Ultrabroadband Electro-Optic Sampling (EOS) with few-cycle optical pulses is known to be an exceptionally sensitive technique to detect electric field amplitudes. By combining this method with dual-comb spectroscopy and with a new class of ultrafast lasers, we perform high-resolution (10-80 MHz, 0.0003-0.0027 cm-1) spectroscopic measurements across the whole frequency range of 1.5 to 45 THz (6.6–200 μm), excluding the strongly absorbing Reststrahlen band of lattice resonances at 4.5–9 THz, with an instantaneous spectral coverage exceeding an octave (e.g., 9–22 μm). As a pump source, we use a pair of mutually coherent low-noise frequency combs centered at 2.35 μm produced by mode-locked solid-state Cr:ZnS lasers. To produce a molecular ‘sensing’ comb in the long-wave infrared region, one of the two driving combs is frequency down-converted via Intrapulse Difference Frequency Generation (IDFG) in ZGP or GaSe nonlinear crystals. The second driving comb is frequency doubled in a GaSe crystal to produce a near-IR comb for EOS. A low intensity and phase noise of our dual-comb system allows capturing a vast amount of comb-mode resolved (mode spacing 80 MHz) spectral information (⪆200,000 comb lines) at up to a video rate of 69 Hz. This result was also facilitated by high IDFG conversion efficiency (e.g., ⪆10% in ZGP crystal). Our long-wavelength IR measurements with low-pressure gases: ethanol, isoprene, and dimethyl sulfide reveal spectroscopic features that had never been explored before.
By using a compact duo of few-cycle Cr:ZnS lasers as a pump source and using optical rectification in ZGP crystal, we demonstrate high-resolution dual-comb spectroscopy in the long-wave infrared (LWIR) region. By recording a sequence of 1500 interferograms, we resolved the comb modes with the finesse exceeding 1000; LWIR spectra of several molecules including methanol, nitrous oxide, and ammonia were recorded in real time (1-10 sec) with 80-MHz (comb spacing) resolution with ~300,000 spectral points (comb modes), and the signal-to-noise ratio of ~100.
We present our recent results on the producing of ultra-broadband frequency combs in the mid-IR – THz range and their applications in dual-comb spectroscopy (DCS) that feature: sub-Doppler resolution, sensitivity down to part-per-billion level, and up to video-rate acquisition speed. We describe two techniques for generating frequency combs: (i) subharmonic generation in a sync-pumped optical parametric oscillator (OPO), which converts the carrier of ultrafast pulses to half that frequency and augments the spectrum to more than one octave, and (ii) optical rectification from few-optical-cycle 2.4- μm mode-locked lasers that produces a frequency downconverted output in the whole range from 1.5 to 50 THz and with the frequency span up to two octaves (e.g. 7.5–30 μm). We describe two approaches to the spectroscopic technique: (i) DCS with two mutually coherent mid-IR beams, and (ii) DCS with electro-optic sampling (EOS).
We report a technique for generation of ultra-broadband coherent femtosecond continua in the infrared. The laser architecture is based on the Cr:ZnS–GaSe and Cr:ZnS–ZGP tandem arrangements that enable simultaneous amplification of ultrashort middle IR pulses and augmentation of pulses’ spectrum via a chain of intrapulse three-wave mixings. The first part of the tandems is based on a single-pass polycrystalline Cr:ZnS amplifier, which is optically pumped by off-the-shelf continuous wave Er-doped fiber laser and outputs 2-cycle pulses with multi-Watt average power at 80 MHz repetition rate, at the central wavelength 2.5 μm. The second stage of the tandems comprises a GaSe or ZGP crystals configured for intrapulse difference frequency generation. The Cr:ZnS–GaSe tandem has allowed us to achieve multi-octave 2–20 μm continuum with 2 W power in the range 2–3 μm and power in excess of 20 mW in the important range 3–20 μm. On the other hand, Cr:ZnS–ZGP tandem features long-wave infrared (6–12 μm) output pulses with record braking sub-Watt power level. Last but not least, Cr:ZnS–GaSe and Cr:ZnS–ZGP IR sources have small footprints and are easily convertible to the optical frequency combs with low carrier-to-envelope timing jitter.
We present a new platform for mid-infrared (MIR) dual-comb spectroscopy, based on a pair of ultra-broadband subharmonic optical parametric oscillators (OPOs) pumped by two phase-locked thulium-fiber combs. Our system provides fast (7 ms for a single interferogram), moving-parts-free, simultaneous acquisition of 350,000 spectral data points, spaced by 115-MHz intermodal interval over 3.1–5.5 μm spectral range. Parallel detection of 22 trace molecular species in a gas mixture, including isotopologues containing such isotopes as 13C, 18O, 17O, 15N, 34S, 33S and 2H (deuterium), with part-per-billion sensitivity and sub-Doppler resolution has been demonstrated. We also show that by utilizing Kerr-lens mode-locked Cr:ZnS lasers operating at λ≈2.35 μm one can create MIR frequency combs spanning almost two octaves in wavelength.
Plasmonic Field Effect Transistor detectors (first proposed in 1996) have emerged as superior room temperature terahertz (THz) detectors. Recent theoretical and experimental results showed that such detectors are capable of subpicosecond resolution. Their sensitivity can be greatly enhanced by applying the DC drain-to-source current that increases the responsivity due to the enhanced non-linearity of the device but also adds 1/f noise. We now propose, and demonstrate a dramatic responsivity enhancement of these plasmonic THz pulse detectors by applying a femtosecond optical laser pulse superimposed on the THz pulse. The proposed physical mechanism links the enhanced detection to the superposition of the THz pulse field and the rectified optical field. A femtosecond pulse generates a large concentration of the electron-hole pairs shorting the drain and source contacts and, therefore, determining the moment of time when the THz induced charge starts discharging into the transmission line connecting the FET to an oscilloscope. This allows for scanning the THz pulse with the strongly enhanced sensitivity and/or for scanning the response waveform after the THz pulse is over. The experimental results obtained using AlGaAs/InGaAs deep submicron HEMTs are in good agreement with this mechanism. This new technique could find numerous imaging, sensing, and quality control applications.
We propose a vertical spiral phase corrector for ring cavity surface emitting (RCSE) quantum cascade lasers (QCLs), which will allow achievement of near-Gaussian generated beam profile. A problem with RCSE QCLs is their donutshaped intensity distribution with a node along the symmetry axis of the ring. This arises because of the π phase difference for the azimuthally polarized rays emitted from opposite elements of the ring. We theoretically demonstrate that near-Gaussian beams can be achieved with a spiral phase shifter that adds one wavelength of additional optical path in going once around the ring. Various three dimensional lithographic techniques for fabricating such a phase shifter, including a grey scale mask, electron-beam resist dose dependency, and two photon induced photopolymerization, are considered. Ring cavity QCLs with the proposed phase corrector will feature better beam quality, larger power, and better resistance to radiative damage in comparison with traditional edge-emitting QCLs.
Robert Peale, Christopher Fredricksen, Andrei Muraviev, Douglas Maukonen, Hajrah Quddusi, Seth Calhoun, Joshua Colwell, Timothy Lachenmeier, Russell Dewey, Alan Stern, Sebastian Padilla, Rolfe Bode
The Planetary Atmospheres Minor Species Sensor (PAMSS) is an intracavity laser absorption spectrometer that uses a mid-infrared quantum cascade laser in an open external cavity for sensing ultra-trace gases with parts-per-billion sensitivity. PAMSS was flown on a balloon by Near Space Corporation from Madras OR to 30 km on 17 July 2014. Based on lessons learned, it was modified and was flown a second time to 32 km by World View Enterprises from Pinal AirPark AZ on 8 March 2015. Successes included continuous operation and survival of software, electronics, optics, and optical alignment during extreme conditions and a rough landing. Operation of PAMSS in the relevant environment of near space has significantly elevated its Technical Readiness Level for trace-gas sensing with potential for planetary and atmospheric science in harsh environments.
The Planetary Atmospheres Minor Species Sensor (PAMSS) is an ultra-trace gas sensor. This paper reports its transition from a Technical Readiness Level of 4 (TRL4) to TRL 5 and an established path forward to TRL6. This report describes tests of PAMSS in chambers that simulate a balloon flight to 30 km. Lessons learned inform a number of improvements, which are being implemented for a balloon flight planned for June 2014.
A mid-infrared intracavity laser absorption spectrometer based on an external cavity multi-mode quantum cascade laser
is combined with a scanning Fabry-Perot interferometer is used as tunable narrow band transmission filter to analyze the
laser emission spectrum. Sensitivity as a trace gas detector at 8.1 micron wavelengths has been demonstrated based on a
weak water vapor line at an absorption coefficient of 1 x 10-5 cm-1. For molecules of reasonably strong absorption cross section (10-17 cm2), this corresponds to a detection limit of 40 ppb.
We present the results of design, fabrication, and characterization of the room-temperature, low electron heat capacity
hot-electron THz microbolometers based on two-dimensional electron gas (2DEG) in AlGaN/GaN heterostructures. The
2DEG sensor is integrated with a broadband THz antenna and a coplanar waveguide. Devices with various patterning of
2DEG have been fabricated and tested. Optimizing the material properties, geometrical parameters of the 2DEG, and
antenna design, we match the impedances of the sensor and antenna to reach strong coupling of THz radiation to 2DEG
via the Drude absorption. Testing the detectors, we found that the THz-induced photocurrent, ΔI, is proportional to the
bias current, I, and the temperature derivative of the resistance and inversely proportional to the area of 2DEG sensor, S.
The analysis allowed us to identify the mechanism of the 2DEG response to THz radiation as electron heating. The
responsivity of our sensors, normalized to the bias current and to unit area of 2DEG, R*= ΔI•S/ (I∙P), is ~ 103 W-1 μm2.
So, for our typical sensor with an area of 1000 μm2 and bias currents of ~ 10 mA, the responsivity is ~ 0.01 A/W. The
measurements of mixing at sub-terahertz frequencies showed that the mixing bandwidth is above 2 GHz, which
corresponds to a characteristic electron relaxation time to be shorter than 0.7 ps. Further decrease of the size of 2DEG
sensors will increase the responsivity as well as allows for decreasing the local oscillator power in heterodyne
applications.
A quantum cascade laser at IR wavelengths with an open external cavity presents an opportunity for spectral sensing of
molecular compounds that have low vapor pressure. The sensitivity of such a system is potentially very high due to
extraordinarily long effective optical paths that can be achieved in an active cavity. We demonstrate here an external
cavity mid-IR QCL molecular absorption sensor using a fixed Fabry-Perot etalon as the spectrum analyzer. The system is
sensitive to the water vapor present in the laboratory air with an absorption coefficient of just 9.6 x 10-8 cm-1. The system
is sensitive enough to detect the absorption coefficient of TNT vapor at room temperature.
Mid-IR spectrometers with adequate resolution for chemical sensing and identification are typically large, heavy, and
require sophisticated non-stationary optical components. Such spectrometers are limited to laboratory settings. We
propose an alternative based on semiconductor micro-fabrication techniques. The device consists of several enabling
parts: a compact broad-band IR source, photonic waveguides, a photon-to-surface-plasmon transformer, a surfaceplasmon
sample-interaction region, and an array of silicon ring-resonators and detectors to analyze the spectrum. Design
considerations and lessons learned from initial experiments are presented.
We present results on design, fabrication, and characterization of hot-electron bolometers based on low-mobility
two-dimensional electron gas (2DEG) in AlInN/GaN and AlGaN/GaN heterostructures. Electrical and optical
characterization of our Hot Electron Bolometers (HEBs) show that these sensors combine (i) high coupling to incident
THz radiation due to Drude absorption, (ii) significant electron heating by the THz radiation due to small value of the
electron heat capacity, (iii) substantial sensitivity of the device resistance to the heating effect. A low contact resistance
(below 0.5 Ω·mm) achieved in our devices ensures that the THz voltage primarily drops across the active region. Due to
a small electron momentum relaxation time, the inductive part of the impedance in our devices is large, so these sensors
can be combined with standard antennas or waveguides. In the capacity of the THz local oscillator (LO) for heterodyne
THz sensing, we fabricated AlGaAs/GaAs quantum cascade lasers (QCLs) with a stable continuous-wave single-mode
operation in the range of 2.5-3 THz. Spectral properties of the QCLs have been studied by means of Fourier transform
spectroscopy. It has been demonstrated that the spectral purity of the QCL emission line doesn't exceed the spectrometer
resolution limit at the level of 0.1 cm-1 (3 GHz). Discrete spectral tuning can be achieved using selective devices; fine
tuning can be done by thermally changing the refractive index of the material and by applied voltage. Compatibility of
the low-mobility 2DEG microbolometers with QCLs in terms of LO power requirements, spectral coverage, and cooling
requirements makes this technology especially attractive for THz heterodyne sensing.
Intracavity Laser Absorption Spectroscopy (ICLAS) at IR wavelengths offers an opportunity for spectral sensing with
sufficient sensitivity to detect vapors of low vapor pressure compounds such as explosives. Reported here are key
enabling technologies for this approach, including multi-mode external-cavity quantum cascade lasers and a scanning
Fabry-Perot spectrometer to analyze the laser mode spectrum in the presence of a molecular intracavity absorber.
Reported also is the design of a compact integrated data acquisition and control system. Applications include military
and commercial sensing for threat compounds, chemical gases, biological aerosols, drugs, and banned or invasive plants
or animals, bio-medical breath analysis, and terrestrial or planetary atmosphere science.
A spectral sensing method with sufficient sensitivity to detect vapors of low vapor-pressure compounds such as
explosives would have great promise for defense and security applications. An opportunity is Intracavity Laser
Absorption Spectroscopy (ICLAS) at IR wavelengths. Our approach is based on multi-mode external-cavity quantum
cascade lasers and a scanning Fabry-Perot spectrometer to analyze the laser mode spectrum in the presence of a narrow
band intracavity absorber. This paper presents results of numerical solution of laser rate equations that support
feasibility of kilometer effective active-cavity path lengths and sensitivity to concentrations of 10 ppb. This is
comparable to the saturated vapor pressure of TNT. System design considerations and first experimental results are
presented at 10 and 70 μm wavelengths.
Gate-voltage tunable plasmon resonances in the two dimensional electron gas of high electron mobility transistors
(HEMT) fabricated from the InGaAs/InP and AlGaN/GaN materials systems are reported. Gates were in the form of a
grating to couple normally incident THz radiation into 2D plasmons. Narrow-band resonant absorption of THz radiation
was observed in transmission for both systems in the frequency range 10 - 100 cm-1. The fundamental and harmonic
resonances shift toward lower frequencies with negative gate bias. Calculated spectra based on the theory developed for
MOSFETs by Schaich, Zheng, and McDonald (1990) agree well with the GaN results, but significant differences for the
InGaAs/InP device suggest that modification of the theory may be required for HEMTs in some circumstances.
Pronounced resonant absorption and frequency dispersion associated with an excitation of collective 2D plasmons have
been observed in terahertz (0.5-4THz) transmission spectra of grating-gate 2D electron gas AlGaN/GaN HEMT (high
electron mobility transistor) structures at cryogenic temperatures. The resonance frequencies correspond to plasmons
with wavevectors equal to the reciprocal-lattice vectors of the metal grating, which serves both as a gate electrode for the
HEMT and a coupler between plasmons and incident terahertz radiation. The resonances are tunable by changing the
applied gate voltage, which controls 2D electron gas concentration in the channel. The effect can be used for resonant
detection of terahertz radiation and for "on-chip" terahertz spectroscopy.
Transmittance spectra of solid and vapor samples of trinitrotoluene (TNT) in the spectral range 0.6 to 10 THz at
resolutions up to 1 GHz are reported. Uniform solid samples of ~100 &mgr;m thickness gave stronger absorption and more
resolved structure than previous studies. New absorption lines for TNT solid below 100 cm-1 are reported. A heated 10
m multpass White cell was used for spectroscopy of the vapor. Strong absorption bands yield unexpectedly large
absorption cross sections for the anticipated saturated vapor pressure at the cell temperature, leaving their assignment to
TNT in doubt. These results indicate that path lengths exceeding 10 m and temperatures higher than 40 C, or
significantly higher instrumental sensitivity, are needed for sensing of TNT vapor in the spectral range 0.6 to 10 THz.
A scanning Fabry-Perot transmission filter composed of a pair of dielectric mirrors has been demonstrated at millimeter
and sub-millimeter wavelengths. The mirrors are formed by alternating quarter-wave optical thicknesses of silicon and
air in the usual Bragg configuration. Detailed theoretical considerations are presented for determining the optimum
design. Characterization was performed at sub-mm wavelengths using a gas laser together with a Golay cell detector and
at mm-wavelengths using a backward wave oscillator and microwave power meter. High resistivity in the silicon layers
was found important for achieving high transmittance and finesse, especially at the longer wavelengths. A finesse value
of 411 for a scanning Fabry-Perot cavity composed of three-period Bragg mirrors was experimentally demonstrated.
Finesse values of several thousand are considered to be within reach. This suggests the possibility of a compact terahertz
Fabry-Perot spectrometer that can operate in low resonance order to realize high free spectral range while simultaneously
achieving a high spectral resolution. Such a device is directly suitable for airborne/satellite and man-portable sensing
instrumentation.
A concept for a terahertz laser in vapor-phase-grown homoepitaxial GaAs with spatially periodic doping profile was theoretically explored. Monte Carlo simulation of hole transport in multilayer delta-doped p-GaAs/GaAs structures in crossed electric and magnetic fields was performed to investigate possibilities of the terahertz amplification on intervalence-band light-to-heavy hole transitions. The results are compared to those calculated for uniformly doped bulk p-GaAs and recently proposed p-Ge/Ge structures. The improvement in the gain for delta-doped p-GaAs structures is about ~2-3 times over bulk p-GaAs. Terahertz laser generation in the considered GaAs device concept appears feasible, as is growth of structures with active thicknesses sufficient to support quasioptical cavity solutions at 100 μm vacuum wavelengths. Potential applications for the considered laser device include sensing of chem/bio agents and explosives, biomedical imaging, non-destructive testing, and communications.
A recently proposed THz laser concept in homoepitaxially grown p-Ge with layered doping is reviewed. Prospects for realizing a similar design in Si or GaAs are considered.
Calculated terahertz gain for periodically delta-doped p-Ge films with vertical and in-plane transport and an orthogonal magnetic field are compared. Gain as a function of structure period, doping concentration, field strength, and temperature is calculated using distributions determined from Monte Carlo simulations. Both transport schemes achieve spatial separation of light holes from impurity layers and the majority of heavy holes, which significantly increases light hole lifetime and gain compared with bulk p-Ge lasers. For in-plane transport, an optimum doping period of 1-2 μm and a 10-fold increase in gain over bulk p-Ge are found. For vertical transport, the optimum period is 300-400 nm, and the gain increase found of 3-5 times bulk values is more modest. However, it is found that gain can persist to higher temperatures (up to 77 K) for vertical transport, while the in-plane transport scheme appears limited to 30-40 K.
A direct-write pulsed Nd:yttrium-aluminum-garnet laser treatment in an aluminum-containing gas was applied to the polished surface of an undoped Ge wafer. After KOH etching to remove metallic aluminum deposited on the surface, secondary ion mass spectroscopy (SIMS) revealed ~60-200 nm penetration for Al at a concentration of ~1017 cm-3. Atomic force microscopy showed that surface roughness is much less than the measured penetration depth. Laser doping of Ge is a potential low cost, selective-area, and compact method, compared with ion-implantation, for production of high current ohmic contacts in Ge and SiGe opto-electronic devices.
A new geometry for the intersubband THz laser on delta-doped multi-layer Ge thin films with in-plane transport of carriers in crossed electric and magnetic fields is proposed. A remarkable increase of the gain compared to existing bulk p-Ge lasers is based on spatial separation of light and heavy hole streams, which helps to eliminate scattering of light holes on ionized impurities and the majority of heavy holes. Inversion population and the gain have been studied using Monte-Carlo simulation. The terahertz transparency of a CVD-grown delta-doped Ge test structure has been experimentally studied by intracavity laser absorption spectroscopy using a bulk p-Ge laser. A practical goal of this study is development of a widely tunable (2-4 THz) laser based on intersubband hole transitions in thin germanium films with the gain sufficient to operate at liquid nitrogen temperatures.
A far-infrared p-type germanium laser with active crystal prepared from ultra pure single-crystal Ge by neutron transmutation doping (NTD) is demonstrated. Calculations show that the high uniformity of Ga acceptor distribution achieved by NTD significantly improves average gain. The negative factor of stronger ionized impurity scattering due to high compensation in NTD Ge is shown to be unremarkable for the gain at moderate doping concentrations sufficient for laser operation. Experimentally, this first NTD laser is found to have lower current-density lasing threshold than the best of a number of melt-doped laser crystals studied for comparison.
We report on development of a turn-key, cryogen-free bulk p-Ge laser which is broadly tunable over the range 1.5 to 4.2 THz. A 4 K closed cycle refrigerator was used to eliminate the need for liquid cryogen. A SmCo permanent magnet assembly provides the necessary magnetic field for the laser. A customized high voltage (HV) power supply and
thyratron pulser were developed to replace the stack of general electronics previously used to operate the laser.
Multi-layer mirrors capable of >99.9% reflectivity at ~100 micron wavelengths were constructed using thin silicon etalons separated by empty gaps. Due to the large difference between the index of refraction of silicon (3.384) and vacuum (1), calculations indicate that only three periods are required to produce 99.9% reflectivity. The mirror was assembled from high purity silicon wafers, with resistivity over 4000 ohm-cm to reduce free carrier absorption. Wafers were double side polished with faces parallel within 10 arc seconds. The multi-layer mirror was demonstrated as a cavity mirror for the far-infrared p-Ge laser.
Monte Carlo simulation of carrier dynamics and far-infrared absorption was performed to test the importance of electron-electron interaction in selectively doped multi-layer p-Ge laser at high doping concentration. The laser design exploits the known widely tunable mechanism of THz amplification on inter-sub-band transitions in p-Ge, but with spatial separation of carrier accumulation and relaxation regions, which allows remarkable enhancement of the gain. The structure consists of doped layers separated by 200 - 500 nm of pure-Ge. Vertical electric field (~ 1 - 2 kV/cm) and perpendicular magnetic field (~ 1 T) provide inversion population on direct intersubband light- to heavy-hole transitions. Heavy holes are found to transit the undoped layers quickly and to congregate mainly around the doped layers. Light holes, due to tighter magnetic confinement, are preferably accumulated within the undoped layers, whose reduced ionized impurity scattering rates allow higher total carrier concentrations, and therefore higher gain, in comparison to bulk p-Ge lasers. Preliminary results of the calculations show a possibility of laser operation at liquid nitrogen temperatures. Device design and diagnostics of CVD grown structure are presented. Combination of total internal reflection and quasi-optical cavity design provides high laser cavity Q.
Optical quenching of the THz inter-sub-band p-Ge laser (tunable in the wavelength range 70-200 micron with ~1W output power) by Nd:YAG laser radiation has been investigated. YAG laser pulses were coupled into a p-Ge laser cavity through a SrTiO3 laser mirror, which is highly reflecting at cryogenic temperatures for THz frequencies and transparent for visible and near-IR light. Fast quenching of the p-Ge laser emission intensity was observed and attributed to free carrier absorption by optically generated electron-hole pairs in a thin layer of the active p-Ge crystal end surface. The effect also occurs when the interband absorption is confined to optically stimulated intracavity Si or GaAs spacers, which are transparent in the far-IR, placed between the SrTiO3 laser mirror and the active crystal end face. Such fast quenching of the p-Ge laser might be used to sharpen the trailing edge of the far-IR emission pulse for time-resolved or cavity-ring-down spectroscopic applications. Direct-gap semiconductor spacers might be used as fast, optically controlled intracavity modulators for active mode-locking.
Monte Carlo simulation of carrier dynamics and far-infrared absorption in a selectively-doped p-type multi-layer Ge structure with vertical transport was performed to test a novel terahertz laser concept. The design exploits the known mechanism of THz amplification on intersubband transitions in p-Ge, but with spatial separation of light hole accumulation regions from doped regions, which allows remarkable enhancement of the gain. The structure consists of doped layers separated by 300-500 nm gaps of pure-Ge. Vertical electric field (~ 1-2 kV/cm) and perpendicular magnetic field (~ 1T) provide inversion population on direct intersubband light- to heavy-hole transitions. Heavy holes are found to transit the undoped layers quickly and to congregate mainly around the doped layers. Light holes, due to tighter magnetic confinement, are preferably accumulated within the undoped layers. There the relatively small ionized impurity and electron-electron scattering rates allow higher total carrier concentrations, and therefore higher gain, than in bulk crystal p-Ge lasers. In contrast to GaAs-based THz quantum cascade lasers (QCL), the robust design and large structure period suggest that the proposed Ge structures might be grown by the technologically-advantageous chemical vapor deposition (CVD) method. The ability of CVD to grow relatively thick structures will simplify the electrodynamic cavity design and reduce electrodynamic losses in future THz lasers based on the presented scheme.
A neutron transmutation doped (NTD) far-infrared p-Ge laser crystal and a melt-grown p-Ge laser are analyzed and compared. Though the doping level in the NTD active crystal is twice lower than optimal, the laser performance is comparable to that produced from high-quality melt-grown crystals because of superior dopant uniformity. Compensation was examined by comparing results of neutron activation analysis with majority carrier concentration. Study of impurity breakdown electric field reveals better crystal quality in NTD. The current saturation behavior confirms the expected higher doping uniformity over melt grown laser rods.
An etched silicon gold plated lamellar mirror is demonstrated as a fixed-wavelength intracavity selector for the far-infrared p-Ge laser, facilitating spectroscopic applications. The depth of the selective mirror, which defines the laser operation wavelength, can be precisely controlled during the etching process. The third-order Fabry-Perot resonance of this selector yields an active cavity finesse of at least 0.06.
Far-infrared p-Ge laser operation in an active crystal prepared by transmutation doping is demonstrated for the first time. Though saturated current density in the prepared active crystal is twice lower than optimal, the laser performance is comparable to that of good lasers made from commercially produced melt grown p-Ge. The current saturation behavior of this material confirms the expected higher doping uniformity over melt grown laser rods.
A thin two-side polished silicon etalon is demonstrated as a fixed-wavelength intracavity selector for the far-infrared p-Ge laser. The active cavity finesse is ~ 0.1. The wavelength position and spectral purity are maintained over a wide range of laser operating fields. A p-Ge laser with such a selector may find application in chemical sensing, THz imaging, or non-destructive testing.
New experimental results are presented for the far-infrared p-Ge laser that enhance its prospects for application to secure satellite and short-range terrestrial free-space communications on a THz carrier. An optical means of gain modulation has been discovered that may potentially permit far-IR pulse generation via active mode-locking with low drive power. A compact high-field permanent-magnet assembly is demonstrated for applying the magnetic field required for laser operation without need of liquid helium. Compact light-weight laser-excitation electronics have been designed to run off a low voltage direct current supply.
Effect of amplification of far-IR radiation on light hole cyclotron resonance in Ge and InSb under the optical pumping by CO2 laser radiation has been calculated using the quantum mechanical model of valence band states in strong magnetic field. The model is based on 6 by 6 Luttinger Hamiltonian for valence band including split-off hole subband. We have found strong resonant dependence of pumping efficiency on magnetic field that is explained by quantum resonance of intersubband absorption of CO2 laser radiation. It was shown that at the optimal magnetic fields the cross-section of the gain can reach 2 X 10-14 cm2 for pumping power density 2 divided by 4 MW/cm2.
The operating principles and experimental results concerning far-IR lasers based on the intersubband hot holes optical transitions in crossed electric and magnetic fields as well as on the optically excited intracenter shallow impurity states are reviewed and discussed. The analysis of the state of the art and the possible directions of the development of p-Ge hot hole intersubband transitions laser and the result of the recent theoretical and experimental investigations of new THz media based on impurity transitions in Si doped by phosphorus are presented.
Wavelength dependent properties of the p-Ge THz laser are reported for pulsed as well as for mode locked operation. The original small mirror laser outcoupler has been replaced by a mesh outcoupler, resulting in clear improvements of laser action. The optical output has been analyzed using a grating spectrometer and fast Schottky diode detectors. FOr 0.25 <EQ B <EQ 0.6T, 170-185 micrometers emission occurs. Laser action starts at short wavelength; during the pulse, longer wavelength components gain intensity, until simultaneous emission across the whole band occurs. With the mesh outcoupler instead of a small mirror, the small signal gain is found to increase, for instance from 0.015 cm-1 to 0.04 cm-1 at 172 micrometers . With the rf field modulation applied, 770 MHz mode locking of the laser is achieved at 172 micrometers , yielding a train of 100 ps FWHM pulses. For 0.5 <EQ B <EQ 1.4T, 75-120 micrometers emission is observed, dependent on both B and E field. Time-and wavelength dependence is complicated; often an oscillatory behavior of spectral components is seen. Although this effect complicates the formation of stable pulse trains under mode locked conditions, 140 ps pulses have been produced.
Electric control of the separation between two interleaved pulse trains from a far-IR p-Ge laser, which is actively mode-locked by rf gain modulation at the second harmonic of the roundtrip frequency, is demonstrated by changing the electric bias at the rf contacts. A suggested application is telemetry using pulse-separation modulation. Optimal operation of the laser on the light-to-heavy-hole transition requires strong, perfectly crossed electric and magnetic fields, but the experimental data reveal a electric-field component along the magnetic field caused by space-charge effects inside the laser crystal, even when the applied fields are perfectly orthogonal. Monte Carlo simulations together with a Poisson solver are used to discuss the various mechanisms behind these effects and to find the electric field inside the laser crystal. These calculations agree reasonably well with experimental data obtained so far, and show not only the significant impact that charging can have on the output of actively mode-locked p-Ge lasers, but also suggest that they strongly influence the average gain of p-Ge lasers in general.
The p-Ge hot hole laser is as yet the only solid state tunable laser with a strong emission in the THz frequency range. Monte Carlo simulations have shown that modelocking of the laser on the intervalence band transition should be possible by gain modulation through the application of an appropriate rf electric field. Recently we did observe for the first time the generation of 200 picosecond pulses in the high frequency (approximately equals 100 cm-1) emission range. Now also pulses as short as 100 ps have been observed in the low frequency regime (approximately equals 50 cm-1). A detailed study of the wavelength dependent optical output of the laser has been started now for (normal) pulsed -- as well as for active mode locked operation. Results on pulse shape and small signal gain in the low frequency (equals low magnetic field) regime are given.
Theoretical proposals concerning submillimeter and far-infrared activity based on shallow acceptors states optical transitions in p-Ge and p-Si semiconductors are discussed. Preliminary experimental investigations will be presented.
The mode spectrum of the p-Ge far-infrared intersubband transition hot hole laser, with several types ofresonator cavity, has been investigated using both grating and Schottky diode spectroscopy. Narrow-bandlasing, with continuous wavelength tunability from 75 pm to 110 pm, has been realised due to intracavityfrequency selection.
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