of the opera-tion bandwidth to the operating voltage for the 10 dB modu-
lation depth, is higher than 33 GHz/V.
When scanning the frequency of the injected signal with
amplitude in the range 0.8–1.4 V, it was found that the pack-
aged RTD optical modulator connected through a coaxial
line 30 cm long, produced optical pulses with a full width at
half maximum ~FWHM! of approximately 33 ps, at frequen-
FIG. 2. Streak camera traces of the modulator optical response associated
with locking of the RTD oscillation to the fundamental mode of a package
with two different transmission lines. ~a! Trace corresponding to 14 GHz
optical modulation for a packaged device with a CPW transmission line 5
mm long. ~b! Trace of the 8 GHz modulator optical response for a packaged
device with a CPW transmission line 7 mm long.
FIG. 3. ~a! Schematic diagram of the electrical circuit. ~b! Streak camera
measurement of the 33 ps pulse optical response, showing modulation up to
18 dB.
1199Appl. Phys. Lett., Vol. 74, No. 9, 1 March 1999 Figueiredo et al.cies also associated with the device operation as a relaxation
oscillator.7 Figure 3~b! displays the streak camera trace of an
optical pulse with 33 ps FWHM. According to Ref. 7, the
pulses are caused by the discharge of the RTD capacitance
when the RTD switches from the second positive differential
resistance ~PDR! region to the first PDR region. On a longer
time scale other streak camera results indicate a pulse repeti-
tion rate around 166 MHz, further confirmed by analysis of
the optical signal with a 2 GHz bandwidth photodetector.
The oscillation wave forms were square wavelike. For a 15
cm long coaxial line the pulse repetition rate was around 340
MHz. The trace in Fig. 3~b! shows a maximum modulation
depth of 18 dB, which suggests that the RTD now switches
between two astable points well into the PDR regions. If we
follow the explanation of Ref. 7 and the inset in Fig. 3~b!,
when starting at point A ~the upper dwell zone of the relax-
ation oscillator! the collector is fully depleted and the electric
field is high, giving low transmission. A return pulse forces
the trajectory down below the valley, turning up a few ps
later, toward point B ~the lower dwell zone!. During switch-
ing, the RTD capacitance first discharges, strongly decreas-
ing the electric field across the depleted region, which gives
rise to high transmission. The RTD capacitance starts to re-
charge causing the electric field to rise to the astable value
associated with the first dwell zone. Transmission at point B
is higher than at point A, because the collector region is now
not fully depleted. Switching from B to A depletes the entire
collector region, recharging the RTD capacitance and mak-
ing the transmission drop again. This drop is not evident in
Fig. 3~b! because of the short time scale operation of the
streak camera required to resolve the 33 ps pulse.
In conclusion, integrating a RTD with an optical wave-
guide is a relatively easy way of combining a wide band-width electrical amplifier with an electroabsorption modula-
tor, opening up the possibility of a variety of modes of
operation. A RTD optical waveguide modulator in a relax-
ation oscillator configuration has been demonstrated, provid-
ing optical modulation up to 14 GHz when a 0.4 V amplitude
rf signal around 1 GHz is injected. Relaxation oscillator op-
eration of the RTD produces optical pulses as short as 33 ps
and with a modulation depth up to 18 dB, for a range of
amplitudes and frequencies of the injected signal. At this
stage, development of the device concept appears to be a
promising route toward a high-speed, low power, optoelec-
tronic modulator, with operation extended to 1550 nm pos-
sible by using the InAlGaAs quaternary system.
J. M. L. Figueiredo acknowledges FCT-PRAXIS XXI-
Portugal for his Ph.D. grant.
1 E. R. Brown, J. R. Soderstrom, C. D. Parker, L. J. Mahoney, K. M.
Molvar, and T. C. McGill, Appl. Phys. Lett. 58, 2291 ~1991!.
2 E. Ozbay, D. M. Bloom, D. H. Chow, and J. N. Schulman, IEEE Electron
Device Lett. 14, 400 ~1993!.
3 S. C. Kan, S. Wu, S. Sanders, G. Griffel, and A. Yariv, J. Appl. Phys. 69,
3384 ~1991!.
4 A. F. Lann, E. Grumann, A. Gabai, J. E. Golub, and P. England, Appl.
Phys. Lett. 62, 13 ~1993!.
5 S. C. Kan, P. J. Harshman, K. Y. Lau, Y. Wang, and W. I. Wang, IEEE
Photonics Technol. Lett. 8, 641 ~1996!.
6 T. S. Moise, Y.-C. Kao, C. L. Goldsmith, C. L. Schow, and J. C. Camp-
bell, IEEE Photonics Technol. Lett. 9, 803 ~1997!.
7 E. R. Brown, C. D. Parker, S. Verghese, M. W. Geis, and J. F. Harvey,
Appl. Phys. Lett. 70, 2787 ~1997!.
8 S. Verghese, C. D. Parker, and E. R. Brown, Appl. Phys. Lett. 72, 2550
~1998!.
9 S. G. Mc Meekin, M. R. S. Taylor, and C. N. Ironside, IEE Proc.: Opto-
electron. 143, 12 ~1996!.
10 S. G. McMeekin, M. R. S. Taylor, B. Vo¨gele, C. R. Stanley, and C. N.
Ironside, Appl. Phys. Lett. 65, 1076 ~1994!.


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Optical modulation in a resonant tunneling relaxation oscillator

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