MANY MOLECULAR SPECIES THAT COMPRISE THE INTERSTELLAR MEDIUM HAVE STRONG SPECTRAL FEATURES IN THE 2-5 THZ RANGE AND HETERODYNE SPECTROSCOPY IS REQUIRED TO OBTAIN ~KM/S VELOCITY RESOLUTION TO RESOLVE THEIR COMPLICATED LINESHAPES AND DISENTANGLE THEM FROM THE BACKGROUND. UNDERSTANDING THE KINETICS AND ENERGETICS WITHIN THE GAS CLOUDS OF THE INTERSTELLAR MEDIUM IS CRITICAL TO UNDERSTANDING STAR FORMATION PROCESSES VALIDATE THEORIES OF GALACTIC EVOLUTION AND TO PROBE PROTOPLANETARY DISKS. THE NEXT FRONTIER FOR HETERODYNE SPECTROSCOPY IS THE 2-6 THZ REGION - A SPECTRAL RANGE WHICH IS WELL MATCHED TO THE USE OF TERAHERTZ QUANTUM-CASCADE (QC) LASERS AS LOCAL OSCILLATORS (LOS). THIS PROPOSAL LOOKS BEYOND THE STATE-OF-THE-ART TO THE DEVELOPMENT OF LARGE FORMAT HETERODYNE ARRAYS WHICH CONTAIN ON THE ORDER OF 20-200 ELEMENTS. LO POWERS ON THE ORDER OF 10-100 MW DELIVERED IN A HIGH-QUALITY GAUSSIAN BEAM WILL BE NEEDED TO PUMP THE MIXER ARRAY - NOT ONLY BECAUSE OF THE MIXER POWER REQUIREMENT BUT TO ACCOUNT FOR LARGE LOSSES IN LO COUPLING AND DISTRIBUTION. LARGE FORMAT HETERODYNE ARRAY INSTRUMENTS ARE ATTRACTIVE FOR A DRAMATIC SPEEDUP OF MAPPING OF THE INTERSTELLAR MEDIUM PARTICULARLY ON AIRBORNE PLATFORMS SUCH AS THE STRATOSPHERIC OBSERVATORY FOR INFRARED ASTRONOMY (SOFIA) AND ON LONG DURATION BALLOON PLATFORMS WHERE OBSERVATION TIME IS LIMITED. IN OUR RECENT WORK WE DEMONSTRATED A NEW ARCHITECTURE FOR TERAHERTZ QUANTUM-CASCADE (QC) LASERS CAPABLE OF DELIVERING SCALABLE OUTPUT POWER A NEAR-DIFFRACTION LIMITED OUTPUT BEAM: THE QUANTUM-CASCADE VERTICAL-EXTERNAL-CAVITY-SURFACE-EMITTING-LASER (QCVECSEL). THE ENABLING TECHNOLOGY FOR THIS PROPOSED LASER IS AN AMPLIFYING METASURFACE REFLECTARRAY WHICH IS MADE UP OF A SPARSE ARRAY OF LOW-QUALITY-FACTOR ANTENNA-COUPLED SUB-CAVITIES LOADED WITH QC GAIN MATERIAL. THE SUB-CAVITIES ON THE METASURFACE RADIATE COHERENTLY INTO A HIGH-QUALITY-FACTOR EXTERNAL CAVITY MODE WHICH SETS THE BEAM SHAPE AND ALLOWS FOR SCALABLE POWER COMBINING. IN PREVIOUS WORK WE MADE TWO KEY PROOF-OF-CONCEPT DEMONSTRATIONS: (A) A 3.4 THZ QC-VECSEL WHICH EMITTED CONTINUOUS-WAVE POWER OF 5 MW IN A SINGLE-MODE AND IN A HIGH-QUALITY BEAM AT A TEMPERATURE OF 83 K WITH A WALL PLUG EFFICIENCY OF APPROXIMATELY 0.13% AND (B) THE ABILITY TO CONTINUOUSLY TUNE THE FREQUENCY OF QC-VECSEL BY CHANGING THE CAVITY LENGTH ALBEIT WITH INCONSISTENT BEAM PATTERN POWER LEVELS AND OCCASIONAL MULTI-MODING. OUR OVERARCHING RESEARCH GOAL IN THIS SUCCESSOR PROPOSAL IS DEMONSTRATE THE QC-VECSEL AS A CREDIBLE FREQUENCY-AGILE LOCAL-OSCILLATOR. IN THE FIRST THEME WE PROPOSE TO ACHIEVE 10X IMPROVEMENT IN CONTINUOUS-WAVE WALL-PLUG EFFICIENCY FROM ~0.1% TO 1% AT 77 K TO ALLOW GENERATION OF 10-100 MW WITH 1-10 W POWER DISSIPATION AT FREQUENCIES OF 2.7 AND 4.7 THZ. IN THE SECOND THEME WE PROPOSE TO DEMONSTRATE ROBUST AND REPEATABLE TUNABILITY OF A SINGLE MODE OVER 15% OF ITS CENTER FREQUENCY NEAR 2.7 THZ WHILE MAINTAINING BEAM QUALITY AND POWER. IN THE THIRD THEME WE WILL DEMONSTRATE FREQUENCY-LOCKING OF THE QC-VECSEL TO A MICROWAVE REFERENCE OVER ITS TUNING RANGE WHICH IS NECESSARY TO STABILIZE THE LO OUTPUT TO RESOLVE COMPLICATED LINESHAPES AND ALLOW LONG RECEIVER INTEGRATION TIMES. THIS WILL BE A CRITICAL DEMONSTRATION AS THE VECSEL CAVITY IS VERY DIFFERENT THAN THE MONOLITHIC THZ QC-LASERS THAT HAVE BEEN FREQUENCY/PHASE LOCKED IN THE PAST. FURTHERMORE DEMONSTRATION OF THIS LEVEL OF PERFORMANCE IN TERMS OF POWER EFFICIENCY BEAM QUALITY AND TUNABILITY WILL BE FIRSTS FOR ANY TYPE OF STABILIZED THZ QC-LASER UNDER CONSIDERATION FOR A LOCAL OSCILLATOR AND WOULD ENABLE NEW POSSIBILITIES FOR FREQUENCY-AGILE HETERODYNE INSTRUMENTS THAT COULD ACCESS MULTIPLE LINES OF INTEREST. ADDITIONALLY IN THE COURSE OF THE PROGRAM WE WILL DEMONSTRATE THE FIRST QC-VECSELS AT 4.7 THZ WHICH IS CLOSE TO THE IMPORTANT OI LINE 4.745 THZ (A MAJOR COOLANT FOR PHOTO-DISSOCIATION REGIONS IN MOLECULAR CLOUDS).