Single axial-mode selection in a far-infraredp-Ge …

1 Single axial -mode selection in a far-infraredp-Ge laserA. V. Muravjov, S. H. Withers, H. Weidner, and R. C. StrijbosDepartment of Physics, University of Central Florida, Orlando, Florida 32816S. G. Pavlov and V. N. ShastinInstitute for the Physics of Microstructures, Russian Academy of Sciences, Nizhny Novgorod,Russia, 603000R. E. Pealea)Department of Physics, University of Central Florida, Orlando, Florida 32816~Received 6 December 1999; accepted for publication 14 February 2000! Single axial -mode operation of the pulsed far-infraredp-Ge laser with an intracavity Fabry Perottype frequency selector has been observed by means of Fourier-transform spectroscopy.

2 A spectralresolution better than 1 GHz has been achieved on an ordinary continuous-scan spectrometer usingthe event-locked technique for pulsed emission sources. A laser active-cavity finesse of at least unityhas been directly confirmed from the measured emission spectral width. Analysis of the envelope ofthe corresponding interferogram suggests that the finesse exceeds 10. 2000 American Instituteof Physics.@S0003-6951~00!00215-1#A practical goal for the far-infrared~far-IR!p-Ge laser,which operates over the wide frequency range from 50 to140 cm21~ THz!

3 , is tunable Single line or singlemode generation. Operation on magnetic-field tunablecyclotron-resonance lines1and use of intracavity frequencyselection2 5have made considerable progress toward thisgoal. However, Single axial -mode generation, althoughsuggested,2has not been demonstrated until emission inp-Ge lasers occurs on direct op-tical transitions between light- and heavy-hole valence sub-bands in bulkp-Ge at liquid-helium temperatures in strongcrossed electricEand magneticBfields. Population inver-sion is built up via light hole accumulation at the optimalratioE/B, when heavy holes repeatedly emit an optical pho-non after being accelerated beyond the threshold energy 37meV, but light holes move on closed cyclotron orbits belowthis threshold and have a much longer lifetime.

4 The usualpulse duration is a few microseconds with usual peak powersof 1 10 W. A large gain bandwidth (Dv/v;1 ) makes thisactive medium promising for wide range frequency selection , the usual spectrum ofp-Gelaser emission has a width up to 30 cm21. This broad, mul-timode band is tunable in the range 80 140 cm21by chang-ing the appliedEandBfields. At low fields, the band sepa-rates into discrete emission lines, related to shallow-impuritytransitions, down to 50 cm21. The longitudinal mode sepa-ration for a typicalp-Ge laser with an active crystal length ofabout 4 cm and mirrors attached to the ends of the crystal is; cm21~;1 GHz!

5 Thus, in the high-field broadbandregime, the laser generates up to 1000 longitudinal to the results of narrow-band heterodynespectroscopy,6the width of each mode is on the order of 1 MHz, which is defined only by the limitations of the laserpulse duration~1 4ms!. This result is supported by transientrecordings using a fast detector and oscilloscope, where itwas observed that the intensity pattern, which is periodic atthe cavity round-trip frequency, persists throughout the en-tire laser pulse 10 Previous broadband spectro-scopic studies2of wavelength selection using an externalcavity lacked sufficient resolution to distinguish individualaxial modes.

6 The purpose of this letter is to demonstrate,using broadband spectroscopy with higher resolution thanpreviously employed, that thep-Ge laser with such a cavitycan generate a Single axial mode. An active-cavity finesse~defined as the axial mode interval divided by the cavityresonance bandwidth!11greater than unity is conclusivelydemonstrated from the spectral width of the observed emis-sion line. Analysis of the envelope of the corresponding in-terferogram strongly suggests that the finesse is at least means that no satellite spectral emission lines, such astransverse modes, occur farther than;100 MHz from thecentral recently developed method for spectroscopy of low-duty-cycle pulsed emission sources was used in this technique, called event-locked Fourier spectroscopy~ELFS!

7 ,12permitsuseofanordinarycommercialconti nuous-scan Fourier spectrometer with the full spectralrange and resolution allowed by that instrument, even forsources with repetition rates of just 10 Hz and pulse dura-tions of only microseconds, as holds for thep-Ge laser. Todate, ELFS has been used to characterize energy transfer inthe pulsed near-IR luminescence spectra of rare-earth dopedlaser crystals,13 15determine the power spectrum of new quantum fountain lasers emitting at 15mm and pumped at10mm by a pulsed CO2laser,16measure the time resolvedphotoluminesence spectrum of Si strained undu-lated multiquantum-well structures,17and characterize thespectrum ofp-Ge lasers in nonselective.

8 19 ELFShas permitted broadband spectroscopy ofp-Ge laser emis-sion with at least three times better spectral resolution thanpreviously construction of the selective resonator for thep-Gea!Electronic mail: PHYSICS LETTERSVOLUME 76, NUMBER 1510 APRIL 200019960003-6951/2000/76(15)/1996/3/$ 2000 American Institute of PhysicsDownloaded 19 Oct 2002 to Redistribution subject to AIP license or copyright, see is shown in Fig. 1. The operation of such cavities hasbeen discussed ,3 The cavity consisted of a mm3p-Ge crystal and a mm Si spacer withevaporated Al stripes, which are mm wide witha1mmperiod.

9 The backmirror was a tunable Fabry Perot cavitymachined from brass, where the piston is adjusted by a pre-cision screw. The diameter of the output mirror was smallerthan the active crystal cross section to allow output of thegenerated emission. The size of the Si and Ge crystals de-fines the cavity length and gives a mode spacing of 1099 MHz or cm21~ , !.The laser crystal was cut from Single -crystal, Ga-dopedGe with a concentrationNA5731013cm23. The ends of thecrystal were polished and made parallel to each other with 30arc sec accuracy and with better than flatness.

10 Thelateral sides of the crystal were left unpolished to suppresstransverse modes. Crystallographic orientation was such thatBi@110#and Electric field pulses wereapplied to the laser from a thyratron pulser via ohmic Alcontacts evaporated on the mm2lateral sides of thecrystals. The entire system was inserted in a superconductingsolenoid and cooled by liquid helium. The radiation wasguided to the spectrometer emission port by a brass published technical description of ELFS can besummarized as an electronics and software so-lution, ELFS introduces no~or trivial!